Risk-Based Classiﬁcation of Radioactive and Hazardous...

NCRP Report No. 139

Risk-Based Classification ofRadioactive and HazardousChemical Wastes

Recommendations of theNATIONAL COUNCIL ON RADIATIONPROTECTION AND MEASUREMENTS

Issued December 31, 2002

National Council on Radiation Protection and Measurements7910 Woodmont Avenue, Suite 400 / Bethesda, Maryland 20814

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Library of Congress Cataloging-in-Publication Data

Risk-based classification of radioactive and hazardous chemical wastes /National Council on Radiation Protection and Measurements.

p. cm. — (NCRP report ; no. 139)Includes bibliographical references and index.ISBN 0-929600-72-X1. Radioactive waste disposal—Risk assessment. 2. Hazardous wastes—

Risk assessment. 3. Hazardous wastes—Classification. I. National Councilon Radiation Protection and Measurements. II. Series.TD898.14.R57 R5725 2002363.72�89—dc21 2002033766

Copyright © National Council on RadiationProtection and Measurements 2002

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Preface

This Report presents recommendations of the National Councilon Radiation Protection and Measurements (NCRP) on a hazardouswaste classification system that would apply to any waste containingradionuclides or hazardous chemicals. This work faced two majorchallenges. The first was to justify the need to replace the separateclassification systems for radioactive and hazardous chemical wastesin use at the present time. The second was to make the Reportaccessible to different audiences with different levels of expertise inareas of waste classification and health risk assessment. It wasparticularly apparent to the Committee that prepared this Reportthat the radioactive waste community was not generally familiarwith laws and regulations governing chemical waste managementand disposal, approaches to classification of chemical wastes, andmethods of risk assessment for hazardous chemicals. As a conse-quence, this Report is lengthy and contains extensive discussions ofexisting waste classification systems and their deficiencies, methodsof risk assessment, and approaches to risk management, not allof which may be of interest to particular audiences. Although thehazardous waste classification system proposed in this Report issimple in its concepts and principles, the technical issues and thehistory underlying waste classification are complex. NCRP believesthat an appreciation of these complexities should be helpful in under-standing the need of a new system and its benefits.

To address the need of various audiences to understand this Reportat different levels of detail, the Report consists of three essentiallyself-contained parts: a short Synopsis, an extended Technical Sum-mary, and the main Report. The Synopsis presents a brief descriptionof the proposed waste classification system, essentially in the form ofan overview for legislators and other executive-level decision makers.The aim is to show that the system is simple in principle andconcepts, and to illustrate its benefits. The Technical Summary(Section 1) presents an extended discussion of existing hazardouswaste classification systems, difficulties with these systems, andthe proposed classification system. The aim is to fully describe theproposed system and its rationale and benefits, but without muchof the background information on technical and historical details thatsupport the proposal. Many audiences may find that the Technical

iii

iv / PREFACE

Summary meets their needs. The main Report (Sections 2 to 8)presents the complete record of this work, without assuming thepresence of the Technical Summary.

This Report was prepared by Scientific Committee 87-2 on WasteClassification Based on Risk. Serving on Scientific Committee 87-2were:

Allen G. Croff, ChairmanOak Ridge National Laboratory

Oak Ridge, Tennessee

Members

Michael J. Bell Dennis J. PaustenbachInternational Atomic Energy Exponent�

Agency Menlo Park, CaliforniaVienna, Austria Vern C. RogersYoram Cohen Rogers & AssociatesUniversity of California Engineering CorporationLos Angeles, California Salt Lake City, Utah

Leonard C. Keifer Andrew Wallo, IIIU.S. Environmental Protection U.S. Department of Energy

Agency Washington, D.C.Washington, D.C.

David C. KocherSENES Oak Ridge, Inc.Oak Ridge, Tennessee

NCRP Secretariat

E. Ivan White, Senior Staff Scientist

The Council wishes to express its appreciation to the Committeemembers for the time and effort devoted to the preparation of thisReport.

Thomas S. TenfordePresident

Contents

Preface ........................................................................................ iii

Synopsis ..................................................................................... 1

1. Technical Summary ............................................................ 51.1 Introduction ................................................................... 51.2 Purpose and Scope of Study ......................................... 61.3 Summary of Existing Waste Classification Systems .. 7

1.3.1 Radioactive Waste Classification in the UnitedStates ................................................................... 71.3.1.1 Classification of Fuel-Cycle Wastes ...... 81.3.1.2 Subclassifications of Fuel-Cycle Wastes... 131.3.1.3 Other Radioactive Wastes ..................... 131.3.1.4 Exempt Radioactive Wastes .................. 141.3.1.5 Deficiencies in the Radioactive Waste

Classification System ............................. 151.3.2 Other Radioactive Waste Classification

Systems ............................................................... 171.3.3 Classification of Hazardous Chemical Wastes . 201.3.4 Comparison of Classification Systems for

Radioactive and Hazardous Chemical Wastes . 221.3.5 Mixed Radioactive and Hazardous Chemical

Wastes ................................................................. 241.4 Approach to Development of a New Waste

Classification System .................................................... 251.4.1 Basic Elements of Hazardous Waste

Classification System ......................................... 261.4.2 Assumptions in Developing the Waste

Classification System ......................................... 271.4.3 Challenges in Developing a Waste

Classification System ......................................... 281.5 Development of the Recommended Waste

Classification System .................................................... 291.5.1 Risk Index for Waste Classification .................. 291.5.2 Generic Exposure Scenarios for Waste

Classification ....................................................... 321.5.3 Determination of Allowable Risk or Dose ......... 331.5.4 Recommended Framework for Risk-Based

Waste Classification ........................................... 37

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1.5.4.1 Exempt Waste ........................................ 371.5.4.2 Low-Hazard Waste ................................ 411.5.4.3 High-Hazard Waste ............................... 43

1.5.5 Calculation of the Risk Index ............................ 441.5.5.1 Measure of Risk for Carcinogens .......... 441.5.5.2 Estimates of Probability Coefficients

for Carcinogens ...................................... 441.5.5.3 Thresholds for Deterministic Effects .... 461.5.5.4 Risk Index for Mixtures of Hazardous

Substances .............................................. 481.5.5.4.1 Risk Index for Mixtures of

Substances That CauseStochastic Effects(Carcinogens) .......................... 48

1.5.5.4.2 Risk Index for Mixtures ofSubstances That CauseDeterministic Effects(Noncarcinogens) .................... 49

1.5.5.4.3 Use of the Composite RiskIndex in Classifying Waste .... 50

1.6 Implications of the Recommended WasteClassification System .................................................... 511.6.1 Classification of Existing Hazardous Wastes ... 511.6.2 Subclassification of Basic Waste Classes .......... 521.6.3 Legal and Regulatory Implications ................... 53

1.7 Further Development of the Recommended WasteClassification System .................................................... 54

2. Introduction ......................................................................... 572.1 Foundations and Directions ......................................... 57

2.1.1 Definition of Waste Classification ..................... 582.1.2 Purpose of Waste Classification ........................ 602.1.3 Bases for Waste Classification ........................... 622.1.4 Shortcomings of Current Waste Classification

Systems ............................................................... 642.1.5 Focus on Classification of Waste ....................... 662.1.6 Classification of Waste for Purposes of

Disposal ............................................................... 662.2 Limits and Relationships .............................................. 67

2.2.1 Regulatory Implications ..................................... 672.2.2 Risk Management ............................................... 672.2.3 Waste Classification in a Continuum of Waste

Compositions ....................................................... 682.2.4 Subclassifications of Basic Waste Classes ........ 68

CONTENTS / vii

2.2.5 Site-Specific Risk ................................................ 692.2.6 Ecological and Other Potential Impacts ........... 69

2.3 Conceptual Framework of This Report ....................... 70

3. Technical Background on Risk Assessment and RiskManagement ......................................................................... 72

3.1 Assessment of Risk ....................................................... 733.1.1 Definition of Risk ................................................ 733.1.2 Types of Responses from Exposure to

Hazardous Substances ....................................... 743.1.3 Definition of Risk Assessment ........................... 753.1.4 Risk Assessment Process ................................... 75

3.1.4.1 Hazard Identification ............................. 763.1.4.1.1 Radiation Hazard

Identification ........................... 763.1.4.1.2 Chemical Hazard

Identification ........................... 763.1.4.2 Dose-Response Assessment ................... 883.1.4.3 Exposure Assessment ............................ 883.1.4.4 Risk Characterization ............................ 923.1.4.5 Risk Management .................................. 94

3.1.5 Use of Risk Assessment in Risk-Based WasteClassification ....................................................... 953.1.5.1 Risk Assessment of a Generic Site ....... 953.1.5.2 Dose-Response Relationships ................ 99

3.2 Assessment of Responses from Exposure toHazardous Substances .................................................. 993.2.1 Assessment of Responses from Exposure to

Hazardous Chemicals ......................................... 1003.2.1.1 Basis for a Dose-Response Assessment ... 1003.2.1.2 Dose-Response Assessment for Chemicals

That Cause Deterministic Effects ........ 1023.2.1.2.1 Dose-Response Concepts ........ 1033.2.1.2.2 Safety Factor Approach for

Chemicals That CauseDeterministic Effects ............. 104

3.2.1.2.3 Selection of the Database ........ 1053.2.1.2.4 Determination of the

Reference Dose ....................... 1063.2.1.2.5 Selection of Uncertainty and

Modifying Factors .................. 1083.2.1.2.6 Assigning Confidence Levels .... 1093.2.1.2.7 Mathematical Modeling and

the Benchmark Dose Method 109

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3.2.1.3 Dose-Response Assessment for ChemicalsThat Cause Stochastic Effects .............. 1113.2.1.3.1 Introduction to Mathematical

Modeling for Chemicals ThatCause Stochastic Effects ........ 112

3.2.1.3.2 Statistical Models ................... 1133.2.1.3.3 Benchmark Dose Method ....... 1153.2.1.3.4 Pharmacokinetic Models ........ 1173.2.1.3.5 Biologically-Based Models of

Cancer ..................................... 1193.2.1.3.6 Use of Stochastic Modeling

Results .................................... 1203.2.1.4 Characterization of Dose-Response

Estimates ................................................ 1223.2.1.5 Uncertainties and Deficiencies in

Dose-Response Assessment ................... 1233.2.1.5.1 Uncertainties in Dose-

Response Assessment ............. 1243.2.1.5.2 Deficiencies in Dose-Response

Assessment ............................. 1253.2.2 Assessment of Responses from Radiation

Exposure .............................................................. 1293.2.2.1 Deterministic Responses from

Radiation Exposure ............................... 1313.2.2.2 Databases and Methods of Dose-

Response Assessment for StochosticEffects ...................................................... 131

3.2.2.3 Measures of Radiation-InducedResponses ................................................ 1343.2.2.3.1 Measures of Deterministic

Responses ................................. 1343.2.2.3.2 Measures of Stochastic

Responses ................................. 1343.2.2.3.3 Effective Dose .......................... 138

3.2.3 Comparison of Dose-Response Assessments forRadionuclides and Chemicals .............................. 1403.2.3.1 Deterministic Responses ......................... 1413.2.3.2 Stochastic Responses ............................... 142

3.3 Approaches to Risk Management for Radionuclidesand Hazardous Chemicals That Cause StochasticEffects ............................................................................. 1453.3.1 Radiation Paradigm for Risk Management of

Stochastic Responses ........................................... 1463.3.2 Chemical Paradigm for Risk Management of

Stochastic Responses ........................................... 150

CONTENTS / ix

3.3.3 Comparison of the Radiation and ChemicalParadigms ............................................................ 155

3.3.4 Reconciliation of the Radiation and ChemicalParadigms ............................................................ 158

3.3.5 Application of Risk Management Paradigms toWaste Classification ............................................. 160

3.4 Summary ........................................................................ 160

4. Existing Classification Systems for Hazardous Wastes 1654.1 Classification and Disposal of Radioactive Waste ......... 166

4.1.1 Background .......................................................... 1664.1.2 Radioactive Waste Classification in the United

States .................................................................... 1674.1.2.1 Introduction ............................................. 1674.1.2.2 Early Descriptions of Radioactive Waste

Categories ................................................ 1724.1.2.2.1 Liquid Wastes .......................... 1724.1.2.2.2 Solid Wastes ............................. 1734.1.2.2.3 Summary of Bases for Early

Descriptions of RadioactiveWastes ...................................... 175

4.1.2.3 Classification and Disposal of Wastesfrom the Nuclear Fuel Cycle ................... 1754.1.2.3.1 High-Level Waste and Spent

Fuel .......................................... 1764.1.2.3.2 Transuranic Waste .................. 1824.1.2.3.3 Low-Level Waste ...................... 1874.1.2.3.4 Uranium or Thorium Mill

Tailings .................................... 1914.1.2.3.5 Characteristics of the System

for Classification and Disposalof Fuel-Cycle Waste ................ 192

4.1.2.4 Naturally Occurring and Accelerator-Produced Radioactive Material ............. 194

4.1.2.5 Exempt Radioactive Waste ................... 1964.1.2.5.1 Concepts and Definitions ....... 1964.1.2.5.2 Exemption Levels for

Radioactive Waste .................. 1974.1.2.5.3 NCRP Recommendation on a

Negligible Individual Dose ...... 1994.1.2.5.4 Summary of Exemptions for

Radioactive Waste in theUnited States .......................... 199

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4.1.2.6 Proposals for Alternative RadioactiveWaste Classification Systems ............... 2004.1.2.6.1 NRC Discussion on Definition

of High-Level Waste ............... 2004.1.2.6.2 Generally Applicable Waste

Classification SystemProposed by Kocher andCroff ......................................... 200

4.1.2.6.3 Generally Applicable WasteClassification SystemProposed by Smith andCohen ....................................... 202

4.1.2.6.4 Waste Classification SystemProposed by LeMone andJacobi ...................................... 203

4.1.3 IAEA Recommendations on Radioactive WasteClassification and Exemption Principles .......... 2044.1.3.1 Recommendations on Waste

Classification .......................................... 2044.1.3.2 Recommendations on Exemption

Principles ................................................ 2084.1.4 Comparison of the United States and IAEA

Radioactive Waste Classification Systems ........ 2094.2 Classification and Disposal of Hazardous Chemical

Waste .............................................................................. 2114.2.1 Classification System for Hazardous Chemical

Waste Under the Resource Conservation andRecovery Act ......................................................... 2114.2.1.1 Description of EPA’s Hazardous Waste

Classification System ............................. 2124.2.1.2 Discussion of EPA’s Hazardous Waste

Classification System ............................. 2144.2.1.3 State Programs ...................................... 216

4.2.2 Treatment and Disposition of HazardousChemical Waste .................................................. 217

4.3 Regulation of Mixed Radioactive and HazardousChemical Waste ............................................................. 2194.3.1 Introduction ........................................................ 2204.3.2 Establishing Dual Regulation of Mixed Waste 2214.3.3 Facilitating Compliance with Dual Regulation

of Mixed Low-Level Waste ................................. 2244.3.4 Dual Regulation of Other Fuel-Cycle Wastes .. 230

4.3.4.1 High-Level Waste .................................. 2304.3.4.2 Transuranic Waste ................................ 2314.3.4.3 Uranium or Thorium Mill Tailings ...... 232

CONTENTS / xi

4.3.5 Dual Regulation of Naturally Occurring andAccelerator-Produced Radioactive MaterialWaste ................................................................... 232

4.3.6 Summary of Mixed Waste Issues ...................... 2334.4 NCRP Recommendations Relevant to Waste

Classification ................................................................. 2354.4.1 Recommendations on Radiation Protection of

the Public ............................................................ 2354.4.1.1 Radiation Dose Limits ........................... 2354.4.1.2 Negligible Individual Dose .................... 2374.4.1.3 Application of NCRP

Recommendations to WasteClassification .......................................... 237

4.4.2 Comparative Carcinogenicity of IonizingRadiation and Chemicals ................................... 237

4.5 Summary ........................................................................ 239

5. Desirable Attributes of a Waste Classification System .. 2435.1 Risk-Based ..................................................................... 2435.2 Exemption ...................................................................... 2465.3 Comprehensive .............................................................. 2475.4 Consistent ...................................................................... 2485.5 Intrinsic .......................................................................... 2505.6 Comprehensible ............................................................. 2515.7 Quantitative ................................................................... 2535.8 Compatible ..................................................................... 2545.9 Flexible ........................................................................... 254

6. Principles and Framework for a Comprehensiveand Risk-Based Hazardous Waste ClassificationSystem .................................................................................... 256

6.1 Issues of Risk Assessment and Risk Management ..... 2586.1.1 Measures of Response from Exposure to

Hazardous Substances ....................................... 2586.1.1.1 Measures of Response for Substances

Causing Deterministic Responses ........ 2596.1.1.2 Measures of Response for Substances

Causing Stochastic Responses .............. 2596.1.1.2.1 Incidence ................................. 2596.1.1.2.2 Fatalities ................................. 2616.1.1.2.3 ICRP’s Total Detriment ......... 261

6.1.1.3 Recommendations on Selection of aMeasure of Response ............................. 262

6.1.2 Dose-Response Relationships ............................. 263

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6.1.2.1 Deterministic Responses ....................... 2646.1.2.2 Stochastic Responses ............................. 265

6.1.3 Exposure Scenarios for Waste Classification ... 2666.1.4 Approaches to Risk Management ...................... 2686.1.5 Legal and Regulatory Constraints .................... 269

6.2 Framework for Risk-Based Waste Classification ........ 2706.2.1 Framework of the Proposed Waste

Classification System ......................................... 2706.2.2 Framework for Waste Classification ................. 271

6.2.2.1 Exempt Waste ........................................ 2726.2.2.2 Nonexempt Waste .................................. 273

6.2.2.2.1 Low-Hazard Waste ................. 2736.2.2.2.2 High-Hazard Waste ............... 274

6.3 Development of the Risk Index for IndividualHazardous Substances .................................................. 2746.3.1 Establishing Allowable Risks or Doses of

Individual Substances ........................................ 2766.3.1.1 Establishing Allowable Doses of

Substances That Cause DeterministicResponses ............................................... 2766.3.1.1.1 Dose Corresponding to a

Negligible Risk ....................... 2766.3.1.1.2 Dose Corresponding to an

Acceptable Risk ...................... 2776.3.1.2 Establishing Allowable Risks or Doses

of Substances That Cause StochasticResponses ............................................... 2786.3.1.2.1 Establishing a Negligible

Risk or Dose ............................ 2786.3.1.2.2 Establishing an Acceptable

Risk or Dose ............................ 2796.3.2 Developing Exposure Scenarios for Purposes

of Waste Classification ....................................... 2806.3.2.1 Exposure Scenarios for Classifying

Exempt Waste ........................................ 2816.3.2.2 Exposure Scenarios for Classifying

Low-Hazard Waste ................................ 2816.3.2.3 Classification as High-Hazard Waste ... 282

6.3.3 Application of the Modifying Factor inRisk Index ........................................................... 283

6.4 Development of the Composite Risk Index forMultiple Substances ...................................................... 2856.4.1 Risk Indexes for Mixtures of Hazardous

Substances ........................................................... 285

CONTENTS / xiii

6.4.1.1 Risk Index for Multiple SubstancesThat Cause Stochastic Responses ........ 286

6.4.1.2 Risk Index for Multiple SubstancesThat Cause Deterministic Responses ... 288

6.4.2 Composite Risk Index for All HazardousSubstances ........................................................... 291

6.4.3 Implications of the Framework for Calculatingthe Risk Index ..................................................... 292

6.4.4 Example Calculations of the Risk Index .......... 2936.4.5 Establishing a Waste Classification System

Based on the Framework and Risk Index ........ 2956.4.5.1 Process of Implementing the Waste

Classification System ............................. 2956.4.5.2 Time When Waste Should Be

Classified ................................................ 2976.4.5.3 Time Frame for Risk Assessment in

Classifying Waste .................................. 2986.4.5.4 Implementation of the Waste

Classification System Over Time .......... 3006.4.6 Shortcomings and Advantages of the Risk

Index .................................................................... 3006.5 Expected Classification of Existing Wastes ................ 301

6.5.1 Wastes Expected to be Classified as Exempt ... 3026.5.2 Wastes Expected to be Classified as Low-Hazard .. 3026.5.3 Wastes Expected to be Classified as High-

Hazard ................................................................. 3046.6 Subclassification of Basic Waste Classes .................... 3056.7 Future Development Needs for Risk-Based Waste

Classification ................................................................. 3086.7.1 Standardization of Nomenclature ..................... 3096.7.2 Approaches to Estimating Dose-Response

Relationships for Radionuclides andHazardous Chemicals ......................................... 3096.7.2.1 Approaches to Estimating Dose-

Response Relationships for SubstancesThat Cause Stochastic Responses ........ 310

6.7.2.2 Approaches to Estimating Dose-Response Relationships for SubstancesThat Cause Deterministic Responses ... 311

6.7.3 Allowable Risks from Exposure to SubstancesThat Cause Stochastic or DeterministicEffects .................................................................. 312

6.7.4 Selection of Exposure Scenarios ........................ 3136.7.5 Legal and Regulatory Development Needs ....... 314

6.8 Summary of the Proposed Risk-Based WasteClassification System .................................................... 317

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7. Implications of the Recommended Risk-Based WasteClassification System ......................................................... 322

7.1 Example Applications of the Risk-Based WasteClassification System .................................................... 3227.1.1 General Approach to the Example Applications .. 323

7.1.1.1 Exempt Waste ........................................ 3247.1.1.2 Low-Hazard Waste ................................ 3257.1.1.3 High-Hazard Waste ............................... 3267.1.1.4 Development of Examples ..................... 326

7.1.2 Consideration of Exempt Wastes ...................... 3267.1.2.1 Radioactive Wastes ................................ 3277.1.2.2 Hazardous Chemical Wastes ................ 328

7.1.3 DOE Low-Level Radioactive Waste .................. 3287.1.3.1 Classification by Calculation of Total

Dose ......................................................... 3287.1.3.2 Classification Using Pre-Established

Limiting Concentrations ....................... 3297.1.3.3 Classification Using Pre-Established

Limiting Concentrations andEnhanced Access .................................... 330

7.1.3.4 Alternative Exposure Scenarios ............ 3317.1.4 Average Commercial Low-Level Radioactive

Waste ................................................................... 3327.1.5 Typical Uranium Mill Tailings .......................... 3337.1.6 Residues from Processing of High-Grade

Uranium Ore ....................................................... 3357.1.7 Mixed Waste: Electric Arc Furnace Dust ......... 336

7.1.7.1 Introduction to Analysis ........................ 3367.1.7.2 Evaluation as Exempt Waste ................ 3397.1.7.3 Approach to Example Analysis ............. 3397.1.7.4 Deterministic Risk Index for

Hazardous Chemical Constituents ....... 3397.1.7.5 Stochastic Risk Index for Hazardous

Chemical Constituents .......................... 3427.1.7.6 Stochastic Risk Index for

Radionuclides ......................................... 3437.1.7.7 Calculation of the Composite

Risk Index .............................................. 3447.1.7.8 Consideration of Alternative

Assumptions ........................................... 3447.1.8 Hazardous Chemical Waste ............................... 3467.1.9 Discussion of Example Analyses ....................... 347

7.2 Legal and Regulatory Ramifications ............................ 3487.2.1 Establishment of an Exempt Waste Class ....... 348

CONTENTS / xv

7.2.2 Elimination of Source-Based WasteClassifications ..................................................... 349

7.2.3 Recognition of Permanent Disposal ofHazardous Chemical Wastes ............................. 349

7.2.4 Establishing the Potential for High-HazardChemical Wastes ................................................ 350

7.2.5 Elimination of the Mixed Waste Category ....... 3517.2.6 Elimination of the Category of Waste

Containing Naturally Occurring andAccelerator-Produced Radioactive Material .... 352

7.2.7 Impact on Subclassification of Waste Classes .. 352

8. Conclusions and Recommendations .............................. 3548.1 Conclusions .................................................................... 3548.2 Recommendations .......................................................... 358

Glossary ...................................................................................... 360

Acronyms ................................................................................... 377

References ................................................................................. 379

The NCRP .................................................................................. 401

NCRP Publications .................................................................. 410

Index ........................................................................................... 420

Synopsis

This Report presents recommendations of the National Councilon Radiation Protection and Measurements (NCRP) on a new systemfor classifying waste that contains hazardous substances, eitherradionuclides or hazardous chemicals. NCRP’s recommendationsincorporate three principles.

First, the classification system is generally applicable toany waste that contains radionuclides, hazardous chemicals,or mixtures of the two. Over the last two decades, the separateand quite different systems for classifying and managing radioactiveand chemical wastes have led to considerable difficulties in managingmixed wastes that contain both types of hazardous substances. Forthe most part, however, the development of different approaches toclassifying and managing waste was not driven by differences inthe properties of radionuclides and hazardous chemicals or theirpotential risks to human health. Thus, there is no evident needof separate systems for classifying and managing radioactive andchemical wastes.

Second, waste that contains hazardous substances is classi-fied based on considerations of health risks to the public thatarise from waste disposal. The existing classification systems forradioactive and chemical wastes in the United States are not basedprimarily on considerations of health risks to the public. Rather,classification of hazardous wastes has been based primarily on thesource of the waste or the presence of particular hazardous sub-stances. The absence of risk-based waste classifications has had anumber of undesirable ramifications:

● Some wastes are managed more stringently and at higher costthan warranted by the health risks they pose. For example,lower-activity high-level radioactive wastes have been managedat high cost as if they require disposal in a geologic repositoryto protect public health, even though they are less hazardousthan some low-level wastes that are managed at much less costand are considered acceptable for near-surface disposal. Otherwastes may be managed less stringently than warranted, suchas wastes from mining and energy exploitation activities thatcontain elevated levels of hazardous chemicals, especially heavymetals, and naturally occurring radionuclides.

1

2 / SYNOPSIS

● Some wastes that pose similar health risks are placed in differ-ent classes (e.g., high-level, transuranic, and high-activity,longer-lived low-level radioactive wastes), whereas wastes thatpose risks ranging from innocuous to highly hazardous may beplaced in the same class (e.g., Class-A and greater-than-Class-Clow-level radioactive wastes, diluted and highly concentratedlisted hazardous chemical wastes). Such inconsistencies do notseem sensible to a nonexpert, and they may increase suspicionand mistrust of authorities by the public, which can only exac-erbate the already difficult challenge of managing hazardouswastes.

Third, the hazardous waste classification system includesan exempt class of waste. Waste in this class would pose a suffi-ciently low risk that it could be managed in all respects as if it werenonhazardous material. Wastes that contain low levels of hazardoussubstances have been exempted on a case-by-case basis, but existingwaste classification systems do not include generally applicableexemption principles. As a consequence, many slightly contaminatedwaste materials are managed at considerable cost as if they werehazardous. If an exempt class of waste were established, recyclingand reuse of exempt materials, such as scrap metals at nuclearfacilities, could be considered, thus saving valuable resources forother needs and reducing impacts on the environment.

Based on these principles, the hazardous waste classification sys-tem recommended by NCRP includes three classes of waste: exempt,low-hazard, and high-hazard waste. Each waste class is defined inrelation to the type of disposal system (technology) that is expectedto be generally acceptable in protecting public health as follows:

● Exempt waste: any waste containing hazardous substancesthat is generally acceptable for disposition as nonhazardousmaterial (e.g., disposal in a municipal/industrial landfill for non-hazardous waste).

● Low-hazard waste: any nonexempt waste that is generallyacceptable for disposal in a dedicated near-surface facility forhazardous wastes.

● High-hazard waste: any nonexempt waste that generallyrequires a disposal system more isolating than a dedicated near-surface facility for hazardous wastes (e.g., a geologic repository).

Given these conceptual definitions, NCRP recommends that theboundaries of waste classes should be quantified in terms of limits

SYNOPSIS / 3

on concentrations of hazardous substances based on the followingprinciples:

● Waste would be classified as exempt if the concentrations ofhazardous substances are sufficiently low that it poses no morethan a negligible risk to a hypothetical inadvertent intruderat a municipal/industrial landfill for nonhazardous waste. Anegligible risk, or the associated dose, is a value so low thatfurther efforts at risk reduction generally are unwarranted (e.g.,an excess lifetime cancer risk less than about 10�4 or doses ofnoncarcinogenic hazardous substances substantially less thannominal thresholds for induction of health effects in the generalpopulation).

● Waste that exceeds concentration limits for exempt waste wouldbe classified as low-hazard if it poses no more than an acceptable(i.e., barely tolerable) risk to a hypothetical inadvertent intruderat a dedicated near-surface disposal facility for hazardouswastes, with the important condition that an acceptable riskor dose used to determine low-hazard waste should be substan-tially higher than a negligible risk or dose used to determineexempt waste.

● Waste that exceeds concentration limits for low-hazard wastewould be classified as high-hazard.

NCRP recognizes that potential risks to residents near waste dis-posal sites are a primary consideration in determining acceptabledisposal practices for hazardous wastes. Because such risks arehighly site-specific, they do not provide a suitable basis for generallyclassifying waste. However, NCRP expects that nearly all exemptand low-hazard waste would be acceptable for disposal using theintended technology at well chosen sites when potential risks tonearby residents are considered in the process of licensing or permit-ting specific disposal facilities.

Based on suitable precedents for defining negligible and acceptable(barely tolerable) risks to the public from exposure to hazardoussubstances, the proposed classification system should be largely con-sistent with current classifications of hazardous wastes and plansfor their disposal. For example: most low-level radioactive waste andhazardous chemical waste would be classified as low-hazard; andmost high-level, transuranic, and high-activity, longer-lived low-level radioactive waste would be classified as high-hazard. However,some hazardous chemical wastes, especially wastes that contain highconcentrations of heavy metals, might not be generally acceptablefor near-surface disposal and, thus, would be classified as high-hazard. The existing classification system for hazardous chemical

4 / SYNOPSIS

waste does not include such a class, and there are no planned alterna-tives to near-surface disposal for highly hazardous chemical wastes.

NCRP believes that the proposed hazardous waste classificationsystem offers significant advantages compared with the classificationsystems currently used in the United States. In addition to address-ing deficiencies in the existing classification systems for radioactiveand hazardous chemical wastes and promoting more cost-effectivemanagement of all hazardous wastes, the proposed approach to clas-sification of waste based on risk is simple and understandable. Theclear association of hazardous waste classification with requirementsfor protection of public health, which is lacking in the existing classi-fication systems, should serve to increase public confidence in wastemanagement and disposal activities.

1. Technical Summary

1.1 Introduction

Waste is any material that has insufficient value to justify furtherbeneficial use, and thus must be managed at a cost. Wastes thatcontain hazardous substances, either radionuclides or toxic chemi-cals, are generated by many human activities. Management anddisposal of these wastes must be conducted in ways that protecthuman health. Because hazardous wastes vary widely in their com-positions and concentrations of hazardous substances and in theirpotential impacts on human health, the need to protect human healthis met most efficiently by use of a variety of technological approachesto waste management and disposal, rather than a single approachfor all wastes.

Management and disposal of the wide variety of hazardous wasteshas been aided by the development of waste classification systems.The term waste classification refers to broadly defined waste cate-gories related, for example, to properties of waste materials, poten-tial risks to human health that arise from waste management ordisposal, or the source of the waste. Ideally, hazardous wastes inthe same class should pose similar risks to human health and, thus,require similar approaches to safe management and disposal.

The primary purpose of waste classification systems is to facilitatedevelopment of efficient strategies for waste management and dis-posal, such as planning for waste treatment and disposal capacity,by identifying wastes that could be managed and disposed of safelyusing essentially the same technologies. Waste classification alsofacilitates communication among organizations that generate, man-age, or dispose of waste, regulatory authorities, and the public.

NCRP emphasizes, however, that waste classification does notprovide a substitute for establishing requirements on treatment anddisposal of specific wastes at specific sites, requirements on remedia-tion of contaminated sites, or decisions by regulatory authoritiesabout the acceptability of any such activities. The acceptability ofparticular waste management or disposal activities must be based onsite-specific assessments of risks posed by well characterized wastes.Waste classification, although useful, can only inform the process of

5

6 / 1. TECHNICAL SUMMARY

selecting safe and cost-effective approaches to waste managementand disposal.

Over the last several decades, separate classification systems havebeen developed for radioactive and hazardous chemical wastes basedon a variety of considerations, the most prevalent being the sourceof the waste. These classification systems have served their intendedpurpose of facilitating development of health-protective strategiesfor waste management and disposal reasonably well. However, theyhave exhibited a number of shortcomings and undesirable ramifica-tions, which indicate that a new approach to classification of hazard-ous wastes would be beneficial.

1.2 Purpose and Scope of Study

This Report is concerned with classification of hazardous wastes.Wastes are materials deemed to have no further beneficial use totheir present custodian, although these materials may be useful toothers. Unless otherwise indicated, the term ‘‘hazardous’’ as usedin this Report refers to the presence of radionuclides, hazardouschemicals, or both. This term also may refer to certain characteristicsof materials that pose a hazard, such as ignitability, corrosivity,or reactivity.

The primary purpose of this Report is to present NCRP’s recom-mendations on classification of hazardous wastes. The Report isdirected at a multidisciplinary audience with different levels of tech-nical understanding in the fields of radiation and chemical riskassessment and radioactive and chemical waste management. A newhazardous waste classification system is proposed that differs fromthe existing classification systems for radioactive and hazardouschemical wastes in two fundamental respects. First, hazardouswaste would be classified based on considerations of health risks tothe public that arise from disposal of waste. Hazardous waste wouldnot be classified based, for example, on its source. Second, the classi-fication system would apply to any hazardous waste, and separateclassification systems for radioactive and hazardous chemical wasteswould not be retained. In the proposed system, waste would be classi-fied based only on its properties, and the same rules would apply inclassifying all hazardous wastes.

The objective of the study presented in this Report was to addressdifficulties (elaborated, for example, in Sections 1.3.1.5 and 1.4) thathave arisen from use of the existing classification systems for radio-active and hazardous chemical wastes. An important impetus for

1.3 SUMMARY OF EXISTING WASTE CLASSIFICATION SYSTEMS / 7

the proposed classification system is the difficulties that have beenencountered in managing and disposing of so-called ‘‘mixed waste’’that contains radionuclides and hazardous chemicals. The proposedsystem also addresses shortcomings of the existing classificationsystems, such as the lack of general principles for exempting wastesthat contain small amounts of hazardous substances from regulatorycontrol as hazardous material.

This Report is concerned with classification of hazardous wastefor purposes of disposal. However, the principles and conceptsembodied in the waste classification system could be applied in classi-fying hazardous materials for any other purpose. The classificationsystem is intended to be applied to hazardous waste prior to disposal.It is not intended to be applied to screening or ranking of contami-nated sites, including existing hazardous waste disposal sites,because these activities involve site-specific considerations that can-not be included in a generally applicable waste classification system.However, any wastes exhumed from contaminated sites that thenrequire disposal would be included in the waste classification system.

This Report presents the foundations and technical principles fordevelopment of a generally applicable and risk-based hazardouswaste classification system. Recommendations on suitableapproaches to establishing boundaries of different waste classes arediscussed; these boundaries could be expressed, for example, in termsof limits on concentrations of hazardous substances. However, aparticular implementation of the proposed waste classification sys-tem in terms of quantifying the boundaries of different waste classesis not presented.

1.3 Summary of Existing Waste Classification Systems

This Section summarizes the separate classification systems thathave been developed for radioactive and hazardous chemical wastes.Impacts of the two classification systems on management and dis-posal of mixed wastes are also described.

1.3.1 Radioactive Waste Classification in the United States

The existing classification system for radioactive waste in theUnited States is depicted in Figure 1.1. This classification system isin the form of a hierarchy of basic waste classifications and wastesubclassifications.


Fig. 1.1. Current radioactive waste classification system in the UnitedStates.

In the first level of the hierarchy, radioactive waste that arisesfrom operations of the nuclear fuel cycle (i.e., from processing ofuranium or thorium ores and production of nuclear fuel, any uses ofnuclear reactors, and subsequent utilization of radioactive materialused or produced in reactors) is distinguished from radioactive wastethat arises from any other source or practice. The latter type ofwaste is referred to as NARM (naturally occurring and accelerator-produced radioactive material), which includes any radioactive mate-rial produced in an accelerator and NORM [naturally occurringradioactive material not subject to regulation under the AtomicEnergy Act (AEA)].

The distinction between nuclear fuel-cycle and NARM wastes isbased on the definitions of source, special nuclear, and byproductmaterials in AEA, which apply only to radioactive materials associ-ated with operations of the nuclear fuel cycle, and the developmentof federal laws and regulations for management and disposal ofradioactive waste that apply only to waste containing radioactivematerials defined in AEA. This distinction originated in the nationalsecurity and safeguards aspects of the nuclear weapons programestablished under AEA, but it is not based on differences in radiologi-cal properties of fuel-cycle and NARM wastes or on differences inrequirements for their safe management and disposal.

1.3.1.1 Classification of Fuel-Cycle Wastes. As indicated in thesecond level of the hierarchy in Figure 1.1, radioactive waste that


arises from operations of the nuclear fuel cycle in the United Statesis divided into five basic classes called spent fuel, high-level waste,transuranic waste, low-level waste, and uranium or thorium milltailings. The current definitions of these waste classes and theintended type of disposal system (technology) for each class are sum-marized in Table 1.1.

The key to the classification system for nuclear fuel-cycle wastesin the United States is the definition of high-level waste, becausethe definitions of transuranic waste and low-level waste depend onthis definition. High-level waste is defined based on its source ratherthan its properties. The important properties of high-level wasteinclude high concentrations of shorter-lived fission products, result-ing in high levels of decay heat and external radiation, and highconcentrations of long-lived, alpha-emitting transuranium radionu-clides. At the present time, any waste that resembles high-levelwaste in its radiological properties but does not arise directly inreprocessing of spent fuel is not classified as high-level waste.

In addition to being source-based, the definition of high-level wasteis only qualitative, because minimum concentrations of fission prod-ucts (or minimum levels of decay heat or external radiation) andminimum concentrations of long-lived, alpha-emitting transuraniumradionuclides are not specified. Thus, the definition is ambiguous,as evidenced by several case-by-case decisions by regulatory authori-ties to exclude from high-level waste certain incidental wastes thatarise in fuel reprocessing (e.g., fuel cladding and ion-exchange beds)or further processing of reprocessing waste (e.g., salts produced indecontamination of liquid wastes). Any ambiguity in the definitionof high-level waste results in a similar ambiguity in the definitionsof transuranic waste and low-level waste.

Another important feature of the classification system for nuclearfuel-cycle wastes in the United States is the definition of low-levelwaste only by exclusion; there is no definition of what low-level wasteis, only a definition of what it is not. As a result, in contrast to theearliest descriptions of low-level waste prior to the establishment ofdefinitions in law, this class is not restricted to waste that containsrelatively low concentrations of radionuclides compared with high-level waste. Rather, low-level waste can range from virtually innocu-ous to highly hazardous over long time frames.

The classification system for nuclear fuel-cycle wastes in theUnited States can be characterized in the following way. First, as aconsequence of the definition of high-level waste as waste from fuelreprocessing, all waste classes, including mill tailings, are definedbased essentially on the source of the waste, rather than its radiologi-cal properties, and most of the definitions are not explicit in regard


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to the primary constituents of the waste or its properties. Second,the definitions of all waste classes, including transuranic waste, arequalitative; i.e., the definitions are not expressed strictly in termsof limits on concentrations of radionuclides or other radiologicalproperties. Third, the definitions are not based primarily on consid-erations of risks to human health, because waste in different classescan have similar radiological properties and pose similar healthrisks (e.g., spent fuel, high-level waste, transuranic waste, and high-activity, longer-lived low-level waste).

As indicated in Table 1.1, the different classes of waste from thenuclear fuel cycle normally are intended for disposal either in ageologic repository or in a near-surface facility. The intention todispose of spent fuel, high-level waste, and transuranic waste in ageologic repository is based primarily on assessments of long-termhealth risks to the public posed by the high concentrations of long-lived, alpha-emitting radionuclides. Certain high-activity, longer-lived low-level wastes, including greater-than-Class-C commerciallow-level waste (see Figure 1.1), also are intended for disposal in ageologic repository, based in part on assessments which indicatethat disposal in a near-surface facility would pose an unacceptablehealth risk to individuals who might inadvertently intrude onto adisposal site after loss of institutional control.

Most low-level waste, except high-activity, longer-lived waste thatis anticipated to be produced in small volumes, is intended for dis-posal in a near-surface facility. The acceptability of near-surfacedisposal for most low-level waste is based primarily on assessmentsof the long-term performance of such facilities, which indicate thatthe health risks to the public, including future inadvertent intruders,should be acceptable.

Uranium or thorium mill tailings also are intended for near-surface disposal. In contrast to low-level waste, however, this inten-tion is based mainly on a judgment by regulatory authorities thatdisposal of the very large volumes of these wastes in undergroundfacilities would be impractical. Most mill tailings are intended fordisposal in situ at uranium or thorium processing sites where thewastes were generated. Because inadvertent intrusion into a tailingspile would result in unacceptable health risks to an intruder andpossibly to individuals residing nearby, due to the high concentra-tions of radium and high emanation rates of radon, an intention tomaintain perpetual institutional control over tailings piles to preventintrusion is an important factor in protecting public health.

Thus, assessments of long-term health risks to the public, includ-ing future inadvertent intruders at near-surface disposal sites, areimportant in selecting disposal technologies for radioactive wastes


that arise from operations of the nuclear fuel cycle. However, asnoted previously, the definitions of the different classes of fuel-cyclewaste are not based on considerations of risks that arise from wastedisposal but are based essentially on the source of the waste, withthe result that waste in different classes can pose similar risks andrequire similar disposal technologies.

1.3.1.2 Subclassifications of Fuel-Cycle Wastes. As shown in thethird level of the hierarchy in Figure 1.1, transuranic waste andlow-level waste are further divided into different subclasses. Thesubclassification of transuranic waste as contact handled or remotelyhandled is based on the level of external radiation in contact witha waste package. This subclassification is related to requirementsfor protection of workers during waste operations, but it is not relatedto requirements for protection of the public following disposal.

The subclassification of low-level waste applies mainly to commer-cial waste intended for disposal in facilities licensed by the U.S.Nuclear Regulatory Commission (NRC) or an Agreement State.Waste designated as Class A, B, or C is generally acceptable for near-surface disposal in accordance with requirements for each subclassspecified by NRC in Title 10, Part 61 of the Code of Federal Regula-tions (10 CFR Part 61) or compatible Agreement State requirements.Greater-than-Class-C waste, which contains the highest concentra-tions of radionuclides with half-lives of about 30 y or greater, requiresdisposal in a geologic repository, unless disposal elsewhere isapproved on a case-by-case basis. The subclassification of commerciallow-level waste is based on assessments of health risks to the publicthat arise from near-surface disposal, especially potential risks toinadvertent intruders at disposal sites after an assumed loss of insti-tutional control.

1.3.1.3 Other Radioactive Wastes. As shown in the second levelof the hierarchy in Figure 1.1, radioactive waste that does not arisefrom operations of the nuclear fuel cycle (NARM waste) is dividedinto waste that contains NORM and radioactive waste produced inan accelerator. This division is not formally defined in federal lawsor regulations but is based mainly on the different sources and prop-erties of the two types of waste. NORM waste often resembles ura-nium or thorium mill tailings in having large volumes but relativelylow concentrations of radionuclides, although some radium wastesoccur in small volumes and are highly concentrated. Accelerator-produced waste resembles some forms of low-level waste in that itcontains mainly short-lived radionuclides and relatively low concen-trations of longer-lived radionuclides.


Federal regulations governing NARM waste have not been estab-lished, and there is no coordinated federal policy for their disposal(see Table 1.1). Thus, commercial NARM waste currently is regu-lated only by the states. States generally regulate accelerator-produced waste as if it were low-level waste. Several approacheshave been taken in regulating commercial NORM waste, particularlywastes produced in mining, energy exploitation, and other industrialactivities. Some states do not currently regulate these forms ofNORM waste as radioactive waste. States that do regulate NORMwaste generally specify concentrations of radium below which thematerials are exempt from regulation as radioactive waste, but theconcentrations of radium that distinguish regulated and exemptNORM waste vary from state to state. The distinction between regu-lated and unregulated (including exempt) NORM waste is indicatedin the third level of the hierarchy in Figure 1.1.

The U.S. Department of Energy (DOE) is responsible for manage-ment and disposal of NARM waste associated with any of its activi-ties. Large volumes of NORM waste that contains relatively lowconcentrations of radionuclides generally are managed in the sameway as uranium or thorium mill tailings. Accelerator-produced wasteand small volumes of concentrated NORM waste generally are man-aged as low-level waste.

1.3.1.4 Exempt Radioactive Wastes. The radioactive waste classi-fication system in the United States does not include a general classof exempt waste (see Table 1.1). Rather, many products and materi-als that contain small amounts of radionuclides (e.g., specified con-sumer products, liquid scintillation counters containing 3H and 14C)have been exempted from requirements for use or disposal as radio-active material on a case-by-case basis. The various exemption levelsare intended to correspond to low doses to the public, especiallycompared with dose limits in radiation protection standards for thepublic or doses due to natural background radiation. However, theexemption levels are not based on a particular dose, and potentialdoses to the public resulting from use or disposal of the exemptproducts and materials vary widely.

A general class of exempt radioactive waste would include anywaste containing sufficiently small amounts of radionuclides thatthe materials could be managed and disposed of as if they werenonradioactive and still provide adequate protection of humanhealth. An important benefit of establishing a general class of exemptradioactive waste would be a reduction in the resources required forwaste treatment and disposal. Classification of waste as exempt alsowould allow consideration of beneficial uses of the materials.


1.3.1.5 Deficiencies in the Radioactive Waste Classification System.The classification system for radioactive waste in the United Statessummarized in Table 1.1 is based primarily on the earliest descrip-tions of different classes of waste that arises from chemical reprocess-ing of spent nuclear fuel and subsequent processing of nuclearmaterials that were developed beginning in the late 1950s. Thesewastes were considered to be the most important in regard to poten-tial radiological impacts on workers.

This classification system has served its intended purpose of aidingdevelopment of strategies for management and disposal of nuclearfuel-cycle wastes reasonably well. Federal programs for spent fuel,high-level waste, and transuranic waste are well established, dis-posal of transuranic waste in a dedicated geologic repository hasbegun, and investigations of a recommended geologic repository sitefor disposal of spent fuel and high-level waste are underway. Low-level waste and uranium or thorium mill tailings have been managedand disposed of under federal and state programs. In addition, statesand DOE have taken responsibility for accelerator-produced wasteand some NORM wastes.

As summarized below, however, the classification system thatencompasses nuclear fuel-cycle and NARM waste also has exhibiteda number of deficiencies that call into question its continued suit-ability.

● The radioactive waste classification system is complex, it is nottransparent to the public, who are increasingly involved in deci-sions about management and disposal of waste, and it is notunderstandable by anyone but a studied expert.

● The classification system lacks a set of principles for determiningwhen a waste contains sufficiently small amounts of radionu-clides that it can be exempted from regulatory control as radioac-tive material. The lack of a general class of exempt wasteincreases in importance as the resources required for manage-ment and disposal of radioactive waste increase compared withthe resources required for management and disposal of thesematerials as nonradioactive waste, and it may foreclose possiblebeneficial uses of slightly contaminated materials.

● The distinction in law between nuclear fuel-cycle and NARMwaste is completely artificial with respect to radiological proper-ties of wastes and requirements for their safe management anddisposal. This distinction cannot be defended on grounds of pro-tection of human health.

● The classification system for fuel-cycle waste is increasinglyunable to accommodate in a logical and defensible manner newer


forms of waste that were not envisioned when the classificationsystem was first developed. For example, some wastes resemblehigh-level waste or transuranic waste in their radiological prop-erties and are intended for disposal in a geologic repository, butthey must be classified as low-level waste because they are notproduced directly in fuel reprocessing or they do not containsufficient concentrations of long-lived, alpha-emitting transura-nium radionuclides. Conversely, some reprocessing wastes, afterdecades of storage and further processing, now contain such lowconcentrations of radionuclides that they would be generallyacceptable for near-surface disposal in accordance with NRCrequirements for low-level waste, but these wastes must be clas-sified as high-level waste and sent to a geologic repository unlessNRC determines otherwise on a case-by-case basis.

● The classification system for fuel-cycle waste is essentially quali-tative. As a result, there is substantial ambiguity about whethersome wastes should be classified as high-level waste, transuranicwaste, or low-level waste. This ambiguity has led to needlessdisputes about classification of specific wastes that are largelyunrelated to important issues of protecting human health.

● The definition of low-level waste is particularly problematic.Contrary to the common meaning of ‘‘low-level’’ and the meaningof this term when this waste class was first defined, low-levelwaste can contain high concentrations of shorter-lived andlonger-lived radionuclides similar to those in high-level waste,as well as relatively low concentrations of any radionuclide.Thus, the definition of low-level waste is not related to its radio-logical properties or to requirements for safe management anddisposal. The definition only by exclusion also may foster mis-trust by the public because the simple question of what low-level waste is cannot be given a direct answer.

The root cause of these deficiencies is a classification system fornuclear fuel-cycle waste that is based primarily on the source of thewaste, rather than its radiological properties or assessments of risksto human health that arise from waste management or disposal,with the result that wastes in different classes can have similarradiological properties and require similar technologies for safe man-agement and disposal. All of these deficiencies, except the lack of ageneral class of exempt waste, are overcome by basing decisionsabout management and disposal of specific wastes at specific siteson the radiological properties of waste, rather than its classification.However, the need to separate decisions about suitable approachesto waste management and disposal from considerations of waste


classification makes the existing radioactive waste classification sys-tem in the United States difficult to defend on logical or technicalgrounds.

1.3.2 Other Radioactive Waste Classification Systems

As part of this study, proposed radioactive waste classificationsystems that differ from the existing classification system in theUnited States were reviewed and evaluated. Of particular interestis the classification system currently recommended by the Interna-tional Atomic Energy Agency (IAEA). This classification system andthe disposal options for each waste class are summarized in Table 1.2.The basic waste classification system consists of exempt waste, low-and intermediate-level waste, and high-level waste.

The radioactive waste classification system recommended by IAEAdiffers from the existing classification system in the United Statesin the following respects.

● The basic waste classification system includes a general class ofexempt waste, which is defined in terms of a dose to an individualmember of the public, resulting from waste disposal, that isregarded as negligible.

● The basic waste classification system does not distinguishbetween radioactive waste associated with the nuclear fuel-cycleand other waste; i.e., fuel-cycle and NARM wastes are includedin the same classification system.

● High-level waste is defined in terms of its radiological properties,rather than its source. Thus, this class includes waste fromsources other than chemical reprocessing of spent nuclear fuelwith radiological properties similar to those of reprocessingwaste.

● Concentrations of shorter-lived radionuclides in low- and inter-mediate-level waste are limited by the criterion on thermalpower density (decay heat). There is no such restriction on low-level waste as defined in the United States.

● The definitions of waste classes are linked to some degree withintended disposal technologies. This linkage is particularlyapparent in the definitions of exempt waste and high-level waste.However, not all waste classes are defined in relation to intendeddisposal technologies, because low- and intermediate-level wastecould be acceptable for near-surface disposal or require disposalin a geologic repository, depending on the radiological propertiesof the waste and requirements imposed by national authorities.


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● Waste that contains long-lived, naturally occurring radionu-clides, including uranium and thorium mill tailings, is not partof the basic waste classification system, in contrast to the classi-fication system for nuclear fuel-cycle waste in the United States.However, both classification systems recognize that manage-ment and disposal of these wastes require special considerations,due primarily to their very large volumes.

In considering whether waste would be exempt, the specified dosecriterion normally would be applied only to waste that containsmainly man-made radionuclides. Since average annual doses to thepublic due to long-lived, naturally occurring radionuclides in theirundisturbed state are considerably above 10 �Sv, this dose could notbe used as the criterion to exempt large volumes of waste that con-tains these radionuclides. As indicated in Table 1.2, large volumesof waste that contains low levels of naturally occurring radionuclidesalso could be exempted from regulation as radioactive material, butthe exemption levels normally would correspond to doses consider-ably higher than the criterion used to define the exempt waste classcontaining mainly man-made radionuclides.

1.3.3 Classification of Hazardous Chemical Wastes

Management and disposal of many wastes that contain hazardouschemicals are regulated by the U.S. Environmental ProtectionAgency (EPA) under authority of the Resource Conservation andRecovery Act (RCRA). In the classification system for hazardouschemical wastes specified in 40 CFR Part 261, waste is classified ashazardous by its characteristics or by listing.

● Waste is classified as hazardous by characteristics if it is ignit-able, corrosive, reactive, or toxic, as defined by EPA.

A waste is hazardous by characteristics based on its physicaland chemical properties. Ignitable, corrosive, or reactive wastesmust be treated to remove these characteristics prior to disposal.Properly treated ignitable, corrosive, and reactive wastes, if theyare not otherwise hazardous, are no longer considered hazard-ous. The toxicity characteristic is defined in terms of the leach-ability of specified organic compounds and heavy metals. Toxicwastes also must be treated to remove this characteristic priorto disposal using such methods as destruction of organic materi-als by incineration or incorporation in an immobilizing wasteform (e.g., cement). However, in contrast to ignitable, corrosive,


or reactive wastes, a properly treated toxic waste may still beconsidered hazardous (e.g., if it contains heavy metals that areimmobilized but not destroyed by treatment).

● Waste is classified as hazardous by listing if it contains anyamount of specified materials from nonspecific sources (the so-called ‘‘F’’ list), specified materials from specific sources (the ‘‘K’’list), or specified chemicals from any source (the ‘‘P’’ and ‘‘U’’lists).

A listed hazardous waste cannot be rendered nonhazardous bytreatment or by dilution or mixing with nonhazardous materials.EPA has issued proposals to establish exemption levels for listedwastes that contain small amounts of hazardous substances, butsuch exemption provisions are not yet established in regulations.Thus, a listed hazardous waste can be exempted from RCRArequirements only by the process of ‘‘delisting.’’ EPA also hasexempted certain materials that contain hazardous substancesto allow their beneficial use (e.g., ash and sludge from coal-burning power plants, sewage sludge).

All wastes classified as hazardous under RCRA, including properlytreated toxic waste that is still considered hazardous, are intendedfor disposal in near-surface facilities regulated under Subtitle C ofRCRA. EPA has developed detailed technical requirements on wastetreatment and the siting, design, operation, and closure of disposalfacilities. Thus, when viewed in relation to intended disposal techno-logies, there is basically only one class of hazardous chemical waste,regardless of the amounts of hazardous substances present; i.e., awaste either is hazardous or it is not.

Some states (e.g., California, Washington) have defined a categoryof extremely hazardous waste, and extremely hazardous substancesare specified by EPA under the Emergency Response and CommunityRight-to-Know Act. Under RCRA and state regulations, however,requirements on waste treatment and disposal generally do not dis-tinguish between extremely hazardous waste and any other hazard-ous chemical waste.

Requirements on treatment and disposal of hazardous chemicalwaste under RCRA, especially the intention to limit contaminationof groundwater, are based to some extent on considerations of risksto public health and the environment posed by waste. However,requirements on waste treatment and the siting, design, operation,and closure of disposal facilities are not based on long-term projec-tions of the ability of disposal systems to limit releases of hazardoussubstances to the environment, nor is any consideration given to


potential risks to inadvertent intruders at disposal sites after lossof institutional control. Rather, in addition to the detailed technicalrequirements on waste treatment and disposal that apply at anysite, the approach to protection of public health and the environmentunder RCRA relies on monitoring of releases from disposal facilities,especially in groundwater, corrective actions should releases exceedspecified limits, and an intention to maintain institutional controlover disposal sites for as long as the waste remains hazardous.

RCRA also governs disposal of nonhazardous waste in municipal/industrial landfills. This type of waste includes household trash,various industrial wastes, and characteristically hazardous wastethat has been properly treated and is no longer considered hazard-ous. In current EPA regulations implementing Subtitle D of RCRA,requirements on siting, design, operation, and closure of landfillsfor nonhazardous waste are similar to requirements that apply tohazardous waste disposal facilities regulated under Subtitle C. Thus,if general principles for exempting wastes that contain hazardouschemicals from RCRA requirements for hazardous waste were estab-lished, the primary benefits would likely include the reduction ofresources expended on waste treatment, transportation, and storageand the reduced commitment to maintaining institutional controlat disposal sites following closure.

The system for classification and disposal of hazardous chemicalwaste developed by EPA under RCRA does not apply to all wastesthat contain hazardous chemicals. For example, wastes that containdioxins, polychlorinated biphenyls (PCBs), or asbestos are regulatedunder the Toxic Substances Control Act (TSCA). In addition, thecurrent definition of hazardous waste in 40 CFR Part 261 specificallyexcludes many wastes that contain hazardous chemicals from regu-lation under RCRA, including certain wastes produced by extraction,beneficiation, and processing of various ores and minerals or explora-tion, development, and use of energy resources. Thus, the wasteclassification system is not comprehensive, because many potentiallyimportant wastes that contain hazardous chemicals are excluded,and it is not based primarily on considerations of risks posed bywastes, because the exclusions are based on the source of the wasterather than the potential risk.

1.3.4 Comparison of Classification Systems for Radioactive andHazardous Chemical Wastes

The existing classification systems for radioactive and hazardouschemical wastes in the United States and approaches to disposal of


these wastes were developed largely independently. As a result,there are important differences in the two classification systems andin approaches to waste disposal, but there also are similarities.

The similarities are of the following kinds. First, neither classifi-cation system includes a general class of exempt waste. Second,neither classification system is comprehensive, because the classifi-cation system for radioactive waste distinguishes between fuel-cycleand NARM waste and the classification system for hazardous chemi-cal waste excludes many potentially important wastes that containhazardous chemicals. Third, any waste must be managed and dis-posed of in a manner that is expected to protect public health andthe environment. In addition, the approach to disposal of hazardouschemical waste under RCRA, which emphasizes monitoring ofreleases from disposal facilities and an intention to maintain institu-tional control over disposal sites for as long as the waste remainshazardous, is applied to disposal of uranium or thorium mill tailingsunder AEA.

There also are two important differences. First, the classificationsystem for radioactive waste from the nuclear fuel cycle includesdifferent classes that are defined based essentially on the source ofthe waste. In addition, some classes of fuel-cycle waste (e.g., high-level waste) often, but not always, contain higher concentrations ofradionuclides than other classes (e.g., low-level waste) and, thus,pose a greater hazard in waste management and disposal. The classi-fication system for hazardous chemical waste does not distinguishbetween hazardous wastes based on their source, with the exceptionof the ‘‘K’’ list of wastes from specific sources. Additionally, hazardouschemical wastes are not further classified based on their relativehazard (i.e., there is only one class of hazardous chemical waste).

Second, different types of disposal systems are intended to beused for radioactive wastes (e.g., near-surface facilities or geologicrepositories), whereas only a single type of disposal system (a near-surface facility) is used for all hazardous chemical wastes, regardlessof the potential risks posed by the waste. Furthermore, acceptabledisposals of all radioactive wastes except mill tailings are determinedbased primarily on long-term projections of potential impacts onpublic health, including potential impacts on future inadvertentintruders in the case of near-surface facilities. In contrast, theapproach to protecting public health in disposal of hazardous chemi-cal waste emphasizes monitoring of releases, corrective actions inthe event of unacceptable releases, and maintenance of institutionalcontrol over disposal sites, rather than long-term projections ofpotential impacts on public health.


1.3.5 Mixed Radioactive and Hazardous Chemical Wastes

The definition of solid waste in RCRA specifically excludes source,special nuclear, and byproduct materials as defined in AEA. There-fore, radioactive constituents of wastes that arise from operationsof the nuclear fuel cycle are excluded from regulation as hazardouswaste under RCRA.

The term ‘‘mixed waste’’ refers mainly to waste that contains radio-nuclides regulated under AEA and hazardous chemical waste regu-lated under RCRA. Dual regulation of mixed waste has no effect onclassification, management, and disposal of the hazardous chemicalcomponent or on classification of the radioactive component. Theeffects of dual regulation of mixed waste on management and dis-posal of the radioactive component are summarized as follows:

● Technical requirements on treatment and disposal of spent fuel,high-level waste, and transuranic waste established under AEAshould be largely unaffected by the presence of waste classifiedas hazardous under RCRA. Some of these wastes meet technology-based treatment standards for hazardous chemical waste estab-lished by EPA (e.g., vitrified high-level waste is an acceptablewaste form under RCRA). Alternatively, a finding that disposalof the radioactive component of the waste complies with applica-ble environmental standards established by EPA under AEAcan serve to exempt the disposal facility from prohibitions ondisposal of restricted hazardous chemical wastes under RCRA[e.g., disposal of mixed transuranic waste at the Waste IsolationPilot Plant (WIPP)].

● Management and disposal of hazardous chemical waste underRCRA is based on detailed and prescriptive technical require-ments that apply to any facility for waste treatment, storage,or disposal, whereas management and disposal of low-levelradioactive waste is more flexible because AEA allows consider-ation of waste- and site-specific factors. As a consequence, accept-able approaches to management and disposal of mixed low-levelwaste probably will be determined primarily by RCRA require-ments, unless exempt levels of hazardous chemicals are estab-lished that render the waste nonhazardous under RCRA.

● EPA regulations developed under AEA specify that operationsand closure at uranium or thorium mill tailings sites must con-form to RCRA requirements on hazardous waste. These require-ments acknowledge the presence of hazardous chemicals in milltailings, especially heavy metals.

1.4 DEVELOPMENT OF A NEW CLASSIFICATION SYSTEM / 25

Thus, dual regulation of mixed waste does not present insurmount-able technical obstacles to waste disposal. The main technical imped-iment to successful management and disposal of mixed waste hasbeen the difficulty in obtaining operating permits for treatment,storage, and disposal facilities under RCRA, especially facilities formixed low-level waste at DOE sites. Similar considerations apply towaste that contains radionuclides regulated under AEA, especiallylow-level waste, and hazardous chemicals regulated under otherenvironmental laws (e.g., TSCA).

The exclusion of radioactive materials from regulation underRCRA does not apply to NARM. However, the current definition ofhazardous waste in 40 CFR Part 261 does not include NARM waste,and the definition specifically excludes many wastes associated withproduction or use of energy and mineral resources that contain ele-vated levels of naturally occurring radionuclides compared withaverage background levels. These potentially important NARMwastes, which also may contain elevated levels of heavy metals, thusare not regulated under RCRA and issues of dual regulation mayarise when NARM waste is mixed with waste regulated under RCRA.

1.4 Approach to Development of a New WasteClassification System

NCRP’s recommendations on classification of hazardous wastesare intended to address deficiencies and inconsistencies in the sepa-rate systems for classification and disposal of radioactive and hazard-ous chemical wastes in the United States summarized previously.The most important of these include:

● the lack of general principles for exempting wastes that containsmall amounts of radionuclides or hazardous chemicals fromregulatory control as hazardous material;

● the ambiguities and logical inconsistencies in the radioactivewaste classification system, especially the difficulties with thesource-based classifications of wastes that arise from operationsof the nuclear fuel cycle and the artificial distinction betweenfuel-cycle and NARM wastes;

● the source-based exclusions of potentially important wastes thatcontain hazardous substances from regulation as hazardouswaste;

● the potential problem that the classification system for hazard-ous chemical waste does not distinguish between wastes thatpose a greater or lesser hazard and, thus, that disposal of allsuch wastes in near-surface facilities may not ensure long-term


protection of public health in the absence of permanent institu-tional control over disposal sites; and

● the use of different disposal systems for radioactive wastes (e.g.,near-surface facilities or geologic repositories) that are selectedbased in part on long-term projections of risks to public healthposed by the waste, but the use of a single disposal system(i.e., a near-surface facility) for all hazardous chemical wasteswithout due consideration of the long-term health risks posedby the waste.

NCRP’s approach to addressing these difficulties is to develop asingle hazardous waste classification system that is comprehensiveand risk-based.

1.4.1 Basic Elements of Hazardous Waste Classification System

NCRP’s recommendations on classification of hazardous wastesare based on two principles. First, a classification system should begenerally applicable to any waste that contains radionuclides,hazardous chemicals, or mixtures of the two (i.e., the system shouldbe comprehensive). Second, waste that contains hazardous sub-stances should be classified based on considerations of health risksto the public that arise from waste disposal, because permanentdisposal is the intended disposition of materials having no furtheruse.

Based on these principles, the essence of NCRP’s recommendationsis that waste that contains radionuclides or hazardous chemi-cals should be classified in relation to the types of disposalsystems (technologies) that are expected to be generallyacceptable in protecting public health. Specifically, the classifi-cation system developed in this Report includes three classes ofwaste defined as follows:

● Exempt waste: any waste containing hazardous substancesthat is generally acceptable for disposition as nonhazardousmaterial (e.g., disposal in a municipal/industrial landfill for non-hazardous wastes).

● Low-hazard waste: any nonexempt waste that is generallyacceptable for disposal in a dedicated near-surface facility forhazardous wastes.

● High-hazard waste: any nonexempt waste that generallyrequires a disposal system more isolating than a dedicated near-surface facility for hazardous wastes (e.g., a geologic repository).

1.4 DEVELOPMENT OF A NEW CLASSIFICATION SYSTEM / 27

Thus, the basic elements of the proposed classification system are,first, that there should be a general class of waste that containssufficiently small concentrations of radionuclides or hazardous chem-icals that it can be exempted from regulatory control as hazardousmaterial and, second, that there should be two classes of nonexemptwaste that contain increasing concentrations of hazardous substancesand require dedicated disposal systems that provide increased wasteisolation.

The definition of exempt waste requires further elaboration.Although this Report is concerned with classification of waste forpurposes of disposal, NCRP recognizes that some materials thatcontain only low concentrations of regulated hazardous substancesmay have beneficial uses if they could be exempted from regulatorycontrol as hazardous material.

Thus, NCRP intends that exempt waste could be used ordisposed of in any manner allowed by laws and regulationsaddressing disposition of nonhazardous materials. However,waste that would be exempt for purposes of disposal wouldnot necessarily be exempt for purposes of beneficial use aswell. Exemption of materials that contain hazardous substances toallow beneficial use also should be based on considerations of healthrisks to the public. However, limits on the amounts of hazardoussubstances that could be present in exempt materials intended fora particular beneficial use could be substantially lower than thelimits for disposal as exempt waste, due to differences in exposurescenarios for the two dispositions, and disposal may be the onlyallowable disposition of some exempt materials based on considera-tions of risk. In addition, exempt materials may consist of trash,rubble, and residues from industrial processes that have no beneficialuses and must be managed as waste.

Based on these considerations and the purpose of this study, dis-posal is the only disposition discussed in developing recom-mendations on exemption of waste that contains smallamounts of hazardous substances based on risk. Considerationof dispositions of exempt materials other than disposal as nonhazard-ous waste is beyond the scope of this study. However, the principlesused in this Report to define exempt waste based on risk also couldbe used to define exempt material for any other purpose.

1.4.2 Assumptions in Developing the Waste Classification System

Given the basic elements of a new waste classification systemdescribed in the previous section, NCRP proceeded with developmentof the system on the basis of several assumptions.


● The recommendations on waste classification should focus onconcepts, principles, and approaches to implementation. Recom-mendations on approaches to using assumed limits on risk ordose to establish quantitative boundaries of waste classesexpressed as limits on concentrations of hazardous substanceswould be presented, and precedents that could be used to definethe assumed limits on risk or dose and to assess risk or dose forpurposes of waste classification would be discussed. However,specific recommendations on values of any such limits and manyof the considerations involved in establishing them would notbe given, because this is properly the role of policy makers andregulatory authorities.

● The waste classification system should be based on the distinctconcepts of negligible and acceptable (i.e., barely tolerable) risksto the public that arise from waste disposal. Precedents for speci-fying negligible or acceptable risks that could be used in classify-ing waste, such as other NCRP recommendations, would be cited,but specific recommendations would not be presented in thisReport.

● Legal impediments to development of a new waste classificationsystem would be ignored. These include, for example, the distinc-tion between radioactive waste that arises from operations ofthe nuclear fuel-cycle and NARM waste, which is based on pro-visions of AEA, the distinction between radioactive and hazard-ous chemical wastes, which is based on provisions of AEA andRCRA, and the provision in the National Energy Policy Actthat prohibits NRC from establishing a general class of exemptradioactive waste.

Thus, NCRP’s recommendations focus on the technical foundationsfor a generally applicable and risk-based waste classification system.

1.4.3 Challenges in Developing a Waste Classification System

Development of a generally applicable and risk-based waste classi-fication system presents a number of technical challenges.

● The classification system must apply to waste that contains carci-nogenic and noncarcinogenic hazardous substances. Therefore,classification of waste based on risk must take into account thedifferent forms of the assumed dose-response relationships forthese two types of substances (response proportional to dose, with-out threshold, for carcinogens; threshold for noncarcinogens).

1.5 DEVELOPMENT OF THE RECOMMENDED SYSTEM / 29

● The current approach to risk management (control of exposures)for hazardous chemicals differs from the approach for radionu-clides under AEA. In particular, the two approaches to riskmanagement attach different meanings to the terms ‘‘accept-able’’ and ‘‘unacceptable’’ commonly used to describe the signifi-cance of health risks.

● The term ‘‘dose’’ has different meanings for radionuclides andhazardous chemicals. In assessments of risk to human health,this term generally refers to energy imparted to tissue and itsbiological significance for radionuclides, but it usually refers tomass intakes for hazardous chemicals.

● The measure of risk (health-effect endpoint) calculated in riskassessments often differs for radionuclides and chemical carcino-gens. Fatalities is the measure of risk most often used for radio-nuclides, but cancer incidence is generally used for chemicalcarcinogens.

● The approach to estimating health risks differs for radionuclidesand hazardous chemicals in regard to the degree of conservatismincorporated in the assumed probabilities of an adverse healtheffect per unit dose and the number of organs at risk that aretaken into account.

● Development of a waste classification system based on considera-tions of risks to the public requires assumptions about genericexposure scenarios (i.e., exposure scenarios that are generallyapplicable at any disposal site).

Each of these issues is addressed in presenting NCRP’s recommenda-tions on classification of hazardous waste in the following section.

1.5 Development of the Recommended WasteClassification System

1.5.1 Risk Index for Waste Classification

For the purpose of developing the waste classification systemdescribed in Section 1.4.1, a simple method of evaluating risks tothe public posed by radionuclides and hazardous chemicals in wasteis needed. The term ‘‘risk’’ generally refers to the probability of harm,combined with the potential severity of that harm. In the context ofhazardous waste disposal, risk is the probability of a response in anindividual or the frequency of a response in a population taking into


account: (1) the probability of occurrence of processes and eventsthat could result in release of hazardous substances to the environ-ment and the magnitude of such releases, (2) the probability thatindividuals or populations would be exposed to the hazardous sub-stances released to the environment and the magnitude of suchexposures, and (3) the probability that an exposure would producea response. In classifying waste based on risk, however, exposuresare assumed to occur according to postulated scenarios, and the onlycomponent of the probability of harm considered in estimating riskis the probability of a response from a given exposure.

NCRP recommends that risks to hypothetical individuals at wastedisposal sites should be evaluated in classifying waste, as describedin the following section, and that the risk to an individual that arisesfrom disposal of any hazardous substance be expressed in the form ofa dimensionless risk index (RI). The risk index for the ith hazardoussubstance (RIi) is defined in terms of the risk that arises from disposalof that substance relative to a specified allowable risk for an assumedtype of disposal system (e.g., municipal/industrial landfill for disposalof exempt waste) as:

RIi � Fi(risk from disposal) i

(allowable risk) i, (1.1)

where F is a modifying factor (F � 0) that can depend on the par-ticular hazardous substance and in each case the index i appliesto the ith hazardous substance involved. The risk in the numeratoris evaluated using generic exposure scenarios appropriate tothe sumed type of disposal system for the particular waste class ofconcern.

The modifying factor (F) in Equation 1.1 is intended to representany considerations of importance to a decision about the generalacceptability of waste disposal using an assumed technology, otherthan those directly incorporated in the calculated risk from disposaland the specified allowable risk. This factor can take into account,for example, the general design of a disposal facility, general require-ments on waste packages and waste forms, the volume of waste, theintended emplacement of waste as it would affect credible exposurescenarios, the probability of occurrence of an assumed exposure sce-nario, and uncertainties in the assessment of risk that arises fromdisposal and in the data required to evaluate Equation 1.1. Themodifying factor is exemplified by assumptions used by NRC indeveloping the concentration limits for near-surface disposal of thesmall volumes of Class-C low-level waste in 10 CFR Part 61. Theselimits incorporate assumptions about the probability that exposuresto Class-C waste would occur according to a postulated scenario and


the ability of waste emplacements and engineered barriers to delayexposures to Class-C waste that were not used in developing theconcentration limits for the much larger volumes of Class-A low-level waste. The modifying factor also can incorporate considerationsof risk management, such as the cost-benefit of different optionsfor disposal of specific wastes, considerations of levels of naturallyoccurring hazardous substances (e.g., uranium, radium, arsenic) insurface soil and their associated health risk to the public, and anyother judgments of importance to waste classification.

Risk is not always a useful measure of health impact in evaluatingthe risk index, because risk is not proportional to dose when a hazard-ous substance is assumed to have a threshold dose-response relation-ship. For this type of substance, the risk is presumed to be zero atany dose below a nominal threshold. Since the allowable dose of suchsubstances should always be less than the threshold in order toprevent the occurrence of adverse responses, expressing the riskindex in terms of risk would result in an indeterminate value whenthe dose is below the threshold and, more importantly, a lack ofdistinction between doses near the nominal thresholds and lowerdoses of much less concern. For any hazardous substance, includingcarcinogens for which risk is assumed to be proportional to dosewithout threshold, a generally useful form of risk indexes (RIi) is interms of dose:

RIi � Fi(dose from disposal) i

(allowable dose) i, (1.2)

where the index i applies to the ith hazardous substance involved.The difference in the meaning of ‘‘dose’’ for radionuclides and hazard-ous chemicals described in Section 1.4.3 is unimportant as long asthe same meaning is used for a given hazardous substance in thenumerator and denominator of Equation 1.2.

Given the definition of risk indexes (RIi) in Equation 1.1 or 1.2and assuming that risks from exposure to the different hazardoussubstances in waste are additive, waste classes are defined by therequirement on each waste class and associated disposal system that:

�i

RIi � 1. (1.3)

Adding risk indexes (RIi) for noncarcinogenic substances and com-bining risk indexes (RIi) for carcinogenic and noncarcinogenic sub-stances requires care, however, due to the assumed forms of thedose-response relationships. The evaluation of Equation 1.3 for mix-tures of hazardous substances is described in Section 1.5.5.4.


1.5.2 Generic Exposure Scenarios for Waste Classification

The numerator in Equation 1.1 or 1.2 is calculated using genericscenarios for exposure of individual members of the public that arisefrom waste disposal. Two types of exposure scenarios can be consid-ered: (1) scenarios involving release of hazardous substances froma disposal facility and exposure of individuals at locations beyondthe boundary of the disposal site; or (2) scenarios involving exposureof individuals who inadvertently intrude onto a disposal site, includ-ing scenarios involving permanent residence on a disposal site orother unrestricted access after an assumed loss of institutionalcontrol.

NCRP recommends that generic scenarios for exposure of hypo-thetical inadvertent intruders at disposal sites should be used inclassifying waste. This recommendation is based on two considera-tions. First, scenarios for inadvertent intrusion can be applied to anassumed type of disposal system at any site, whereas scenarios forexposure of members of the public due to release and transport ofhazardous substances to locations beyond the boundary of a disposalfacility are highly site-specific and, thus, are not appropriate for usein generally classifying waste.

Second, generic and site-specific assessments of near-surface dis-posal facilities for radioactive waste have shown that allowable dosesto hypothetical inadvertent intruders usually are more restrictive indetermining acceptable disposals than allowable doses to individualsbeyond the boundary of the disposal site. This conclusion is based onpredictions that concentrations of radionuclides in the environment(e.g., ground-water) at locations beyond the site boundary usuallyshould be far less than the concentrations at the disposal site towhich an inadvertent intruder could be exposed, owing to such fac-tors as the limited solubility of some radionuclides, the partitioningof radionuclides between liquid and solid phases, and the dilutionin transport of radionuclides in water or air beyond the site boundary.More people are likely to be exposed beyond the site boundary thanon the disposal site, but acceptable disposals of radioactive waste innear-surface facilities have been based on assessments of dose toindividuals, rather than populations.

The recommendation that generic scenarios for exposure of hypo-thetical inadvertent intruders should be used in classifying wasteis consistent with the approach used by NRC in 10 CFR Part 61 toestablish different subclasses of low-level radioactive waste that aregenerally acceptable for near-surface disposal (Class-A, -B, and -Cwaste) or are generally unacceptable for near-surface disposal(greater-than-Class-C waste). Such scenarios have not been used to


determine acceptable disposals of hazardous chemical waste in near-surface facilities regulated under RCRA. However, scenarios similarto those developed by NRC have been used in risk assessments atsites contaminated with hazardous chemicals or radionuclides thatare subject to remediation under the Comprehensive EnvironmentalResponse, Compensation, and Liability Act (CERCLA). Generic sce-narios for inadvertent intrusion to be used in classifying waste shouldbe credible for an assumed type of disposal system at any site. Appro-priate scenarios for inadvertent intrusion are discussed further inSection 1.5.4.

1.5.3 Determination of Allowable Risk or Dose

Evaluation of the risk index (RI) in Equation 1.1 or 1.2 requiresassumptions about allowable risks or doses from waste disposal tobe used in defining the different waste classes (see Section 1.4.1).These assumptions should be based on an understanding of differ-ences in the approaches to risk management for radionuclides andhazardous chemicals embodied in current laws and regulations,including the different meanings that have been attached to theterms ‘‘acceptable’’ and ‘‘unacceptable’’ commonly used to describethe significance of health risks.

The approach to risk management for radionuclides, when theyare regulated under AEA, incorporates a limit on acceptable dose(and therefore risk) and a requirement that doses be reduced belowthe limit as low as reasonably achievable (ALARA), economic andsocial factors being taken into account; this approach conforms toNCRP’s recommendations on radiation protection. In this approach,risks to individuals are divided into three categories of significance,which are commonly termed ‘‘unacceptable,’’ ‘‘acceptable,’’ and‘‘negligible.’’

● The term ‘‘unacceptable’’ is used to describe excess lifetime can-cer risks from exposure to radionuclides greater than a valuein the range of about 10�1 to 10�3, the particular value dependingon the exposure situation. Such risks normally must be reducedregardless of cost or other circumstances and, thus, are properlyinterpreted as intolerable (de manifestis).

● The term ‘‘acceptable’’ is used to describe risks below intolerablelevels that also are ALARA. Risks just below ‘‘unacceptable’’levels are regarded as barely tolerable and normally should bereduced substantially based on the ALARA principle. Risks thatare ALARA may vary from one exposure situation to another;


i.e., a risk that is ALARA is not a predetermined result thatapplies to all sources and practices.

● The term ‘‘negligible’’ is used to describe risks so low that furtherefforts at risk reduction using the ALARA principle generallyare unwarranted; i.e., the risks are de minimis. However, achiev-ing a negligible risk is not the goal of ALARA, and a risk thatis ALARA (‘‘acceptable’’) can be substantially above negligiblelevels.

A negligible dose or risk from exposure to radionuclides has not beenestablished in regulations under AEA. However, based on recommen-dations of NCRP and IAEA, excess lifetime cancer risks on the orderof 10�4 or less generally could be considered negligible.

The approach to risk management for hazardous chemicals devel-oped under several environmental laws (e.g., Safe Drinking WaterAct, RCRA, CERCLA) essentially is the opposite of the approach toregulating radionuclides under AEA described above. The approachfor hazardous chemicals incorporates goals for acceptable risk andallowance for an increase (relaxation) in risks above the goals basedprimarily on considerations of technical feasibility and cost. In thisapproach, which also applies to radionuclides when they are regu-lated under laws addressing hazardous chemicals, risks or doses toindividuals are divided into two categories of significance, which arecommonly termed ‘‘acceptable’’ and ‘‘unacceptable.’’

● The term ‘‘acceptable’’ is used to describe excess lifetime cancerrisks in the range of about 10�4 to 10�6, the particular valuedepending on the exposure situation, or intakes of noncarcino-gens less than EPA’s reference doses (RfDs).

● The term ‘‘unacceptable’’ is used to describe lifetime cancer risksor intakes of noncarcinogens greater than ‘‘acceptable’’ levels.

RfDs are estimates of daily intakes of noncarcinogenic substancesthat are expected to be without an appreciable risk of deleterioushealth effects in sensitive population groups (e.g., children, theelderly). An RfD usually is derived from the highest dose withoutadverse effect in studies in humans or animals, referred to as theno-observed-adverse-effect level (NOAEL), using one or more safetyand uncertainty factors that depend on the nature and quality ofthe data. Some RfDs are derived from the lowest dose at which asignificant increase in adverse effects is observed, referred to as thelowest-observed-adverse-effect level (LOAEL), using an additionaluncertainty factor that accounts for the uncertainty in extrapolatingfrom a LOAEL to a NOAEL. RfDs are widely used in health protec-tion of the public, but they do not represent threshold doses of noncar-cinogenic hazardous chemicals.


In the approach to risk management for hazardous chemicals,risks termed ‘‘acceptable’’ are properly interpreted as negligible,because action to reduce risks at these levels generally is notrequired. However, risks termed ‘‘unacceptable’’ are not necessarilyintolerable because risks above ‘‘acceptable’’ levels often are permit-ted (e.g., in remediating contaminated sites under CERCLA). Rather,‘‘unacceptable’’ refers to risks sufficiently high that risk reductionmust be considered, but action to reduce risk is required only to theextent feasible. This approach does not explicitly include the conceptof an intolerable risk that normally must be reduced regardless ofcost or other circumstances. These considerations apply to noncarcin-ogens as well as carcinogens, owing to the large safety and uncer-tainty factors often applied in deriving RfDs.

Based on these discussions, the commonly used terms ‘‘acceptable’’and ‘‘unacceptable’’ clearly do not have the same meanings in thedifferent approaches to risk management for radionuclides and haz-ardous chemicals. ‘‘Acceptable’’ risks or doses for hazardous chemi-cals generally correspond to negligible levels for radionuclides,whereas ‘‘acceptable’’ risks or doses for radionuclides can be wellabove negligible levels, provided they are ALARA. For hazardouschemicals, ‘‘unacceptable’’ essentially means ‘‘non-negligible’’ andthis term does not distinguish between risks or doses so high thatthey are intolerable, where reductions normally would be requiredregardless of cost or other circumstances, and lower risks or dosesabove negligible levels where reductions are required only to theextent feasible. For radionuclides, ‘‘unacceptable’’ refers to risks ordoses well above negligible levels that are intolerable under normalcircumstances. These differences in meanings are important inunderstanding the approaches to risk management for radionuclidesand hazardous chemicals, and they are summarized in Table 1.3.

NCRP recognizes that both of the approaches to risk managementdescribed above are valid. NCRP believes, however, that theapproach to risk management for radionuclides is more transparentin depicting how risk management decisions are made, because itexplicitly includes a range of risks between negligible and intolerablelevels where risks are managed based on the ALARA principle. In theapproach to risk management for hazardous chemicals, the ALARAprinciple is only implicit in the proper interpretation of ‘‘unaccept-able’’ given in Table 1.3, and the term ‘‘unacceptable’’ as commonlyused in this approach is easily misinterpreted as referring to intolera-ble risks. The clear separation between negligible and intolerablerisks in the approach to risk management for radionuclides is partic-ularly relevant to waste classification because it allows the use ofdistinctly different levels of risk in classifying waste. Specifically,


TABLE 1.3—Differences in interpretations of ‘‘acceptable’’ and‘‘unacceptable’’ risks in approaches to risk management for

radionuclides and hazardous chemicals.a

Interpretation in Risk Interpretation in RiskDescription of Management for Management for Hazardous

Risk Radionuclidesb Chemicalsc

‘‘Acceptable’’ Risks are below Risks are negligibleintolerable (de minimis); further(de manifestis) levels reduction of risksand are ALARAd usually need not be

considerede

‘‘Unacceptable’’ Risks are intolerable; Risks are aboverisks normally must be negligible levels;reduced regardless of reduction of risks mustcost or other be considered but iscircumstancesf required only to the

extent feasibleg

a Interpretations of commonly used terms also apply to dose when controlof exposures is based on dose rather than risk; dose generally is used fornoncarcinogenic hazardous chemicals and often is used for radionuclides.

b Interpretations apply to control of exposures to radionuclides under AEA,but not to control of exposures to radionuclides under other environmen-tal laws.

c Interpretations also apply to control of exposures to radionuclides whenthey are regulated under laws addressing hazardous chemicals.

d Excess lifetime cancer risks considered intolerable have values in therange of about 10�1 to 10�3 or greater, depending on the exposure situation,and are well above risks considered negligible (e.g., excess lifetime risks onthe order of 10�4 or less). Risks that are ALARA depend on the particularexposure situation, and achieving a negligible risk is not the goal of ALARA.

e Excess lifetime cancer risks considered negligible have values in therange of about 10�4 to 10�6 or below, depending on the exposure situation;intakes of noncarcinogenic hazardous chemicals less than RfDs are consid-ered negligible.

f Risks also are considered unacceptable if they are below intolerable levelsbut are not ALARA.

g Approach to risk management for hazardous chemicals does not explicitlyinclude concept of an intolerable risk that normally must be reduced regard-less of cost or other circumstances.

with reference to the definitions of waste classes in Section 1.4.1, anegligible risk can be used to classify exempt waste and a substan-tially higher acceptable (barely tolerable) risk can be used to classifylow-hazard waste. Recommendations on establishing negligible and


acceptable risks for purposes of waste classification are discussedfurther in the following section.

1.5.4 Recommended Framework for Risk-Based WasteClassification

NCRP’s recommendations on a framework for a risk-based classi-fication system that is applicable to any waste containing radionu-clides or hazardous chemicals are described in Table 1.4 and depictedin Figure 1.2. This framework follows from the assumption thatwaste classes should be defined in relation to types of disposal sys-tems that are expected to be generally acceptable in protecting thepublic (Section 1.4.1), the definition of the risk index for any hazard-ous substance in terms of the risk that arises from disposal of thatsubstance relative to a specified allowable risk for a particular wasteclass and associated disposal system (Section 1.5.1), the recommen-dation on the type of generic exposure scenario that should be usedfor purposes of waste classification (Section 1.5.2), and recognitionof the distinction between a negligible and an acceptable risk(Section 1.5.3). The different waste classes are discussed in the fol-lowing paragraphs.

1.5.4.1 Exempt Waste. Waste classified as exempt would be regu-lated as if it were nonhazardous, and would be generally acceptablefor disposition as nonhazardous material (e.g., disposal in a municipal/industrial landfill). As noted in Section 1.4.1, disposal is the onlydisposition of exempt materials considered in this Report. Limits onconcentrations of hazardous substances in exempt waste would bederived based on an assumption that the risk or dose to a hypotheti-cal inadvertent intruder at a disposal site should not exceed negligi-ble levels. The use of a negligible risk or dose to determine exemptwaste is based on an assumption that a disposal facility for nonhaz-ardous waste could be released for unrestricted use by the publicsoon after the facility is closed.

Negligible risks or doses used to classify exempt waste could beestablished based on a variety of considerations, consistent withthe different approaches to risk management for radionuclides andhazardous chemicals described in Section 1.5.3. For noncarcinogenichazardous chemicals, NCRP recommends that a negligible doseshould be set at a small fraction (e.g., 10 percent) of a nominalthreshold for deterministic responses in humans; the recommendedapproach to estimating this threshold is described in Section 1.5.5.3.For radionuclides, NCRP has recommended that an annual effective


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dose of 10 �Sv, which corresponds to an estimated lifetime fatalcancer risk of about 4 � 10�5 for an assumed exposure time of 70 y,is a negligible individual dose for any source or practice; this dosealso was used by IAEA to define an exempt class of radioactivewaste (see Table 1.2). Similarly, EPA has proposed that a negligiblelifetime risk of about 10�5 could be used to exempt waste that con-tains chemical carcinogens from requirements for disposal as hazard-ous waste under RCRA.

As an alternative, RfDs established by EPA could be used to definenegligible doses of noncarcinogenic hazardous chemicals. RfDs usu-ally are derived from NOAELs or LOAELs by applying a safety anduncertainty factor between 100 and 10,000 depending on the natureand quality of the data, with values of at least 100 being mostcommon. Thus, RfDs are intended to be well below thresholds fordeterministic responses in humans. However, NCRP believes thatRfDs should not be used without presenting NOAELs or LOAELsused to derive the values. In addition, when RfDs for importantwaste constituents are derived using large safety and uncertainty


factors, thus indicating that the quality of the data is poor, NCRPbelieves that further studies should be undertaken to reduce uncer-tainties in the nominal threshold in humans, to avoid introducingundue levels of conservatism in classifying waste. To promote consis-tency in waste classification, NCRP believes that it would be desir-able to define negligible doses of all noncarcinogens at approximatelythe same fraction of the nominal thresholds in humans.

Negligible risks or doses for radionuclides and chemical carcino-gens also could be established based on considerations of unavoidablerisks from natural background. Since the average lifetime risks fromexposure to natural background radiation and naturally occurringchemical carcinogens each are about 10�2, a negligible risk could beset at a small fraction (e.g., one percent) of the average backgroundrisk. Such a risk should be less than the variability in the backgroundrisk at any location due to differences in living habits. The negligibleindividual dose for radiation discussed above is consistent with thisalternative, because an annual effective dose of 10 �Sv is about onepercent of the dose due to natural background radiation, exclud-ing radon.

Exemption of waste that contains naturally occurring hazardoussubstances warrants further consideration because the negligiblerisks or doses for carcinogens described above may correspond toexemption levels that are less than background levels in soil or rock.This could be the case, for example, for radium, thorium, and arsenic.As a consequence, exemption of virtually any waste derived fromearthen materials could be precluded, even when the concentrationsof naturally occurring hazardous substances are not enhanced byhuman activities. In order to provide a practical system for exempt-ing such wastes, NCRP believes that exemption levels for naturallyoccurring hazardous substances in waste should be based on consid-erations of background levels in surface soil and their associatedhealth risks to the public, in addition to the negligible risks used toestablish exemption levels for man-made substances. These addi-tional considerations could be incorporated in the modifying factor(F) in Equation 1.1, which can be waste- and substance-specific.

Disposal facilities for nonhazardous waste (e.g., municipal/indus-trial landfills) normally are constructed without substantial engi-neered barriers, such as a rock cover or cement waste forms, thatwould deter inadvertent intrusion into waste, and the waste itselfoften is in a readily accessible physical form. Therefore, in determin-ing exempt waste, scenarios for inadvertent intrusion involving per-manent occupancy of disposal sites and normal human activitiesthat could access waste would be appropriate. Examples includeexcavation in the construction of homes and permanent residence on


a site at any time after an excavation. Furthermore, these scenariosshould be assumed to occur at the time of facility closure, becauseinstitutional control is not expected to be maintained at disposalsites for nonhazardous waste for a substantial period of time there-after. These assumptions would apply to any allowable means ofdisposal of exempt waste on or near the land surface. An assumptionthat exempt waste would be sent to a landfill for nonhazardous wastepermitted under Subtitle D of RCRA is not required.

1.5.4.2 Low-Hazard Waste. Waste classified as low-hazard wouldbe generally acceptable for disposal in a dedicated near-surface facil-ity for hazardous wastes. Limits on concentrations of hazardoussubstances in low-hazard waste would be derived based on anassumption that the risk or dose to a hypothetical inadvertentintruder at a disposal site should not exceed acceptable (barely toler-able) levels.

Acceptable risks or doses used to determine low-hazard wasteshould be substantially higher than the negligible risks or dosesused to determine exempt waste (see Section 1.5.3). As a result,limits on concentrations of hazardous substances in low-hazardwaste generally should be substantially higher than in exempt waste.The use of higher risks or doses in classifying low-hazard waste canbe justified on the following grounds. Institutional control is plannedto be maintained over dedicated near-surface disposal sites for haz-ardous wastes until the sites can be released for unrestricted useby the public. Furthermore, allowable risks or doses used to defineconditions for unrestricted release should be similar to the valuesthat would be used to determine exempt waste. Therefore, prior tounrestricted release of a site, scenarios for inadvertent intrusioninto near-surface disposal facilities for low-hazard waste should beregarded as accidental occurrences. Risks to inadvertent intruders,taking into account the probability that assumed exposure scenarioswould occur with a specified duration of exposures, should be compar-able to or less than risks resulting from unrestricted access to near-surface disposal sites for exempt waste.

Acceptable (barely tolerable) risks or doses used to classifylow-hazard waste could be established based on a variety of consider-ations, consistent with the different approaches to risk manage-ment for radionuclides and hazardous chemicals described inSection 1.5.3. For noncarcinogenic hazardous chemicals, NCRP rec-ommends that an acceptable dose should be set at a nominal thresh-old for deterministic responses in humans obtained as described inSection 1.5.5.3, or slightly below the threshold (e.g., by a factor oftwo or three) if an additional margin of safety is warranted. For


radionuclides, the limit on annual effective dose to individual mem-bers of the public of 1 mSv recommended by NCRP, which corres-ponds to an estimated lifetime fatal cancer risk of about 4 � 10�3

for an assumed exposure time of 70 y, provides a suitable precedent.The limits on concentrations of radionuclides in waste that is gen-erally acceptable for near-surface disposal established by NRC in10 CFR Part 61, based on scenarios for inadvertent intrusion follow-ing an assumed loss of institutional control at 100 y after disposal,also could be used. An acceptable (barely tolerable) risk from expo-sure to chemical carcinogens has not been considered by EPA, butits value should be about the same as for radionuclides.

As an alternative, multiples of RfDs established by EPA could beused to define acceptable (barely tolerable) doses of noncarcinogenichazardous chemicals, because RfDs normally are intended to be wellbelow nominal thresholds for deterministic responses in humans.However, the cautions about using RfDs discussed in the previoussection, especially when RfDs are based on data of poor quality, alsoapply in establishing acceptable doses. As in establishing negligibledoses of noncarcinogens, NCRP prefers an approach in which accept-able doses are based directly on nominal thresholds in humans andapplication of small safety factors, as appropriate, to promote trans-parency and consistency in waste classification.

Acceptable risks or doses for radionuclides and chemical carcino-gens also could be established based on considerations of unavoidablerisks from natural background; as noted previously, these lifetimerisks are about 10�2. For example, an acceptable risk could be setat a value corresponding approximately to the geographical variabil-ity in the background risk, because people normally do not considerthis variability in deciding where to live.

The assumed disposal systems for exempt waste and low-hazardwaste both involve near-surface disposal, and either type of wasteoften would be emplaced sufficiently close to the surface that inadver-tent intrusion into the waste could occur as a result of normal humanactivities. However, there are differences in the two types of disposalsystems that should be taken into account in developing appropriatescenarios for inadvertent intrusion. Disposal facilities for low-hazardwaste frequently include engineered barriers to deter inadvertentintrusion, impenetrable waste forms, or deliberate emplacement ofmore hazardous wastes at locations where access to the waste duringnormal human activities would be less likely. Most importantly, asnoted previously, current plans call for institutional control to bemaintained over hazardous waste disposal sites for a considerableperiod of time after facility closure, which allows for substantial


decay of many radionuclides and the possibility of chemical transfor-mations of other hazardous substances to less hazardous forms priorto the time permanent occupancy of disposal sites by the public couldoccur. Inadvertent intrusion could occur during the institutionalcontrol period, but credible scenarios would differ from scenariosinvolving permanent occupancy of disposal sites in regard to therelevant exposure pathways and the duration of exposures.

The role of institutional control over near-surface disposal sites isparticularly important in cases of very large volumes of waste, suchas uranium mill tailings and wastes from mining and milling of oresto extract nonradioactive materials, that contain concentrations ofnaturally occurring hazardous substances (e.g., radium, heavy met-als) far above background levels in Earth’s crust. For such wastes,the risk to an inadvertent intruder often would be well above anylevel that could be considered acceptable if permanent occupancy ofnear-surface disposal sites could occur. In the case of uranium milltailings, however, disposal in facilities located well below the groundsurface had been deemed impractical due to the volumes of wasteinvolved. Rather, the intention is to maintain perpetual institutionalcontrol over near-surface disposal sites to prevent scenarios for inad-vertent intrusion involving permanent site occupancy. Similarconsiderations could apply to large volumes of other mining andmilling wastes.

In developing generic scenarios for inadvertent intrusion intonear-surface disposal facilities used to determine limits on concen-trations of hazardous substances in exempt and low-hazard waste,consideration must be given to the question of how far into the futurethe scenarios should be applied, as well as the earliest time at whichthe scenarios could occur. This issue arises because the potentialrisk posed by some radionuclides (e.g., uranium) increases with time,due to the long-term buildup of radiologically significant decay prod-ucts, and some hazardous chemicals could be transformed over timeinto more hazardous forms. NCRP believes that scenarios for inad-vertent intrusion used to classify waste should be applied over atime period consistent with the time period for applying standardsfor protection of members of the public beyond the boundaries ofwaste disposal sites.

1.5.4.3 High-Hazard Waste. Waste classified as high-hazardwould contain such high concentrations of radionuclides or hazard-ous chemicals that it would not be generally acceptable for near-surface disposal in a dedicated facility for hazardous waste, butwould require disposal in a facility located well below the groundsurface. At the present time, geologic repositories are intended for


disposal of most radioactive waste that is not generally acceptablefor near-surface disposal (see Table 1.1 and Section 1.3.1.1), butthere are no planned alternatives to near-surface disposal for highlyhazardous chemical wastes.

An important characteristic of acceptable disposal facilities forhigh-hazard waste is that inadvertent intrusion into a facility, suchas by drilling, must be unlikely. Therefore, assessments of risk ordose to hypothetical inadvertent intruders based on exposure scenar-ios that are assumed to occur do not provide a suitable basis fordetermining acceptable disposals in facilities located well below theground surface.

1.5.5 Calculation of the Risk Index

The risk index for any hazardous substance in Equation 1.1 or 1.2(see Section 1.5.1) is calculated based on assumed exposure scenariosfor hypothetical inadvertent intruders at near-surface waste disposalsites and a specified negligible risk or dose in the case of exemptwaste or acceptable (barely tolerable) risk or dose in the case of low-hazard waste. Calculation of the risk index also requires consider-ation of the appropriate measure of risk (health-effect endpoint),especially for carcinogens, and the appropriate approaches to esti-mating the probability of a stochastic response per unit dose forcarcinogens and the thresholds for deterministic responses for non-carcinogens. Given a calculated risk index for each hazardous sub-stance in a particular waste, the waste then would be classified usingEquation 1.3.

1.5.5.1 Measure of Risk for Carcinogens. The health-effect end-point most often calculated in risk assessments for radionuclides iscancer fatalities, whereas cancer incidence normally is calculatedfor chemical carcinogens. In principle, the same measure of riskshould be used for all carcinogens in calculating the risk index.However, since about half or more of most cancers are fatal, thedifference between cancer fatalities and cancer incidence usually isonly about a factor of two or less. Such small differences generallyshould be unimpotant in classifying waste.

1.5.5.2 Estimates of Probability Coefficients for Carcinogens. Thenominal probabilities of a stochastic response (primarily cancers)per unit dose used in risk assessments, which are referred to in thisReport as probability coefficients, normally differ for radionuclidesand chemical carcinogens in regard to the degree of conservatismincorporated in the assumed values and the number of organs ortissues at risk that are taken into account.


The nominal probability coefficient for radionuclides normallyused in radiation protection is derived mainly from maximum likeli-hood estimates (MLEs) of observed responses in the Japanese atomic-bomb survivors. A linear or linear-quadratic dose-response model,which is linear at low doses, is used universally to extrapolate theobserved responses at high doses and dose rates to the low doses ofconcern in radiation protection. The probability coefficient at lowdoses also includes a small adjustment that takes into account anassumed decrease in the response per unit dose at low doses anddose rates compared with the observed responses at high doses anddose rates.

In contrast, nominal probability coefficients for chemical carcino-gens are derived from upper 95 percent confidence limits of observedresponses at high doses, mainly in studies in animals. In some stud-ies, the difference between the upper 95 percent confidence limitand MLE of the observed responses at high doses is an order ofmagnitude or more. Furthermore, several models have been used toextrapolate the observed responses to the low doses of concern inhealth protection of the public, with the result that estimated proba-bility coefficients at low doses can differ by several orders of magni-tude depending on the extrapolation model chosen.

Thus, the nominal probability coefficients at low doses of chemicalcarcinogens could be considerably more conservative (more likely tooverestimate risk) than the probability coefficient for radionuclides.As a result, potential risks posed by chemical carcinogens could begiven a disproportionate weight in classifying waste.

For the purpose of classifying waste that contains radionuclides,NCRP reaffirms use of the nominal probability coefficient for fatalcancers (i.e., the probability of a fatal cancer per unit effective dose)of 0.05 Sv�1 normally assumed in radiation protection of the public.For chemical carcinogens, NCRP believes that MLEs of probabilitycoefficients obtained from the linearized, multi-stage model shouldbe used in classifying waste, in order to provide reasonable consis-tency with the probability coefficient for radionuclides. The use ofMLEs for chemical carcinogens usually will result in substantiallylower probability coefficients than the use of upper 95 percent confi-dence limits.

The use of MLEs of probability coefficients, rather than upperconfidence limits (UCLs), to classify waste can be justified, in part,on the grounds that the assumed exposure scenarios for hypotheticalinadvertent intruders at waste disposal sites are expected to beconservative compared with likely on-site exposures at future times.However, uncertainties in probability coefficients should still be con-sidered in classifying waste. When risk is calculated using MLEs of


probability coefficients, judgments about allowable risk, which canbe substance-specific, should take uncertainties in probability coeffi-cients into account, along with such other factors as judgments aboutthe quality of the data on dose-response, desired margins of safetyin protecting the public, and the cost-benefit of different choices.This approach would provide a clear separation between risk assess-ment and risk management aspects of waste classification. Riskassessment would focus on central estimates of risk for assumedexposure scenarios, and risk management decisions could incorpo-rate any desired degrees of conservatism in protecting the publicbeyond those embodied in the assumed scenarios.

In risk assessments for radionuclides, the nominal probabilitycoefficient is applied to the effective dose, which takes into accountdoses and stochastic responses in all irradiated organs or tissues.In contrast, probability coefficients for chemical carcinogens oftenare based on observed responses in a single organ and the possibilityof significant responses in multiple organs is not taken into account.This difference cannot be eliminated at the present time, in partbecause the probability coefficients for most chemicals are based onstudies in animals and the organs in which cancers are seen in thestudy animals often do not correspond to the organs at greatest riskin humans. However, the extent of underestimation of risks fromexposure to chemical carcinogens is unlikely to be large when cancerspresumably are induced only at sites of deposition in the body. Fora few chemical carcinogens, the probability coefficients are based onobserved responses in multiple organs.

1.5.5.3 Thresholds for Deterministic Effects. In controlling expo-sures to substances that induce deterministic health effects, the goalis to prevent such effects by limiting doses to levels considered safe,i.e., to levels below those known to cause adverse effects. Thus, thegoal is to achieve zero risk. For radionuclides and noncarcinogenichazardous chemicals, dose limits for the public are established byapplying safety and uncertainty factors to nominal threshold dosesestimated from studies in humans or animals. Thus, doses consid-ered safe are substantially less than no-effects levels observed inthe studies. For radionuclides, deterministic dose limits are unim-portant in routine health protection of the public, and should beunimportant in classifying waste, because the limit on annual effec-tive dose of 1 mSv from exposure to all man-made sources combined,which is intended to limit stochastic effects, generally ensures thatequivalent doses in any organ or tissue will be substantially lessthan the limits intended to prevent deterministic effects. Therefore,the approach to estimating thresholds for deterministic effects for the


purpose of classifying waste should be important only for hazardouschemicals.

For noncarcinogenic hazardous chemicals, NCRP believes that thethreshold for deterministic effects in humans should be estimatedusing EPA’s benchmark dose method, which is increasingly beingused to establish allowable doses of noncarcinogens. A benchmarkdose is a dose that corresponds to a specified level of effects in astudy population (e.g., an increase in the number of effects of 10percent); it is estimated by statistical fitting of a dose-response modelto the dose-response data. A lower confidence limit of the benchmarkdose (e.g., the lower 95 percent confidence limit of the dose thatcorresponds to a 10 percent increase in number of effects) then isused as a point of departure in establishing allowable doses.

Consistent with EPA’s benchmark dose method, NCRP believesthat a suitable representation of the threshold for deterministiceffects in virtually all humans is a dose that is a factor of 10 lowerthan the lower confidence limit of the benchmark dose obtained ina high-quality human study or a dose that is a factor of 100 lowerthan the lower confidence limit of the benchmark dose obtained ina high-quality animal study. The reduction by a factor of 10 whendata in humans are available takes into account the need to protectsensitive population groups (e.g., children, the elderly). This reduc-tion is consistent with the approach used in radiation protection ofthe public, where deterministic dose limits are set at a factor of 10lower than nominal thresholds for deterministic radiation effects inadults. The further reduction by a factor of 10 when data are avail-able only in animals takes into account that the animals may be lesssensitive than humans. The recommended approach acknowledgesthe considerable uncertainty in estimating the highest dose at whichno significant effects would be observed in humans. However, theapproach is not unduly conservative and, thus, should not give dis-proportionate weight to noncarcinogenic hazardous chemicals, com-pared with radionuclides and chemical carcinogens, in classifyingwaste.

In traditional toxicological methods of determining virtually safedoses of hazardous chemicals, nominal thresholds for deterministicresponses in humans are estimated based on a NOAEL obtained inhuman or animal studies. In most high-quality studies, NOAEL isapproximately the same as the lower confidence limit of the bench-mark dose that corresponds to a 10 percent increase in the numberof responses. Thus, as an alternative to the benchmark dose method,the nominal threshold in humans could be set at a factor of 10 or100 lower than NOAEL obtained in a high-quality human or animalstudy. However, the benchmark dose method preferred by NCRP


generally should provide a more reliable estimate of the highest doseat which no effects would be observed, mainly because the methodmakes use of the full range of data on dose-response, rather than asingle data point (NOAEL). The benchmark dose method can alsoaddress difficulties that arise when a NOAEL is not obtained in ahigh-quality study or is not included in a data set.

Given the nominal threshold for deterministic effects in virtuallyall humans estimated as described above, NCRP believes that thenegligible and acceptable (barely tolerable) doses of noncarcinogenichazardous chemicals used in classifying waste should be set at appro-priate fractions of the nominal thresholds (see Sections 1.5.4.1and 1.5.4.2). NCRP’s preferred approach is transparent in pre-senting the nominal threshold in humans, and it encourages theuse of reasonably consistent safety factors for all noncarcinogens inestablishing negligible and acceptable doses.

1.5.5.4 Risk Index for Mixtures of Hazardous Substances. For thepurpose of developing a comprehensive and risk-based hazardouswaste classification system, a simple method of calculating the riskposed by mixtures of radionuclides and hazardous chemicals isneeded. The method should account for the linear, nonthresholddose-response relationships for radionuclides and chemical carcino-gens (stochastic effects) and the threshold dose-response rela-tionships for noncarcinogenic hazardous chemicals (deterministiceffects).

NCRP believes that a conceptually simple composite risk indexfor mixtures of hazardous substances can be developed that providesan adequate representation of risk for the purpose of waste classifi-cation. The composite risk index is written in terms of separate riskindexes for substances that induce stochastic (s) and deterministic(d) effects as:

RIj � RI sj � RI d

j , (1.4)

where j is an index indicating whether the denominator in the riskindex (RI) (see Equation 1.1) represents a negligible or acceptablerisk (i.e., whether a material is being evaluated for classificationas exempt or low-hazard waste). The recommended approaches toevaluating Equation 1.4 are described in the following sections.

1.5.5.4.1 Risk index for mixtures of substances that cause stochasticeffects (carcinogens). The risk index for mixtures of substances thatcause stochastic effects (radionuclides and chemical carcinogens)takes into account the risk in all organs or tissues, and it assumesthat the risk in any organ is independent of the risk in all otherorgans. The risk index for mixtures of substances causing stochasticeffects can be represented as:


RI sj � �

i�

r�

T

Fi(risk from disposal) s

i, j, r,T

(allowable risk) si, j, r,T

, (1.5)

where the index T denotes the different organs (tissues) at risk, rdenotes the different stochastic health effects of concern (cancers andsevere hereditary effects), and i again denotes each hazardous sub-stance. The index j described following Equation 1.4 is included in thenumerator, as well as the denominator, to indicate that the exposurescenarios for disposal of exempt waste can differ from those for dis-posal of low-hazard waste (see Section 1.5.4.2). In accordance withEquation 1.2, the risk index (RI) for mixtures of substances causingstochastic effects can be evaluated in terms of dose rather than risk.

In practice, Equation 1.5 can be greatly simplified. For radionu-clides, doses in all organs and tissues and the different health effectsof concern are incorporated in the effective dose, and calculation ofthe risk index for mixtures is reduced to a single sum over all radionu-clides of the ratio of a calculated effective dose from exposure to eachradionuclide to the allowable effective dose applicable to the particularwaste class (disposal technology) of concern. Furthermore, the denom-inator in the risk index normally would be the same for all radionu-clides, and any differences in judgments about an allowable effectivedose for different wastes in the same class could be included in themodifying factor which can be radionuclide-specific. For hazardouschemicals, substance-specific probability coefficients incorporateinformation on risks in single or, in a few cases, multiple organs andthe stochastic effects of concern. Therefore, calculation of the riskindex for mixtures again is reduced to a single sum over all substancesof the ratio of a calculated to an allowable risk or dose.

1.5.5.4.2 Risk index for mixtures of substances that cause determin-istic effects (noncarcinogens). The risk index for mixtures of sub-stances causing deterministic effects (hazardous chemicals only)takes into account the threshold dose-response relationships fordeterministic effects in any organ. The risk index for mixtures alsoassumes, first, that doses in any organ due to exposures to multiplesubstances are additive even though the deterministic effectsinduced in that organ may not be the same for each substance and,second, that the threshold doses in any organ are independent ofdoses in any other organ. Based on these assumptions, the risk index(RI) for mixtures of substances causing deterministic effects, whichgenerally should be expressed in terms of dose rather than risk (seeSection 1.5.1), can be represented as:

RI dj � INTEGER �MAXT �

i�

r

Fi(dose from disposal) d

i, j, r,T

(allowable dose)di, j, r,T

� , (1.6)


where MAX is a function yielding the maximum value of a set ofnumbers and INTEGER is a function yielding the truncated integervalue of a number. All indexes in this equation have been describedpreviously, except r denotes the different deterministic health effectsof concern.

The procedure for evaluating Equation 1.6 is the following. Foreach substance, the critical organ or organs in which deterministiceffects are assumed to occur are identified, and the ratio of a calcu-lated dose to those organs in the assumed exposure scenario to theallowable dose to those organs for the waste class of concern isobtained. If a particular substance is assumed to induce determinis-tic effects in more than one organ, this ratio is calculated for allorgans at risk from exposure to that substance. Then, for each organ,the ratios of calculated to allowable doses are summed over all sub-stances that induce deterministic effects, without regard for anydifferences in the health effects induced in that organ by the differentsubstances, and the maximum of the summed ratios in any organis selected. Use of the MAX function is based on the assumptionthat induction of deterministic effects in any organ is independentof doses to other organs. Finally, the highest risk index in any organis truncated using the INTEGER function. This operation takesinto account the assumption that the risk of a deterministic responseis zero if the dose to each organ is less than the allowable dose inthe denominator of the risk index. The modifying factor (F) is allowedto depend on the particular hazardous substance of concern, butits value often would be the same for all substances that causedeterministic effects.

1.5.5.4.3 Use of the composite risk index in classifying waste. Giventhe risk indexes for mixtures of substances causing stochastic ordeterministic effects calculated using Equations 1.5 and 1.6, respec-tively, the composite risk index for all hazardous substances is calcu-lated using Equation 1.4. This procedure assumes that induction ofstochastic effects is independent of exposures to substances causingdeterministic effects, and vice versa.

In accordance with Equation 1.3 (see Section 1.5.1) and as indi-cated in Figure 1.2 (see Section 1.5.4), classification of waste wouldproceed in the following way. First, if the composite risk index isless than unity when the denominator represents a negligible riskand the numerator is evaluated using an exposure scenario appro-priate to disposal of nonhazardous waste, the waste would be classi-fied as exempt, but the waste would be nonexempt if the compositerisk index is unity or greater. Then, for nonexempt waste, if the

1.6 IMPLICATIONS OF THE RECOMMENDED SYSTEM / 51

composite risk index is less than unity when the denominator repre-sents an acceptable (barely tolerable) risk and the numerator isevaluated using an exposure scenario appropriate to disposal in adedicated near-surface facility for hazardous waste, the waste wouldbe classified as low-hazard, but the waste would be classified ashigh-hazard if the composite risk index is unity or greater.

As emphasized in Section 1.1, the recommended approach to clas-sifying waste does not provide a basis for establishing waste accep-tance criteria at specific disposal sites. NCRP expects, however, thatwaste classified as exempt or low-hazard in accordance with its rec-ommendations should be acceptable for disposal in the associatedtype of disposal facility at well-chosen sites.

1.6 Implications of the Recommended WasteClassification System

The recommended risk-based waste classification system hasimportant implications in three areas: (1) the resulting classificationof existing radioactive and hazardous chemical wastes, (2) subclassi-fication of the basic waste classes, and (3) changes in existing lawsand regulations that would be required to implement such a classifi-cation system.

1.6.1 Classification of Existing Hazardous Wastes

As part of this study, NCRP investigated how the recommendedwaste classification system would affect the current classificationsof radioactive and hazardous chemical wastes. The results of thisinvestigation are summarized as follows:

● Substantial quantities of waste that contains small amounts ofradionuclides or hazardous chemicals could be exempted fromregulatory control as hazardous waste.

● Most radioactive waste currently classified as low-level wasteand most hazardous chemical waste would be classified as low-hazard waste, based on the expectation that these wastes wouldbe generally acceptable for disposal in dedicated near-surfacefacilities for hazardous wastes. A possible exception is hazardouschemical waste that contains relatively high concentrations ofheavy metals, which could be classified as high-hazard waste.

● Most uranium and thorium mill tailings that contain elevatedlevels of naturally occurring radionuclides could be classifiedas low-hazard waste, but only under conditions of perpetual


institutional control. In the absence of institutional control overdisposal sites, most mill tailings would be classified as high-hazard waste. Management and disposal of the large volumesof uranium mill tailings would continue to require separate con-siderations regardless of how this type of waste is classified,because regulatory authorities have judged that disposal of thesewastes below ground is impractical and unrestricted release oftailings piles could result in unacceptable health risks to thepublic. Similar considerations could apply to other wastes withlarge volumes that contain elevated levels of hazardous sub-stances, especially heavy metals (e.g., wastes from mining orprocessing of ores to obtain nonradioactive materials).

● Most radioactive waste currently classified as spent fuel, high-level waste, or transuranic waste, and most low-level waste cur-rently subclassified as greater-than-Class-C would be classifiedas high-hazard waste, based on the expectation that thesewastes usually would require disposal in a geologic repositoryor other disposal system providing a substantially greater degreeof waste isolation than a near-surface facility. However, someof these wastes that contain relatively low concentrations ofradionuclides could be classified as low-hazard waste.

Based on this investigation, the waste classification system recom-mended by NCRP appears to be largely consistent with the currentclassification systems for radioactive and hazardous chemical wastesand with plans for their disposal. Therefore, implementation of thenew waste classification system should not be unduly disruptive orcostly. In addition, the possibility that substantial quantities ofwaste currently classified as radioactive or chemically hazardouscould be exempted from regulatory control, and thus managed asnonhazardous waste or considered for beneficial use, could result insignificant cost savings without increasing risks to public health bymore than a negligible amount.

1.6.2 Subclassification of Basic Waste Classes

Various wastes that would be classified as low-hazard or high-hazard in accordance with NCRP’s recommendations may have sig-nificantly different physical, chemical, radiological, or toxicologicalproperties. To facilitate efficient management of wastes having dif-ferent properties, subclassification of these waste classes may bedesirable. For example, uranium mill tailings and other similarwastes with very large volumes could be distinguished from wasteswith similar properties but much smaller volumes in subclassifying

1.6 IMPLICATIONS OF THE RECOMMENDED SYSTEM / 53

low-hazard waste, because large waste volumes necessitate a differ-ent approach to management and disposal. Similarly, it may bereasonable to subclassify smaller volumes of low-hazard waste thatcontains varying concentrations of hazardous substances, in a man-ner similar to the system for subclassifying low-level radioactivewaste developed by NRC in 10 CFR Part 61.

NCRP believes that subclassification of the basic waste classeswould be appropriate as long as it is based on properties of wastethat are related to health risks from disposal or considerations of thecost-benefit of different options for waste management and disposal.Other factors that have influenced waste classification in the pastshould not be used as a basis for waste subclassification. For example,the present distinction between radioactive waste that arises fromoperations of the nuclear fuel cycle and NARM waste should not bemaintained in subclassifying waste, because this distinction is basedsolely on the source of the waste rather than significant differencesin health risks from waste disposal or considerations of cost-benefitin waste management and disposal.

1.6.3 Legal and Regulatory Implications

Development of the generally applicable and risk-based waste clas-sification system recommended by NCRP would have a number ofimportant implications with regard to current laws and regulations:

● A general class of exempt waste, which could be regulated asnonhazardous material, would be established. Development ofan exempt class of waste that contains low levels of hazardoussubstances has been controversial and currently is banned bylaw in the case of radioactive waste. Some radioactive and haz-ardous chemical wastes have been exempted on a case-by-casebasis, but general principles for exempting radioactive or haz-ardous chemical wastes have not been established. In spite ofthese difficulties, however, a meaningful risk-based waste classi-fication system must include a general class of exempt waste.

● The present difficulties with management and disposal of mixedradioactive and hazardous chemical wastes, which result fromdual regulation of these materials under AEA and RCRA orother laws (e.g., TSCA) and the different approaches to wastemanagement and disposal under the various laws, would beaddressed by including all radioactive and hazardous chemicalwastes in the same waste classification system. This approachto waste classification would require changes in existing lawsand regulations that apply to mixed waste.


● Inclusion of NARM waste in the same classification system withradioactive waste that arises from operations of the nuclearfuel cycle would require a change in the scope of AEA, becausemanagement and disposal of commercial NARM waste cannotbe regulated under AEA.

● Under current laws and regulations, many radioactive and haz-ardous chemical wastes are classified based on their source,rather than their radiological or toxicological properties. Devel-opment of a risk-based waste classification system would requireelimination of source-based waste classifications.

● The recommended waste classification system would affect cur-rent disposal practices for hazardous chemical waste in twoways. First, the new system calls for risk assessments over longtime frames in deciding whether waste is generally acceptablefor near-surface disposal. Second, it allows the possibility thatwaste containing the highest concentrations of hazardous chemi-cals might be classified as high-hazard waste and, thus, wouldgenerally require a disposal system considerably more isolatingthan the type of near-surface facility currently used for all haz-ardous chemical waste.

1.7 Further Development of the Recommended WasteClassification System

The waste classification system presented in this Report wouldapply to all radioactive and hazardous chemical wastes from anysource, and it would be based on considerations of health risks tothe public that arise from waste disposal. The recommended classifi-cation system differs from the existing waste classification systemsin three respects: radioactive and hazardous chemical wastes wouldbe included in the same classification system; all waste would beclassified based on its properties, rather than its source; and theclassification system would include a general class of exempt waste.

Given these differences, NCRP believes that replacement of theexisting waste classification systems by the classification systemrecommended in this Report should be undertaken carefully overtime, and in recognition that the existing systems for waste classifi-cation and waste management, despite their shortcomings, havewith few exceptions been more than adequate in protecting humanhealth. In establishing a new hazardous waste classification systemthat would be an improvement over the existing systems, there is a

1.7 FURTHER DEVELOPMENT OF THE RECOMMENDED SYSTEM / 55

need to ensure that current approaches to management and disposalof radioactive and hazardous chemical wastes would not be undulydisrupted. While some of the benefits of the recommended wasteclassification system could be obtained with incremental changes tothe existing system (e.g., the addition of exempt classes of radioactiveand hazardous chemical wastes), many other benefits, such as classi-fication of waste based on risk and transparency of the system, willrequire full implementation.

Many details would need to be considered in developing a newwaste classification system based on the framework presented inthis Report. Assumptions about generic scenarios for exposure ofhypothetical inadvertent intruders at waste disposal sites to be usedin classifying waste and the time frames for applying the scenarioswould be required. Decisions would need to be made about negligibleand acceptable (barely tolerable) doses or risks that would be usedin classifying waste as exempt or low-hazard, respectively.

Inconsistencies in current approaches to cancer risk assessmentfor radionuclides and hazardous chemicals would need to be consid-ered and resolved in developing a comprehensive waste classificationsystem. Foremost among these is the difference between cancer riskestimates for radionuclides, which are based on MLEs of observedrisks and a standard model for extrapolating observed risks at highdoses to the low doses of concern in health protection of the public,and risk estimates for chemical carcinogens, which are based onupper 95 percent confidence limits of observed risks and have beenderived using different risk-extrapolation models that can result inrisk estimates at low doses that differ by several orders of magnitude.Another issue requiring consideration is the difference in the measureof risk normally used in cancer risk assessments, i.e., fatal cancers forradionuclides but cancer incidence for chemical carcinogens.

For noncarcinogenic hazardous chemicals, an important issuerequiring consideration is the most suitable approach to estimatingnominal thresholds for deterministic health effects in humans. Con-sideration also needs to be given to the appropriate magnitude ofsafety and uncertainty factors that should be applied to nominalthresholds in determining negligible and acceptable doses of noncar-cinogens. Deterministic effects from exposure to radionuclidesshould not be important in classifying waste.

In addition to the effort required to develop a comprehensive andrisk-based hazardous waste classification system based on NCRP’srecommendations, several legal and regulatory impediments wouldneed to be addressed. However, the resulting classification systemwould be more transparent and understandable than the separateclassification systems for radioactive and hazardous chemical wastes


in use at the present time. In addition, the new system would belargely consistent with the existing classification systems in regardto the intended disposal technologies for the different waste classes.Such a classification system could lead to greater acceptance by thepublic of waste management and disposal activities.

2. Introduction

The purpose of this Report is to set forth the technical principlesand framework for a comprehensive and risk-based hazardous wasteclassification system. In this context, waste is any material that hasinsufficient value to justify further beneficial uses, and thus mustbe managed at a cost. Hazardous waste is waste that can be harmfulto biological organisms, due to the presence of radioactive substancesor chemicals that are deemed hazardous, to the extent that it must beregulated. Hazardous waste excludes material that is simply useless(e.g., typical household trash). This work is comprehensive becauseit considers all hazardous wastes irrespective of their source.1

NCRP undertook a study of waste classification because of theimportance and visibility of hazardous waste management in theUnited States coupled with the observation that the existing classifi-cation systems for hazardous wastes are increasingly complex andinefficient. This determination led to the independently conceivedalternative approach to hazardous waste classification described inthis Report.

2.1 Foundations and Directions

This Section presents basic definitions and concepts necessary toundertake a detailed discussion of waste classification (more detailedtechnical background is provided in Section 3). It also describes thescope of this Report to indicate why further discussion of a numberof issues that are important to waste classification is not requiredbecause they are outside the scope of this study.

1 Biohazardous wastes are not considered because they are conventionally renderednonhazardous before disposal according to guidelines of EPA (1986a).

57

58 / 2. INTRODUCTION

2.1.1 Definition of Waste Classification

The generic life cycle of materials containing radionuclides or haz-ardous chemicals is shown in Figure 2.1. The current approach towaste management is to prevent the generation of hazardous wasteby substituting nonhazardous input materials to the extent practica-ble. Generation of hazardous waste also may be reduced by recycling

Fig. 2.1. Life cycle of hazardous materials.

2.1 FOUNDATIONS AND DIRECTIONS / 59

of hazardous materials, either within the generating facility (e.g.,treatment of contaminated waste water and reuse of the purifiedwater within the facility) or externally (e.g., recycling of lead con-tained in automobile batteries). To the extent that these practicesare not implemented for whatever reason, hazardous wastes result.

Waste management includes any activities associated with thedisposition of waste products after they have been generated. Opera-tions in waste management typically include:

● A wide variety of treatment technologies to reduce the volume,change the physical or chemical form (e.g., incineration, solidifi-cation of a liquid waste, neutralization of acidic or basic waste),and suitably package the waste for subsequent managementsteps.

● Storage (defined as holding a waste with the intent to retrieveit for further management operations) awaiting the accumula-tion of an economic quantity of material for subsequent steps(e.g., a full truck load of waste), allowing for decay of radioactivematerials, or awaiting the development of appropriate treatmentor disposal facilities.

● Transportation from generation to treatment or storage facilitiesand eventually to disposal facilities.

● Disposal of the waste by emplacing it in isolating surroundings,with no intent to retrieve it, for the purpose of preventing thehazardous substances from reaching the biosphere in unaccept-able amounts (this does not mean that the waste is irretrievableif a particular disposal method does not prove satisfactory).

An important purpose of waste management is to dispose of haz-ardous waste safely and cost-effectively. If waste disposal is not cost-effective within the constraint of protecting human health and theenvironment, then resources would be required that could better bespent on other beneficial activities.

For the purposes of this Report, waste classification is defined asa grouping of wastes having similar attributes related to disposal.For example, one might seek to group highly toxic and long-livedwastes in one class destined for disposal in a geologic repository andlower-toxicity or shorter-lived wastes in another class destined fordisposal in a regulated near-surface facility. A waste classificationsystem could be expressed in terms of waste characteristics thatdefine the boundaries between classes and the rules for using thesedefining characteristics. For example, a waste that contains morethan 100 units of a hazardous substance per cubic meter of wastemight be in waste Class X, a waste with 10 to 100 units per cubicmeter in waste Class Y, and waste with less than 10 units per cubic


meter in waste Class Z. Waste also could be classified for purposesother than disposal (e.g., transportation or storage), but such classi-fications are not addressed in this Report.

2.1.2 Purpose of Waste Classification

The purpose of waste classification is to provide guidance at con-ceptual and operational levels on appropriate approaches to wastemanagement and disposal for many kinds of waste exhibiting widelyvarying potential hazards. However, it may not be immediately obvi-ous why a waste classification system is needed. Referring toFigure 2.1, it would appear possible to simply accumulate wasteuntil a disposal facility is available and then send the waste to thefacility. While there are many reasons for classifying waste (IAEA,1994), there are two major reasons why accumulating waste andsending it to a single disposal facility is sufficiently impractical soas to require that wastes be classified.

First, without waste classification there would be one large com-posite waste stream that is the aggregate of many input streams.These input streams could range from essentially nonhazardous (e.g.,household and industrial trash) to highly hazardous (e.g., high-levelradioactive waste). The composite stream would have a very largevolume because of the large amounts of nonhazardous trash, and itwould be managed on the basis of its higher-hazard substances,which could require the use of technologies for treatment (e.g., high-integrity packaging, vitrification) and disposal (e.g., engineered andmonitored near-surface disposal, geologic repository) that are muchmore expensive than what is needed for nonhazardous trash. Theneed to manage waste based on the characteristics of constituentsthat pose the highest hazard would require the siting and construc-tion of an impractically large number of treatment and disposalfacilities to handle the large volume of aggregated waste. Thisapproach would entail costs far beyond those required to protecthuman health and the environment.

A second driving force for waste classification, which cannot beadequately reflected in Figure 2.1, is the sequential nature of wastemanagement activities that take place over an extended period oftime:

● A wide range of hazardous waste is being generated and somemanagement actions must be performed almost immediately(e.g., containment of hazardous materials, assuring proper stor-age). However, disposal is often many years in the future, asevidenced by the fact that a geologic repository for the most


hazardous radioactive wastes will not begin operations untilmany decades after these wastes were first generated. As aconsequence of timing differences for various waste managementoperations, decisions must be made now concerning actions thatanticipate the disposal technology that will eventually be used,and these decisions must be clearly communicated to ensureconsistency and continuity. Particularly important in this regardare decisions concerning the co-mingling of various wastestreams,2 dilution of hazardous wastes,3 exemption of wastematerials from requirements for management as hazardouswaste,4 the relevant regulations and regulatory agency, and thenature of any immediate treatment in anticipation of disposal.

● If it is assumed that there will be multiple waste disposal tech-nologies available to waste generators (e.g., municipal/industriallandfill, near-surface engineered facility, geologic repository), itis necessary to decide which wastes are generally acceptable foreach type of disposal facility so that planners can determine therequired capacity. Furthermore, planning for treatment facili-ties requires knowledge of the expected amounts of variouswastes in different classes because the treatment required forwastes is usually determined by disposal requirements.

● Early knowledge of the cost of management operations for differ-ent waste classes allows engineers to optimize the design ofwaste generating facilities and treatment operations (e.g., pro-duction of a large volume of lower-hazard waste versus a smallvolume of higher-hazard waste). For example, knowledge of thecosts of transporting and disposing of low-level and high-levelradioactive wastes at DOE sites allows decisions to be madeconcerning the extent to which costly separations technologyshould be used to reduce the amount of the more expensive high-level waste.

● Early knowledge of the types of facilities needed in the manage-ment sequence for each class of waste is necessary to plan for andestablish appropriate regulations. Waste management facilities

2 It is usually less costly to manage a unit of lower-hazard waste than one with ahigher hazard. Thus, a generator would not want to create a large volume of high-hazard waste by adding a smaller volume of high-hazard waste to a large volume oflow-hazard waste.

3 In the United States, it is deemed unacceptable to dilute a waste that poses ahigh-hazard for the purpose of reducing the hazard, including doing so by combiningwastes of different hazard, unless such combination eliminates an inherently hazard-ous characteristic (e.g., ignitability, corrosivity).

4 It is desirable to identify and segregate exempt wastes at the earliest possibletime to avoid unnecessary expenditure of resources to further manage them and toavoid subsequent contamination with hazardous materials.


are expensive, and regulations need to be established in advanceto provide public confidence that will allow resources to be com-mitted to facility construction and operation, to provide the basisfor facility design (e.g., effluent controls), and to allow promptcommencement of facility operations.

In summary, a hazardous waste classification system is neededbecause (1) disposal of the composite unclassified waste would beprohibitively expensive and (2) the differences in timing betweenwaste generation (now) and the development of treatment and dis-posal facilities (the future) require that wastes be segregated inanticipation of cost-effective means of waste management and dis-posal. Waste classification also allows consistent communication ofthe information needed to develop adequate treatment and disposalcapacity and to develop appropriate regulations.

2.1.3 Bases for Waste Classification

Numerous formal waste classification systems, or, equivalently,boundaries between classes of waste and rules for using them, havebeen developed over the years (see Section 4 for an extensive discus-sion). The bases for the boundaries also are numerous, with thefollowing being the most common:

● physico-chemical properties (for example, strong acid or base,pyrophoric)

● facility or process (i.e., source) generating the waste● composition of the waste, especially its hazardous constituents● environmental persistence or rate of degradation and decay● environmental mobility or availability● toxicity of hazardous substances in the waste (i.e., probability

and severity of adverse health effects resulting from ingestionor inhalation of a unit amount of material)

Other factors that are sometimes taken into account in classifyinghazardous waste include the quantity of the waste, containment(packaging) of the waste, potential for exposure to hazardous sub-stances, control of potential releases, environmental and humanhealth risks, economic factors, and sociopolitical aspects (IAEA,1994). Most of these factors are, at best, indirectly related to risk(e.g., a material that does not degrade rapidly does not necessarilypose a larger risk) and, at worst, are unrelated to risk (e.g., wastefrom a particular source being treated as if it were hazardous irre-spective of its constituents and their concentrations).


As implied by Figure 2.1, the objective of a waste managementsystem is to identify generally acceptable methods for disposition ofhazardous wastes such that risks to human health and the environ-ment will be sufficiently low. Given the need to categorize wastes(see previous section), it makes sense to define the categories so thatthe wastes in each category would pose roughly equivalent risksfollowing disposition. The consequence of this is that wastes withina category could be managed in the same way to ensure adequateprotection of human health and the environment while not commit-ting resources to excessively protective measures. That is, the idealwaste classification system should be based on considera-tions of risk management. Unfortunately, achieving this idealsolution while trying to establish a practical waste classificationsystem faces some major obstacles:

● Risk assessment (i.e., calculation of risk) is a complex, multi-step process, and the results usually have a significant degreeof uncertainty because of limitations in data and in the modelsof environmental and biological systems. In addition, for pur-poses of generally classifying waste, risk assessment must begeneric; i.e., it is not intended to apply to disposition of a specificwaste in a specific manner at a specific site.

● Establishing the boundaries in a risk-based waste classificationsystem requires that one or more values of acceptable risk bespecified. The values of acceptable risk are then used to establishthe values of parameters that define the boundaries of the differ-ent waste classes. The process of establishing the value(s) ofacceptable risk is part of risk management. Risk managementis an essential aspect of establishing a waste classification sys-tem, but it has an important nontechnical component thatreflects societal values.

● Existing waste classification systems now codified in law, regula-tion, and commerce evolved from times when risks that arisefrom waste disposal could only be evaluated qualitatively, dueto inadequate knowledge of the long-term performance of wastedisposal systems and the dose-response relationships for radio-nuclides and hazardous chemicals. Establishing a new, risk-based waste classification system will require changes in currentlaws and regulations. Despite the benefits of such a unified,transparent system, change is likely to be resisted because oflegal and regulatory inertia.

Despite the challenges described above, NCRP believes that riskis the appropriate primary basis for a waste classification system,and risk will be used as the basis for the work described in this


Report; thus, the phrase ‘‘risk-based waste classification system.’’The desirability of basing a waste classification system on risk hasbeen recognized for many years (DOE, 1980). While ‘‘risk’’ will bedefined more precisely in subsequent sections, the following generaldefinition is useful at this point (Garrick and Kaplan, 1995):

Risk is composed of:

● What can go wrong (an undesirable event)?● How likely is it (probability)?● What are the consequences (e.g., probability of induction of

cancer)?

This means that development of a risk-based waste classificationsystem must consider the events that could result in exposing biologi-cal organisms (e.g., humans) to hazardous substances placed in adisposal facility, the probability that each event will occur, and theconsequences of the event if it does occur.

2.1.4 Shortcomings of Current Waste Classification Systems

Given that waste classification systems presently exist, it is rea-sonable to ask whether an effort to develop the foundations of a newsystem would be beneficial. The short answer (expanded in Sections 4and 5) is ‘‘yes’’ for the following reasons:

● For many wastes, there is no practical classification sys-tem for establishing a boundary (e.g., amount of hazard-ous substances) below which the waste is considered tobe nonhazardous. This means that large volumes of wasteare managed at considerable cost because the waste cannot beconclusively shown to contain no hazardous substances or, evenmore difficult, to contain an amount of hazardous substances(e.g., uranium) no greater than was initially present in a materialbefore its use by humans.

● Significant portions of existing waste classification sys-tems are not based on the primary objective of ensuringthat the risk that arises from waste disposal is acceptable.The best example of this is wastes that are classified based solelyon the nature of the generating process or facility (e.g., high-level radioactive waste, chemical wastes from certain indus-tries), irrespective of the content and concentration of hazardoussubstances. This results in resources being used unnecessarilyon lower-risk situations when they could be better applied tohigher-risk situations (hazardous waste disposal or otherwise).For example, billions of dollars have been spent in managing


DOE’s high-level wastes as if they were among the most hazard-ous of all radioactive wastes. However, the concentrations ofhazardous substances in some of these wastes are similar tothose in low-level radioactive waste that is normally intendedfor disposal in near-surface facilities. In contrast, some chemicalwastes that are highly hazardous, compared with other wastes,and nondegradable are being sent to near-surface disposal facili-ties. Both of these situations occur largely because of the source-based aspects of existing waste classification systems.

● To the extent that risk is used as a basis for waste classifi-cation, it is not used consistently. Different values for accept-able risk are assumed for different hazardous waste disposalsituations. In addition, a variety of surrogate measures (e.g.,ingestion toxicity, total radioactivity) having varying relation-ships to risk have been used to classify wastes.

● The requirements for managing hazardous chemicalwaste are sufficiently different from those for radioactivewaste that treatment and disposal of waste that containsboth types of substances is greatly impeded. Large volumesof waste that contains hazardous chemicals and radionuclides(referred to as ‘‘mixed waste’’) are presently being stored becausethe inconsistency in regulations has resulted in inadequatetreatment and disposal capacity.

● The existing waste classification systems are becomingincreasingly complex as additional waste streams areincorporated into a patchwork system that is not basedon a consistent set of principles. Some wastes are classifiedbased on their source (i.e., the nature of the process or facilitythat produces them), some based on their composition, and somebased on their physico-chemical characteristics.

● Some wastes are defined by exclusion (i.e., by what theyare not), not on the basis of their properties or associatedrisks. Low-level radioactive waste is defined as waste that isnot high-level waste, spent fuel, transuranic waste, or uraniumor thorium mill tailings. Because the excluded wastes are definedby their source, rather than their properties, the definition oflow-level waste is not based on properties of the waste andwastes in this class can vary from essentially innocuous to highlyhazardous over long time frames.

● Waste classification systems are not transparent or defen-sible. There exist numerous classification systems for differentwastes having a variety of bases and implementation rules thatare not tied to any consistent set of principles. As a consequence,the overall classification of hazardous waste is not transparent to


anyone but the most knowledgeable of experts. Its defensibility ishighly questionable because of the potential for inconsistenciesamong waste classification systems that provide fuel for legalchallenges.

2.1.5 Focus on Classification of Waste

This Report is concerned with classification of waste. However,NCRP’s assumption that hazardous waste should be classified basedon considerations of health risks posed by its constituents also couldbe used in classifying hazardous materials for such other purposesas transportation or their beneficial use in commerce.

The waste classification system developed in this Report includesa general class of exempt waste. Waste in this class would containsufficiently small amounts of hazardous substances that it could bemanaged in all respects as if it were nonhazardous (e.g., as householdtrash). NCRP intends that exempt materials could be used ordisposed of in any manner allowed by laws and regulationsaddressing disposition of nonhazardous materials. However,exempt waste would not necessarily be exempt for purposesof beneficial use without further analysis of the risks associ-ated with anticipated uses. Materials could be exempted for pur-poses of disposal or beneficial use based on similar considerationsof acceptable risk. However, based on differences in exposure scenar-ios for the two dispositions, limits on the amounts of hazardoussubstances that could be present in exempt materials intended forbeneficial use could be substantially lower than the limits for disposalas exempt waste. Thus, disposal may be the only allowable disposi-tion for some exempt materials based on considerations of risk. Inaddition, some exempt materials may consist of trash, rubble, andresidues from industrial processes that would have no beneficialuses and must be managed as waste.

Based on these considerations and the purpose of this study, therecommended approach to defining an exempt class of wastethat contains low levels of hazardous substances focuses ondisposal as the intended disposition of exempt material. Con-sideration of other dispositions of exempt material (e.g., recycling,reuse in commerce) is beyond the scope of this study. However, theprinciples used to exempt waste for purposes of disposal based onrisk could be used to exempt such materials for any other purpose.

2.1.6 Classification of Waste for Purposes of Disposal

The classification system for hazardous wastes developed in thisReport is intended to be applied to waste prior to disposal. It is not

2.2 LIMITS AND RELATIONSHIPS / 67

intended to be applied to screening or ranking of contaminated sites,including sites at which hazardous wastes have previously beendisposed. Site screening or ranking involves site-specific considera-tions that cannot be taken into account in a generally applicablewaste classification system. However, remediation of contaminatedsites may involve exhuming hazardous materials that require dis-posal elsewhere, and such wastes would be included in the proposedclassification system.

2.2 Limits and Relationships

The discussions in Section 2.1 outline the logic leading to the needfor, and scope of, a risk-based waste classification system. Despitethis presentation, there is the potential for confusion and misunder-standing concerning the limits of developing the foundations of arisk-based waste classification system and its relationship to otheraspects of waste management. The following sections address theselimits and relationships.

2.2.1 Regulatory Implications

This Report culminates in the presentation of the principles andframework for a comprehensive and risk-based hazardous wasteclassification system. NCRP does not propose a particular implemen-tation of the proposed classification system (e.g., a particular quanti-fication in terms of limits on concentrations of hazardous substancesin each waste class); this is most appropriately left to governmentalpolicy organizations. The relationship of the proposed risk-basedwaste classification system to existing regulations is discussed inSection 7.2.

2.2.2 Risk Management

Establishment of a risk-based waste classification system requiresthat one or more levels of acceptable risk be specified. A determina-tion of acceptable risks depends on societal values, and is a taskappropriately left to governmental policy makers and the public. Asa result, this Report will not attempt to select or justify specificvalues for acceptable risk. However, in Sections 6 and 7, values of


acceptable risk taken from the literature, including NCRP recom-mendations, are used to illustrate application of the proposed risk-based waste classification system.

2.2.3 Waste Classification in a Continuum of Waste Compositions

Concentrations of hazardous substances in waste occur as a contin-uum, ranging from extremely dilute to extremely concentrated,rather than in discrete, well-separated amounts. Given that risksfrom waste disposal generally are proportional to the concentrationsof hazardous constituents, it is not obvious how one can justify theestablishment of boundaries separating waste classes. However, thisapproach can be justified by recognizing that waste disposal technol-ogies intended to ensure acceptable risks to the public are discretein regard to their expected capabilities for isolating waste from thehuman exposure environment. There are just a few types of disposaltechnologies that are generally available, including (1) municipal/industrial landfills, (2) regulated near-surface disposal facilities forhazardous wastes located where human intrusion can readily occurin the absence of institutional control and water that may becomecontaminated with hazardous substances is relatively accessible,and (3) highly isolating disposal facilities located where humanintrusion is much less likely and water that may become contami-nated would be less accessible (e.g., geologic repositories).

Thus, boundaries of waste classes can be established by determin-ing waste properties that would result in an acceptable risk if wastedisposal were to occur using one of the few available technologies.If a waste is not generally acceptable for disposal using a giventechnology, then a more isolating technology normally would berequired. The fact that a waste just exceeding a boundary wouldbe sent to a more confining facility than a waste just within theboundary can be reconciled by the fact that the design and analysisof disposal facilities is intended to be conservative (i.e., to provideincreased margins of safety below regulatory requirements), and thecapability of the less confining facility to maintain risks at or belowan acceptable level should actually extend beyond the boundary. Inaddition, to the extent that the waste classification system is flexible,regulators can accommodate special cases in which the concentra-tions of contaminants in wastes are near the boundaries.

2.2.4 Subclassifications of Basic Waste Classes

There often are valid reasons for developing subclassifications ofbasic waste classes (i.e., classifications subordinate to those based

2.2 LIMITS AND RELATIONSHIPS / 69

on risk that arises from waste disposal). Waste subclassificationsare typically required for engineering purposes during pre-disposaloperations and may be desirable to account for differing waste char-acteristics (e.g., waste forms) in a disposal context. For example, inwaste operations, there is a clear need to distinguish between liquidand solid wastes and between heat-generating wastes and those withinsignificant heat generation. Subclassifications based on differentcompositions of hazardous substances, resulting in different require-ments for waste treatment to allow use of a particular disposaltechnology, also are justifiable. An example is the subclassificationof low-level radioactive waste destined for near-surface disposalestablished by NRC (1982a; 1982b), which depends on the concentra-tions of different radionuclides in the waste and includes differingrequirements on packaging and disposal of the different subclassesin the same type of facility.

The principles of waste classification presented in this Report donot address a framework for subclassification of hazardous wastes.However, the relationship of existing subclassifications to the pro-posed framework is discussed.

2.2.5 Site-Specific Risk

A risk-based waste classification system must focus on the inher-ent characteristics of waste, representative facilities, and genericevents, because the system necessarily presumes that specific dis-posal sites and related waste treatment and disposal technologieshave not yet been identified and characterized. NCRP emphasizesthat the principles, framework, and implementation details of a risk-based waste classification system do not provide a substitute for site-specific risk assessments. The two most important cases wheresite-specific risk must be estimated are (1) an assessment of risk forthe spectrum of actual wastes at a specific disposal site for thepurpose of establishing site-specific waste acceptance criteria, and(2) an assessment of risk posed by a prior waste disposal at a sitefor the purpose of determining whether the risk is unacceptable and,thus, whether remedial action is required at the site.

2.2.6 Ecological and Other Potential Impacts

In developing a risk-based approach to waste classification, NCRPhas focused exclusively on the potential for significant adverse healtheffects in humans. However, there are other potential adverse impacts


that can result from disposal of radioactive and hazardous chemicalwastes. These include adverse impacts on flora and fauna, damageto cultural or natural resources, and sensory impacts. With regardto radionuclides, standards for radiation protection of the public alsoare believed to be protective of nonhuman biota (IAEA, 1976; 1979;1992; NCRP, 1991). However, some chemicals may pose greater risksto biota than to humans. An example is selenium, which is toxic tocattle at levels that are not generally a problem for humans. Inaddition, hazardous substances may accumulate in the environmentin ways that could result in much higher doses to biota than tohumans, especially in aquatic systems.

In most cases, evaluations of ecological impacts are site-specificand, as a consequence, are not considered when establishing a gener-ally applicable waste classification system. These impacts normallyare addressed in disposal site selection, design, and operation, andthey may be used in establishing waste acceptance criteria for thesite. To the extent that ecological impacts can be evaluated generi-cally, NCRP believes that the principles and framework for risk-based waste classification presented in this Report are sufficientlyflexible to take them into account.

2.3 Conceptual Framework of This Report

This Report is directed at a multidisciplinary audience with differ-ent levels of technical understanding. NCRP recognizes that readershaving expertise in areas of radiation risk assessment and radioac-tive waste management may not be as knowledgeable about riskassessment and waste management for hazardous chemicals, andvice versa. Therefore, one of the aims of this Report is to presentdiscussions on technical issues relevant to risk assessment and wasteclassification in sufficient detail to allow readers having differenttechnical backgrounds to understand these issues without havingto refer to other sources of information. The following summarizesthe conceptual framework of the Report to provide initial points ofreference for the various discussions.

Section 3 provides technical background related to risk-basedwaste classification, including:

● general background on the standard risk assessment process(NAS/NRC, 1983)

● more detailed information on the aspect of risk assessment ofcentral import to this Report; namely, the assessment of adversehealth effects in humans resulting from exposure to hazardoussubstances

2.3 CONCEPTUAL FRAMEWORK OF THIS REPORT / 71

● approaches to cancer risk management for radionuclides andhazardous chemicals

Section 4 presents detailed information on existing classificationsystems for radioactive and hazardous chemical wastes, the relation-ships between waste classification and requirements for wastedisposal, and the impacts of waste classification systems on manage-ment and disposal of mixed wastes. This Section also summarizesprevious NCRP recommendations relevant to waste classification.

Section 5 discusses the desirable attributes of a waste classificationsystem and evaluates present classification systems with respectto these attributes. These discussions essentially summarize therationale for the development of a comprehensive and risk-basedhazardous waste classification system.

Section 6 then establishes and discusses the principles and frame-work for a comprehensive and risk-based hazardous waste classifi-cation system in a number of steps:

● The conceptual foundation of the system is first established byaddressing such issues as its focus, dose-response relationships,measures of response, and the applicable risk managementparadigm;

● The framework for a risk-based waste classification system isthen proposed;

● A risk index for waste classification to be used in conjunctionwith the framework is then developed, the combination of theseconstituting the recommended risk-based waste classificationsystem; and

● Issues related to subclassification of basic waste classes, incorpo-ration of conservative assumptions in applying the system, andfuture development needs regarding waste classification arediscussed.

Section 7 then addresses the implications of the recommendedrisk-based waste classification system. By assuming key parameters(e.g., values of acceptable risk, characteristics of exposure scenarios)and applying the system to a variety of example waste streams, thequestion of how existing wastes would be classified in the new systemis investigated. This Section also summarizes the legal and regula-tory ramifications of the proposed hazardous waste classificationsystem.

Section 8 summarizes NCRP’s conclusions and recommendationson waste classification. This is followed by an extensive glossary anda list of references.

3. Technical Background onRisk Assessment andRisk Management

Assessment of risks to human health posed by hazardous wastesand decisions about acceptable risks posed by hazardous wastes (i.e.,risk management) are essential to the development of a risk-basedwaste classification system. The purpose of this Section is to providetechnical information concerning risk assessment for radionuclidesand hazardous chemicals and approaches to risk management forthe two types of substances. It begins by defining risk in generalterms and in terms relevant to disposal of hazardous waste, and bydescribing the process by which risks that arise from waste disposalwould be assessed. The discussion then focuses on aspects of riskassessment concerned with estimating the probability that a signifi-cant adverse health effect, called a response, will result from a hypo-thetical exposure to a hazardous substance. This issue is discussedseparately for radionuclides and hazardous chemicals because sig-nificantly different approaches have been used to estimate responseprobabilities for the two types of substances. This Section concludeswith a discussion of the different approaches to risk managementused in controlling exposures of the public to radionuclides and haz-ardous chemicals.

The discussions on risk assessment, particularly the approachesto assessing the probability of a response from a given exposure(dose-response assessment), are presented in considerable detail toallow readers who are knowledgeable about risk assessments forionizing radiation and the data that support them to become familiarwith risk assessments for hazardous chemicals, and vice versa. Thesignificant differences in the approaches to dose-response assess-ment for radionuclides and hazardous chemicals constitute a majorissue requiring resolution in establishing a comprehensive and risk-based waste classification system. Therefore, even for readers knowl-edgeable about issues of risk assessment, the comparisons of thedifferent approaches presented in Section 3.2.3 are important to anunderstanding of the waste classification system developed in thisReport.

72

3.1 ASSESSMENT OF RISK / 73

Similar considerations apply to the discussions of approaches torisk management in Section 3.3. Readers who are knowledgeableabout principles of radiation protection may not be familiar with thedifferent approach to health protection used for hazardous chemicals,and vice versa, and an understanding and resolution of the differentapproaches to risk management is important in developing a compre-hensive and risk-based waste classification system.

3.1 Assessment of Risk

3.1.1 Definition of Risk

The term ‘‘risk’’ as used in this Report refers to the probability ofharm, combined with the potential severity of that harm. In thecontext of impacts on human health resulting from disposal of haz-ardous waste, ‘‘risk’’ is the probability of a response in an individualor the frequency of a response in a population taking into account(1) the probability of occurrence of processes and events that couldresult in release of hazardous substances to the environment andthe magnitude of such releases, (2) the probability that individualsor populations would be exposed to the hazardous substancesreleased to the environment and the magnitude of such exposures,and (3) the probability that an exposure would produce a response.For example, ‘‘risk’’ refers to the probability that a member of thepublic living near a waste disposal site will develop a certain typeof cancer as a result of emplacement of hazardous substances at thesite. When expressed as a probability, risk is a number between zeroand one, without units. In this Report, all values are risks to anindividual over a normal lifetime. Risk can be calculated for individ-ual radioactive and chemical substances in waste and for specificpathways by which release and exposure might occur. These compo-nent risks can be combined to yield an overall risk that arises fromdisposal of waste.

Different measures of response can be used in estimating risk.For example, risk could refer to the probability of occurrence (inci-dence) of a particular response or the probability that death willresult. The probabilities of these two endpoints will rarely be thesame, because some adverse effects will be cured by medical treat-ment or the receptor will die by some other means before death iscaused by exposure to a hazardous substance.

In environmental health, risk is typically expressed in such termsas ‘‘the estimated incremental lifetime cancer risk to an individual

74 / 3. TECHNICAL BACKGROUND

that might live near a disposal site containing carcinogen A is onein 100,000 (0.00001 or 10�5).’’ This means in effect that, absentuncertainties in the estimated risk, one of every 100,000 peoplehaving the specified relationship (e.g., geographic proximity, livinghabits) to the disposal site would be expected to develop a specificcancer due to exposure to carcinogen A during their lifetime thatthey would not have developed if they were not located near the site.

3.1.2 Types of Responses from Exposure to Hazardous Substances

Two types of responses from exposure to hazardous substances,called stochastic or deterministic,5 are of concern in risk assessment.The two types of responses are distinguished by the characteristicfeatures of the dose-response relationship, i.e., the relationshipbetween the dose of a hazardous substance and the probability (orfrequency) of a response.

Stochastic responses are those for which the probability, but nottheir severity, is a function of dose, without threshold. Because ofthe long latency period between exposure and the expression of astochastic response, the existence of a causal relationship betweendose and response can only be inferred on statistical grounds based,for example, on knowledge of the background incidence of theresponse of concern in unexposed populations. Severe hereditary(genetic) and many carcinogenic (e.g., genotoxic) responses are con-sidered to be stochastic.

Deterministic responses are those for which the severity varieswith dose and for which a threshold usually exists. In some toxicologytexts, this type of response is called a graded response, to reflectboth the increase in incidence of the response and the increase inits severity that usually are observed as the dose increases abovethe threshold. If the dose does not exceed a certain threshold, theprobability of occurrence of a particular response is presumed to bezero. Deterministic responses often occur soon after exposure, anda causal relationship between dose and response in such cases iseasily established if the dose is sufficiently high. Deterministicresponses resulting from exposure to chemical toxicants include, forexample, increased protein in the urine, birth defects and sterility,

5 In common usage, the term ‘‘carcinogenic’’ often is used instead of ‘‘stochastic’’because the vast preponderance of substances having a stochastic relationship ofadverse biological effect to dose are carcinogens. Similarly, ‘‘noncarcinogenic’’ is com-monly used instead of ‘‘deterministic.’’


effects on the nervous system, and liver damage. Exposure to radia-tion can result in such deterministic responses as cataract in the lensof the eye, skin erythema, cell depletion in bone marrow resultingin hematological deficiencies, cell damage in the gonads leading toimpairment of fertility, and damage to blood vessels or connectivetissue in many organs. For some hazardous chemicals that exhibitcarcinogenic responses, the dose-response relationship appears tobe deterministic in character, in that an increase in the probabilityof cancer is not observed until certain doses are reached for a specificperiod of time. These substances are referred to as nongenotoxicbecause they do not affect deoxyribonucleic acid (DNA).

3.1.3 Definition of Risk Assessment

If one conducts a literature search on the term ‘‘risk assessment,’’a lengthy list of publications on a range of topics will be produced(NAS/NRC, 1983; 1994; Paustenbach, 1995), because this term hasbeen used to describe estimates of the likelihood of a number ofunwanted events. These include, for example, industrial explosions,workplace injuries, failures of machine parts, natural catastrophes,injury or death as a result of voluntary activities or lifestyle, diseases,and death from natural causes.

For the purposes of this Report, a risk assessment is a writtendocument wherein all the pertinent scientific information regardingthe risk that arises from disposal of hazardous waste is assembled,critiqued, and interpreted. The goal of the assessment generallycould be to calculate the likelihood of responses in humans, aquaticor terrestrial biota, or ecological systems that arise from disposal ofhazardous wastes. In this Report, however, the focus is on assess-ment of health risks in humans (see Section 2.2.6). The magnitudeof the risk depends on both the potency of hazardous substancesand the amount of exposure, which is a function of the duration ofexposure and the concentrations of hazardous substances.

3.1.4 Risk Assessment Process

Estimates of risks to human health resulting from disposal ofhazardous wastes will nearly always be calculated values based onmodels. Even if health effects were to occur in the future, they arelikely to be unobservable in the background of similar effects fromall causes. Therefore, mathematical predictions of risks are required.In general, risk assessment is the process by which toxicology data


collected from animal studies and human epidemiology are combinedwith information about the amount of exposure to quantitativelypredict the likelihood that a particular response will be seen ina specific human population (Paustenbach, 1995). The four stepsinvolved in calculating risk are conventionally referred to as hazardidentification, dose-response assessment, exposure assessment, andrisk characterization (NAS/NRC, 1983; 1994; Paustenbach, 1995).These steps in the risk assessment process, as they would conceptu-ally be applied to hazardous waste classification, are shown inFigure 3.1.

3.1.4.1 Hazard Identification. The process of determiningwhether exposure to a particular substance at any dose can causea response in a biological organism and, if so, the type(s) of response iscalled hazard identification. Hazard identification typically involvesdoses of a substance that are much higher than would actually beexperienced in routine exposures of the public, including exposuresresulting from waste disposal. Once the hazardous nature of a sub-stance is determined, the results are documented and the hazardidentification process need not be repeated for other applications.

3.1.4.1.1 Radiation hazard identification. The hazard identifica-tion process is trivial in the case of radiation, because all types ofionizing radiation are assumed to be hazardous and, thus, all radionu-clides are assumed to be hazardous substances (see Section 3.2.2).While some responses may not occur at low doses (e.g., damage tothe lens of the eye), other responses are assumed to occur with someprobability at any dose (e.g., cancer induction).

3.1.4.1.2 Chemical hazard identification. In contrast to radiation,most chemicals are thought not to be hazardous to human healthat a sufficiently low dose. In the United States, the process of deter-mining whether a chemical is hazardous relies upon principles estab-lished by EPA. These principles are used extensively, but notuniversally, in other countries. This Section describes the generalprinciples used by EPA to identify hazardous chemicals. Hazardidentification is related to the process of dose-response assessmentfor hazardous chemicals discussed in Section 3.2.1.

Characterization and classification of chemical toxicity is complexbecause of the many possible responses a chemical might induceand the variability of the dose required to yield a response. Toxicresponses can include acute effects on the function of various organsor long-term effects such as cancer. Occurrence of a response maybe deterministic or stochastic. EPA treats chemicals showing deter-ministic responses as if there is a threshold below which there is no


Fig. 3.1. Risk assessment process as applied to waste classification.

observable response. The basis of this assumption is EPA’s under-standing of homeostatic, compensating, detoxifying, and adaptivemechanisms. The response occurs only after these mechanisms fail.For example, a toxicant must damage many nephrons in the kidneybefore clinical signs of kidney failure appear. In contrast, EPA usu-ally treats carcinogenic and mutagenic responses as having a sto-chastic relationship to dose, but there are some exceptions to this.


Evidence of possible toxicity in humans comes primarily from twosources: long-term animal tests and epidemiologic investigations.Results from these studies are supplemented with available informa-tion from short-term tests, pharmacokinetic studies, comparativemetabolism studies, structure-activity relationships, and other rele-vant toxicologic studies. The question of how likely it is that a sub-stance is a human toxicant is answered in the framework of a weight-of-evidence judgment. Such a judgment involves consideration of thequality and adequacy of the data and the kinds and consistency ofresponses induced by a suspected hazardous substance. There arethree major steps in characterizing the weight of evidence for toxicityin humans: (1) characterization of the evidence from human studiesand animal studies individually; (2) combination of the characteriza-tions from these two types of studies into an indication of the overallweight of evidence; and (3) evaluation of all supporting informationto determine if the overall weight of evidence should be modified.

Although there is the potential for a complex, multi-dimensionaldiscussion in this area, the identification of chemicals that causedeterministic responses is discussed first, followed by a discussion ofidentifying chemicals that cause stochastic responses. An additionalcomplexity is that a material may be hazardous due to its physicaland chemical form. Thus, an additional section discusses the identi-fication of hazardous chemical wastes, as opposed to their hazardouschemical constituents per se.

Identification of Chemicals That Cause Deterministic Responses.Hazardous chemicals having a threshold in the dose-response rela-tionship are identified using the following process:

● consider toxic responses● select the critical response● select principal study (human or laboratory animal)● judge ability of study to predict human toxicity● judge appropriateness of route of administration, nature of expo-

sure, and the approach used in each study● consider overall weight of evidence from principal and support-

ing studies

There are significant judgmental aspects involved in this process.The following discussion of hazard identification is taken fromEPA (1987a).

In hazard identification, EPA considers the adverse toxic responsesfrom all studies. A chemical may cause a variety of adverse effectsdepending on the magnitude of the dose and the duration of exposure.These may range from clearly defined effects, such as death, to more


subtle biochemical, physiological, or pathological changes. Theeffects seen may also differ depending on whether the exposure isacute, subchronic, or chronic. EPA gives primary attention to thesignificant adverse effect that is seen at the lowest dose or that isof greatest biological significance to humans.

EPA defines the critical response (often called the critical effector the most sensitive toxic endpoint) as the significant adverse effect,or its known precursor, that occurs at the lowest dose. When selectingthe critical response, EPA depends on professional judgment indetermining whether an observed effect constitutes a response. EPAconsiders the statistical and biological significance of the effect whenmaking this judgment. However, EPA gives precedence to biologicalsignificance and does not consider a statistically significant changelacking biological significance to be a response. For example, EPAhas concluded that male rat nephropathy is not an appropriate effectto consider in a human health risk assessment because humans lackthe precursor that produces the kidney damage. Other effects thatEPA judges to be biologically insignificant include a decrease in bodyweight compared with controls of less than 10 percent, a change inliver weight of less than 20 percent without significant histopatholog-ical changes, and minor changes in clinical chemistry values thatwill not affect the physiological well-being of the animal. However,EPA relies on its review of all the data before deciding whetheran effect is biologically significant and adverse, thus constituting aresponse to be considered further.

Principal studies are those that are the most significant for deter-mining whether a chemical is potentially toxic in humans. Thesestudies are of two types: studies of human populations and studiesusing laboratory animals. EPA also uses the principal studies in thedose-response assessment (see Section 3.2.1).

Human data often are useful in showing the presence of a response.When human studies provide information on the dose associatedwith toxicity, EPA gives priority to appropriately documented stud-ies in the dose-response assessment.

In epidemiologic studies, the investigator attempts to control andmeasure, within limits, recognized confounding factors. Case reportsand acute doses showing severe adverse effects provide support forthe choice of the critical response. However, these sources are oftenof limited utility in showing a quantitative relationship betweendose and response. Epidemiologic and clinical studies may containdose-response information that EPA can use in estimating responseprobabilities, but EPA must determine that the method of quantify-ing exposure is appropriate.


In the absence of adequate human data, EPA selects data fromlaboratory animals, generally mammals. The animals used mostoften are the rat, mouse, rabbit, guinea pig, hamster, dog, and mon-key. In a typical laboratory animal study, the investigator carefullycontrols the doses of the toxicant and reduces exposure to othertoxicants. Laboratory animal studies also reduce problems associ-ated with heterogeneity of exposed populations. When using thesedata, EPA must extrapolate from laboratory animals to humans andmust account for human heterogeneity.

When reviewing animal studies, EPA makes judgments on theability of the study to predict the potential for toxicity in humans.EPA tries to select data from the species that is most relevant tohumans using the most defensible biological rationale. EPA oftenwill use comparative pharmacokinetic data for this decision. Forexample, dogs and rodents differ in their ability to excrete organicacids. Since humans resemble rodents more closely than dogs in thisability, studies in dogs to test the toxicity of organic acids may notpredict the response in humans. Therefore, in these cases, studiesin dogs are inappropriate as the basis for determining potentialhuman toxicity.

In the absence of a clearly most relevant species, EPA uses themost sensitive mammalian species (i.e., the species showing toxicityat the lowest dose). EPA makes this judgment because there is noassurance that humans are not at least as sensitive as the mostsensitive species tested.

In addition to the principal studies, supporting studies are usedin evaluating chemical toxicity. These studies provide supportive,rather than definitive, information and can include data from avariety of sources. For example, studies of different durations or indifferent species may confirm the choice of the critical response.Metabolic and other pharmacokinetic studies can provide insightsinto the mechanism of action of a chemical. By comparing the metabo-lism of the chemical in the laboratory animal and in humans, EPAmight be able to estimate equitoxic doses. In vitro studies can provideinsights into the chemical’s potential for biological activity. Theknown toxicity of a structurally related compound and the use ofstructure-activity relationships can also provide clues to the chemi-cal’s possible toxicity.

EPA usually approaches hazard identification for a chemical withrespect to a particular route of exposure (e.g., oral or inhalation).The most appropriate studies for assessing the toxicity of a chemicalby a particular route of exposure are those in which the investigatoradministers the chemical by that route.


In many cases, the data for a chemical do not include detailedtesting for all routes of exposure. The toxicity of the chemical maydepend on the route of exposure because of differences in mechanismof action, biochemistry, or absorption. For example, hexavalent chro-mium (Cr�6) can cause lung cancer when inhaled but is not assumedto be carcinogenic when ingested since it is converted to trivalentchromium (Cr�3), an essential dietary component, in the stomach.However, EPA’s judgment is that toxicity from one route of exposuresuggests the potential for toxicity from another route, unless convinc-ing evidence exists to the contrary. EPA considers potential differ-ences in absorption or metabolism resulting from different routesof exposure. Whenever EPA has relevant data (e.g., comparativemetabolism studies), EPA describes the quantitative effects of thesedifferences on the chemical’s toxicity.

The amount, frequency, and duration of exposure may vary consid-erably in different laboratory or epidemiologic studies. For studiesin laboratory animals, investigators use a variety of conditions ofexposure, typically an acute dose over 1 to 14 d, a subchronic doseover 90 d, or a chronic dose over 1 to 2 y. Dosing schedules are eithersingle, intermittent, or continuous. EPA uses information from allof these types of studies in hazard identification. For example, overtneurotoxicity shown in high-dose, acute studies reinforces the findingof subtle neurological changes in low-dose, chronic studies.

EPA gives special attention to studies involving low doses that arecontinuous and chronic, because such studies reflect the conditions ofexposure for which EPA is trying to protect the public. Continuousexposure at low doses can elicit responses absent in studies involvinghigh, short-term doses. Common mechanisms for this behaviorinclude an accumulation of toxicants during chronic exposure orexceeding the repair capacity of a particular organ. If the chronicdose is below that resulting in toxicity, then EPA assumes that notoxicity will occur from any equivalent dose of shorter duration.

An ideal study approach attempts to clearly delineate a hypothesisand follow a carefully prescribed protocol. In addition, the investiga-tor provides a clear reporting of the data and describes the analysisto support the conclusions. Listed below are some of the factors thatEPA considers in its review of a study:

● identity of the substance(s) under study● test species and its similarities to humans● sex and age of test animals● use of proper controls● number of animals and doses tested● spacing and choice of dose levels


● possible alteration in metabolism at high doses● types of observations and methods of analysis● nature of pathological changes

As the final step in hazard identification, the risk assessor consid-ers the weight of evidence available from the principal and supportingstudies. The results from different studies are examined to determinewhether a consistent, plausible picture of toxicity emerges. Some ofthe factors that add weight to the evidence that the chemical posesa hazard to humans include:

● similar effects shown in studies by different investigators● similar effects shown across sex, strain, and species● similar effects shown across different routes of exposure● clear evidence of a dose-response relationship● a plausible relationship between data on metabolism, a postu-

lated mechanism of action, and the effect of concern● similar toxicity shown by structurally related compounds

Identification of Chemicals That Cause Stochastic Responses. Thefollowing discussion on identification of chemicals that cause stochas-tic responses is based on guidelines issued in 1987 (EPA, 1987a);these guidelines have been used in most risk assessments. Newguidelines were proposed in 1996 (EPA, 1996a), but they have notbeen issued in final form. Differences between the two guidelinesare discussed at the end of this Section.

Hazard identification for chemicals that cause stochastic responsesis concerned with the process of determining whether exposure to asubstance has the potential to increase the incidence of stochasticresponses. Hazard identification should include a review of the fol-lowing information to the extent that it is available.

1. Physico-chemical properties, and routes and patterns of expo-sure. Parameters relevant to identifying stochastic responsesinclude physical state, physical and chemical properties, andexposure pathways in the environment.

2. Structure-activity relationships. Relevant structure-activityrelationships can support or argue against the potential toxicityof a substance. These relationships are used to predict the toxic-ity or the chemical and physical properties of a substance basedon its similarity in chemical structure with other substanceswith known toxicity or other properties (Enslein, 1988).

3. Metabolic and pharmacokinetic properties. This part of the haz-ard assessment should summarize relevant metabolic informa-tion. Such information as whether the substance is direct-acting


or requires conversion to a reactive carcinogenic (e.g., electro-philic) substance, metabolic pathways for such conversions,macromolecular interactions, and fate (e.g., transport, storage,and excretion), as well as species differences, should be dis-cussed and critically evaluated. Pharmacokinetic propertiesdetermine the biologically effective dose and may be relevantto hazard identification and other aspects of stochastic riskassessment.

4. Toxicologic effects. Other toxicologic effects that are relevant tothe evaluation of the stochastic response of interest should besummarized. Interactions with other hazardous substances andwith lifestyle factors (e.g., smoking) should be discussed. Pre-chronic and chronic toxicity evaluations, as well as other testresults, may yield information on target organ effects, patho-physiological reactions, and pre-neoplastic lesions that bear onthe evaluation of the toxicity of substances causing stochasticresponses. Dose-response and time-to-response analysis ofthese reactions may also be helpful.

5. Short-term tests. Tests for point mutations, numerical andstructural chromosome aberrations, DNA damage/repair, andin vitro transformation provide supportive evidence of stochas-tic responses and may give information on potential mecha-nisms of action. A range of tests for each of the above responseshelps to characterize the response spectrum of a substance.

Short-term in vivo and in vitro tests that can give an indicationof initiation and promotion activity may also provide supportiveevidence for a particular stochastic response. However, lack ofpositive results for genetic toxicity does not necessarily providea basis for discounting positive results in long-term animalstudies.

6. Long-term animal studies. Transplacental and multigenera-tional studies of stochastic responses, in addition to moreconventional long-term animal studies, can yield useful infor-mation about the toxicity of hazardous substances. Criteria forthe technical adequacy of animal studies have been publishedin references provided by EPA (1987a) and should be used tojudge the acceptability of individual studies.

It is recognized that chemicals that induce benign tumors caninduce malignant tumors, and that benign tumors can progressto malignant tumors. Therefore, as a conservative measure, theincidence of benign and malignant tumors often is combined.For example, EPA generally will consider the combination ofbenign and malignant tumors to be scientifically defensible


unless the benign tumors are not considered to have the poten-tial to progress to the associated malignancies of the samehistogenic origin. If an increased incidence of benign tumors isobserved in the absence of malignant tumors, EPA considerssuch information to be limited evidence of carcinogenicity inmost cases.

The weight of evidence that a substance is likely to be carcino-genic in humans increases when there is (1) an increase in thenumber of tissue sites affected by the substance, (2) an increasein the number of animal species or sexes showing a stochasticresponse, (3) an increase in the number of experiments and dosesshowing a stochastic response, (4) a clear-cut dose-response rela-tionship as well as a high level of statistical significance of theincreased responses in treated subjects compared with controls,(5) a dose-related shortening of the time-to-incidence or time-to-death for the response, and (6) a dose-related increase in theproportion of malignant responses.

Long-term animal studies using doses at or near the maximumtolerated dose (MTD) are used to ensure adequate power for thedetection of toxicity (EPA, 1987a). Negative long-term animalstudies at doses above MTD may not be acceptable if animalsurvival is so impaired that the sensitivity of the study is sig-nificantly reduced below that of a conventional chronic animalstudy at MTD. Positive studies at doses above MTD should becarefully reviewed to ensure that the responses are not dueto factors that do not operate at doses below MTD. Evidenceindicating that high doses alter responses by indirect mecha-nisms that may be unrelated to responses at lower doses shouldbe dealt with on an individual basis.

Stochastic responses under conditions of the experiment shouldbe reviewed carefully with respect to the relevance of the evi-dence to humans (e.g., the occurrence of bladder tumors in thepresence of bladder stones and implantation site sarcomas).Interpretation of animal studies is aided by the review of targetorgan toxicity and other effects (e.g., changes in the immuneand endocrine systems) that may be noted in pre-chronic orother toxicologic studies. Time- and dose-related incidence ofpre-neoplastic lesions may also be helpful in interpreting ani-mal studies.

To evaluate toxicity of substances causing stochastic responses,the primary comparison is responses in exposed animals rela-tive to responses in contemporary matched controls. Historical


control data often are valuable and could be used along withconcurrent control data in the evaluation of responses (EPA,1987a). For the evaluation of rare responses, even smallresponse rates may be significant compared with historicalcontrols.

Data from all long-term animal studies are considered in theevaluation of toxicity. A positive response in one species/strain/sex is not generally negated by negative results in anotherspecies/strain/sex. However, replicate negative studies that areessentially identical to a positive study may indicate that thepositive results are spurious.

Evidence of toxicity should be based on an observation of statis-tically significant responses in specific organs or tissues. Appro-priate statistical analysis should be performed on data fromlong-term studies to help determine whether the responses arerelated to exposure to the study substance or possibly dueto chance. This analysis should include, at a minimum, a sta-tistical test for trend, including appropriate corrections fordifferences in survival. The weight to be given to the levelof statistical significance (the p-value) and to other availableinformation is a matter of scientific judgment. A statisticallysignificant excess of responses of all types in the aggregate, inthe absence of a statistically significant increase in any individ-ual response, should be regarded as minimal evidence of toxicityunless there are persuasive reasons to the contrary.

7. Human studies. Epidemiologic studies provide unique informa-tion about the responses of humans who have been exposed tosubstances suspected of being hazardous. Descriptive epidemio-logic studies are useful in generating hypotheses and providingsupporting data but can rarely be used to make a causal infer-ences. Analytical studies of the case-control or cohort variety,on the other hand, are especially useful in assessing risks tohumans.

Criteria for the adequacy of epidemiologic studies are well rec-ognized (Monson, 1990). They include, for example, properselection and characterization of exposed and comparisongroups, adequacy of the duration and quality of follow-up,proper identification and characterization of confounding fac-tors, attention to potential methodologic biases, appropriateconsideration of latency effects, valid ascertainment of thecauses of morbidity and death, and the ability to detect specificresponses. The statistical power to detect a particular response


should be included in the assessment when it can be calculated.The weight of the epidemiologic evidence for toxicity depends,among other things, on the type of analysis and on the magni-tude and consistency of the response. The weight of evidenceincreases rapidly with the number of adequate studies thatshow comparable results on populations exposed to the samesubstance under different conditions.

Epidemiologic studies are inherently capable of demonstratingan association between exposure to a given agent and a disease,thereby allowing estimation of the dose-response relationship,only when the increase in occurrence of the disease is substan-tially above the background incidence. Negative results fromsuch studies, while reassuring, cannot prove the absence oftoxicity. However, negative results from a well-designed andwell-conducted epidemiologic study that contains usable dataon doses can serve to define upper limits on the possibility ofa response. Such results are useful if animal evidence indicatesthat the substance is potentially toxic in humans.

Taking into account the available information, the overall weightof evidence for carcinogenicity of a chemical is classified by EPA intofive groups:

Group A: Human carcinogenGroup B: Probable human carcinogenGroup C: Possible human carcinogenGroup D: Not classifiable as to human carcinogenicityGroup E: Evidence of noncarcinogenicity in humans

Substances judged to be in Group A or B generally are regarded assuitable for quantitative risk assessment. Substances judged to bein Group C normally are regarded as suitable for quantitative riskassessment, but judgments about this may be made on a case-by-case basis. Substances judged to be in Group D or E are not subjectedto quantitative risk assessment.

In the hazard identification process for chemicals that cause sto-chastic effects described above (EPA, 1987a), the weight-of-evidenceclassification is determined primarily by observations of tumors inanimals or humans. Other information about the properties of achemical, structure-activity relationships for other chemicals thatcause stochastic effects, and the influence of a chemical on the carcin-ogenic process often is limited and plays only a modulating role inthe weight-of-evidence classification based on tumor findings.

The approach to hazard identification in the proposed revision ofthe cancer risk assessment guidelines (EPA, 1996a) differs from the


1987 guidelines in two respects. First, the five weight-of-evidenceclasses (Groups A through E) are replaced by three categories, whichinclude standard descriptors of conclusions about the carcinogenicityof a substance in humans and a brief narrative description of theinformational basis for that conclusion. The standard descriptors ofthe three categories are ‘‘known/likely,’’ ‘‘cannot be determined,’’ and‘‘not likely.’’ The narrative explains the kinds of evidence and howthey fit together in drawing conclusions. As an example, the narra-tive describes the carcinogenic potential by different routes of expo-sure, and it may include a description that a substance is likely tobe carcinogenic by one route (e.g., inhalation) and not likely to becarcinogenic by another route (e.g., ingestion).

Second, instead of basing the classification mainly on tumor find-ings in animals or humans, with other information playing only amodulating role in the classification, the conclusion about the weightof evidence for carcinogenicity is reached in a single step, whereinall the information is considered together. This change recognizesthe growing sophistication of research methods, particularly in theirability to study modes of action of carcinogenic substances at cellularand subcellular levels, as well as toxicokinetic and metabolic pro-cesses. If such information is largely unavailable, cancer risk assess-ments under the proposed new guidelines will not differ significantlyfrom assessments under the earlier guidelines.

Identification of Hazardous Chemical Wastes. The foregoing discus-sions in this Section have considered the process of identifying sub-stances that are hazardous to human health and the nature of anytoxic effects. Two additional concerns arise in identifying hazardouschemical wastes.

First, a waste (or any other material) may be hazardous due toits physical and chemical properties, rather than the presence ofhazardous substances. For example, a material that is readily explo-sive or reactive (e.g., hydrogen gas, liquid sodium metal) clearlyconstitutes a hazard even though the constituent substances them-selves may not be hazardous to human health. EPA has identifiedwastes as hazardous if they are ignitable, corrosive, or reactive.

Second, not all chemical wastes that contain hazardous substancesare deemed to be hazardous. EPA considers wastes that containcertain hazardous substances (heavy metals and organic compounds,including carcinogens and noncarcinogens) not to be hazardous ifthe leachability of the substances from the waste form is limited.This characterization of waste as nonhazardous is based on EPA’sjudgment that potential risks to humans resulting from disposal ofthe waste would not exceed acceptable levels.


The identification of hazardous chemical waste based on its physi-cal and chemical characteristics is discussed in Section 4.2.

3.1.4.2 Dose-Response Assessment. Determining the relationshipbetween the dose of a hazardous substance and the probability of aspecific response is called dose-response assessment.6 This aspect ofrisk assessment is needed to extrapolate from responses observedin experiments or incidents involving high doses to the much lowerpotential doses relevant to waste disposal and other routine exposuresituations. Dose-response assessment is a major issue in establishingthe foundations of a risk-based waste classification system, and itis discussed in detail in Section 3.2.

3.1.4.3 Exposure Assessment. In exposure assessment, the popu-lation potentially exposed to hazardous substances and the pathwaysand routes through which exposure could occur are specified, andthe magnitude, duration, and timing of the doses people mightreceive are quantified. The approach to exposure assessment forhazardous waste disposal can range from very sophisticated andcomplex (e.g., Wilson et al., 1994) to a multiplication of simple factors(e.g., Dornsife, 1995; EG&G, 1982; EPA, 1989; Smith et al., 1980).Exposure assessment for waste disposal is itself a multi-step process,and is discussed below.

1. Describe the conditions of waste disposal. Wastes in specifiedphysical and chemical forms and having certain compositionsor ranges of compositions of hazardous substances are assumedto be emplaced in certain ways in a disposal site having speci-fied characteristics. The disposal site can be a real location orgeneric with hypothetical characteristics typical of real sites.The exposure assessment usually assumes that disposal opera-tions have been completed and the site is closed, although the

6 In health risk assessments for ionizing radiation, the term ‘‘dose’’ generally refersto the energy imparted to organs or tissues from exposure to radionuclides or othersources of radiation modified by a quantity that represents the biological effectivenessof different radiation types, and the dose-response relationship in a particular organor tissue gives the probability of an adverse health effect as a function of dose (e.g.,X excess cancers per sievert). For hazardous chemicals, however, ‘‘dose’’ usually refersto mass intake, rather than an impact on an organ or tissue, and the dose-responserelationship usually is an exposure-response relationship (e.g., Z excess cancers permilligram of a toxic chemical ingested per kilogram of body weight per day). Thisdifference is a result of the existence of a unifying measure of radiological impact onhumans (i.e., the sievert) and the absence of such a measure for hazardous chemicals,which can impact biological organisms in many ways. In spite of this difference, thephrase ‘‘dose-response’’ will be used in discussing both radionuclides and hazard-ous chemicals.


assessment is most often conducted before operations com-mence as a part of the licensing or permitting process.

2. Describe possible mechanisms by which hazardous substancescould be released from a disposal facility. A credible series ofprocesses and events that could result in release of hazardoussubstances from the disposal site to a portion of the environmentthat is accessible to humans and the probability that theseprocesses and events would occur, often called a release sce-nario, is developed. Release scenarios for waste disposal facili-ties generally should include considerations of inadvertenthuman intrusion resulting from normal activities, such as exca-vation or drilling, as well as releases to air and groundwaterdue to natural processes and events.

Processes and events normally considered in developing releasescenarios are shown in Figure 3.2. The thought process involvesconceiving of ways in which the barriers to release of hazardoussubstances, such as waste containers or control of groundwater,might be compromised or circumvented. Examples of poten-tially important release scenarios include: (1) infiltration ofwater, which degrades waste containers, dissolves hazardoussubstances, and transports them to a location where exposureof humans can occur; and (2) inadvertent drilling into the wastesite, resulting in hazardous material being brought to the sur-face where an intruder can be exposed.

3. Characterize possible mechanisms of exposure to hazardous sub-stances. The pathways by which hazardous substances releasedfrom a disposal facility can be transported through the bio-sphere and the resulting routes of human exposure are speci-fied, often along with their respective probabilities. To estimateexposures of humans at assumed receptor locations, dilution ofcontaminants by transport in air or water as well as concentra-tion by various means, such as precipitation and uptake byintermediate biological organisms consumed by humans, mustbe considered. An example of the potentially complex web ofexposure pathways is shown in Figure 3.3.

4. Develop models for scenarios and acquire data. The releaseand exposure scenarios described above are evaluated throughmodeling. The models embody the mathematical interrelation-ships of the possible steps in each scenario. Simplifications andapproximations usually are introduced to reflect limitations inknowledge and data or the results of previous risk assessmentsthat show certain scenarios and pathways to be negligible. Theresult often is a series of models describing (1) degradation of


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containers and release of hazardous substances or access towaste by inadvertent human intruders, (2) transport to thebiosphere, and (3) biospheric transport and exposure ofhumans. Many models also include data to estimate doses andassociated responses. Exogenous data, such as container corro-sion rates, solubility and mobility of hazardous substances, andconsumption rates of foodstuffs by humans, are required to usethe models. As a result of risk being defined as the probabilityof a response, consideration of the probability of the processes


and events in the release and exposure scenarios is required.In some risk assessments, probabilities of scenarios may beconsidered only qualitatively. For example, scenarios judged tobe credible are assumed to occur with a probability of one,whereas scenarios judged not to be credible are assigned aprobability of zero. This approach often is taken when the pur-pose of the assessment is to evaluate compliance with require-ments on facility performance, rather than to estimate expectedrisks. In more complex analyses, probabilities of scenarios maybe specified quantitatively and the input data or probabilitiesof processes and events may be specified as distributions thatare statistically sampled to yield a distribution of risks.

5. Exercise the models to calculate exposure and risk. The modelsare typically implemented using computer programs, which areexercised using the acquired data. Simpler programs will calcu-late an exposure and associated response resulting from a wastedisposal situation and multiply it by the probability of the ini-tiating process or event, yielding an estimate of the risk forthat situation. More complex programs calculate a distributionof risks based on a range of probabilities for the initiating pro-cesses and events and the input data. A summary depictionof the probabilistic exposure and risk assessment process asapplied to waste disposal is shown in Figure 3.4 (Garrick andKaplan, 1995). It should be recognized that probabilistic assess-ments are appropriate for use at specific sites and when thepurpose is to estimate expected risks. For the purpose of classi-fying waste, however, risk assessment must be nonsite-specific(generic) and is necessarily much simpler.

3.1.4.4 Risk Characterization. As emphasized in the preceding dis-cussions of hazard identification (Section 3.1.4.1), dose-response assess-ment (Section 3.1.4.2), and exposure assessment (Section 3.1.4.3),calculating risk involves numerous assumptions and simplifications,including significant extrapolation of data on dose-response. Riskcharacterization provides the capstone of a risk assessment by inte-grating and interpreting the information developed in these steps,identifying limitations and uncertainties in the models and dataused to estimate human health risks, and then communicating theresults appropriately (NAS/NRC, 1994).

Integration of the results of the first three steps in a risk assess-ment typically results in a quantitative estimate of risk. Estimated


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risks can be expressed in a variety of ways, such as incidence orfatalities, deterministic or probabilistic, individual or population,occupational or public, relative or absolute. Comparison of estimatedrisks with other relevant risks to provide perspective also is a partof risk characterization.

An important part of risk characterization is an interpretation ofthe results of a risk assessment, particularly in regard to evaluatingthe impact of uncertainties in the different steps of the assessmenton the significance of the estimated risk. The appropriate treatmentof uncertainty depends on the purpose of the assessment. If thepurpose is to estimate actual or expected risks, the uncertainty anal-ysis should consider the variability of the estimated risk about ameasure of central tendency, such as a standard deviation of themean. However, if the purpose of the assessment is to evaluatecompliance with regulatory requirements, such as a specified levelof acceptable risk, the uncertainty analysis should identify thoseparameters or assumptions which, when varied over their assumedranges, could change the decision about compliance. Some aspectsof uncertainty can be quantified, such as the distribution functionsthat result from probabilistic risk calculations (see Figure 3.4). How-ever, for many important aspects of risk assessment, uncertaintiescan only be characterized qualitatively (see discussion of ‘‘weight ofevidence’’ in Section 3.1.4.1.2). No distinction is made here betweenthe type of uncertainty that results from an observed variability ina natural or biological system and the type of uncertainty that resultsfrom lack of knowledge about the behavior of a system (NAS/NRC,1994). Uncertainty due to lack of knowledge about system behavioris likely to be particularly important in risk assessments of wastedisposal systems. This type of uncertainty is difficult to quantifyobjectively.

3.1.4.5 Risk Management. Not part of the risk assessment processper se is the process by which the results of risk assessments areintegrated with other information to make decisions about the needfor, method of, and extent of risk reduction or limitation. This processis referred to as risk management. In a waste classification context,risk management would involve evaluating estimates of risks thatarise from waste disposal and their attendant uncertainties withrespect to values of acceptable risk, resulting in decisions aboutclassification and acceptable methods of disposal (see Figure 3.1).Values of acceptable risk may be expressed using surrogates, such asrelease limits or maximum acceptable concentrations of hazardous


substances in waste; such surrogates are calculated from the valuesof acceptable risk based on assumed exposure scenarios. The riskmanagement process may involve consideration of socioeconomic andinstitutional factors, as well as the results of risk assessment. Riskmanagement is a very important aspect of establishing a waste clas-sification system and is discussed further in Section 3.3.

3.1.5 Use of Risk Assessment in Risk-Based Waste Classification

Based on the discussions in Section 3.1.4, the process of estimatingthe risk that arises from disposal of a hazardous waste at a specificsite is relatively straightforward from a conceptual standpoint. Inpractice, however, implementation of the risk assessment processcan be difficult owing to several factors, including incomplete under-standing and data concerning release, transport, exposure (dose),and response to hazardous substances; uncertainties in the data;and simplifications necessary for the models to be usable. However,since this Report is concerned with establishing the conceptual foun-dations of a waste classification system based on nonsite-specific(generic) risk assessments, the complexities of implementation canbe greatly reduced. The two most important issues in implementinga generic risk assessment for purposes of waste classification involveassumptions about release and exposure scenarios and assumptionsabout dose-response relationships. These issues are introduced inthe following two sections.

3.1.5.1 Risk Assessment of a Generic Site. The primary purposeof waste classification is to allow a grouping of wastes destined forthe same disposal technology, so that waste can be managed beforedisposal with this objective in mind. The characteristics of specificdisposal sites and site-specific waste acceptance criteria usually arenot known at the time waste is being classified. The issue resultingfrom this situation is how to perform the risk assessment for disposalnecessary to classify waste when waste may be sent to differentdisposal sites having unknown but potentially very different charac-teristics (e.g., arid versus humid).

There are three potentially significant pathways through whichexposure to hazardous substances emplaced in a waste disposal facil-ity can occur:

● Dissolution and transport of hazardous substances by ground-water. Release of hazardous substances into groundwater is rele-vant to virtually all hazardous substances, although it may not


be significant for many substances that are short-lived, insolu-ble, or greatly retarded during migration. Water is assumed toinfiltrate the disposal facility from above (downward percolationof precipitation) or the side (lateral flow of water into the waste-bearing area). This water degrades waste containers, solubilizeshazardous substances, and flows away from the disposal facilityto a point where the contaminated water enters the biosphereand is assumed to be accessible to plants, animals, and humans.This release pathway is very sensitive to the site selected,because the amounts of precipitation and groundwater varywidely and the behavior of the water and hazardous substancescontained therein are strongly dependent on the physical andchemical characteristics of the soil and rock at the site.

● Release of hazardous substances directly to the atmosphere. Haz-ardous substances can be transported to the atmosphere as sus-pended particles or gases. However, the atmospheric releasepathway usually is significant only for a limited number of haz-ardous substances that are volatile at temperatures common inthe environment, including volatile organic chemicals and a fewgaseous radionuclides, such as volatile 3H and 14C compoundsand radon. Volatile substances typically are contained in a wastedisposal facility by the use of caps composed of plastic, clay, orother impermeable materials. A site-specific risk assessmentnormally assumes that these caps fail or are compromised,resulting in the release of volatile substances. Because contain-ment is provided primarily by engineered barriers, releases tothe atmosphere are not very sensitive to the specific disposalsite being considered.

● Inadvertent human intrusion into hazardous waste. Inadvertenthuman intrusion is relevant to disposal of virtually all hazardoussubstances, especially in near-surface facilities. Typical scenar-ios assume that an unknowing individual (1) digs or drills intothe waste and brings some of it to the surface where it is thenavailable for dispersal and uptake, or (2) lives on the disposalsite after waste has been exhumed or the cover removed, andconsumes contaminated plant and animal products. Scenariosfor inadvertent intrusion usually are assumed to occur aftersome period of active institutional control over the disposal site,which is typically 100 to 300 y. Intrusion scenarios are not verysensitive to site-specific parameters because the nature of intru-sion (by digging or drilling) effectively bypasses the site-specificprotection features, such as small amounts of groundwater,


impermeable soils, or the presence of engineered barriers torelease and transport by natural processes.7

Thus, for purposes of waste classification, it appears possible toassess the risk from the intrusion and atmospheric release pathwaysat a generic hazardous waste disposal site. However, a genericassessment of risk from the water release pathway normally wouldincorporate assumptions that would be extremely conservative formany sites (e.g., the amount of water infiltration and travel timesof hazardous substances to a nearby well).

Fortunately, risk-based waste classification becomes possiblebecause the risk from hazardous waste disposal, especially in near-surface facilities, usually is dominated by the risk from inadvertentintrusion onto a disposal site after an assumed loss of institutionalcontrol, given an assumption that postulated scenarios for intrusionwould occur. For disposal of radioactive waste in near-surface facili-ties, generic assessments (NRC, 1982b) have shown that, for mostradionuclides, disposal limits based on the need to protect inadver-tent intruders are more restrictive than limits based on requirementsfor protection of off-site individuals. That is, except in unusual casesof highly soluble, long-lived radionuclides, the calculated dose perunit amount of a radionuclide in disposed waste is substantiallyhigher for hypothetical inadvertent intruders than for off-site indi-viduals who are assumed to be exposed to contaminants released towater or air. The key feature of these analyses is that transport ofcontaminants from a disposal facility into the environment by thewater pathway generally results in considerable reductions in con-centrations compared with the concentrations in the disposal facilityitself and, thus, potential exposures of off-site individuals comparedwith inadvertent intruders. Similar analyses have not been per-formed for hazardous chemicals, but the same general result shouldbe obtained for substances that are not highly soluble.

Thus, as a practical matter, it is possible to establish a risk-basedhazardous waste classification system by focusing on intrusion sce-narios that are essentially generic. In reaching this conclusion, it is

7 Inadvertent human intrusion continues to be controversial because it does noteffectively discriminate among sites. Many view intrusion as something to beaddressed by including certain features in the disposal system to discourage intrusionand warn the unknowing, but that intrusion scenarios should not be used as a basisfor establishing site suitability. However, intrusion scenarios continue to be used asa basis for assessing the acceptability of waste disposal sites in most countries,including the United States, and it is used in this context in this Report. For furtherinformation, the reader is referred to a report by the Nuclear Energy Agency (NEA,1995).


recognized that there will be cases where disposal of relatively solu-ble contaminants should be limited based on an analysis of transportin water to off-site receptor locations, rather than an analysis ofintrusion scenarios. However, such cases would be accounted forin establishing site-specific waste acceptance criteria, and the fewexceptions should not negate the usefulness of a waste classificationsystem based on generic analyses of scenarios for inadvertent intru-sion. This is especially the case if analyses of intrusion scenariosare based on the conservative assumption that all contaminants arehighly immobile and, thus, are retained in the disposal facility untilthe time intrusion is assumed to occur.

Given the conclusion that the concept of a hypothetical inadvertentintruder at a waste disposal site is useful for purposes of classifyingwaste, a multitude of scenarios might be considered. Although thedevelopment of scenarios for inadvertent intrusion for the purposeof waste classification is properly the role of regulatory authorities,NCRP believes that the types of scenarios commonly assumed inrisk assessments of near-surface disposal sites for low-level radioac-tive and hazardous chemical wastes (EPA, 1989; NRC, 1982b) or inscreening of contaminated sites (NCRP, 1996; 1999a; NRC, 1996a)are appropriate. These types of intrusion scenarios involve suchactivities as building homes at a site, excavation or drilling intowaste and exhuming its contents, and cultivation of the site byresident homesteaders after waste has been exhumed or uncovered.Although other intrusion scenarios might be envisioned, these proba-bly capture the plausible range of scenarios relevant to classificationof hazardous waste. Furthermore, they are likely to result in conser-vative estimates of risk compared with intrusion scenarios thatmight actually occur at any site, because the assumed scenariosusually involve pessimistic assumptions about the quantities andconcentrations of contaminated materials to which an intruderwould be exposed, the number of exposure pathways, and expo-sure times.

The dominance of the risk to inadvertent intruders at near-surfacewaste disposal sites allows the use of this type of scenario to developa risk-based waste classification system. However, NCRP recognizesthat exposures of the public and protection of the environment alsoare of concern in determining acceptable disposal practices at specificsites. The potential for off-site releases of hazardous substances isthe primary reason that classification of waste based on risks tohypothetical inadvertent intruders does not obviate the need for site-specific risk assessments to determine waste acceptance criteria inthe form of limits on disposal of particular hazardous substances.

3.2 EXPOSURE TO HAZARDOUS SUBSTANCES / 99

3.1.5.2 Dose-Response Relationships. The primary objective ofthis study is to set forth the foundations of a risk-based waste classi-fication system that applies to hazardous chemicals and radionu-clides. Most aspects of the risk assessment process that provide thebasis for establishing this system are conceptually the same forchemicals and radionuclides, although the specific data (e.g., solubili-ties) may differ. One important exception is the assumed relationshipof the probability of a response to a unit dose of a substance thatcauses stochastic effects, which is called the dose-response relation-ship.8 There are important conceptual differences in the way thisrelationship has been defined and used for hazardous chemicals andradionuclides, and these differences could pose a major impedimentto development of a risk-based waste classification system thatapplies to both types of substances on a consistent basis. Thesedifferences are elucidated in the following section.

3.2 Assessment of Responses from Exposure toHazardous Substances

For any hazardous substance, estimates of the relationship of doseto response in humans are based on either animal or human data.For example, only about 20 of the approximately 300 chemical carcin-ogens regulated by EPA have dose-response relationships based onhuman data from epidemiologic studies; the remainder are basedon animal bioassays. In contrast, the dose-response relationships forradiation are based primarily on the results of human epidemio-logic studies.

The doses of hazardous substances at which responses can beobserved in humans or animals are higher (sometimes by large fac-tors) than doses relevant to waste disposal and other routine expo-sure situations. Therefore, most dose-response relationships at thelow doses of interest in protection of human health are calculatedrather than measured; they are based not only on scientific data butalso on various assumptions and extrapolation models which, whilescientifically plausible, cannot yet be subjected to empirical study

8 In general, the relationship between dose and response can be represented by avariety of functional forms. At low doses of substances that cause stochastic effects,the dose-response relationship usually is assumed to be linear and, thus, can beexpressed as a single probability coefficient. This coefficient is frequently referred toas a ‘‘risk’’ (or potency factor or unit risk factor or slope factor) in the literature.However, it is really the response (consequence) resulting from a dose of a hazardoussubstance, and it should not be confused with ‘‘risk’’ as defined and used in this Report.


and validation. Indeed, the vast majority of responses alleged to beassociated with low doses of hazardous substances are not generallytestable with any practical epidemiologic study. Despite this diffi-culty, regulatory authorities make decisions based on the possibilitythat hypothetical responses predicted to occur at low doses may bereal. Such actions are based partly on legal requirements and partlyon prudence. In short, the rationale for making decisions based onsuch evidence is that the body of scientific information, assumptions,and extrapolation models used to develop dose-response relation-ships is considered sufficiently revealing on the question of risk tohumans to prompt control measures. The following sections discussdose-response relationships for hazardous chemicals and radionuclides.

3.2.1 Assessment of Responses from Exposure toHazardous Chemicals

Once the hazard identification process is completed for a chemical(see Section 3.1.4.1), a judgment is made concerning the appropriate-ness of conducting a dose-response assessment. If such an assess-ment is judged appropriate, based on the observation of sufficientresponses in humans or animals, the process shown in Figure 3.1is undertaken. First, the basis for the assessment must be estab-lished, as described in Section 3.2.1.1. The dose-response assessmentthen is undertaken using one of several approaches described inSections 3.2.1.2 and 3.2.1.3, depending on the available data, natureof the responses (deterministic or stochastic), and applicable regulatoryguidance. Characterization of the dose-response assessment is dis-cussed in Section 3.2.1.4. Deficiencies and uncertainties are an inevita-ble part of this process, and these are discussed in Section 3.2.1.5.

3.2.1.1 Basis for a Dose-Response Assessment. At a minimum, theresults of the hazard identification process are available as a basisfor dose-response assessment. Additional data obtained from knownoccupational exposures or studies conducted specifically for the pur-pose of dose-response assessment may also be available.

If available, adequate human epidemiologic data are preferredover data from animal studies. If there are adequate data on dosesreceived in a well-designed and well-conducted negative epidemio-logic study, it may be possible to obtain an upper-bound estimate ofthe response probability from that study. Estimates of upper boundsobtained from negative studies on animals, if available, also shouldbe presented as supporting evidence.

In the absence of appropriate human studies, data from an animalspecies expected to respond most like humans should be used, if this


type of information is available. When several studies on a givensubstance are available, which may involve different species, strains,and sexes given several doses by different routes of exposure, thefollowing approach to selecting the data sets is used: (1) the responsedata are separated according to organ affected and response type;(2) all biologically and statistically acceptable data sets are pre-sented; (3) the range of the dose-response estimates is presentedwith due regard to biological relevance, particularly in the case ofanimal studies, and appropriateness of the route of exposure; and(4) the biologically acceptable data set from long-term animal studiesshowing the greatest sensitivity is generally given the greatestemphasis, again with due regard to biological and statistical consid-erations, because human sensitivity could be as high as the mostsensitive responding animal species, in the absence of evidence tothe contrary.

When the route of exposure in the species from which the dose-response information is obtained differs from the route occurring inenvironmental exposures, the considerations used in making theroute-to-route extrapolation must be carefully described. All assump-tions should be presented along with a discussion of the uncertaintiesin the extrapolation. Whatever procedure is adopted in a given case,it must be consistent with the existing metabolic and pharmacoki-netic information on the chemical, such as the absorption efficiencyin the gut and lung, doses to target organs, metabolic toxification ordetoxification processes, and changes in placental transport through-out gestation for transplacental toxicity.

When two or more significantly elevated response sites or typesare observed in the same study, extrapolations may be conductedon selected sites or types. These selections will be made on biologicalgrounds. To obtain an estimate of the total response probability inanimals with two or more response sites or types showing signifi-cantly elevated occurrence, response probabilities often are pooledand used for extrapolation. Pooling of data results in an estimate oftotal risk that is applied to humans without regard for the particularorgans or tissues at risk in humans. Such pooling increases thestatistical power of a study, and it takes into account that observedcancers in study animals often do not correspond to cancers expectedto occur in humans. Quantitative extrapolations of the dose-responserelationship generally will not be made on the basis of totals thatinclude response sites without statistically significant elevations.

Chemical agents are not expected to increase the incidence ofcancer in all, or even many, organs or tissues. Rather, it is thoughtthat certain agents can cause an increase in the incidence of cancerin a single organ or, in some cases, two or three related tissues. This


is the rationale for conducting quantitative risk assessments basedon those organs or tissues in which there is a statistically relatedincrease in cancer incidence between study populations and controlgroups, and generally only in those cases where there is an increasein tumor response with increasing dose. When more than one organhas an increased cancer incidence, several permutations of the dataoften are evaluated and the one that yields the strictest and lowestvirtually safe dose is used for purposes of regulatory risk assessmentand health protection. For example, if malignant tumors in the kid-ney and bladder are observed, and both carcinomas and adenomasare present in both organs, the arrangement of the data (e.g., the sumof both benign and malignant tumors in the kidney) that producesthe lowest ‘‘safe’’ dose is the one used. There is no standard set ofassumptions about which organs should be considered, the tumortypes that can be added, the degree of dose-relatedness that mustaccompany the tumor incidence, or which animal model is preferred.

Benign tumors generally should be combined with malignanttumors for the purpose of estimating the dose-response relationship,unless the benign tumors are not considered to have the potentialto progress to the associated malignancies of the same histogenicorigin. The contribution of the benign tumors to the total responseshould be indicated.

Since responses at the low dose levels of concern in routine expo-sures of the public cannot be measured directly in animal or humanepidemiologic studies, a number of approaches have been developedto extrapolate from high to low doses. Different extrapolationapproaches may fit the observed data reasonably well but lead tolarge differences in the projected responses at low doses (see Sec-tion 3.2.1.5.2).

3.2.1.2 Dose-Response Assessment for Chemicals That Cause Deter-ministic Effects. For hazardous chemicals that cause deterministiceffects and exhibit a threshold in the dose-response relationship, thepurpose of the dose-response assessment is to identify the dose of asubstance below which it is not likely that there will be an adverseresponse in humans. Establishing dose-response relationships forchemicals that cause deterministic effects has proved to be complexbecause (1) multiple responses are possible, (2) the dose-responseassessment is usually based on data from animal studies, (3) thousandsof such chemicals exist, and (4) the availability and quality of data arehighly variable. As a consequence, the scientific community has neededto devise and adhere to a number of methods to quantify the mostimportant (low or safe dose) part of the dose-response relationship.


There are two possible approaches to estimating the human ‘‘safe’’dose for chemicals that cause deterministic effects: the use of safetyand uncertainty factors and mathematical modeling. The former consti-tutes the traditional approach to dose-response assessment for chemi-cals that induce deterministic effects. Biologically-based mathematicalmodeling approaches that more realistically predict the responses tosuch chemicals, while newer and not used as widely, hold promise toprovide better extrapolations of the dose-response relationship belowthe lowest dose tested.

3.2.1.2.1 Dose-response concepts. Dose-response assessment forhazardous chemicals that can cause deterministic effects begins withthe toxicology data developed during the hazard identification stepdescribed in Section 3.1.4.1.2. In many cases, hazard identificationand dose-response assessment occur simultaneously. For each chem-ical, the critical response (a specific response in a specific organ) isidentified in the hazard identification process. Using the availabledata for the critical response, one of the following is established:

● A no-observed-adverse-effect level (NOAEL), which is the high-est administered dose at which no biologically significantincrease in the frequency or severity of the critical responsebetween the exposed population and its control in the most sensi-tive test species is identified. The study may show some effectat this dose but the effect is not deemed a response because EPAjudges that it is not adverse or is not a precursor to a specificadverse effect severe enough to be considered a response. Ifseveral doses in a study are NOAELs or if several studies indi-cate different NOAELs, EPA focuses on the highest dose withoutadverse effect; this leads to the common usage of the termNOAEL to mean the highest dose without significant adverseeffect.

● A lowest-observed-adverse-effect level (LOAEL), which is thelowest administered dose of a chemical at which there is a biolog-ically significant increase in frequency or severity of the criticalresponse between the exposed population and its control.

LOAEL is, of course, higher than NOAEL.NOAEL (or LOAEL if NOAEL is not available) is used as a point

of departure to calculate a reference dose (RfD), which is the highestdose of the chemical at which no statistically significant adverseeffects are expected in the most sensitive humans. RfD of a particularsubstance is calculated from NOAEL (or LOAEL) by applying oneor more safety and uncertainty factors. RfD usually is 100 to 1,000times lower than NOAEL or 1,000 to 10,000 times lower than


LOAEL, although a substantially smaller safety and uncertaintyfactor may be used when RfD is derived from a NOAEL obtainedin a high-quality study in humans. The schematic relationship ofNOAEL, LOAEL, and RfD is illustrated in Figure 3.5. Although RfDsare widely used in health protection of the public, it is important tounderstand that they do not represent thresholds for deterministicresponses in humans.

3.2.1.2.2 Safety factor approach for chemicals that cause determin-istic effects. Traditional toxicologic procedures for chemicals thatcan induce deterministic effects, which are assumed to have a thresh-old dose, define RfD for humans or animals as some fraction ofNOAEL. This fraction is determined by establishing safety factorsto account for weaknesses and uncertainties in the data and in theextrapolation from animals to humans. In the safety factor approach,doses below RfD are assumed not to result in a response becausethey are below the threshold of toxicity (Dourson and Stara, 1983;Renwick and Lazarus, 1998; Weil, 1972).

Fig. 3.5. Illustration of relationships of NOAEL, LOAEL, and RfD.


EPA bases its procedures for estimating RfD on several assump-tions, the most basic of which is that a threshold exists in the dose-response relationship for the critical response. If the dose is abovethe threshold (not the same as RfD) and is of sufficient duration,EPA considers that the chemical will cause the response in somesegment of the exposed population. The U.S. Food and Drug Adminis-tration uses a similar approach to identify safe levels of exposure tofood additives and certain residues. Studies on many substanceshave shown that before toxicity occurs, the chemical must depletea physiological reserve or overcome the various repair capacities inthe human body (Klaassen et al., 1996).

A second major assumption is that an RfD adequately protectsparticularly susceptible humans (e.g., infants and children, theelderly or infirm). To account for these population groups, EPAincludes an uncertainty factor that represents the variation in thethresholds among individuals. For some chemicals, however, a fewpersons show evidence of hypersensitivity or chemical idiosyncrasy(NAS/NRC, 1994), and there is concern over whether using RfD tolimit exposure will protect these subgroups.

A third important assumption relates to selecting the criticalresponse. EPA assumes that if the dose is below that required tocause the most sensitive response, then other deterministicresponses will not occur. However, if other responses have shallowerslopes in the dose-response curves near their thresholds, estimatingRfD on the basis of the critical response may not be sufficientlyprotective to preclude a noncritical response from occurring. For thisreason, EPA may use information on the slopes of dose-responsecurves to determine the critical response and the number of safetyfactors to be applied, although EPA rarely does so.

Lastly, EPA usually assumes that using animals of different agesdoes not affect NOAEL and LOAEL. This assumption ignores possi-ble differences in toxicity among animals of different ages.

3.2.1.2.3 Selection of the database. The types of studies that makeup a complete database for estimating an RfD of high confidencefor chemicals causing deterministic effects from data in laboratoryanimals include:

● Two adequate chronic toxicity studies in different mammalianspecies by oral exposure;

● One adequate multi-generation reproductive toxicity study in amammalian species by oral exposure; and

● Two adequate developmental toxicity studies in different mam-malian species by oral exposure.


These studies may show a need for special studies to assess, forexample, neurotoxic or immunotoxic responses. In such cases, thedatabase may not be complete without the special studies.

EPA does not consider data on developmental toxicity, standingalone, as an adequate basis for estimating RfD. Investigators useacute or short-term exposures for these studies. Therefore, they are oflimited use in estimating the threshold for deterministic responses.However, if developmental toxicity is the critical response for a chem-ical with a complete database, EPA will derive RfD from that study.

EPA uses the following hierarchy for choosing the study, species,and NOAEL used to calculate an RfD:

● The most appropriate NOAEL for the critical response from awell-conducted study in humans is identified.

● The most appropriate NOAEL for the critical response from awell-conducted study in a laboratory animal species that mostresembles humans is chosen.

● The most sensitive species and study is identified. EPA basesthis decision on a comparison of the available NOAELs andLOAELs. However, the data from one study may be unusual orinconsistent when compared with the results of other studies.If there is a convincing scientific argument that the responsewill not occur in humans, EPA may discount the unusual datain these cases.

A subchronic (90 d) bioassay in a mammalian species by oralexposure is the recommended minimum data for estimating an RfD.The study must meet EPA’s minimum standards of quality. Ideally,the study should identify a NOAEL and a LOAEL. In the absence ofthese minimum data, EPA assigns the chemical to a ‘‘not verifiable’’group, and EPA then seeks or waits for additional data before esti-mating RfD for that chemical.

3.2.1.2.4 Determination of the reference dose. EPA determinesRfD from NOAEL or LOAEL obtained from an animal study or,occasionally, a human study using the following equation:

RfD � NOAEL or LOAEL/(UF � MF), (3.1)

where UF is an uncertainty factor that accounts for uncertaintiesin extrapolating from the experimental data and MF is a modifyingfactor (see Table 3.1).

The product of UF and MF is the safety factor, which also is referredto as the safety and uncertainty factor since it accounts for uncertain-ties in the data.


TABLE 3.1—Guidelines for the use of uncertainty and modifyingfactors in deriving an RfD (EPA, 1989).

Sub-Factor Issue Addressed Value and Comments

Uncertainty Factor (UF): H � A � S � L � D

H Interhuman Generally use a factor of 10 whenextrapolating from experimental resultsin studies using prolonged exposure ofaverage healthy humans. EPA intendsthat this factor account for variation insensitivity in humans.

A Experimental: Generally use a factor of 10 whenanimals to humans extrapolating from results of chronic

studies on experimental animals. EPAintends that this factor account foruncertainty involved in extrapolatingfrom laboratory animals to humans.

S Subchronic to chronic Generally use a factor of 10 whenextrapolating from results of subchronicstudies on experimental animals orhumans. EPA intends that this factoraccount for uncertainty involved inextrapolating from subchronic NOAELsto chronic NOAELs.

L NOAEL to LOAEL Generally use a factor of 10 whenderiving an RfD from a LOAEL, insteadof a NOAEL. EPA intends that thisfactor account for uncertainty involvedin extrapolating from a LOAEL to aNOAEL.

D Incomplete database Generally use a factor of 10 when dataare incomplete. EPA intends that thisfactor account for inability of a study toadequately address all adverse effects.This factor reflects extent of informationon the chemical to judge its toxicity inchronic, reproductive, anddevelopmental settings.

Modifying Factor (MF)

Other uncertainties Use professional judgment to determineand limitations a factor which is �0 and �10. The size

of this factor depends on uncertaintiesin the study and database not explicitlytreated in uncertainty factor. Defaultvalue is one.


RfD is useful as a reference point for identifying whether a particu-lar dose to humans poses a significant health hazard. Doses belowRfD are not likely to cause any deterministic responses and are oflittle regulatory concern. As the frequency of exposure that exceedsRfD increases, the chance increases that the dose may cause aresponse. However, EPA cannot conclude that doses below RfD willnot result in a response in a few individuals, or that doses aboveRfD will result in a response in any individual.

3.2.1.2.5 Selection of uncertainty and modifying factors. Thechoice of an uncertainty factor for a chemical that can cause deter-ministic effects is based on case-by-case judgment. This factor shouldaccount for the shortcomings and uncertainties in the scientific data.

As noted in Table 3.1, EPA often uses up to a factor of 10 for eachof five areas of uncertainty in establishing an RfD from a NOAELor LOAEL. In practice, however, the magnitude of any uncertaintyfactor is dependent on professional judgment. If EPA has resolveduncertainties in all areas, which usually is not the case, an uncer-tainty factor of one is used to estimate RfD. When uncertaintiesexist in one, two, or three areas, EPA can use an uncertainty factorof 10, 100, or 1,000, respectively. When uncertainties exist in fourareas, EPA often uses an uncertainty factor of 3,000. When uncer-tainties exist in five areas, EPA might use an uncertainty factor of10,000. The justification for reducing the uncertainty factor in thelatter two situations is EPA’s knowledge of interrelationships amongthe various areas of uncertainty. In these cases, the multiplicationof four or five factors of 10 is likely to yield an unnecessarily low RfD.

EPA occasionally uses an uncertainty factor of less than 10 if theavailable data reduce the need to account fully for a particular areaof uncertainty. For example, if the lowest dose tested shows only aminor adverse effect, EPA might assign an intermediate value tothe uncertainty factor. The usual intermediate value is three, whichis the geometric mean of one (the lowest theoretical factor) and 10,rounded to one significant digit. This rounding procedure reflectsthe expected precision of the process. EPA uses the geometric meanbecause it judges that the biological processes involved are likely toshow a log-normal probability distribution. EPA might use a valueof three for the database factor if studies on developmental or repro-ductive toxicity are missing. EPA might also use a value of three forthe subchronic-to-chronic factor if the database includes occupa-tional studies of long duration (e.g., 7 to 15 y).

EPA uses a modifying factor as an occasional adjustment in esti-mating an RfD. EPA intends that this factor account for areas ofuncertainty not explicitly addressed by the usual factors. Its value


may be less than one but not more than 10; the default value is one.EPA assigns the value using the same rules as for the uncertaintyfactors (i.e., 0.3, 1, 3, or 10).

EPA uses a modifying factor when the usual uncertainty factorsdo not fully account for uncertainties in the data. For example, thefewer the number of laboratory animals used in a group, the morelikely it is that the study will show a NOAEL. In such a case, EPAmight raise the usual uncertainty factor of 100 to 300 to account forthis deficiency. While this increase is scientifically reasonable, thepublic might perceive the change as arbitrary. EPA intends to avoidthis perception by using a modifying factor. Thus, EPA might usean uncertainty factor of 100 and a modifying factor of three to arriveat an RfD.

3.2.1.2.6 Assigning confidence levels. EPA assigns confidence lev-els (low, medium or high) to the principal study, the database, andRfD. When assessing the level of confidence in the principal study,EPA considers several factors. One is the adequacy of study designincluding the route of exposure, sample size, duration of the study,analytical techniques, and biological responses measured. Otherfactors considered include the demonstration of a dose-responserelationship and potential differences in response among differentspecies. If EPA determines that a subchronic study meets high stand-ards of quality, the study will receive a high confidence rating. EPAgives a low confidence rating to the database for a chemical lackingsupporting studies in other species and lacking studies on reproduc-tive or developmental toxicity.

The confidence in RfD is a composite of the confidence in theprincipal study and in the database. In assigning confidence to RfD,EPA gives precedence to confidence in the database. If the principalstudy has a medium confidence rating and the database a low confi-dence rating, EPA assigns low confidence to RfD. The confidence levelis a part of dose-response characterization discussed in Section 3.2.1.4.

3.2.1.2.7 Mathematical modeling and the benchmark dose method.Mathematical models may be applied to data on dose-response toreduce the uncertainty in identifying a reliable (statistically valid)NOAEL for chemicals inducing deterministic effects or, alterna-tively, an ED10 (the benchmark dose), which is the dose at which10 percent of the study animals are expected to show a response(Krewski et al., 1989; Moolgavkar et al., 1999). Examples of relevantcurve-fitting models include the probit and Weibull (Moolgavkaret al., 1999; Park and Snee, 1983; Paustenbach, 1995). These modelstake into account the uncertainty in dose-response data obtained


from animal studies and use ‘‘best-fit’’ procedures to construct adose-response curve. Sometimes these models are combined withphysiologically-based pharmacokinetic (PB-PK) models (see Section3.2.1.3.4) to predict the response across the doses tested (Reitzet al., 1996).

Since about 1995, EPA and other agencies have begun to usethe so-called benchmark dose method to estimate NOAEL and ED10

(Barnes et al., 1995; Crump, 1984; 1995; EPA, 1995a). As illustratedin Figure 3.6, a statistical fit of a dose-response model to the dose-response data is used to identify an ED10, which is the central esti-mate of the dose that results in a response in 10 percent of the studyanimals. The lower 95 percent confidence limit of the benchmarkdose (LED10) then is used as the point of departure for establishingallowable exposures to chemicals that cause deterministic effects,in a manner similar to the approach of determining RfDs fromNOAELs by using safety and uncertainty factors. For example, a

Fig. 3.6. Illustration of use of benchmark dose method to estimate nomi-nal thresholds for deterministic effects in humans. The benchmark dose(ED10) and LED10 are central estimate and lower confidence limit of dosecorresponding to 10 percent increase in response, respectively, obtained fromstatistical fit of dose-response model to dose-response data. The nominalthreshold in humans could be set at a factor of 10 or 100 below LED10,depending on whether the data are obtained in humans or animals (see textfor description of projected linear dose below point of departure).


nominal threshold for deterministic responses in humans could beestimated as a dose that is a factor of 10 or 100 less than the lowerconfidence limit of the benchmark dose, depending on whether thedata are obtained in humans or animals, and allowable doses couldbe established by applying additional safety and uncertainty factorsto the nominal threshold.

The rationale supporting use of ED10 as the benchmark dose isthat a 10 percent response is at or just below the limit of sensitivityin most animal studies. Use of the lower confidence limit of thebenchmark dose, rather than the best (maximum likelihood) esti-mate (ED10), as the point of departure accounts for experimentaluncertainty; the difference between the lower confidence limit andthe best estimate does not provide information on the variability ofresponses in humans. In risk assessments for substances that inducedeterministic effects, a dose at which significant effects are notobserved is not necessarily a dose that results in no effects in anyanimals, due to the limited sample size. NOAEL obtained using moststudy protocols is about the same as an LED10.

The benchmark dose method was developed to overcome difficul-ties with determining NOAEL based on dose-response data. Thepotential advantages of the method include the following (Crump,1984; 1995):

● The benchmark dose method makes use of all the dose-responsedata by fitting a dose-response model to the data, whereas thedetermination of a NOAEL generally involves a comparison ofresponses at discrete and well separated doses with responsesin control subjects.

● The benchmark dose reflects sample size more appropriatelythan a NOAEL because small studies tend to result in smallerbenchmark doses, whereas the opposite is the case for NOAELs.

● A NOAEL is constrained to be one of the administered doses,but this is not the case with the benchmark dose method.

● A benchmark dose can be defined from a data set that does notinclude a NOAEL.

● The determination of a NOAEL generally involves dose datathat are categorized into distinct groups, but this categorizationis arbitrary in some studies. Grouping of data into distinct dosecategories is not required in the benchmark dose method.

The benchmark dose method can also be applied to chemicals thatcause stochastic effects (Section 3.2.1.3.3). This is indicated by theprojected linear response at doses below LED10 in Figure 3.6.

3.2.1.3 Dose-Response Assessment for Chemicals That Cause Sto-chastic Effects. For hazardous chemicals that do not have a thresh-old in the dose-response relationship, which is currently believed to


be true for genotoxic carcinogens, the purpose of a dose-responseassessment is to extrapolate the data obtained at high doses tothe low-dose region of concern in routine exposure situations. Thisextrapolation is usually accomplished with mathematical modelsthat are thought to be conservative (i.e., likely to overestimate thetrue responses).

Although dose-response assessments for deterministic and sto-chastic effects are discussed separately in this Report, it should beappreciated that many of the concepts discussed in Section 3.2.1.2for substances that cause deterministic effects apply to substancesthat cause stochastic effects as well. The processes of hazard identi-fication, including identification of the critical response, and develop-ment of data on dose-response based on studies in humans or animalsare common to both types of substances. Based on the dose-responsedata, a NOAEL or a LOAEL can be established based on the limitedability of any study to detect statistically significant increases inresponses in exposed populations compared with controls, eventhough the dose-response relationship is assumed not to have athreshold. Because of the assumed form of the dose-response rela-tionship, however, NOAEL or LOAEL is not normally used as apoint of departure to establish ‘‘safe’’ levels of exposure to substancescausing stochastic effects. This is in contrast to the common practicefor substances causing deterministic effects of establishing ‘‘safe’’levels of exposure, such as RfDs, based on NOAEL or LOAEL (orthe benchmark dose) and the use of safety and uncertainty factors.

3.2.1.3.1 Introduction to mathematical modeling for chemicals thatcause stochastic effects. Given the assumption of a nonthresholddose-response relationship for chemicals causing stochastic effects(genotoxic), mathematical modeling is essential in estimating theresponse at doses below levels where dose-response data are avail-able, or in estimating the dose that would be ‘‘safe,’’ i.e., a dosecorresponding to an ‘‘acceptable’’ level of health risk (probabilityof a stochastic response) established by regulators. The followingsections discuss dose-response modeling for chemicals that causestochastic effects. Except for biologically-based models of cancerinduction, the various models discussed also can be applied in dose-response assessment for substances that cause deterministic effects,because the models do not depend on the particular form of the truedose-response relationship.

Because of the statistical and biological problems inherent in theidentification of a true no-effect level in any study of dose-response,most mathematical models for chemicals that cause stochastic effectshave eliminated the concept of a threshold dose below which no


response would be expected. Cornfield (1977) and Schaffer (1981),for example, have discussed kinetic models for cancer induction thatlead to the existence of thresholds under steady-state conditions,but these have not been used in regulatory decision making in theUnited States. Other investigators have argued that numerous bio-logical protective mechanisms surely exist for all mechanisms bywhich cancers can be initiated (Bus and Gibson, 1995), and that atsome low dose a practical threshold for even the most potent carcino-gen must exist (Gold et al., 1998; Stokinger, 1977). The basis forassuming the absence of a threshold is that one cannot dismiss thepossibility of a response being induced, for example, if a reactivemetabolite were produced and it then interacted with DNA (Brown,1978); if this hypothesis is accepted, there is some probability of aresponse no matter how small the dose.

3.2.1.3.2 Statistical models. A number of statistical dose-response extrapolation models have been discussed in the literature(Krewski et al., 1989; Moolgavkar et al., 1999). Most of these modelsare based on the notion that each individual has his or her owntolerance (absorbed dose that produces no response in an individual),while any dose that exceeds the tolerance will result in a positiveresponse. These tolerances are presumed to vary among individualsin the population, and the assumed absence of a threshold in thedose-response relationship is represented by allowing the minimumtolerance to be zero. Specification of a functional form of the distribu-tion of tolerances in a population determines the shape of the dose-response relationship and, thus, defines a particular statisticalmodel. Several mathematical models have been developed to esti-mate low-dose responses from data observed at high doses (e.g.,Weibull, multi-stage, one-hit). The accuracy of the response esti-mated by extrapolation at the dose of interest is a function of howaccurately the mathematical model describes the true, but unmea-surable, relationship between dose and response at low doses.

For the most frequently used low-dose models, the ‘‘multi-stage’’and ‘‘one-hit,’’ there is an inherent mathematical uncertainty in theextrapolation from high to low doses that arises from the limitednumber of data points and the limited number of animals tested ateach dose (Crump et al., 1976). The statistical term ‘‘confidence lim-its’’ is used to describe the degree of confidence that the estimatedresponse from a particular dose is not likely to differ by more thana specified amount from the response that would be predicted by themodel if much more data were available. EPA and other agenciesgenerally use the 95 percent upper confidence limit (UCL) of thedose-response data to estimate stochastic responses at low doses.


By using UCL and assuming that the model accurately reflects thedose-response relationship at low doses, there is only a five percentchance that the true response is higher than the response predictedby the model.

UCL takes into account measurement uncertainty in the studyused to estimate the dose-response relationship, such as the statisti-cal uncertainty in the number of tumors at each administered dose,but it does not take into account other uncertainties, such as therelevance of animal data to humans. It is important to emphasizethat UCL gives an indication of how well the model fits the data atthe high doses where data are available, but it does not indicate howwell the model reflects the true response at low doses. The reasonfor this is that the bounding procedure used is highly conservative.Use of UCL has become a routine practice in dose-response assess-ments for chemicals that cause stochastic effects even though a bestestimate (MLE) also is available (Crump, 1996; Crump et al., 1976).Occasionally, EPA will use MLE of the dose-response relationshipobtained from the model if human epidemiologic data, rather thananimal data, are used to estimate risks at low doses. MLEs havebeen used nearly universally in estimating stochastic responses dueto radiation exposure.

Although rarely presented in a dose-response assessment, innearly all cases the lower bound on the incremental probability ofa response will be zero or less (see Figure 3.7). That is, the statisticalmodel that accounts for the uncertainty in the results of an animalstudy also accommodates the possibility that no response may occurat low doses and that, in fact, there may be fewer responses (e.g.,cancers) than observed in the control population at some low doses.The possibility of reduced responses at low doses also is shown bythe lower confidence limit of data on radiation-induced cancers insome organs of humans including, for example, the pancreas, pros-tate, and kidney (Thompson et al., 1994).

The question of whether MLE or UCL of the dose-response rela-tionship should be used to estimate health risks to the public forthe purpose of regulatory decision making has been debated by manyinvestigators (Bailar et al., 1988; Crump et al., 1976; Finkel, 1994;Sielken, 1985; Sielken et al., 1994). Although good reasons for adopt-ing one approach or the other have been advanced, chemical riskassessments generally use UCLs and radiation risk assessmentsgenerally use MLEs. The difference between the two estimates usu-ally is not trivial for chemicals that cause stochastic effects, and itcan be of great regulatory significance. For example, for a numberof chemicals that have been examined, UCL will rarely predictresponses less than five times greater than those predicted by MLE,


Fig. 3.7. Typical relationship of best estimate and statistical bounds ofincremental probability of an adverse response from exposure to a hazardouschemical (Sielken, 1987).

and the predicted responses based on UCL can often be as much as200 times greater than estimates based on MLE (Paustenbach,1995). Finkel (1989; 1994) and others have maintained that thisamount of possible conservatism or prudence is justified in light ofthe possible severity of the hazards, our lack of knowledge aboutmechanisms of action of most carcinogens, and the largely unknowneffects of exposure to mixtures of hazardous substances.

3.2.1.3.3 Benchmark dose method. In recent years, confidence inthe ability of statistical curve-fitting models to accurately predictcancer incidence in humans based exclusively on data obtained fromanimal studies has lessened (Crump, 1996). The reasons are mani-fold and include that (1) the pharmacokinetic behavior of chemicalsin humans is often different than in animals, (2) the study animals


do not live as long as humans, (3) MTDs in animals have littlerelationship to the doses that humans normally would experience,(4) biological repair mechanisms in humans are different than inanimals, and (5) the study animals are in-bred with a predispositionto developing tumors.

As a result of this lack of confidence, recent cancer guidelines(EPA, 1996a) include a recommendation that it would be appropriateto use the benchmark dose method discussed in Section 3.2.1.2.7,rather than traditional low-dose extrapolation models, whenattempting to predict cancer risks at low doses. The benchmark dosemethod acknowledges that traditional models are only an approxi-mation of the plausible response at low doses. In this method, LED10

(the lower 95 percent confidence limit of a dose associated with anincrease in response of 10 percent) is considered to be within therange of observation, without any significant extrapolation (see Fig-ure 3.6). The dose-response relationship for chemicals that causestochastic effects often is assumed to be essentially linear at dosesbelow LED10.

The benchmark dose method is particularly useful when the modeof action of a chemical that causes stochastic effects is thought tobe nonlinear. In these circumstances, the response is assumed todecrease more rapidly than linearly with decreasing dose. Alterna-tively, the mode of action may theoretically have a threshold; forexample, the carcinogenicity of a substance may be a secondary effectof its toxicity or of an induced physiological change that is itself athreshold phenomenon.

In the benchmark dose method, observed responses are not extrap-olated to give an estimate of the probability of a response at low doses.Instead, a margin of exposure (MOE) analysis is used to evaluate thedegree of concern about different levels of exposure (CRARM, 1997;EPA, 1996a). MOE usually is defined as LED10 divided by the level ofexposure. For example, if an individual routinely ingests 10 �g (kg d)�1

and LED10 is 1,000 �g (kg d)�1, MOE is 100. Larger margins ofexposure indicate exposures of lesser concern, and vice versa.

The benchmark dose method and MOE analyses are essentiallythe same for substances that cause stochastic or deterministic effects.For both types of substances, the point of departure in the dose-response curves for purposes of protecting human health is a doseat which some response is expected, either LED10 or some otherhuman equivalent dose or concentration as the data support. Forstochastic responses (e.g., cancers), the point of departure when ani-mal data are used is a human equivalent dose or concentration


obtained based on interspecies dose adjustment or toxicokineticanalysis.

3.2.1.3.4 Pharmacokinetic models. An important advance in riskassessment for hazardous chemicals has been the application ofpharmacokinetic models to interpret dose-response data in rodentsand humans (EPA, 1996a; Leung and Paustenbach, 1995; NAS/NRC,1989; Ramsey and Andersen, 1984). Pharmacokinetic models canbe divided into two categories: compartmental or physiological. Acompartmental model attempts to fit data on the concentration of aparent chemical or its metabolite in blood over time to a nonlinearexponential model that is a function of the administered dose of theparent. The model can be rationalized to correspond to different‘‘compartments’’ within the body (Gibaldi and Perrier, 1982).

PB-PK models, sometimes referred to as biologically-based disposi-tion models, allow for accurate extrapolation of rodent data to esti-mate human dose-response relationships (Paustenbach, 1995).PB-PK models, unlike compartmental models, have the capabilityof simulating a chemical’s behavior in biological systems. The pur-pose of a PB-PK model is to predict the human dose-response rela-tionship based on animal data by quantitatively estimating thedelivered dose of the biologically relevant chemical species in a tar-get tissue (Andersen et al., 1987; Clewell et al., 1994; Leung andPaustenbach, 1995; Ramsey and Andersen, 1984).

Models based on the PB-PK approach differ from compartmentalmodels in that they incorporate actual physiology and the biochemi-cal behavior of the hazardous chemical in the test animal. Insteadof compartments defined by the experimental data, actual organ andtissue groups are modeled. After a conceptual model of the chemical’sbehavior is developed, mass-balance differential equations are writ-ten to describe the behavior in each biologically relevant compart-ment. These models can be adapted to accommodate such biologicalprocesses as nonlinear tissue binding, Michaelis-Menten elimina-tion, parallel organ-specific elimination, enzyme induction andinhibition, biliary recycling, diffusional resistance across cell mem-branes, and the number and affinity of receptors (Andersen andKrishnan, 1999; Andersen et al., 1987). Biokinetic models similar toPB-PK models for hazardous chemicals are widely used in estimatingradiation doses to different organs or tissues following intakes ofradionuclides into the body (see Section 3.2.2), although the modelsfor radionuclides do not include biochemical processes that may beimportant for hazardous chemicals other than heavy metals.

Physico-chemical processes and blood flows are modeled in thePB-PK approach (Ramsey and Anderson, 1984). Use of physico-chemical and biochemical rate constants of the subject hazardous


chemical allows the creation of a model that has greater predictivepower than traditional compartmental models. Because actual meta-bolic parameters are used, dose extrapolations can be carried out overranges in which saturation of metabolism occurs. Most importantly,because known physiological parameters are used, a different sub-stance can be modeled by replacing the appropriate rate constants.Different routes of administration can be accounted for by addingequations describing the nature of the new route. PB-PK modelshave been developed for at least 60 industrial chemicals, includingkepone, methylene chloride, styrene, perchloroethylene, polychlori-nated dibenzofurans, carbon tetrachloride, dioxane, methyl chloro-form, methylene chloride, ethylene dichloride and dioxin (Leungand Paustenbach, 1995). For illustrative purposes, the conceptualmodel for carbon tetrachloride is shown in Figure 3.8 (Paustenbachet al., 1988).

The power of the PB-PK approach was demonstrated by Andersenet al. (1987) in an analysis of stochastic responses to low doses ofmethylene chloride and by Reitz et al. (1996) with vinyl chloride.Based on consideration of biological factors that occur only at highdoses, these studies showed that in order to predict the low-doseresponse in humans, the model must correct for a number of meta-bolic, physiological, and pharmacokinetic factors. One of the keyfactors that PB-PK approaches can take into account is saturablemetabolic pathways, which is the reason that many chemicals arecarcinogenic in animal studies but would pose much less of a hazardto humans at low doses. After applying the appropriate correctionsand developing a model that could accurately predict the chemical’sbehavior in rodents and humans, these investigators determinedthat the highest plausible dose associated with a frequency ofresponse of 1 in 1,000,000 was overestimated by more than a factorof 100 when a non-pharmacokinetic approach was used.

In addition to describing complex phenomena needed to quantifyand understand the dose-response relationship, PB-PK models canhelp provide physiological explanations for differences in responsebetween animals and humans. For example, Leung et al. (1990)developed a PB-PK model for dioxin wherein they showed how differ-ences in the number and affinity of receptors in the liver could explainobserved differences in the dose-response relationship in two speciesof mice. Although the investigators did not discuss how this insightmight affect the low-dose extrapolation, they noted that a PB-PKmodel for dioxin should be useful in assessing dose-response relation-ships in humans. A validated model that accounts for differences indioxin concentrations in the liver among species might explain whyrodents appear to be more sensitive to dioxin than humans, since


Fig. 3.8. Schematic of a combination PB-PK and compartmental modelfor inhalation of radiocarbon tetrachloride (14CCl4) (Paustenbach et al.,1988).

rodents have liver concentrations 10 to 100 times higher thanhumans exposed to similar doses (Leung et al., 1990).

3.2.1.3.5 Biologically-based models of cancer. Although the devel-opment and use of PB-PK models represents a significant improve-ment in achieving an understanding of the likely human response tohazardous chemical exposure, owing to their improved extrapolationcapabilities, perhaps an even more promising contribution to sto-chastic dose-response assessment for hazardous chemicals is thedevelopment of biologically-based models of cancer. The differencebetween PB-PK models and more complex biologically-based modelsis that the latter attempt to address factors that are not easilymeasured but influence the tumorigenic progress. These factors


include, for example, cell proliferation, cell death, immunizationrates, and the progress of tumors to malignancy.

The first of these models was developed by Moolgavkar andKnudson (1981); see also Moolgavkar et al. (1988; 1999). Despite thesimplicity of the model compared with the diversity of the diseaseprocesses categorized under the heading of cancer, the model offerssignificant potential. It is a form of the two-stage model of cancerinduction initially described by Armitage and Doll (1954). In thismodel, cancer is explained as the end result of two mutagenic eventsthat correspond to mutations at a critical gene locus. In humans,these mutations are duplicated within the genetic material of thecell. As discussed by Andersen et al. (1987), the first event depictedin Figure 3.9, Step I, produces an intermediate cell type that mayhave different growth characteristics than the normal cell but isnot aggressively malignant. A second irreversible event (Figure 3.9,Step II) is necessary to complete the cell transformation process,alter the second locus of the critical gene, and produce the cancercell that is aggressively malignant and grows into a tumor by clonalexpansion (Figure 3.9, Step III). This model can be used to explainhow genotoxicants alter mutation rates, how cytotoxicants alter celldeath and birth rates of the normal and intermediate cells, andhow promoters convey growth advantages on the intermediate cellpopulations. Unfortunately, because most of the factors in thesemodels cannot be measured at the present time, it is unlikely thatthe models will be very helpful in the foreseeable future.

3.2.1.3.6 Use of stochastic modeling results. EPA and other regu-latory agencies review each dose-response assessment for substancescausing stochastic effects with respect to the evidence on causativemechanisms and other biological or statistical evidence that indi-cates the suitability of a particular mathematical extrapolation. Inthe absence of adequate information to the contrary, the linearizedmulti-stage model is employed (Crump et al., 1976; EPA, 1996a).This model assumes that at each stage in the development of acancer (see Figure 3.9), the response induced by exposure to an agentis additive to the response induced by all other external stimuli atthat stage. The result is a linear dose-response model at low doses.Where appropriate, the results of using other extrapolation modelsmay be useful for comparison with the linearized multi-stage proce-dure. When longitudinal (time-series) data on tumor developmentare available, time-to-tumor models may be used.

The linearized multi-stage procedure leads to a plausible upperlimit to the dose-response relationship at doses below the rangeof observation that is consistent with some proposed mechanisms


Fig. 3.9. Biologically-based model of the cancer induction process usedto estimate the dose-response relationship of chemicals causing stochasticeffects (Andersen et al., 1987).

of carcinogenesis (Allen et al., 1988). However, this model does notnecessarily give a realistic prediction of responses at low doses. Thetrue relationship is unknown, and the response may be as low aszero. The range of responses defined by the upper and lower confi-dence limits given by the chosen model, which may include zero,should be explicitly stated in a dose-response assessment. A proce-dure for obtaining a ‘‘best estimate’’ of the response at a particular


dose within the range of uncertainty defined by the upper and lowerconfidence limits has not yet been established, although the modelcan provide a value of MLE. Best estimates of true responses willbe most feasible when human data are available and the data encom-pass expected exposures.

When pharmacokinetic or metabolic data are available, or whenother substantial evidence on the mechanistic aspects of the caus-ative process exists, a low-dose extrapolation model other than thelinearized multi-stage procedure might be considered more appro-priate on biological grounds. When a different model is used, thedose-response assessment should discuss the nature and weight ofevidence that led to the choice. Since considerable uncertainty willremain concerning the response at low doses, a UCL of the dose-response relationship obtained using the linearized multi-stage pro-cedure also should be presented in most cases when another modelis used.

Estimates of responses at low doses derived from data on labora-tory animals and extrapolated to humans are complicated by a vari-ety of factors that differ among species and potentially affect theresponse to hazardous chemicals. These factors include differencesbetween humans and experimental test animals with respect to lifespan, body size, genetic variability, population homogeneity, exis-tence of concurrent disease, such pharmacokinetic effects as metabo-lism and excretion patterns, and the dosing regimen. These factorsare discussed further in Section 3.2.1.5.

The usual approach to making interspecies comparisons has beento use standardized scaling factors. Commonly employed standard-ized dosage scales include mg kg�1 body weight per day, ppm (partsper million) in the diet or water, mg m�2 body surface area per day,and mg kg�1 body weight per lifetime (Travis et al., 1990). In theabsence of comparative toxicological, physiological, metabolic, andpharmacokinetic data for a given suspect substance that causes astochastic effect, EPA (1996a) takes the position that an extrapola-tion on the basis of body weight to the three-fourths power is mostappropriate.

3.2.1.4 Characterization of Dose-Response Estimates. Characteriza-tion of the estimated dose-response relationship involves a presentationof (1) the dose-response relationship per se and (2) a frameworkto help judge the significance of the relationship. Dose-responsecharacterization includes an evaluation of exposure mechanismsduring data acquisition, as well as how the dose-response relation-ship was established. If the dose-response relationship is linear at


low doses, it may be presented as a unit-dose estimate (e.g., probabil-ity of a response per unit dose), which can be combined with apredicted exposure to hazardous chemicals for the purpose of assess-ing risk.

Numerical estimates of the dose-response relationship at low dosescan be presented in one or more of the following ways.

● Response to a unit dose. Under an assumption of low-dose linear-ity, the response per unit dose is independent of dose, andthe response to a unit dose is often given. Typical units ofdose include ppm or ppb (parts per billion) in food or water, mg(kg d)�1 by ingestion, or ppm or �g m�3 in air.

● Dose corresponding to a given level of response. Presenting thedose corresponding to a given response can be useful, particu-larly when using nonlinear extrapolation models in which theresponse per unit dose depends on the dose.

● Individual and population responses. The dose-response rela-tionship may be characterized either in terms of the lifetimeprobability that an individual exposed to a given dose willdevelop a cancer as a result of that dose, the excess number ofresponses per year in an exposed population, or both.

Whichever method of presentation is chosen, it is critical that thenumerical estimates not be separated from the various assumptionsupon which they are based and their uncertainties. The dose-response characterization should contain a discussion and interpre-tation of the numerical estimates so that the risk manager gainssignificant insight into the extent to which the quantitative estimatereflects the true magnitude of potential human responses. The riskmanager needs to understand that the true human health risk can-not be known with the degree of accuracy reflected in the numeri-cal estimates.

3.2.1.5 Uncertainties and Deficiencies in Dose-Response Assess-ment. Any approach to determining the dose-response relationshipfor hazardous chemicals involves many attendant uncertainties thatlimit its accuracy. In addition, many dose-response assessments suf-fer from deficiencies in the way they are conducted, which furtherdecreases accuracy. These two aspects of dose-response assessment,which in some ways have led to adoption of such conservativeapproaches as large safety factors and UCLs in applying the resultsto health protection of the public, are discussed in the followingtwo sections.


3.2.1.5.1 Uncertainties in dose-response assessment. Severalaspects of dose-response assessment result in significant uncertain-ties in the accuracy of the resulting relationship.

● Dose-response data based largely on animal studies. It is note-worthy that rodent studies now used to predict the dose-responserelationship in humans were never intended for that purpose(Barr, 1988). These studies were designed for purposes of hazardidentification (see Section 3.1.4.1.2) and were not intendedto be the basis for estimating human responses at low doses(Paustenbach, 1995). For example, there usually are significantdifferences between animals and humans with respect to therate at which chemicals are metabolized, distributed, andexcreted, and these are not taken into account when animaltests are designed. Also, animal tissues will frequently responddifferently to toxicants than human tissue.

● Extrapolation to low doses. The doses used in animal tests areso high that they often produce responses that would not occurat doses to which humans might be exposed. Thus, a model ortheory must be used to estimate responses in humans at dosesthat often are a factor of 100 to 1,000 below the lowest animaldose tested or the doses to which humans have been occupation-ally exposed (Krewski et al., 1989; Munro and Krewski, 1981).Extrapolation to low doses is probably the most uncertain aspectof assessing the dose-response relationship for chemicals, espe-cially substances that produce stochastic effects. The responseof humans exposed to many such substances at low doses maywell be negligible because of the presence of protective biologicalmechanisms (Ames, 1987; Ames and Gold, 1995; Bus and Gibson,1995), although this is often difficult or impossible to demon-strate unequivocally.

Low-dose extrapolation models are the backbone of dose-response assessments. Because they can play such a dominantrole in the regulatory process, it is important to understandsome of their characteristics. As shown in Figure 3.10, differentextrapolation models usually fit the data in the observable doseregion in animal tests about equally well (Krewski et al., 1989),but they often give quite different results in the unobserved low-dose region of interest in assessments of risk to human health.The results obtained by extrapolation of the most commonlyused low-dose models usually vary in a predictable manner,because the models use different mathematical equations todescribe the chemical’s likely behavior in the low-dose region.


Fig. 3.10. Illustration of close correlation between observable dose-response data and results of different statistical models but very differentmodel extrapolations into the unobservable response range (Paustenbach,1995).

The one-hit and linearized multi-stage models usually will pre-dict the highest response rates and the probit model the lowest(Paustenbach, 1989a; 1989b).

● Absence of mechanistic understanding. Substantial understand-ing of the mechanisms by which most chemicals cause responsesis not yet available. To date, virtually none of the models usedto identify safe or acceptable doses of contaminants in air, water,or soil have been based on assessments that attempt to accountfor biological phenomena quantitatively (Andersen et al., 1987;Paustenbach, 1995). As a consequence, conservative approachesare usually employed, and this introduces an unknown uncer-tainty in the estimation of the dose-response relationship.

3.2.1.5.2 Deficiencies in dose-response assessment. In addition tothe sources of uncertainty in dose-response assessment describedabove, there are several important deficiencies in the way that the


dose-response relationship for hazardous chemicals has been evalu-ated in most assessments (Paustenbach, 1989a).

● Presenting only the upper bound of the dose-response relation-ship. Bounding techniques are used in dose-response assess-ments in an attempt to account for the statistical uncertaintyin the results of animal tests. However, the degree of potentialconservatism incorporated in the bounding procedure and thefact that a negligible response can be nearly as likely as theestimated upper bound usually are not presented. As a result,risk managers often are not fully aware of the range of equallyplausible response estimates (Sielken, 1985). For example, thecancer probability associated with exposure to chloroform inchlorinated drinking water has been reported to be as high as1 in 10,000, based on an upper-bound estimate obtained fromthe multi-stage model (Reitz et al., 1988). However, using thesame model, MLE is about 1 in 1,000,000 and the lower-boundestimate is virtually zero (about 1 in 10,000,000). Therefore, therange of plausible responses obtained from the model is between1 in 10,000 and zero. When biological factors are considered,such as the pharmacokinetics and weak genotoxicity of chloro-form, the stochastic response associated with low levels of chloro-form in drinking water is most likely to be quite small ornegligible (Corley et al., 1990). Based on such considerations,the establishment of drinking water standards for chloroformhas been particularly controversial (EPA, 2000a).

● Reliance on results of only one mathematical model. Severaldifferent modeling approaches may need to be considered whenestimating responses at low doses (see Section 3.2.1.3.6). Eachmodel can yield results that are plausible, depending on themechanism of action and pharmacokinetics of the chemical, aswell as the characteristics of the dose-response relationship(Krewski et al., 1984; 1989; Sielken, 1985). As a result ofan improved understanding of the genesis of responses andthe shortcomings of statistical models, regulatory agencieshave recently become more willing to consider models thatcan account for chemical-specific phenomena quantitatively(Andersen et al., 1987; Corley et al., 1990; Paustenbach, 1989b).However, support for flexibility in dose-response assessment hasbeen criticized on the grounds that too little is known aboutresponse initiation to regulate in other than a very conservativemanner (Perera, 1984; Perera and Boffetta, 1988; Silbergeld,1988; 1993).Although some investigators have claimed that models lackinglow-dose linearity are not appropriate for substances that cause


stochastic effects, the scientific support for this assertion is notcompelling, especially for nongenotoxic chemicals (Reitz et al.,1988). Except perhaps for chemicals that are known to beinitiators or mutagens, no single statistical model can beexpected to predict the low-dose response with greater certaintythan another. One possible approach to resolving this problemis to present MLE of the dose-response relationship obtainedfrom the two or three models that are considered most plaus-ible, as well as the upper and lower confidence limits. Basedon this information and considering statistical and biologicalfactors, an appropriate dose-response relationship could beselected based on the ‘‘preponderance of evidence.’’ EPAattempted this approach for dioxin, but it failed to gain broadsupport (Paustenbach et al., 1992).

● Not giving sufficient weight to results of epidemiologic studies.There is a widely held belief that epidemiologic studies arealmost never as statistically robust as animal studies and, there-fore, are not very useful (Silbergeld, 1988). This assertion istoo strong because epidemiologic studies can, at the very least,establish the degree of confidence that should be placed in theresults of low-dose extrapolation models (Layard and Silvers,1989). A difficulty with epidemiologic studies at low doses isthe inability to adequately control for potentially confoundingfactors to the extent necessary to exclude spurious observations,either positive or negative. Epidemiologic studies are not capableof detecting increased responses unless the excess relative riskis on the order of 30 to 40 percent or higher.

At the present time, many regulatory agencies and some scien-tists believe that not enough is known about responses at lowdoses of hazardous substances to consider using epidemiologicdata, which typically give less conservative results than extrapo-lation models (Perera and Boffetta, 1988). For example, ques-tions have been raised about the lack of understanding of thecancer process, the possibility that several mechanisms mightoccur simultaneously, and whether the risk from any incremen-tal exposure to a carcinogen is additive to the much larger riskof cancer from all other causes. For some chemicals, investigatorshave noted that even the multi-stage model could underpredictactual responses at moderate doses (Bailar et al., 1988).Although these are legitimate concerns, the available data(Ikeda, 1988; NAS/NRC, 1988a) seem to indicate that the inher-ent conservatism in methods of dose-response assessment usu-ally yields predictions of responses in the low-dose region that


are greater than those indicated by epidemiologic data. Theprimary reasons for this are thought to be the use of the mostsensitive animal species to estimate responses, the use of asurface-area scaling factor, and the inability of models to accountfor the many protective biological mechanisms that operate atlow doses.

● Failure to quantitatively scale data from rodents in predictingresponses in humans. When evaluating most toxicologicaleffects, statisticians and biologists have generally assumed thatat a given dose of a chemical, the response in humans will benearly identical to the response observed in rodents. Thisassumption is usually accurate for deterministic responses. Forsubstances that cause stochastic effects, however, several factorsneed to be considered when trying to predict how humans willrespond compared with rodents. For example, the biological half-life of the chemical in rodents can be expected to be differentfrom that in humans for virtually all chemicals. Often, for agiven chemical, this difference will vary in a predictable mannerbased simply on the ratio of body weight to surface area and/orlife span (D’Souza and Boxenbaum, 1988). This scaling mayalso be valid for those chemicals causing stochastic effects thatrequire activation. Consequently, for regulatory purposes, sur-face-area corrections have been used in an attempt to adjustfor pharmacokinetic differences between rodents and humans.However, there is ample work suggesting that body weight aloneis probably a more valid scaling factor if no compelling informa-tion to the contrary is available (Allen et al., 1988), and EPAnow uses body weight to the three-fourths power as a defaultscaling factor (EPA, 1996a).

As an alternative to relying on simple scaling factors, PB-PKmodels described in Section 3.2.1.3.4 can be used to moreaccurately predict responses in humans based on rodent data(Corley et al., 1990; Reitz et al., 1988; 1996). The benefits of thisapproach have been so impressive that a special symposium washeld by the National Academy of Sciences to encourage its use(Krewski et al., 1987).

● Failure to adjust dose-response estimates by considering biologi-cal information. In many dose-response assessments, potentiallyimportant biological information is not taken into account inselecting an extrapolation model. Examples of information oftennot included when a model is selected are the types of tumors,time to onset, and whether the chemical is genotoxic. Some


chemicals for which this was a critical issue in low-dose extrapo-lation include ethylene oxide, formaldehyde, dioxin, the goitro-gens, trimethylpentane, and nitrilotriacetic acid.

● Use of models that do not respond to dose-response relationship.As discussed by Sielken (1985), it does not seem appropriate tobase important regulatory decisions on the results of modelsthat are minimally responsive to the very costly informationcollected in standard lifetime rodent studies. Two terms arefrequently used to describe the responsiveness of an extrapola-tion model to the data: ‘‘fragile’’ and ‘‘insensitive.’’ A fragile modelvaries too strongly with the data, whereas an insensitive modelvaries little irrespective of the response in rodents. For example,if the predicted probability of a response at low doses variesdramatically depending on whether a single animal in the studydevelops a tumor, then the model is too responsive. On the otherhand, if the probability of a response does not change muchirrespective of the observed tumor incidence, then the model isnonresponsive or insensitive.

Scientists should not be constrained by the insensitivities of thestatistical bounding methodology or the responsiveness of MLEto the data. Instead, decisions should be influenced by biologicalfactors and scientific judgment. Clearly, toxicologists and riskmanagers need to be aware of the potential for a mathematicalmodel to inadvertently over- or underestimate the significance ofthe data because, at times, such a tendency may have a dramaticeffect on the regulatory decision.

● Excessive regulatory constraints. Estimation of the dose-response relationship is also affected by regulatory regimes.Whenever adherence to strict regulatory guidance is required,the potential exists for the dose-response assessment to be soconstrained that it cannot account for information excluded bythe regulations that would dramatically alter the results. Someof the more commonly encountered problems in dose-responseassessment caused by rigid regulatory policy have been dis-cussed elsewhere (Paustenbach, 1989a; 1995).

3.2.2 Assessment of Responses from Radiation Exposure

Estimation of the probability of a response from exposure to radio-nuclides (or any other source of ionizing radiation) is greatly facili-tated by the knowledge that radiation dose is the common measureof insult to any organ or tissue for any exposure situation (e.g., seeNCRP, 1993a; 1993b). All radiation dose or risk assessments are


based on estimates of absorbed dose, given in grays (Gy), which isa physical quantity defined essentially as the energy imparted tomatter by ionizing radiation divided by the mass irradiated. Forpurposes of radiation protection and assessing risk in general terms,the biologically significant dose is assumed to be the equivalent dose,given in sieverts (Sv), which is defined as the average absorbed dosein an organ or tissue (i.e., the total energy absorbed in an organ ortissue divided by its mass) modified by a radiation weighting factorthat accounts for differences in biological effectiveness of differenttypes of radiation. For example, a given absorbed dose of alphaparticles is assumed to result in a greater biological response thanthe same absorbed dose of photons or electrons. The radiation weight-ing factor depends essentially on the density of ionization in matter,which often is given in terms of the linear energy transfer (LET)(ICRP, 1991).

The utility of absorbed dose and equivalent dose in radiation riskassessments is the following. Once the response per unit absorbeddose of low-LET radiation (e.g., photons) in a particular organ ortissue is known, this relationship can be used to estimate theresponse per unit equivalent dose of any type of radiation. Givenknowledge of the response per unit absorbed dose in the differentorgans or tissues of the body, the response from any exposure thencan be estimated based on estimates of equivalent dose in all irradi-ated organs or tissues. The equivalent dose in different organs ortissues from any exposure can be estimated, as described below,using knowledge of the energies and intensities of the ionizing radia-tions of different types emitted in the decay of any radionuclide.Thus, in contrast to the situation for hazardous chemicals, separatestudies to determine the response from exposure to each radionuclideof concern are not needed.

Radiation dose may be delivered to organs or tissues when radionu-clides located outside the body emit penetrating radiations, such asphotons or higher-energy electrons, or when radionuclides are takeninto the body by inhalation, ingestion, or (rarely) absorption throughthe skin. These two means of irradiation often are referred to asexternal and internal exposure. Doses to specific organs or tissuesfrom external exposure to radionuclides in the environment areestimated using complex models of radiation transport (e.g., seeEckerman and Ryman, 1993). Doses from internal exposure are esti-mated using two types of models: (1) models for absorption, deposi-tion, and retention of radionuclides taken into the body, whichdepend on the chemical and physical form of the radionuclide; and(2) dosimetry models involving complex calculations of radiationtransport that give estimates of the dose delivered to each target


organ or tissue per disintegration of a radionuclide in each of itssites of deposition in the body (e.g., lung, bone surfaces, thyroid) orduring transit (e.g., in the gastrointestinal tract or in blood) [seeICRP (1996) and references therein]. The biokinetic models currentlyused to describe absorption, deposition, and retention of any radionu-clide in the body are similar to the compartmental or PB-PK modelsdeveloped for some hazardous chemicals (see Section 3.2.1.3.4), andthey are based on studies in humans and animals.

Given the models for estimating external or internal radiationdoses in specific organs or tissues, the following sections considerthe responses resulting from a given dose by any route of exposure.As is the case with hazardous chemicals, both stochastic and deter-ministic radiation effects can occur.

3.2.2.1 Deterministic Responses from Radiation Exposure. Basedmainly on data in humans, a threshold dose-response relationshipgenerally is assumed for radiation-induced deterministic responses.For purposes of radiation protection, deterministic responses gener-ally are assumed not to occur if the annual equivalent dose is lessthan 150 mSv to the lens of the eye or 500 mSv to any other organsor tissues, including the skin and extremities (ICRP, 1977; 1991;NCRP, 1987a; 1993a). Dose limits for the public intended to ensureprevention of deterministic responses are set at one-tenth of theassumed thresholds, in order to provide an adequate margin of safetyfor nearly all individuals. However, deterministic responses are notexpected to be of concern in routine exposures of the public, becausethe limit on annual effective dose of 1 mSv from exposure to allcontrolled sources combined, which is intended to limit the increasein stochastic responses, should ensure that deterministic responseswould not occur, even when the dose from natural background radia-tion is included (ICRP, 1991; NCRP, 1993a).

3.2.2.2 Databases and Methods of Dose-Response Assessment forStochastic Effects. At radiation doses below levels where cell killingoccurs, a linear-quadratic relationship between dose and stochasticresponses, including cancers and severe hereditary effects, generallyis assumed, based on analyses of data in humans (NAS/NRC, 1988b;1990). The importance of the quadratic term differs for low-LETradiations (photons and electrons) and high-LET radiations (e.g.,alpha particles and neutrons). For purposes of radiation protectionand assessing risk in general terms, a linear dose-response relation-ship, without threshold, generally is assumed in estimating stochas-tic responses at low doses. This Section discusses databases andmethods used in estimating the relationship of radiation-induced


stochastic responses to dose. More detailed discussions are givenelsewhere (NCRP, 1993b; 2001).

A substantial body of data on the frequency of radiation-inducedcancers in humans has been obtained from studies in human popula-tions that received doses considerably above levels of natural back-ground; for recent reviews of these data see IARC (2000; 2001) andUNSCEAR (2000). These populations, and the associated types ofcancers for which information on the dose-response relationshipshas been obtained, include the Japanese atomic-bomb survivors(cancers at many sites), uranium and other underground miners(lung cancer), medical patients irradiated with x rays (leukemia andbreast, thyroid, and bone cancer) or injected with radium or thorium(leukemia, bone and liver cancer), and the radium dial painters (bonecancer). For low-LET radiations (e.g., photons) and for most organsand tissues, the Japanese atomic-bomb survivors are the most impor-tant source of data used to obtain estimates of cancer frequencyper unit dose. These data were obtained under conditions of acuteexternal exposure at high doses and dose rates. The observed dose-response relationships in this population are assumed to apply tointernal exposure, taking into account the different radiation weight-ing factors for high-LET radiations (e.g., alpha particles) as appro-priate, and to exposures at lower doses and dose rates, taking intoaccount information on the dependence of the frequency of responseson dose and dose rate.

The Japanese atomic-bomb survivors also are a potentially impor-tant source of data on the dose-response relationship for severehereditary responses. However, no evidence for inherited geneticeffects has been observed in spite of nearly 50 y of study. In theabsence of data in humans, estimates of the frequency of radiation-induced hereditary responses have been based primarily on datafrom studies in mice.

In all studies of the relationship of radiation-induced stochasticresponses to dose, the derivation of MLEs (mean values) of the dose-response relationships has been emphasized (NAS/NRC, 1988b;1990). Furthermore, for purposes of radiation protection, MLEs ofthe dose-response relationships, rather than UCLs, have beenemphasized in extrapolating the observed dose-response data tolower doses beyond the range of observation (NCRP, 1975; 1999b).

The use in radiation protection of MLEs of the relationships ofstochastic responses to dose, rather than UCLs, is justified on thegrounds that the probability of a response in most individuals is notlikely to be significantly underestimated. Even if the probabilitywere underestimated, the current framework for radiation protection


of the public generally ensures that responses from exposure to man-made sources would be well below any responses from unavoidableexposures to natural background radiation. Features providing suchassurance include a limit on annual effective dose of 1 mSv fromexposure to all controlled sources combined and a requirement tomaintain doses below the limit ‘‘as low as reasonably achievable’’(ALARA). The average annual effective dose from natural back-ground is about 3 mSv (NCRP, 1987b), and the geographical variabil-ity in the background dose is a substantial fraction of the average.In addition, epidemiologic studies have not consistently found evi-dence for increased cancer incidence below a dose on the order of100 to 200 mSv, which is about the same as the average lifetimedose from natural background.

Relationships between dose and stochastic responses obtainedfrom data on human populations are subject to uncertainty resulting,e.g., from (1) uncertainties in doses received, (2) uncertainties inextrapolating data obtained at high doses and dose rates to thelow doses and dose rates of concern in routine exposure situations,(3) incomplete expression of responses in study populations, particu-larly the Japanese atomic-bomb survivors who were young in 1945,resulting in uncertainty in projecting future responses in those popu-lations, (4) differences in the dose-response relationships for differ-ent radiation types and different organs or tissues, and (5) effectsof competing causes of the same response (e.g., smoking) and age atexposure. However, for external exposure of the whole body to low-LET radiations, the uncertainty in the dose-response relationshipfor induction of all cancers in humans at low doses appears to beless than an order of magnitude (EPA, 1994a; 1999a; 1999b; NAS/NRC, 1990; NCRP, 1997). For external exposure at low doses anddose rates, NCRP has estimated that the 90 percent confidence inter-val of the probability coefficient for fatal cancers for lifetime exposureof the United States population is 0.012 to 0.088 Sv�1 (NCRP, 1997).The mean of the uncertainty distribution is 0.040 Sv�1 and themedian (50th percentile) is 0.034 Sv�1.

The conclusion about the uncertainty in the dose-response rela-tionship for radiation stated above takes into account the uncertaintyin extrapolating the data at high doses and dose rates in the Japaneseatomic-bomb survivors to the lower doses and dose rates of concernin routine exposures of the public. The issue of extrapolation to lowdoses and dose rates is a matter of considerable controversy and isan important source of uncertainty (NCRP, 1997; 2001). For purposesof radiation protection, the frequency of responses at low doses anddose rates generally has been assumed to be a factor of two less thanMLE of the frequency of responses in the Japanese atomic-bomb


survivors (ICRP, 1991; NCRP, 1993a). This correction is called thedose and dose-rate effectiveness factor (DDREF). An evaluation ofthe uncertainty in DDREF is presented in another NCRP report(NCRP, 1997).

In spite of uncertainties in the dose-response relationship for radi-ation discussed above, it is generally believed that radiation risksin humans can be assessed with considerably greater confidencethan risks from exposure to most hazardous chemicals that causestochastic effects. The state of knowledge of radiation risks inhumans compared with risks from exposure to chemicals that causestochastic effects is discussed further in Section 4.4.2.

3.2.2.3 Measures of Radiation-Induced Responses. This Sectiondiscusses the measures of response from radiation exposure gener-ally used in radiation protection and assessments of radiation riskin general terms.

3.2.2.3.1 Measures of deterministic responses. Deterministicresponses from radiation exposure are expected to occur only at dosesmuch higher than doses that the general public might experience inroutine exposure situations. The measure of deterministic responsesused in radiation protection generally has been incidence of anadverse effect, although prompt fatalities also are of concern at veryhigh doses (i.e., at doses well above thresholds for nonfatal determin-istic responses). Prompt fatalities are considered, for example, inevaluating the potential consequences of severe radiation accidents.

In radiation protection, incidence is the appropriate measure ofdeterministic response because the goal is to prevent such responsesin any organ or tissue in almost all individuals. No attempts havebeen made to assign different weights to different deterministicresponses, depending on their severity. Rather, all responses consid-ered to be significant to human health are given equal weight inestablishing deterministic dose limits in specific organs or tissues.As mentioned previously, deterministic responses from radiationexposure generally are not of concern in routine exposures of thepublic, because they should be precluded by the dose limit that isintended to ensure an acceptable increase in stochastic responses.

3.2.2.3.2 Measures of stochastic responses. The primary measureof stochastic responses used in radiation protection and radiationrisk assessment by ICRP and NCRP has been fatalities (i.e., fatalcancers and severe hereditary effects). Fatalities have been empha-sized essentially because this was the only health-effect endpointfor which data generally were available, both for study populations


receiving high doses and for the background frequency of cancersand severe hereditary effects from all causes.

Until recently, fatalities (especially latent cancer fatalities) wasthe only measure of stochastic response used in radiation protectionand assessments of radiation risks in general terms (ICRP, 1977;NCRP, 1987a). No consideration was given to radiation-induced non-fatal stochastic responses or to the relative severity of different typesof fatal responses (e.g., the expected length of life lost per fatality).

In its current recommendations on radiation protection, ICRP(1991) developed a quantity called total detriment to describe sto-chastic responses (see also NCRP, 1993a). As summarized below,total detriment includes not only the probability of a fatal cancer orsevere hereditary effect but also a contribution from nonfatal cancersand an additional adjustment that accounts for differences inexpected length of life lost per fatal response in different organs ortissues. The term ‘‘detriment,’’ rather than ‘‘response’’ or ‘‘risk,’’ isused to describe this quantity because (1) the contribution from non-fatal cancers is not simply the probability of a nonfatal cancer,(2) severe hereditary responses are not experienced by exposed indi-viduals but by their progeny, and (3) the adjustment for expectedlength of life lost per fatal response does not represent a probabilityof a fatality or incidence.

ICRP (1991) has acknowledged that the modifications of the proba-bility of a fatal response are necessarily judgmental and somewhatarbitrary, particularly the weight to be given to nonfatal cancersrelative to fatal responses in assessing total detriment. Nonetheless,the following approach to assessing total detriment from radiationexposure for purposes of radiation protection was developed.

ICRP (1991) has recommended that the detriment due to radia-tion-induced stochastic responses in any organ should include, inaddition to the probability of a fatal response, the probability of anonfatal response weighted by the lethality fraction (k). This adjust-ment is used only for cancers because all severe hereditary effectsare assumed to be fatal. If the probability coefficient for fatal cancer(i.e., probability of a fatal cancer per unit equivalent dose) in aparticular organ is denoted by F, then the probability coefficient forcancer incidence in that organ is F/k and the probability coefficientfor nonfatal cancers is F/k � F � (1 � k)F/k. Then, using thelethality fraction (k) to weight the probability coefficient for nonfatalcancers, the contribution to the total detriment from nonfatal cancersis k(1 � k)F/k � (1 � k)F, and the total weighted detriment fromfatal and nonfatal cancers is F � (1 � k)F � F(2 � k). Finally, thetotal detriment in any organ, including the gonads, is obtained bymultiplying the total weighted detriment for fatal and nonfatal


responses by the factor �/l, where � is the expected years of life lostper fatal cancer in that organ or per fatal hereditary effect and lis the average expected years of life lost from all fatal responses.Therefore, in any organ, the total detriment due to stochasticresponses recommended by ICRP (1991) is given by F(�/l)(2 � k).

In estimating the total detriment due to stochastic responses inany organ as described above, the probability coefficient for fatalcancers (F) or severe hereditary responses is based on data inhumans and animals described in Section 3.2.2.2, and the lethalityfraction (k) and relative length of life lost per fatal response (�/l)are based on data on responses from all causes in various nationalpopulations. The values of F, k, and �/l for different organs, aswell as the probability coefficient for severe hereditary responses,assumed by ICRP (1991) and the resulting estimates of total detri-ment, F(�/l)(2 � k), are summarized in Table 3.2. The two entriesfor ‘‘Total’’ in the last row represent the probability coefficient for

TABLE 3.2—Contributions from different organs to total detrimentfrom radiation exposure of a general population.a

RelativeFatal Severe Lethality Length of Total

Cancers Hereditary Fraction Life Lost DetrimentOrgan (F)b Effectsb (k)c (�/l) F(�/l)(2 � k)b

Bladder 30 0.50 0.65 29Bone marrow 50 0.99 2.06 104Bone surface 5 0.70 1.00 6.5Breast 20 0.50 1.21 36Colon 85 0.55 0.83 102Liver 15 0.95 1.00 16Lung 85 0.95 0.90 80Esophagus 30 0.95 0.77 24Ovary 10 0.70 1.12 15Skin 2 0.002 1.00 4Stomach 110 0.90 0.83 100Thyroid 8 0.10 1.00 15Remainder 50 0.71 0.91 59Gonads — 100 — 1.33 133Total 500 — — — 730d

a Adapted from Tables B-19 and B-20 of ICRP (1991).b Values per 10,000 person-Sv.c Assumed fraction of all cancers in adults that are fatal.d The sum has been rounded.


fatal cancers and the total detriment resulting from uniform irradia-tion of the whole body, i.e., when all organs and tissues receive thesame dose.

From the expression for the total detriment given above and thedata in Table 3.2, the following observations can be made. For can-cers that are nearly always fatal (e.g., leukemia from irradiation ofbone marrow), the total detriment is determined essentially by theprobability of a fatal cancer, absent consideration of the relativelength of life lost (�/l), and the contribution from weighted nonfatalcancers is insignificant. For cancers that are rarely fatal (e.g., skinor thyroid cancers), the total detriment exceeds the probability of afatal cancer by no more than a factor of two, based on the assumptionthat nonfatal cancers should be weighted by the lethality fraction(k). In general, if the lethality fraction is less than about 0.1, thetotal detriment essentially is twice the probability of a fatal cancer,independent of the lethality fraction.

For routine exposures of the public, ICRP recommends a totaldetriment per unit equivalent dose from uniform whole-body irradia-tion of 7.3 � 10�2 Sv�1, as shown in Table 3.2. Of this, the recom-mended probability coefficient for fatal cancers is 5.0 � 10�2 Sv�1,or about two-thirds of the total detriment, and the contributionsfrom severe hereditary responses and weighted nonfatal cancers are1.3 � 10�2 Sv�1 and 1.0 � 10�2 Sv�1, respectively. These probabilitycoefficients are summarized in Table 3.3, and their use in radiationprotection is discussed in the following section. As noted previously,the probability coefficient for weighted nonfatal cancers is not thesame as the probability coefficient for incidence of nonfatal cancers.The probability coefficient for fatal cancers also gives the probabilityof a fatal cancer per unit effective dose. The effective dose was devel-oped to describe nonuniform irradiations of the body and is dis-cussed below.

TABLE 3.3—Nominal probability coefficients for stochasticresponses due to radiation exposure of the general public.a

Weighted Nonfatal Severe HereditaryFatal Cancer Cancerb Effects Total Detriment(10�2 Sv�1) (10�2 Sv�1) (10�2 Sv�1) (10�2 Sv�1)

5.0 1.0 1.3 7.3

a Adapted from Table 3 of ICRP (1991).b Probability coefficient does not represent probability of a nonfatal cancer

(see Section 3.2.2.3.2).


3.2.2.3.3 Effective dose. In radiation protection, all organs andtissues at risk from a given exposure are taken into account. Thisis accomplished by calculating a quantity called the effective dose(E), which is defined as a weighted sum (average) of equivalent dosesin all organs and tissues (ICRP, 1991; NCRP, 1993a):

E � �TwTHT, �TwT � 1, (3.2)

where HT is the equivalent dose in organ or tissue (T) and wT is theweighting factor for tissue (T) described below. The equivalent dosein organ or tissue (T) is calculated as:

HT � �RwRDT,R, (3.3)

where wR is the radiation weighting factor for radiation of type (R)described below and DT,R is the average absorbed dose in organ ortissue (T) (i.e., the total energy absorbed divided by the total mass)from radiation (R).

For external exposure, the effective dose represents the dosesreceived in the different organs or tissues during the time of anexposure. However, since intakes of radionuclides continue to delivera dose to target organs or tissues until the radionuclides are removedfrom the body by radioactive decay or biological elimination, evenwith no further intakes, the effective dose for internal exposurerepresents committed doses, i.e., the time-integral of the dose ratesfollowing an acute intake, in the different organs or tissues. Forintakes by an adult, the effective dose used in radiation protectionnormally is the integrated dose over 50 y (i.e., the dose received toage 70 following an acute intake at age 20); for intakes by youngerage groups, the effective dose normally is the integrated dose fromthe age at intake to age 70 (ICRP, 1996).

The wR for a specified type and energy of radiation is chosen torepresent the relative biological effectiveness of that radiation ininducing stochastic responses at low doses. For example, the radia-tion weighting factor is one for photons and electrons of any energyand 20 for alpha particles of any energy (ICRP, 1991; NCRP, 1993a).A complete listing of recommended radiation weighting factors isgiven in Table 1 of ICRP (1991) and Table 4.3 of NCRP (1993a).

The wT used in calculating the effective dose is proportional to thetotal detriment given in Table 3.2 and, thus, takes into account fatalcancers and severe hereditary effects, weighted nonfatal cancers,and the relative severity of all fatal responses. When the whole bodyis irradiated uniformly, the value of wT for a particular organ is thefraction of the total detriment resulting from irradiation of thatorgan. Thus, the effective dose is intended to be proportional to total


detriment for nonuniform irradiations of the whole body, such asoften result from inhalation or ingestion of radionuclides, as wellas for uniform whole-body irradiations; i.e., exposures with equaleffective doses are assumed to correspond to equal total detrimentsregardless of the distribution of doses among different organs andtissues. The tissue weighting factors used in calculating the effectivedose are given in Table 3.4.

Both the effective dose and equivalent doses in specific organs andtissues, from which effective dose is calculated, are intended for useonly in radiation protection (i.e., in setting limits on radiation doseand evaluating compliance with dose limits) or in assessments of riskin general terms, such as for prospective or hypothetical exposuresituations. This caution is warranted because of the approximatenature of wTs used in estimating the effective dose (they are obtainedas rounded values of the total detriment coefficients for specificorgans or tissues given in Table 3.2), as well as the approximatenature of wRs used in estimating equivalent doses in each organ or

TABLE 3.4—Tissue weighting factors used in calculating effective dose.a

Organ wT

Gonads 0.20Red bone marrow 0.12Colon 0.12Lung 0.12Stomach 0.12Bladder 0.05Breast 0.05Liver 0.05Esophagus 0.05Thyroid 0.05Skin 0.01Bone surface 0.01Remainderb 0.05

a Adapted from Table 2 of ICRP (1991).b The ‘‘Remainder’’ category includes the adrenals, brain, upper large intes-

tine, small intestine, kidney, muscle, pancreas, spleen, thymus, and uterus.The wT for remainder normally is applied to the average of the equivalentdoses to these organs and tissues. In those exceptional cases in which asingle one of the remainder organs or tissues receives an equivalent dosein excess of the highest dose in any of the 12 organs for which a weightingfactor is specified, a weighting factor of 0.025 is applied to that organ ortissue and a weighting factor of 0.025 is applied to the average dose in therest of the remainder.


tissue from calculated absorbed doses. Equivalent doses in specificorgans or tissues and the effective dose generally are not intendedfor use in estimating probabilities of stochastic responses from actualexposures. In such cases, it is preferable to use estimates of absorbeddose, data on the relative biological effectiveness in each organ irradi-ated and for each radiation type of concern to the particular exposuresituation, the sex of the exposed individuals, and age at exposure.Assessments of risk from waste disposal for the purpose of developinga generally applicable, risk-based waste classification system providean example of an appropriate use of equivalent doses and the effec-tive dose.

It also is important to note that the total detriment developed byICRP (1991) is intended to be used mainly in obtaining wTs in theeffective dose (the values of wT in Table 3.4 are roughly proportionalto the corresponding total detriments in Table 3.2). However, totaldetriment is not normally used in estimating responses from a giveneffective dose. ICRP and NCRP have continued to emphasize fatalcancers as the health effect of primary concern and have used theprobability coefficient for fatal cancers of 5 � 10�2 Sv�1 given inTable 3.3 for this purpose. Total detriment is not used in estimatingresponses because, as noted previously, the detriment due to nonfatalcancers in Table 3.3 is not the probability of a nonfatal cancer andthe detriment due to severe hereditary effects is not experienced byexposed individuals.

3.2.3 Comparison of Dose-Response Assessments forRadionuclides and Chemicals

The discussions in Sections 3.2.1 and 3.2.2 have indicated thatthere are important differences in the approaches to dose-responseassessment for radionuclides and hazardous chemicals. An under-standing of these differences is important in developing a risk-basedwaste classification system that applies to both types of substances.

A fundamental difference between radionuclides and hazardouschemicals in regard to dose-response assessment is the following.Estimates of responses from exposure to radionuclides can be basedon estimates of absorbed dose and equivalent dose in all organs andtissues, and the dose-response relationships for different organs ortissues obtained from human or animal studies can be applied toany radionuclide and any exposure situation. Separate studies ofresponses from exposure to each radionuclide of concern are notneeded. For hazardous chemicals, however, quantities analogous toabsorbed dose and equivalent dose have not been developed; i.e.,


the physical and biological quantities that relate response to dosedelivered to target tissues have not yet been elucidated. Therefore,the dose-response relationship for most hazardous chemicals canonly be determined by direct observation but cannot readily beinferred from the dose-response relationship for any other substance.Structure-activity relationships and pharmacokinetic models maybe useful in inferring dose-response relationships for some hazardouschemicals based on data for another substance, but these methodshave not been applied widely.

The following sections present a comparison of approaches to esti-mating deterministic and stochastic dose-response relationships forradionuclides and hazardous chemicals.

3.2.3.1 Deterministic Responses. Prevention of deterministicresponses is a basic principle of health protection for both radionu-clides and hazardous chemicals; the goal is to achieve zero probabilityof such responses. Incidence is the primary measure of deterministicresponse for any hazardous substance, although prompt fatalitiesalso are of concern at sufficiently high doses. In risk assessmentsand in establishing deterministic dose limits, no adjustments aremade to take into account, for example, the relative severity of differ-ent responses with regard to consequent reductions in the qualityof life.

For purposes of health protection, the dose-response relationshipsfor deterministic effects from exposure to radionuclides and hazard-ous chemicals are assumed to have a threshold. For either type ofsubstance, the assumed thresholds are based on data for the mostsensitive organ or tissue. However, there are potentially importantdifferences in the way these thresholds are estimated and thenapplied in health protection of the public.

First, the threshold for hazardous chemicals that cause determin-istic effects is assumed for purposes of health protection to representa lower confidence limit, taking into account uncertainties in thedose-response relationship (see Section 3.2.1.2.7). Depending, forexample, on the slope of the dose-response relationship near thethreshold, the chosen steps in the dosing regimen, and the magnitudeof uncertainties in the data, the lower confidence limit of the assumedthreshold can be substantially below MLE. In radiation protection,the estimated thresholds for deterministic effects are based on MLEsof dose-response relationships (ICRP, 1991).

Second, in radiation protection of the public, deterministic doselimits are based mainly on data in humans and normally are set ata factor of 10 below the assumed thresholds. This safety factor isintended to ensure that deterministic responses would be precluded


in almost all individuals, including those that might be unusuallysensitive to radiation. A more conservative approach to health protec-tion of the public normally is taken for hazardous chemicals thatcause deterministic effects, in part because most such substanceshave been studied only in animals. Limits on acceptable dose oftenare defined by RfDs that usually are derived from lower confidencelimits of the assumed thresholds, as represented by NOAELs orlower confidence limits of benchmark doses, by applying severalsafety and uncertainty factors (see Sections 3.2.1.2.5 and 3.2.1.2.7).A safety and uncertainty factor of at least 100 normally is appliedin obtaining an RfD, and this factor may be as much as 5,000 forsome substances.

Based on these differences, the use of RfDs for hazardous chemicalsthat induce deterministic effects to define acceptable exposures ofthe public often may be considerably more conservative (provide asubstantially larger margin of safety) than the dose limits for radia-tion induced deterministic effects. The likely degree of conservatismembodied in RfDs has important implications for establishing limitson allowable exposures to substances causing deterministic effectsfor the purpose of developing a risk-based waste classification sys-tem. Dose limits for deterministic effects for radiation should not beimportant in classifying waste (see Section 3.2.2.1).

3.2.3.2 Stochastic Responses. A basic principle of health protec-tion for both radionuclides and hazardous chemicals is that the prob-ability of a stochastic response, primarily cancers, should be limitedto acceptable levels. For any substance that causes stochasticresponses, a linear dose-response relationship, without threshold,generally is assumed for purposes of health protection. However, theprobability coefficients for radionuclides and chemicals that inducestochastic responses that are generally assumed for purposes ofhealth protection differ in two potentially important ways.

First, the dose-response relationships for radiation used for pur-poses of health protection and the probability coefficients derivedfrom those relationships are intended to be MLEs. In contrast, thedose-response relationships and probability coefficients for chemi-cals that induce stochastic responses are intended to be upper-boundestimates (UCLs), although MLEs also are available. In animal datafrom which the probability coefficients for most chemicals that causestochastic responses are obtained, UCL can be greater than MLEby a factor that ranges from 5 to 100 or more.

Second, the primary measure of stochastic response used in radia-tion protection and in most radiation risk assessments has beenfatalities. In contrast, the measure of response for chemicals causing


stochastic responses generally has been incidence. This differenceresults from the fact that dose-response relationships for radiationwere based on data on radiation-induced fatalities in human popula-tions, whereas the dose-response relationships for most hazardouschemicals that cause stochastic responses have been based on dataon tumor incidence in animal studies. The total detriment, whichtakes into account nonfatal cancers as well as fatalities (ICRP, 1991),also is used in radiation protection. However, total detriment is usedmainly to obtain the tissue weighting factors in the effective doseand is not normally used in radiation risk assessments (see Sections3.2.2.3.2 and 3.2.2.3.3).

Data on radiation-induced cancer incidence in the Japaneseatomic-bomb survivors and comparisons of cancer incidence andmortality in this population are becoming available (Mabuchi et al.,1994; Preston et al., 1994; Ron et al., 1994; Thompson et al., 1994;UNSCEAR, 2000). These data could be used to derive the relation-ship between dose and cancer incidence at low doses, in which casethe same measure of response could be used for radionuclides andstochastic chemicals. However, there are difficulties with using thedata on cancer incidence for radiation, including uncertainties in(1) determining background rates of cancer incidence from all causesin various organs or tissues and (2) applying the results from theJapanese study population to other national populations in whichthe background rates of cancer incidence in some organs may besignificantly different (e.g., the gastrointestinal tract). These con-cerns also apply, of course, to the data on cancer mortality in theatomic-bomb survivors.

Radiation-induced cancer incidence also could be estimated usingcalculations of the probability of cancer incidence per unit activityintake of specific radionuclides by particular ingestion and inhala-tion pathways or the probability per unit activity concentration ofspecific radionuclides in the environment by particular pathwaysof external exposure (Eckerman et al., 1999); probabilities of fatalcancers for the different exposure pathways also have been calcu-lated. These probability coefficients differ from those developed byICRP (see Section 3.2.2.3.2) in that they are calculated with respectto activity of specific radionuclides rather than dose, and they thusbypass the need to estimate the effective dose. For external exposure,the methods used by Eckerman et al. (1999) and ICRP (1991) toestimate responses essentially are equivalent. However, there aresignificant differences in the methods used to estimate responsesfrom intakes of radionuclides, and the results obtained by Eckermanet al. (1999) differ substantially in a few cases (e.g., intakes of 232Th)


from those based on effective doses and probability coefficients devel-oped by ICRP. The method used by Eckerman et al. is methodologi-cally more rigorous (NAS/NRC, 1999a), mainly because the responseestimates are based on calculated dose rates as a function of timeafter intake and age at intake. The approach of estimating responsesfrom intakes of radionuclides based on committed effective doses fordifferent ages at intake (ICRP, 1991) does not properly account forthe distribution of doses over time after intake in cases where along-lived radionuclide decays to radiologically significant progenywhose activity increases over a period of a few years or more.

An additional difference in current approaches to dose-responseassessment for radionuclides and stochastic chemicals is the follow-ing. Radiation is a much more general carcinogen, affecting cancerin many different organs and tissues, than most chemicals. Use ofthe effective dose in radiation protection takes into account all organsand tissues at risk for any exposure situation, regardless of whetherthe whole body is irradiated uniformly or nonuniformly. For mosthazardous chemicals, however, only one organ or tissue at risk istaken into account, and responses in other organs or tissues areignored. In only a few cases are risk estimates for hazardous chemi-cals based on observed responses in multiple organs. The develop-ment of PB-PK models for hazardous chemicals offers the possibilityof estimating doses (concentrations) of chemicals in different organsor tissues, but such models have not yet been widely accepted byregulators. This difference in approaches to dose-response assess-ment for radionuclides and stochastic chemicals cannot be elimi-nated at the present time, in part because the probability coefficientsfor most chemicals are based on studies in animals and the organsin which cancers are seen in study animals often do not correspondto the organs at greatest risk in humans. However, this difference isunlikely to be important when chemicals presumably induce cancersonly at sites of deposition in the body and most hazardous chemicalsare not distributed widely in the body following an intake.

Given the different approaches to dose-response assessment andthe different measures of response normally used for radionuclidesand chemicals that cause stochastic effects, estimates of responsesfrom exposure to the two types of substances clearly are not equiva-lent, and the correspondence of the estimated frequency of responsesto the frequency that might actually be experienced differs substan-tially. Specifically, if the results of experiments indicating chemical-induced stochastic responses in animals are assumed to be indicativeof stochastic responses in humans, estimates of responses for chemi-cals could be considerably more conservative (pessimistic) than esti-mates for radionuclides. This difference is primarily the result of

3.3 APPROACHES TO RISK MANAGEMENT / 145

using best estimates (MLEs) of response probabilities for radionu-clides but upper bounds (UCLs) for chemicals that cause stochasticresponses, because cancer fatalities and cancer incidence do not dif-fer substantially in most organs and tissues (see Table 3.2). Thedifference between MLEs and UCLs of dose-response relationshipsis an important concern in developing a comprehensive and risk-based hazardous waste classification system.

3.3 Approaches to Risk Management forRadionuclides and Hazardous Chemicals That Cause

Stochastic Effects

Risk management is the process by which values of acceptablerisk are established and the results of risk assessments are comparedwith these values, resulting in decisions concerning the acceptabilityof particular practices involving hazardous substances, includingwaste disposal. Risk management often involves consideration ofeconomic, legal, and socio-political factors, and is typically performedby regulatory authorities. Consideration of suitable approaches torisk management clearly is important in establishing a risk-basedwaste classification system.

The acceptable risks for substances that induce stochasticresponses discussed in this Section are values in excess of unavoid-able risks from exposure to the undisturbed background of naturallyoccurring agents that cause stochastic responses, such as manysources of natural background radiation and carcinogenic com-pounds produced by plants that are consumed by humans. Thisdistinction is based on the assumption of a linear, nonthreshold dose-response relationship for substances that cause stochastic responsesand the inability to control many sources of exposure. Risk manage-ment can address exposures to naturally occurring substances thatinduce stochastic responses, but only when exposures are enhancedby human activities or can be reduced by reasonable means.

In contrast, risk management for substances that cause determin-istic effects must consider unavoidable exposures to the backgroundof naturally occurring substances that cause such effects. Based onthe assumption of a threshold dose-response relationship, the riskfrom man-made sources is not independent of the risk from undis-turbed natural sources, and the total dose from all sources must beconsidered in evaluating deterministic risks. In the case of ionizingradiation, thresholds for deterministic responses are well above aver-age doses from natural background radiation (see Section 3.2.2.1)


and background can be neglected. This may not be the case, however,for some naturally occurring chemicals that induce deterministiceffects (e.g., lead). In such cases, exposures to man-made sourcescomparable to background exposures could result in a significantincrease in deterministic risks.

As noted in Section 3.2.3.1, the approaches to management ofdeterministic risks are essentially the same for radionuclides andhazardous chemicals, although the degree of conservatism may differfor the two types of substances. Management of deterministic risksis not discussed further in this Section.

This Section discusses approaches to risk management that areused in protecting the public from exposure to radionuclides andchemicals that cause stochastic responses in the environment. Differ-ent approaches to management of stochastic risks are used for radio-nuclides and chemicals. An understanding of the two approaches,including their differences and ways in which these differences canbe reconciled, is important in developing a comprehensive and risk-based hazardous waste classification system.

The different approaches to management of stochastic risks forradionuclides and hazardous chemicals are referred to in this Reportas the radiation and chemical paradigms (EPA, 1992a). The followingdiscussion of the two paradigms for management of stochasticrisks is adapted from previous papers (Kocher, 1999; Kocher andHoffman, 1991).

3.3.1 Radiation Paradigm for Risk Management of StochasticResponses

The radiation paradigm for management of stochastic risks isapplied to control of radiation exposures under authority of AEA(1954). Thus, this approach to risk management applies only toregulation of radionuclides that arise from operations of the nuclearfuel cycle, but it does not apply to control of radiation exposuresunder authority of any other laws. For example, the radiation para-digm does not apply to regulation of radionuclides in public drinkingwater supplies under authority of the Safe Drinking Water Act (EPA,1975; SDWA, 1974). Radionuclides in drinking water are regulatedin accordance with the chemical paradigm discussed in the follow-ing section.

The radiation paradigm for management of stochastic risks isbased on the fundamental principles of radiation protection devel-oped over many decades by ICRP and NCRP. As stated by NCRP(1993a), these principles include the following:


● the need to justify any activity involving radiation exposure onthe basis that the expected benefits to society exceed the overallsocietal cost (principle of justification)

● the need to ensure that the total societal detriment from suchjustifiable activities or practices is maintained as low as reason-ably achievable (ALARA), economic and social factors beingtaken into account (principle of optimization)

● the need to apply dose limits to individuals to ensure that theprocedures of justification and ALARA do not result in exposuresof individuals or groups of individuals that exceed levels ofacceptable risk (principle of dose limitation)

As depicted in Figure 3.11, the principles of optimization (ALARA)and dose limitation embodied in the radiation paradigm may bethought of as defining a ‘‘top-down’’ approach to management ofstochastic risks. Given that radiation exposures have been justified,the radiation paradigm has two basic elements:

Fig. 3.11. The radiation paradigm for management of stochastic risks(Kocher, 1999).


1. a limit on radiation dose to individuals from exposure to allcontrolled sources combined, corresponding to a maximumallowable risk for any routine exposure situation; and

2. a requirement to reduce exposures to controlled sources as farbelow the limit as reasonably achievable (ALARA).

The proper interpretation of the dose limit is that higher doses (andtheir associated risks) from controlled sources are regarded as unac-ceptable, meaning intolerable. Thus, the dose limit normally mustbe met in routine exposures to controlled sources regardless of cost orother circumstances. Application of the ALARA principle to furthercontrol of exposures takes into account, for example, the cost ofreducing radiation doses in relation to the benefits in health risksaverted and other societal concerns.

Thus, in the radiation paradigm, doses are acceptable if they corre-spond to risks less than the maximum allowable risk and they areALARA; compliance with the dose limit does not, by itself, determineacceptable risks. It is important to emphasize that doses that areALARA may vary from one exposure situation to another; i.e., whatis ALARA is not a pre-determined result that applies to all exposuresituations. As indicated in Figure 3.11, doses that are less than thedose limit but are not ALARA are regarded as barely tolerable andare not considered to be acceptable in most cases.

The radiation paradigm is embodied in current recommendationsof ICRP (1991) and NCRP (1993a) and in radiation protection stan-dards for the public established by NRC (1991) and DOE (1990).These recommendations and standards include a limit on annualeffective dose of 1 mSv to individual members of the public from allsources of routine exposure combined, excluding natural back-ground, indoor radon, and deliberate medical practices. Assuming anominal probability coefficient for fatal cancers of 5.0 � 10�2 Sv�1

(see Table 3.3) and continuous exposure over an average lifetime of70 y, the estimated lifetime fatal cancer risk corresponding to thedose limit is about 4 � 10�3. ICRP (1991) and NCRP (1993a) alsorecommend, however, that the lifetime risk from routine exposureto man-made sources normally should not exceed about 1 � 10�3.Risks from man-made sources having values in the range of(1 to 4) � 10�3 are regarded as barely tolerable, and risks belowthis range are regarded as reasonably achievable in most cases.

The development of many standards that specify dose constraintsfor specific practices or sources at levels well below the annual doselimit of 1 mSv for all controlled sources combined (Kocher, 1988;Mills et al., 1988) is an important means of ensuring that the lifetimecancer risk from exposure to controlled sources normally will not


exceed about 10�3. This type of standard is referred to as a sourceconstraint (ICRP, 1991). Examples include EPA standards for opera-tions of uranium fuel-cycle facilities in 40 CFR Part 190 (EPA, 1977)and NRC’s performance objectives for release of radionuclides fromnear-surface waste disposal facilities in 10 CFR Part 61 (NRC,1982a). In the radiation paradigm, source constraints essentiallyrepresent generic applications of the ALARA principle; i.e., they arebased primarily on judgments by regulatory authorities that thespecified doses are reasonably achievable at any site. In practice,the lifetime fatal cancer risk due to any single controlled source oftenis much less than 10�3 (NCRP, 1987b), due to vigorous site-specificapplications of the ALARA principle beyond the requirements ofsource constraints.

The radiation paradigm also is applied to other situations includ-ing cleanup of sites contaminated with uranium or thorium milltailings, mitigation of indoor radon, remediation of elevated levelsof naturally occurring radionuclides other than radon, and responsesto radiation accidents. In these applications, the maximum accept-able risk has a value in the range of about 10�1 to 10�3 (Kocher, 1999).

In addition to the dose limit that defines a maximum acceptablerisk and the requirement to reduce doses below the limit using theALARA principle, there has long been the concept in radiation protec-tion of a dose so low that the associated risk would be considerednegligible (de minimis), as indicated in Figure 3.11. At such lowdoses, efforts at further reductions in dose using the ALARA princi-ple generally would be unwarranted. A widely discussed de minimisdose for individual members of the public from any man-made sourceis an annual effective dose of 0.01 mSv (IAEA, 1988; NCRP, 1993a).This dose is one percent of the dose limit for individual members ofthe public, and it corresponds to a lifetime fatal cancer risk of about4 � 10�5. The concept of a negligible dose is discussed further inSections 4.1.2.5, 4.1.3.2, and 4.4.1.2; such a dose is not yet incorpo-rated in radiation protection standards for the public in the UnitedStates (DOE, 1990; NRC, 1991).

If the concept of a negligible risk is included, the radiation paradigmfor management of stochastic risks (‘‘top-down’’) depicted in Figure3.11 defines three regions of risk:

1. a risk so high that it is unacceptable (intolerable, de manifestis)and normally must be reduced regardless of cost or other cir-cumstances; i.e., an excess lifetime cancer risk above a valuein the range of about 10�1 to 10�3, the particular value depend-ing on the exposure situation;


2. a risk so low that it generally is negligible regardless of thecost-benefit for dose reduction; i.e., an excess lifetime risk lessthan about 10�4 for any exposure situation; and

3. risks between intolerable and negligible levels that are accept-able (i.e., tolerable) if they are ALARA but are unacceptableotherwise.

It is important to emphasize that achieving a negligible risk is notthe goal of ALARA, because any non-negligible risk between intolera-ble and negligible levels is acceptable if it is ALARA.

3.3.2 Chemical Paradigm for Risk Management of StochasticResponses

The chemical paradigm for management of stochastic risks isapplied to control of exposures to stochastic chemicals under author-ity of several environmental laws. The chemical paradigm alsoapplies to control of radiation exposures when these exposures areregulated under authority of any laws other than AEA.

The chemical paradigm for management of stochastic risks essen-tially is the opposite of the radiation paradigm (‘‘top-down’’) describedin Section 3.3.1, and thus may be thought of as ‘‘bottom-up.’’ Thechemical paradigm depicted in Figure 3.12 has two basic elements:(1) a goal for acceptable risk, and (2) allowance for an increase (relax-ation) in risks above the goal, based primarily on considerationsof technical feasibility and cost. The extent to which the goal foracceptable risk may be relaxed generally depends on the particu-lar situation.

The use of risk goals and allowance for an increase (relaxation)in risks is fundamentally different from the approach in the radiationparadigm of establishing a limit on dose (and therefore risk) andrequiring reductions in dose below the limit based on the ALARAprinciple. Thus, the goal for acceptable risk in the chemical paradigmclearly does not have the same meaning as the limit on acceptablerisk in the radiation paradigm. Indeed, a noteworthy feature of thechemical paradigm is that it does not explicitly incorporate the con-cept of an intolerable risk that normally must be reduced regardlessof cost or other circumstances.

The chemical paradigm also differs from the radiation paradigmin that there are no standards that apply to all controlled sourcesof exposure and all hazardous substances combined, as in radiationprotection standards. Regulations for hazardous chemicals generallyapply only to specific release pathways (e.g., the atmosphere) or


Fig. 3.12. The chemical paradigm for management of stochastic risks(Kocher, 1999).

exposure pathways (e.g., drinking water), or to all release and expo-sure pathways at specific sites (e.g., standards for cleanup of contami-nated sites). In some cases (e.g., standards for atmospheric releasesand drinking water), each hazardous chemical of concern is regulatedseparately, and there is no standard that specifies an acceptable riskfrom exposure all regulated substances combined.

The chemical paradigm for management of stochastic risks isexemplified by standards for contaminants in public drinking watersupplies established by EPA under authority of the Safe DrinkingWater Act (EPA, 1975; SDWA, 1974). EPA must first establish maxi-mum contaminant level goals (MCLGs), which are non-enforceablehealth goals for drinking water that must correspond to levels whereno known or anticipated health effects would occur. Thus, based onthe assumption of a linear, nonthreshold dose-response relationship,MCLG for all known substances that induce stochastic responses,including radionuclides, must be zero. This goal cannot be achievedat any cost. Then, EPA must establish maximum contaminant levels


(MCLs), which are the legally enforceable standards for contami-nants in drinking water. MCLs must be set as close to MCLGs aspossible, taking into account technical feasibility and cost. Thus,MCLGs are the goals for health risk, and MCLs are the allowablerelaxations above the goals.

Under authority of the Safe Drinking Water Act, EPA has estab-lished MCLs for substances that cause stochastic responses, includ-ing radionuclides, in public drinking water supplies. The MCLsusually (but not always) correspond to lifetime cancer risks havingvalues in the range of about 10�4 to 10�6. This range of acceptablerisks from contaminants in drinking water also has been embodiedin other regulations established by EPA, including standards forairborne emissions of radionuclides and chemicals (EPA, 1992a)developed under authority of the Clean Air Act (CAA, 1963), goalsfor cleanup of radionuclides and chemicals at hazardous waste sites(EPA, 1990a) developed under authority of CERCLA (1980), andrequirements for corrective actions at disposal sites for hazardouschemical waste in 40 CFR Part 264 (EPA, 1980a) developed underauthority of RCRA (1976).

It is important to understand that the limits on lifetime cancerrisk in the range of about 10�4 to 10�6 as embodied, for example, indrinking water standards are conceptually different from the doselimit (and associated limit on risk) in radiation protection standardsdescribed in Section 3.3.1. In the radiation paradigm, the dose limitis regarded as necessary for protection of public health and, thus,normally must be met in routine exposure situations regardless ofcost or other circumstances. In contrast, the standards (MCLs) forhazardous substances in drinking water, although they also arelegally enforceable limits, are based primarily on considerations ofcontaminant levels that are reasonably achievable, taking intoaccount technical feasibility and cost, rather than levels that must bemet to protect public health regardless of cost or other circumstances.Thus, drinking water standards and other standards establishedunder the chemical paradigm that embody limits on lifetime cancerrisk in the range of about 10�4 to 10�6 are analogous to the sourceconstraints that are widely used as one means of applying theALARA principle in the radiation paradigm.

As another example, risk goals having a value in the range of 10�4

to 10�6 for cleanup of contaminated sites under CERCLA (EPA,1990a) do not define a limit that must be met without regard forcost or other circumstances, because CERCLA and its implementingregulations (EPA, 1990a) specify many conditions for waiving com-pliance with the goals. Rather, CERCLA risk goals define risks abovewhich action to reduce risk must be considered, but reduction of


risks above the goals is required only to the extent feasible. Further-more, EPA (1991a) has indicated that lifetime cancer risks belowabout 10�4 are properly interpreted as negligible, because action toreduce risks at these levels generally is not required. Indeed, incleanup decisions involving radioactively contaminated sites underCERCLA, the risk levels achieved in many cases have values in therange of about 10�2 to 10�4 (EPA, 1994b) and, thus, are substantiallyabove the goals. This result is indicative of how CERCLA risk goalsshould be interpreted.

The chemical paradigm for management of risks (‘‘bottom-up’’)depicted in Figure 3.12 often is interpreted as defining two regionsof stochastic risk: (1) an ‘‘acceptable’’ risk, i.e., any excess lifetimecancer risk less than a value in the range of about 10�4 to 10�6, theparticular value depending on the exposure situation; and (2) an‘‘unacceptable’’ risk, i.e., any risk greater than an ‘‘acceptable’’ risk.However, as indicated above, this interpretation is misleadingbecause it does not properly convey how risk-management decisionsusually are made in the chemical paradigm. Although risks less thana value in the range of about 10�4 to 10�6 clearly are acceptable,risks at these levels are more properly interpreted as negligiblebecause there usually is no requirement for further reduction ofrisks based, for example, on considerations of technical feasibilityand cost, even when such reductions would be cost-effective. Further-more, ‘‘unacceptable’’ risks (any risks greater than a value in therange of about 10�4 to 10�6) clearly are not intolerable and are notrequired to be mitigated regardless of cost or other circumstances,because risks above these levels often have been permitted basedprimarily on considerations of cost-benefit (e.g., in cleanups ofCERCLA sites).

These conclusions about the proper interpretations of the signifi-cance of different levels of risk in the chemical paradigm are sup-ported, for example, by a review of EPA regulatory decisions priorto 1985 (Travis et al., 1987). As depicted in Figure 3.13, this reviewshowed the following. First, EPA always declined to regulate whenthe risk was less than a value in the range of about 10�4 to 10�6,depending on the size of the exposed population. Thus, EPA clearlyregarded risks at these levels as negligible. Second, when the riskwas greater than a value in the range of about 10�4 to 10�6 but lessthan a value in the range of about 10�2 to 10�3 (the middle regionin Figure 3.13), EPA required risk reduction in some cases butdeclined to do so in others, depending primarily on the cost-benefitfor risk reduction. Thus, risks greater than a value in the range of


about 10�4 to 10�6 clearly are not unacceptable without regard forother circumstances. This interpretation is not changed by morerecent laws and regulations implementing the chemical paradigmfor management of stochastic risks. The upper region in Figure 3.13is discussed in the following section.

The chemical paradigm for risk management also is used in regu-lating exposures to hazardous chemicals that cause deterministiceffects and exhibit a threshold in the dose-response relationship. Forthese substances, RfDs, which are often used to define acceptableexposures, represent negligible doses, because RfDs usually are wellbelow assumed thresholds for deterministic responses in humansand action to reduce doses below RfDs generally is not required.This interpretation is supported by cases where doses above an RfDare allowed when achieving RfD is not feasible. A particular example

Fig. 3.13. Summary of EPA regulatory decisions prior to 1985 onwhether to regulate carcinogenic hazardous materials (Travis et al., 1987);lower-right region comprising high population risks is excluded based onassumed United States population.


involves regulation of thallium in drinking water. In this case, thestandard (MCL) for limiting concentrations in drinking water estab-lished by EPA in 40 CFR Part 141 (EPA, 1975) is substantially abovethe goal (MCLG) that is intended to ensure a negligible dose (a dosebelow RfD) because achieving the goal is not feasible using existingtechnology for water treatment. Thus, RfDs clearly do not defineintolerable doses.

3.3.3 Comparison of the Radiation and Chemical Paradigms

The radiation paradigm for management of stochastic risk (‘‘top-down’’) described in Section 3.3.1 and depicted in Figure 3.11 clearlyis quite different, at least conceptually, from the chemical paradigm(‘‘bottom-up’’) described in Section 3.3.2 and depicted in Figure 3.12.The radiation paradigm essentially involves a limit on ‘‘acceptable’’(meaning barely tolerable) risk and reductions in risk below thelimit using the ALARA principle, whereas the chemical paradigmessentially involves a goal for ‘‘acceptable’’ (meaning negligible) riskand allowance for relaxation of risks above the goal based primarilyon considerations of technical feasibility and cost.

The acceptable risks embodied in the radiation and chemical para-digms—i.e., lifetime cancer risks less than values in the range ofabout 10�1 to 10�3 in the radiation paradigm but less than about10�4 to 10�6 in the chemical paradigm—appear to be inconsistent.This seeming inconsistency, if not properly understood, leads to themisleading and improper conclusion that risk management basedon the chemical paradigm is more stringent (achieves lower levelsof risk). However, this inconsistency is more perceived than real,and it results essentially from the different meanings of ‘‘acceptable’’and ‘‘unacceptable’’ in the two paradigms. The different meaningsof these terms are summarized in Table 3.5 and discussed below.

In the radiation paradigm, ‘‘unacceptable’’ clearly means ‘‘intolera-ble’’ because this term describes risks so high that they normallymust be reduced regardless of cost or other circumstances, and‘‘acceptable’’ is used to describe risks below intolerable levels thatalso are ALARA, i.e., risks that are ‘‘optimized.’’ The radiation para-digm also includes the concept of a risk so low that further reductionsin risk using the ALARA principle would be unwarranted, but suchlow risks are termed ‘‘negligible,’’ rather than ‘‘acceptable,’’ to distin-guish them from higher risks that are acceptable if they are ALARA.

In contrast, ‘‘acceptable’’ in the chemical paradigm usually means‘‘negligible’’ because further reductions in risk usually need not beconsidered even if they would be cost-effective, and ‘‘unacceptable’’


TABLE 3.5—Differences in interpretations of ‘‘acceptable’’ and‘‘unacceptable’’ risks in radiation and chemical paradigms for

management of stochastic risks.a

Description of Interpretation in Radiation Interpretation in ChemicalRisk Paradigmb Paradigmc

‘‘Acceptable’’ Risks are below Risks are negligibleintolerable (de minimis); further(de manifestis) levels reduction of risksand are ALARAd usually need not be

considerede

‘‘Unacceptable’’ Risks are intolerable; Risks are aboverisks normally must be negligible levels;reduced below reduction of risks mustintolerable levels be considered but isregardless of costf required only to the

extent feasibleg

a Differences in interpretations of ‘‘acceptable’’ and ‘‘unacceptable’’ in thetwo paradigms also apply to dose when regulations are expressed in termsof dose rather than risk; dose is commonly used in regulating radionuclidesunder either paradigm.

b Interpretations apply to control of exposures to radionuclides under AEA,but not to control of exposures to radionuclides under other environmen-tal laws.

c Interpretations also apply to control of exposures to radionuclides whenthey are regulated under laws addressing hazardous chemicals.

d Lifetime cancer risks considered intolerable have values in the range ofabout 10�1 to 10�3 or greater, with the particular value depending on theexposure situation, and are well above risks considered negligible (e.g.,lifetime risks having a value less than about 10�4). Risks that are ALARAdepend on the particular exposure situation, and achieving a negligible riskis not the goal of ALARA.

e Lifetime cancer risks considered negligible have values in the range ofabout 10�4 to 10�6 or below, with the particular value depending on theexposure situation.

f Risks also are considered unacceptable if they are below intolerable levelsbut are not ALARA.

g Approach to risk management for hazardous chemicals does not explicitlyinclude concept of intolerable risk that normally must be reduced regardlessof cost or other circumstances.

usually refers to any risks sufficiently high that they are not uncondi-tionally acceptable. That is, while reduction of risks above ‘‘accept-able’’ levels must be considered, risk reduction usually is not requiredunless it would be practicable based, for example, on considerations


of technical feasibility and cost. It clearly is not the case that anyrisks above ‘‘acceptable’’ levels are intolerable.

Thus, perhaps the most important substantive difference in thetwo approaches to management of stochastic risks is that the radia-tion paradigm clearly and explicitly incorporates the concept of anintolerable risk that normally must be reduced in any exposuresituation regardless of cost or other circumstances, whereas thechemical paradigm does not. Only the radiation paradigm explicitlyrecognizes that there is a wide range of risks between intolerableand negligible levels where risk management decisions are madebased on considerations of cost-benefit and other societal concerns(the ALARA principle). The concept of an intolerable risk that is wellabove negligible levels has been acknowledged, at least implicitly, byEPA in regulating hazardous chemicals. As shown in the analysisof case-by-case regulatory decisions prior to 1985 in Figure 3.13,EPA always acted to reduce risks having a value above a range ofabout 10�2 to 10�3, thus defining a de facto maximum tolerable risk.As noted previously, for risks in the middle region of Figure 3.13,reduction of risks was required in some cases, but not in others,based primarily on considerations of cost-benefit (Travis et al., 1987).

As indicated in Figure 3.13 and previous discussions, there is animportant similarity in the radiation and chemical paradigms thatoverrides any differences, both perceived and real. In spite of thedifference in the basic approach to risk management (limits plusALARA in the radiation paradigm, in contrast to goals plus allowancefor relaxation in the chemical paradigm) and in spite of the differentmeanings attached to the terms ‘‘acceptable’’ and ‘‘unacceptable’’ riskin the two paradigms, application of the ALARA principle essentiallyis the basis for almost all risk-management decisions, without regardfor the particular paradigm being applied (Kocher, 1999; NAS/NRC,1999a). Application of the ALARA principle is an explicit requirementin the radiation paradigm, and it has been so successful that thedose limit defining a maximum acceptable (barely tolerable) risk fromexposure to all controlled sources combined essentially plays no rolein regulating routine public exposures to man-made sources. In thechemical paradigm, application of the ALARA principle is the primarybasis for virtually all risk-management decisions, particularly whenthe goal for acceptable (i.e., negligible) risk cannot be achieved atany cost but also when standards have been established that defineallowable exposures for particular situations based on considerationsof technical feasibility and cost (e.g., drinking water standards). Thisis the case even though the term ALARA does not appear explicitlyin laws or regulations that embody the chemical paradigm.


3.3.4 Reconciliation of the Radiation and Chemical Paradigms

Based on discussions similar to those in Section 3.3.3 on the simi-larities and differences in the radiation and chemical paradigmsfor management of stochastic risks, Kocher and Hoffman (1991)developed a proposed regulatory framework that would apply to allsubstances in the environment that cause stochastic effects, in-cluding those that are naturally occurring, and to any routine oraccidental exposure situation. This framework is intended to be con-sistent with the two risk-management paradigms while also address-ing the seeming inconsistencies and ambiguities in their presentapplications.

In the framework for regulating public exposures to all substancesthat cause stochastic effects proposed by Kocher and Hoffman (1991),three regions of risk depicted in Figure 3.14 are defined:

Fig. 3.14. Unified framework for regulating all radionuclides and chemi-cals in the environment that cause stochastic effects proposed by Kocherand Hoffman (1991).


1. a negligible (de minimis) excess lifetime risk having a value inthe range of about 10�4 to 10�6 and below, where risks fromany substance and source of exposure would be so low thatefforts at further risk reduction would be unwarranted;

2. an intolerable (de manifestis) excess lifetime risk having a valuein the range of about 10�1 to 10�3 and above, where reductionof risks normally would be required regardless of cost or othercircumstances; and

3. excess lifetime risks between de minimis and de manifestislevels, where efforts to reduce risk would be based on applica-tion of the ALARA principle, with the proviso that achieving ade minimis risk is not the goal of ALARA.

The use of ranges for the de manifestis and de minimis risks, ratherthan single values, would allow consideration of the size of anexposed population. For example, higher levels could be used whenonly a few individuals are at risk, but lower levels could be usedfor large populations. The use of ranges also would allow consider-able flexibility in accommodating the kinds of subjective societaljudgments involved in applying the ALARA principle to particularexposure situations. Kocher (1999) has shown that this regulatoryframework is consistent with all current regulatory policies for limit-ing routine or accidental exposures of the public to radionuclidesand chemicals that cause stochastic effects.

Perhaps the most important consideration in developing a consis-tent approach to regulating all substances in the environment thatinduce stochastic effects would be to recognize the primary impor-tance of the ALARA principle in risk-management decisions, withoutregard for the particular risk management paradigm being applied.Another important consideration would be to achieve consensus ona clear and unambiguous meaning of the term ‘‘unacceptable’’ inregard to risk of stochastic effects. A consistent regulatory approachwould be greatly aided if ‘‘unacceptable’’ were used only to describean intolerable risk, rather than any risk above negligible levels. Sucha consistent interpretation would address the widespread confusionconcerning the difference between a dose (risk) limit in the radiationparadigm and a risk goal in the chemical paradigm. Similarly, aconsistent approach to regulation would be aided by an understand-ing that ‘‘acceptable’’ means not only that a risk is negligible butalso that a risk is below intolerable levels and is ALARA. Suchinterpretations of these terms would be completely consistent withmost risk management decisions that have been made using theradiation and chemical paradigms.


3.3.5 Application of Risk Management Paradigms toWaste Classification

A proper reconciliation of the radiation and chemical paradigmsfor risk management is important to the development of a compre-hensive and risk-based hazardous waste classification system. Inparticular, the proposed waste classification system developed inSections 6.2 and 6.3 of this Report is based fundamentally on theconcept that an acceptable risk generally can be substantially greaterthan a negligible risk. This distinction is used to define differentclasses of waste that pose an increasing hazard.

3.4 Summary

Section 3 has discussed issues of risk assessment and risk manage-ment for radionuclides and hazardous chemicals. These discussionsprovide important background information for the development ofa comprehensive and risk-based hazardous waste classificationsystem.

A basic premise of this Report is that waste that contains hazard-ous substances should be classified based on considerations of risksresulting from disposal. In the context of waste classification, ‘‘risk’’is the probability that an adverse health effect (response) wouldresult from disposal of hazardous waste, taking into account (1) theprobabilities of all processes and events that could result in exposureof humans, (2) the magnitude of such exposures, and (3) the probabil-ity that an exposure would result in a response. Probabilities ofprocesses or events that could result in exposures may be consideredonly qualitatively (e.g., as credible or non-credible occurrences), butthe probability of a response from a given exposure generally mustbe considered quantitatively. Waste would be classified based onrisk by comparing an estimated risk resulting from disposal of a unitamount or concentration of a hazardous substance using a particulardisposal technology with a specified allowable risk, thus yielding anallowable amount or concentration of the substance in the wasteclass associated with that disposal option.

In classifying waste based on considerations of risks resulting fromdisposal, hypothetical and generic disposal sites or abstractions ofreal sites must be considered, because waste may be classified beforedisposal sites are chosen and multiple disposal sites may be usedfor similar wastes. An important characteristic of waste disposalsystems is that risks resulting from release of hazardous substancesand transport beyond the site boundary are highly site-specific,

3.4 SUMMARY / 161

whereas risks resulting from inadvertent human intrusion into adisposal facility are much less dependent on the disposal site. Fur-thermore, assessments of near-surface waste disposal facilities haveshown that risks to inadvertent intruders generally are higher thanrisks to individuals beyond the site boundary, given an assumptionthat exposures of intruders would occur according to postulated sce-narios. Thus, assessments of risk associated with scenarios for inad-vertent intrusion at waste disposal sites can be used to develop arisk-based waste classification system. Risks to members of the pub-lic beyond the site boundary also are an important consideration indetermining acceptable disposal practices, and these risks are takeninto account in developing site-specific waste acceptance criteria.

A second basic premise of this Report is that a waste classificationsystem should apply to waste that contains radionuclides, hazardouschemicals, or mixtures of the two, and that the approaches to riskassessment and risk management used in classifying waste shouldbe reasonably consistent for the two types of substances. The processof risk assessment per se is quite similar for radionuclides and haz-ardous chemicals, and there are important similarities in the waysthat deterministic and stochastic responses are treated in riskassessment and risk management. Deterministic responses gener-ally are treated by identifying a threshold in the dose-responserelationship and applying safety and uncertainty factors to limitexposures to levels well below the threshold. Furthermore, incidenceis the measure of deterministic response used for all hazardous sub-stances. Similarly, stochastic responses generally are treated byassuming that the probability of a response is linearly proportionalto dose, without threshold, and this relationship is used to establishlimits on exposure that are intended to limit the probability of occur-rence of stochastic responses.

However, as summarized below, there are important differencesin the ways that the dose-response relationships for radionuclidesand hazardous chemicals are used in risk assessment and risk man-agement.

1. In setting limits on exposure intended to prevent the occurrenceof deterministic responses, the safety and uncertainty factorsthat are applied to the assumed thresholds for hazardous chemi-cals that cause deterministic effects usually are considerablylarger (by at least a factor of 10) than the safety factor normallyapplied to the thresholds for deterministic responses from expo-sure to radiation. Furthermore, the assumed threshold usuallyis more conservative for hazardous chemicals than for radiation(i.e., a lower confidence limit of the threshold often is used for


chemicals, but a best estimate generally is used for radiation).These differences result, in part, from the much greater relianceon studies in animals for hazardous chemicals compared withradiation and the uncertainty in applying animal data tohumans.

2. In establishing dose-response relationships for stochasticeffects, primarily cancers, for use in health protection, thereare three differences in the approaches used for radionuclidesand hazardous chemicals:● The dose-response relationship for radionuclides is intended

to be a best estimate, whereas the dose-response relationshipfor chemicals that cause stochastic effects is intended to bean upper-bound estimate (UCL).

● The primary measure of stochastic responses for radio-nuclides used in radiation protection has been fatalities,whereas incidence is the universal measure of stochasticresponses for hazardous chemicals.

● Assessments of stochastic responses for radionuclides takeinto account all organs and tissues at risk from a given expo-sure, whereas assessments for hazardous chemicals thatcause stochastic effects usually are based on observedresponses in a single organ or tissue in study animals.

Deterministic responses from exposure to hazardous chemicalsgenerally are of concern in health protection of the public becausemany of the exposure limits derived from the assumed thresholdsand the applied safety and uncertainty factors fall within the rangeof potential routine exposures. However, the possibility that thelarge safety and uncertainty factors normally used in setting expo-sure limits are quite conservative (pessimistic) could be taken intoaccount in developing a risk-based waste classification system.Deterministic responses from exposure to radionuclides should notbe of concern in health protection of the public or in classifying waste,because the dose limits intended to prevent deterministic responsesare substantially higher than the dose limit intended to limit theoccurrence of stochastic responses.

Stochastic responses from exposure to radionuclides and haz-ardous chemicals generally are of concern in health protection ofthe public and in classifying waste. Of the three differences inapproaches to dose-response assessment identified above, the mostimportant is the use of a best estimate (MLE) of the dose-responserelationship for radionuclides but upper-bound estimates (UCLs) forhazardous chemicals that cause stochastic effects. UCL in the dose-response relationship for chemicals that cause stochastic effects nor-mally exceeds MLE by a factor of 5 to 100 or more. If this difference

3.4 SUMMARY / 163

is not reconciled, unequal weight would be given to radionuclidesand chemicals in classifying waste, and the weight given to chemicalscould be far out of proportion to the potential stochastic risks. Thedifference between using fatalities or incidence as the measure ofstochastic response is unlikely to be important in classifying wastebecause, on average, about 60 to 70 percent of all stochastic responsesare fatal. The difference in approaches to accounting for the organsand tissues at risk would be important only if several organs wereat risk from exposure to chemicals and the probability of a responsewere about the same in all organs at risks. This situation is expectedto occur only rarely, if ever.

An essential consideration in developing a risk-based waste classi-fication system is the levels of acceptable risk that should be assumedin classifying waste. Therefore, an important concern in develop-ing a comprehensive waste classification system is the differentapproaches to management of stochastic risks that have been usedfor radionuclides and hazardous chemicals.

The radiation paradigm for management of stochastic risks isbased on the principles of radiation protection developed by ICRPand NCRP. In this paradigm, stochastic risks are managed by(1) establishing a limit on dose (and therefore risk) from routineexposure to all controlled sources combined, which has the interpre-tation that doses (risks) above the limit normally are intolerableand must be reduced regardless of cost or other circumstances, and(2) requiring that doses be reduced below the limit ALARA, takinginto account cost-benefit and other societal concerns. The approachof establishing a limit and requiring reductions below the limit isreferred to as ‘‘top-down.’’ The radiation paradigm also includes theconcept that there are risks so low that they generally need not bereduced (i.e., the risks are negligible), but this concept has not beenincorporated in laws and regulations in the United States that imple-ment the radiation paradigm.

The chemical paradigm for management of stochastic risks essen-tially is the opposite of the radiation paradigm, and is referred toas ‘‘bottom-up.’’ In this approach, a goal for acceptable risk is estab-lished, but the goal may be increased (relaxed) based, for example,on considerations of cost-benefit and technical feasibility. The degreeof allowable relaxation in the goal for acceptable risk depends onthe exposure situation. Thus, the goal for acceptable risk in thechemical paradigm clearly does not have the same meaning as thelimit on acceptable risk in the radiation paradigm. The chemicalparadigm does not explicitly include the concept of an intolerablerisk that normally must be reduced regardless of cost or other circum-stances, as defined by the dose limit in the radiation paradigm. The


two paradigms for management of stochastic risks also differ in thatregulations established under the chemical paradigm apply only toparticular substances, release or exposure pathways, or sources,whereas the dose limit in the radiation paradigm applies to all con-trolled sources combined.

The two paradigms for management of stochastic risks can bereconciled based on two considerations. The first is a recognitionthat the terms ‘‘unacceptable’’ and ‘‘acceptable’’ in regard to risk havedifferent meanings in the two paradigms. The term ‘‘unacceptable’’means ‘‘intolerable’’ in the radiation paradigm but ‘‘non-negligible’’in the chemical paradigm, whereas ‘‘acceptable’’ essentially meansALARA in the radiation paradigm but ‘‘negligible’’ in the chemicalparadigm. These differences are a particularly important consider-ation in applying the two risk management paradigms to wasteclassification. The second important consideration in reconciling thetwo risk management paradigms is the realization that the ALARAprinciple is the single most important factor in risk managementdecisions for radionuclides and hazardous chemicals, without regardfor the particular risk management paradigm being applied.

4. Existing ClassificationSystems for HazardousWastes

Wastes have been classified for decades for a variety of purposes.This Section discusses the historical development of classificationsystems for radioactive and hazardous chemical wastes and theresulting classification systems in use at the present time. The rela-tionship between waste classification and requirements for disposalof different classes of hazardous waste is emphasized. The frame-work for this discussion is the top-level system for waste classifica-tion in the United States shown in Figure 4.1. Within this framework,it is first determined whether a waste is nonhazardous (e.g., munici-pal waste); these wastes are not addressed in this Report. If a wasteis deemed hazardous, it is so classified due to the presence of radionu-clides or hazardous chemicals. Mixed radioactive and hazardouschemical waste is not a separate class of waste. However, mixedwaste has been an important concern as a result of differences inrequirements for management and disposal of radioactive and haz-ardous chemical wastes. Section 4.1 addresses classification anddisposal of radioactive waste, and is followed by discussions of classi-fication and disposal of hazardous chemical waste in Section 4.2and approaches to management of mixed radioactive and hazardouschemical waste in Section 4.3. Finally, Section 4.4 summarizes previ-ous NCRP recommendations relevant to waste classification.

The discussions of classification of radioactive and hazardouschemical wastes and management of mixed waste in Sections 4.1 to4.3 are presented in considerable detail to facilitate understandingof these issues by readers who may not be knowledgeable in theseareas. The existing hazardous waste classification systems and thehistorical developments underlying them are complex. NCRPbelieves that an appreciation of these complexities is important ingaining an understanding of the need for a new hazardous wasteclassification system and the benefits it would provide.

165

166 / 4. EXISTING CLASSIFICATION SYSTEMS

Fig. 4.1. Top-level waste classification system in the United States.

4.1 Classification and Disposal of Radioactive Waste

This Section discusses the historical development and currentapproaches to classification and disposal of radioactive waste. Classi-fication and requirements for disposal of different radioactive wastesin the United States are emphasized, particularly the relationshipbetween waste classification and requirements for disposal; much ofthis discussion is adapted from a previous paper (Kocher, 1990).Proposals for alternative radioactive waste classification systemsare reviewed. Classification systems developed by the InternationalAtomic Energy Agency (IAEA) and the relationship between wasteclassification and disposal requirements in IAEA recommendationsare discussed in some detail. Waste classification systems developedin other countries are briefly mentioned.

4.1.1 Background

Classification of radioactive waste has been facilitated by two con-siderations. The first is that radiation dose provides a common mea-sure of potential health impacts from exposure to any radionuclideand for any exposure situation (see Section 3.2.2). All classificationsystems for radioactive waste take into account, at least to some

4.1 CLASSIFICATION AND DISPOSAL OF RADIOACTIVE WASTE / 167

degree, potential doses to individuals who might be exposed towaste materials.

A second consideration that has been important, at least implicitly,in developing classification systems for radioactive waste is naturalbackground radiation. The presence of a ubiquitous and unavoidablebackground of radiation and its description in terms of radiationdose provide a measure of the significance of potential exposures ofradiation workers and members of the public to any radioactivewaste. Levels of radiation in waste materials compared with levelsof natural background radiation have played an important role inradioactive waste classification.

4.1.2 Radioactive Waste Classification in the United States

4.1.2.1 Introduction. This Section reviews the classification sys-tem for radioactive waste that has been developed in the UnitedStates. The historical development of radioactive waste classes isemphasized to provide an understanding of the present classificationsystem and an appreciation of why a new system would be beneficial.Descriptions of waste classes developed prior to the current defini-tions in laws and regulations are discussed first, followed by statu-tory and regulatory definitions developed over the last three decades.Requirements for permanent disposal of different classes of radioac-tive waste also are described. The relationship between requirementsfor waste disposal and its classification is important in developingan understanding of the present system for classifying radioactivewaste in the United States. To provide a focus for these discussions,the current definitions of different classes of radioactive waste inthe United States and the intended disposal systems (technologies)for the different waste classes are summarized in Table 4.1.

The use of nuclear reactors to produce fissionable materials fordefense purposes, beginning in the 1940s, and to generate electricpower in the commercial sector, beginning in the 1950s, has beenthe most important source of radioactive waste requiring manage-ment and disposal. In operations of the nuclear fuel cycle,9 radioac-tive waste produced in the commercial sector often is distinguishedfrom waste that arises from atomic energy defense activities, but thisdistinction usually is not important in classifying waste. However,

9 For the purposes of this Report, the term ‘‘nuclear fuel cycle’’ encompasses the produc-tion, utilization, and disposition of the fuel used in fission reactors for electricity genera-tion, research and development, or production of nuclear materials for any purpose, andany byproduct materials that arise from or are associated with these activities.


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radioactive wastes also arise from activities not associated with thenuclear fuel cycle, and the distinction between fuel-cycle and NARMwaste has been important in waste classification.

The present system for classification of radioactive waste in theUnited States may be depicted in the form of a hierarchy shown inFigure 4.2. As indicated at the top of the figure, the first distinctionis between waste associated with the nuclear fuel cycle and anyother radioactive waste. The latter category includes any waste thatcontains naturally occurring radioactive material (NORM) otherthan that associated with the nuclear fuel cycle (e.g., radium wasteproduced in treating drinking water) or radioactive material pro-duced in an accelerator. These two types of radioactive material notassociated with the nuclear fuel cycle are called naturally occurringand accelerator-produced radioactive material (NARM). The distinc-tion between fuel-cycle and NARM waste originates in law, asdescribed below, but is largely artificial with regard to requirementsfor safe management and disposal of waste because this distinctionis based on the source of the waste rather than its radiologicalproperties.

Radioactive wastes that arise from operations of the nuclear fuelcycle are divided into five classes, called spent nuclear fuel, high-level waste, transuranic waste, low-level waste, and uranium orthorium mill tailings. At the present time, NARM wastes are notformally divided into different classes (see Section 4.1.2.4). The divi-sion of all radioactive waste into fuel-cycle and NARM waste andthe division of fuel-cycle waste into five classes constitutes the basicclassification system for radioactive waste in the United States.

Fig. 4.2. Current United States radioactive waste classification system.


The lowest level of the hierarchy in Figure 4.2 represents furtherclassifications of transuranic waste and low-level waste that arisesfrom operations of the nuclear fuel cycle and a further classificationof NORM waste that represents various state regulations specifyingconcentrations of naturally occurring radionuclides, especiallyradium, below which the materials are not regulated as radioactivewaste. This level of the hierarchy is not part of the basic radioactivewaste classification system in the United States that identifies broadcategories of waste. The further classifications of transuranic waste,low-level waste, and NORM waste are used primarily for planningpurposes in developing specific systems for waste management anddisposal, rather than for the purpose of identifying, in general terms,the type of disposal technology that might be acceptable for a broadclass of waste.

The distinction between radioactive waste associated with thenuclear fuel-cycle and NARM waste shown in Figure 4.2 arises fromprovisions of AEA (1954), which governs the production and use ofso-called source, special nuclear, and byproduct materials for defenseand peaceful purposes. Source material is defined as (1) uranium orthorium, or (2) ores that contain more than 0.05 percent by weightof either of these elements, except source material does not includespecial nuclear material. Thus, source material essentially is theraw material from which nuclear fuel is made. Excluding sourcematerial, special nuclear material is defined as (1) plutonium, 233U,or uranium enriched in 233U or 235U, or (2) materials artificiallyenriched by any of these isotopes. Thus, special nuclear material isthe fissionable material used in nuclear reactors or nuclear weapons.Byproduct material is defined as (1) any radioactive material, exceptspecial nuclear material, resulting from production or use of specialnuclear material, and (2) uranium or thorium mill tailings. Becausethese are the only radioactive materials defined in AEA, the Actgoverns classification and disposal of radioactive wastes only if theyarise from operations of the nuclear fuel cycle, but the Act does notgovern classification and disposal of NARM waste. The importantdistinction between fuel-cycle materials and NARM originated inthe security and safeguards aspects of the early nuclear weaponsprogram.

AEA governs the processing and use of source, special nuclear,and byproduct materials in the commercial sector under licensesissued by NRC or Agreement States (states that enter into licensingagreements with NRC). Since all licensing activities of NRC areperformed under authority of AEA, NRC has no licensing authorityover NARM waste generated in the commercial sector (Agreementand non-Agreement States may regulate NARM waste under state


statutes). AEA, as amended by the Energy Reorganization Act of1974 (ERA, 1974) and DOE Organization Act of 1977 (DEOA, 1977),also governs atomic energy defense and research and developmentactivities of DOE. Under AEA, DOE also is responsible for manage-ment and disposal of any NARM waste that arises from its authorizedactivities. EPA could regulate commercial or DOE NARM wasteunder TSCA (1976) or RCRA (1976). EPA also regulates NARMwaste at Superfund sites subject to remediation under CERCLA(1980). However, for all practical purposes, federal regulation ofNARM waste at the present time depends on whether the wasteoriginates in commercial activities, where it is regulated only by thestates, or in DOE activities, in which case it is self-regulated by DOE.

4.1.2.2 Early Descriptions of Radioactive Waste Categories. Thefollowing sections discuss the earliest categories of radioactive wastethat were developed prior to the current legal and regulatory defini-tions of waste classes. These categories applied only to waste thatarises from operations of the nuclear fuel cycle.

4.1.2.2.1 Liquid wastes. Historically, the most important radioac-tive wastes have been liquid wastes that arise from chemical repro-cessing of spent nuclear fuel for defense production purposes, i.e.,for the purpose of extracting plutonium for use in nuclear weapons.These wastes contain varying concentrations of many radionuclides,primarily fission products and long-lived, alpha-emitting transura-nium isotopes.

Three categories of liquid radioactive waste from fuel reprocessing,containing decreasing concentrations of radionuclides, were firstdescribed and used at U.S. Atomic Energy Commission (AEC) pro-duction sites in the late 1950s (Lennemann, 1972). High-level wastecontained the highest concentrations of radionuclides and requiredconfinement and storage in underground tanks. Liquid high-levelwaste contained high concentrations of fission products, including90Sr and 137Cs, and long-lived radionuclides, principally alpha-emit-ting transuranium isotopes such as 239Pu and 241Am. Liquid high-level waste was further categorized as self-boiling or non-boiling.Self-boiling waste was waste with high levels of decay heat thatrequired engineered cooling systems during storage, whereas non-boiling waste was waste with lower levels of decay heat that requiredonly natural cooling during storage. Medium- or intermediate-levelwaste contained lower concentrations of radionuclides than high-level waste and could be released to underground structures or seep-age basins. Finally, low-level waste contained the lowest concentra-tions of radionuclides in liquid reprocessing waste and could bereleased to holding ponds and lagoons or directly to surface waters.


It is important to emphasize that the earliest classification systemfor radioactive waste described above was not based on considera-tions of potential impacts on public health or the environment follow-ing permanent disposal of waste. Rather, these descriptions ofcategories of liquid wastes from reprocessing of spent nuclear fuelwere based primarily on operational requirements for safe handlingand storage of waste, taking into account the widely varying levelsof radioactivity in different waste streams (Lennemann, 1972). Thatis, the primary impetus for waste classification was the need toprotect workers from radiation exposure during waste operations.

Each AEC site that reprocessed spent nuclear fuel developed itsown limits on radionuclide concentrations in the three categories ofliquid waste described above, based on site-specific operating prac-tices and environmental conditions (Beard and Godfrey, 1967;Marter, 1967). At the Hanford site, for example, the following limitswere used to classify liquid wastes from fuel reprocessing at 100 to200 d after discharge from a reactor (Beard and Godfrey, 1967):high-level waste contained total activity concentrations greater than3.7 TBq m�3, intermediate-level waste contained total activity con-centrations between 1.9 MBq m�3, and 3.7 TBq m�3 and low-levelwaste contained total activity concentrations less than 1.9 MBq m�3.Since this classification of liquid wastes was based on the total activ-ity concentration of all radionuclides, classification of these wasteswas based primarily on the concentrations of relatively short-livedfission products. These radionuclides are quite important in protec-tion of workers, but they are relatively unimportant in regard tolong-term impacts on public health and the environment followingpermanent disposal of waste.

4.1.2.2.2 Solid wastes. The development of different categories ofsolid radioactive waste began in 1960 when AEC initiated interimshallow-land burial services for solid wastes generated in the privatesector (e.g., at nuclear power plants) until disposal facilities for commer-cial waste could be developed. The following three categories of solidwaste that was acceptable for shallow-land burial, containing decreas-ing concentrations of radionuclides, were defined (Lennemann, 1967):high-level waste contained total activity concentrations greater than1,300 TBq m�3, intermediate-level waste contained total activity con-centrations between 13 and 1,300 TBq m�3, and low-level waste con-tained total activity concentrations less than 13 TBq m�3. As in theearliest classifications of liquid reprocessing wastes discussed inthe previous section, these descriptions were based primarily onoperational requirements for protection of workers during wastehandling at generating sites, rather than requirements for protection


of public health and the environment at disposal sites. By comparisonwith the descriptions of liquid waste classes at the Hanford sitegiven in the previous section, the earliest descriptions of differentclasses of solid waste were not based on those for the correspondingclasses of liquid waste. The differing descriptions of classes of liquidand solid waste resulted primarily from differences in radionuclidecompositions (i.e., the source of the waste) and methods of handlingliquid and solid wastes.

In the late 1960s, a fourth category of solid waste, first called alpha-bearing waste but later transuranic waste, came into use at AECsites. Transuranic waste arose primarily from processing of materialscontaining plutonium or 233U that were obtained from chemical repro-cessing of spent uranium or thorium fuel.10 This category of solid wastecontained relatively high concentrations of long-lived, alpha-emittingtransuranium radionuclides or 233U but generally lower concentrationsof beta/gamma-emitting fission products than higher-activity liquidwastes from fuel reprocessing. As shown in Figure 4.2, transuranicwaste was further classified as ‘‘contact-handled’’ if it required littleor no shielding or ‘‘remotely-handled’’ if it required shielding or remotehandling to protect workers from high levels of external photon orneutron radiation. The subclasses of transuranic waste thus werebased only on requirements for safe handling and storage but werenot based on requirements for permanent disposal.

In 1970, AEC established a policy that solid waste with concentra-tions of certain alpha-emitting radionuclides, including long-livedtransuranium radionuclides and 233U, greater than 0.4 kBq g�1 wasnot acceptable for shallow-land burial but required storage or burialin a retrievable manner (Hollingsworth, 1970). Transuranic wastethus referred to solid waste with concentrations of alpha-emittingradionuclides greater than 0.4 kBq g�1. The concentration limit forshallow-land burial of solid waste that contains certain alpha-emit-ting radionuclides was based on the higher concentrations of radium,also an alpha-emitting radionuclide, that occur naturally in Earth’scrust. That is, shallow-land burial of waste with concentrations ofalpha-emitting radionuclides less than 0.4 kBq g�1 was regarded asacceptable because the resulting radiation doses to the public shouldnot be significantly greater than the unavoidable dose due to natu-rally occurring radium and its decay products in surface soil. This

10 According to strict interpretation of ‘‘transuranic,’’ these wastes could be so classi-fied only if they contained sufficient amounts of elements having an atomic numbergreater than 92. Despite this, wastes that contained sufficient amounts of 233U andother alpha-emitting non-transuranium radionuclides often were classified and man-aged as transuranic waste because their specific activities (activities per unit mass)are more similar to those of the transuranium elements than to natural uraniumor thorium.


definition of transuranic waste represented the first quantificationof a waste class based primarily on considerations of protection ofpublic health following permanent disposal of solid waste.

As discussed in the previous section, low-level waste originallyincluded any waste with concentrations of radionuclides less thanthose in high-level or intermediate-level waste, and the descriptionsof low-level waste were based primarily on operational requirementsfor protection of workers at waste generating sites, rather thanrequirements for protection of public health and the environmentfollowing permanent disposal. However, as described in more detailin Section 4.1.2.3.3, the definition of low-level waste later was broad-ened to include any radioactive waste that arises from operations ofthe nuclear fuel cycle other than spent nuclear fuel, high-level waste,transuranic waste, or uranium or thorium mill tailings. This defini-tion was no longer related to requirements for safe handling andstorage of waste or permanent disposal, because low-level waste wasno longer restricted to containing relatively low concentrations ofany radionuclides and could include waste destined for differentdisposal systems (e.g., a near-surface disposal facility or a geologicrepository). Indeed, low-level waste could contain very high concen-trations of relatively short-lived beta/gamma-emitting radionuclidesas well as high concentrations of long-lived fission or activation prod-ucts, for example. Thus, low-level waste could require extensiveshielding to protect workers during waste operations or storage anddisposal far below the ground surface to protect public health.

4.1.2.2.3 Summary of bases for early descriptions of radioactivewastes. The earliest descriptions of different classes of radioactivewaste were based primarily on operational requirements for safehandling and storage of liquid wastes that arise from a particularsource, namely, chemical reprocessing of spent nuclear fuel. Thesedescriptions were extended to reflect operational requirements forsafe handling and storage of solid waste as well. Thus, protection ofpublic health and the environment following permanent disposal ofwaste was not a primary consideration in developing the earliestdescriptions of different waste classes. Later on, a description oftransuranic waste was developed and quantified based on considera-tions of protecting public health following shallow-land burial ofsolid waste that contains alpha-emitting radionuclides.

4.1.2.3 Classification and Disposal of Wastes from the Nuclear FuelCycle. This Section discusses the different classes of radioactivewaste that arise from operations of the nuclear fuel cycle that havebeen defined in laws and regulations over the last three decades and


requirements for permanent disposal of the different classes of fuel-cycle waste. The discussions particularly emphasize the relationshipbetween waste classification and requirements for disposal.

4.1.2.3.1 High-level waste and spent fuel. The earliest descriptionsof different classes of liquid and solid radioactive wastes were basedprimarily on requirements for safe operations, rather than require-ments for permanent disposal (Section 4.1.2.2). These descriptionshave greatly influenced the present classification system for radioac-tive waste that arises from operations of the nuclear fuel cycle inthe United States, even though permanent disposal of solid wasteis now the desired endpoint of radioactive waste management. Thedefinition of high-level waste is the most important because thedefinitions of all other classes of fuel-cycle waste, except uraniumor thorium mill tailings, depend on the definition of high-level waste.

Statutory and Regulatory Definitions. The first regulatory definitionof high-level waste was developed by AEC in 1970 and is containedin 10 CFR Part 50, Appendix F (AEC, 1970). Specifically:

High-level waste is the aqueous wastes resulting from operationof a first-cycle solvent extraction system, or equivalent, andconcentrated wastes from subsequent extraction cycles, or equiv-alent, in a facility for fuel reprocessing.

High-level waste thus includes the concentrated wastes that arisefrom reprocessing of commercial or defense nuclear fuel that containvirtually all the fission products and transuranium radionuclides(except plutonium) in spent fuel. However, the definition does notmention the constituents of the waste, and it is only qualitativebecause ‘‘concentrated’’ is not quantified and the minimum fuelburnup that would yield high-level waste is not specified. Althoughthe definition given above referred only to liquid (aqueous) waste,it is clear from further discussions in 10 CFR Part 50, Appendix F(AEC, 1970), that AEC intended that high-level waste also wouldinclude concentrated solid waste derived from liquid high-level wastethat was suitable for permanent disposal.

The definition of high-level waste developed by AEC was basedon the traditional source-based description of high-level waste dis-cussed in Section 4.1.2.2.1; i.e., high-level waste is the primary wastefrom fuel reprocessing. Thus, the definition implies that high-levelwaste (1) produces high levels of decay heat and external radiation,due primarily to the high concentrations of shorter-lived fission prod-ucts, and (2) requires long-term isolation from the biosphere in orderto protect public health (AEC, 1969a), due primarily to the high


concentrations of long-lived, alpha-emitting transuranium radionu-clides. High-level waste was defined in terms of its source, ratherthan its radiological properties, mainly because reprocessing of spentnuclear fuel was the only significant source of waste with theseproperties at that time.

Because the definition of high-level waste developed by AEC wasonly qualitative, there was some ambiguity regarding materials fromfuel reprocessing that would be included in high-level waste. AECand, later, NRC have indicated that, in their view, high-level wastedoes not include the following: (1) metal cladding used to containfuel material and other irradiated and contaminated fuel structuralhardware; (2) incidental wastes that arise in operations of reprocess-ing plants, such as ion-exchange beds or sludges; or (3) incidentalwastes generated in further treatment of high-level waste, such asdecontaminated salts containing substantially lower concentrationsof 90Sr, 137Cs, and plutonium than first-cycle solvent extraction wastes(AEC, 1969b; NRC, 1987). Wastes that have been excluded fromhigh-level waste generally have lower concentrations of fission prod-ucts and transuranium radionuclides than wastes that arise directlyfrom fuel reprocessing. However, general principles, such as limitson concentrations of radionuclides or levels of external radiation,have not been established for excluding incidental wastes that arisefrom fuel reprocessing from high-level waste, and decisions regard-ing classification of such wastes have been made only on a case-by-case basis.

Early statutory definitions of high-level waste are contained in theMarine Protection, Research and Sanctuaries Act of 1972 (MPRSA,1972) and the West Valley Demonstration Project Act of 1980(WVDPA, 1980). These definitions are consistent with the definitiondeveloped by AEC.

NRC’s current regulatory definition of high-level waste is con-tained in 10 CFR Part 60 (NRC, 1983). Specifically:

High-level waste is: (1) irradiated reactor fuel; (2) liquid wastesresulting from operation of a first-cycle solvent extraction sys-tem, or equivalent, and concentrated wastes from subsequentextraction cycles, or equivalent, in a facility for fuel reprocessing;and (3) solids into which such liquid wastes have been converted.

NRC thus has retained the qualitative, source-based definition ofhigh-level waste first developed by AEC, and spent fuel is consideredto be a form of high-level waste.

The Nuclear Waste Policy Act of 1982 [NWPA (1982)] as amendedin 1987, contains the current statutory definitions of spent nuclear


fuel and high-level waste. Spent fuel is defined separately from high-level waste as follows:

Spent nuclear fuel is fuel that has been withdrawn from anuclear reactor following irradiation, the constituent elementsof which have not been separated by reprocessing.

However, spent fuel is not a waste until it is so declared. As in thedefinitions of high-level waste discussed previously, the constituentsof spent fuel and the minimum fuel burnup or concentrations ofradionuclides produced by irradiation are not specified. High-levelwaste then is defined in two parts as:

Clause (A): highly radioactive material from fuel reprocessing,including liquid waste produced directly in reprocess-ing and any solid material derived from such liquidwaste that contains fission products in sufficient con-centrations; and

Clause (B): other highly radioactive material that NRC, consistentwith existing law, determines by rule requires perma-nent isolation.

In the context of NWPA, ‘‘requires permanent isolation’’ means dis-posal in a geologic repository, or in an alternative system that wouldprovide equivalent capabilities for isolation of the waste from thebiosphere, with no intention of retrieving the waste after facilityclosure.

The definition of high-level waste in Clause (A) of NWPA givenabove follows the traditional, source-based description although, forthe first time, the presence of fission products is mentioned explicitly.However, the definition remains qualitative because ‘‘highly radioac-tive’’ material and ‘‘sufficient concentrations’’ of fission products arenot quantified, nor are the minimum concentrations of alpha-emit-ting transuranium radionuclides.

The definition of high-level waste in Clause (B) of NWPA givenabove represents a potentially significant departure from previousdefinitions in that it allows the development of a generally applicabledefinition of high-level waste that is not based on the source of thewaste. However, as in Clause (A), ‘‘highly radioactive’’ and ‘‘requirespermanent isolation’’ in Clause (B) are not quantified.

In 1987, NRC announced its intent to develop a quantitative andgenerally applicable definition of high-level waste in response to thedefinition in Clause (B) of NWPA (NRC, 1987). NRC indicated thatthe definition would specify minimum concentrations of radionu-clides constituting high-level waste and would be based primarily


on analyses of risks from waste management and disposal. In partic-ular, ‘‘highly radioactive’’ in Clause (B) would be defined in terms ofminimum concentrations of shorter-lived radionuclides that producehigh levels of decay heat and external radiation, and ‘‘requires per-manent isolation’’ would be defined in terms of minimum concentra-tions of long-lived radionuclides that require disposal systemsproviding a high degree of isolation from the biosphere (a geologicrepository or equivalent). Thus, while the definition in Clause (B)would apply to waste with radiological properties similar to thoseof high-level waste from fuel reprocessing, the definition would incor-porate these properties explicitly, and the definition essentiallywould be risk-based rather than source-based.

In considering a new definition of high-level waste in accordancewith Clause (B) of NWPA, an important issue for NRC was whetherthis definition should encompass and quantify the traditional,source-based definition in Clause (A). Such a definition would quan-tify ‘‘sufficient concentrations’’ of fission products and the minimumconcentrations of alpha-emitting transuranium radionuclides inhigh-level waste from fuel reprocessing. NRC indicated its preferencethat the definition in Clause (B) should not apply to the primarywastes from fuel reprocessing and that the definition in Clause (A)should continue to apply to all wastes previously considered to behigh-level waste in accordance with source-based definitions(NRC, 1987).

In 1988, NRC announced its intention to abandon efforts to developa quantitative and generally applicable definition of high-level wastein response to Clause (B) of NWPA (NRC, 1988). This decision wasbased on the following argument. First, the definition in Clause (B)should not be applied to the primary wastes from fuel reprocessingdefined in Clause (A). Given this, there is little need for a new defini-tion of high-level waste, given the current institutional frameworkfor management of high-level waste and other radioactive wastesassociated with the nuclear fuel cycle and the small volumes of wastefrom sources other than fuel reprocessing that likely would be definedas high-level waste under Clause (B). Furthermore, considerableeffort would be required to quantify ‘‘requires permanent isolation’’in the context of NWPA on the basis of analyses of risks from wastedisposal and to develop licensing criteria for disposal of higher-activ-ity wastes that do not ‘‘require permanent isolation’’ (disposal in ageologic repository or equivalent) but would not be acceptable fordisposal in a near-surface facility. This effort would be difficult tojustify, given the small amounts of waste involved.

In 1989, NRC confirmed its decision to retain the qualitative,source-based definition of high-level waste and not to define any


radioactive wastes from sources other than fuel reprocessing as high-level waste (NRC, 1989). Thus, all definitions of high-level wastedeveloped in accordance with NWPA apply only to waste from chemi-cal reprocessing of spent nuclear fuel. Waste with similar radiologicalproperties that arises from any other source is not included in high-level waste.

EPA’s current definition of high-level waste was first developedin 1985 (EPA, 1985) and is contained in 40 CFR Part 191 (EPA,1993a). This definition defers to NWPA, and spent fuel is definedseparately from high-level waste as in the Act. Thus, EPA hasadopted the traditional, source-based definition of high-level waste.

DOE also has used the traditional, source-based definition of high-level waste. In contrast to other definitions, the definition adoptedin 1988 was explicit that high-level waste contains transuraniumradionuclides (DOE, 1988a). Later, however, DOE essentiallyadopted the definition in NWPA given above (DOE, 1999a).

In summary, in accordance with current laws and regulations,high-level radioactive waste essentially can be defined as follows:

High-level waste is the primary waste (either liquid or solid)that arises from chemical reprocessing of spent nuclear fuel.

This definition is based on the source of the waste, but certain inci-dental wastes that arise from fuel reprocessing that contain lowerconcentrations of fission products and alpha-emitting transuraniumradionuclides than the primary reprocessing wastes have beenexcluded on a case-by-case basis.

Spent nuclear fuel is a form of high-level waste in some definitions[e.g., NRC’s 10 CFR Part 60 (NRC, 1983)] but not in others [e.g., theNuclear Waste Policy Act (NWPA, 1982)]. This inconsistency is notimportant, because spent fuel and the primary waste from fuel repro-cessing have similar radiological properties and require similar pre-cautions for safe handling, storage, and disposal. Spent fuel is nota waste until it is so declared.

NWPA authorizes NRC to define radioactive materials other thanthe primary waste from fuel reprocessing as high-level waste, butNRC has chosen not to do so. As a consequence, waste from sourcesother than fuel reprocessing with equivalent levels of decay heator external radiation, due to high concentrations of shorter-livedradionuclides, and requiring an equivalent degree of long-term isola-tion from the biosphere for protection of public health (such asdisposal in a geologic repository), due to high concentrations of long-lived radionuclides, are not included in high-level waste. Thus, thedefinition of high-level waste clearly is not risk-based.


Requirements for Disposal. The first requirements for disposal ofcommercial high-level waste were developed by AEC in 10 CFR Part 50,Appendix F (AEC, 1970). AEC specified that liquid high-level wasteshall be converted to dry solids and transferred to a federal reposi-tory, to be designated later, for permanent disposal.

NWPA (1982), as amended, governs disposal of spent fuel andhigh-level waste. This statute established the current DOE programfor disposal of commercial spent fuel and high-level waste in geologicrepositories, subject to licensing by NRC. Its requirements apply toany repository not used exclusively for (1) disposal of defense spentfuel or high-level waste or (2) DOE research and development activi-ties. This statute also specifies that liquid high-level waste fromfuel reprocessing must be converted to a solid prior to permanentdisposal. The 1987 amendments to NWPA designated the site atYucca Mountain, Nevada, as the sole candidate to be studied for itspotential to host the first geologic repository.

NWPA authorizes but does not require disposal of commercialspent fuel and high-level waste in a geologic repository. Indeed, theAct directed DOE to investigate alternative technologies for disposalof these wastes (e.g., subseabed disposal), but DOE is not authorizedto construct or operate alternative disposal facilities. The Act alsocalled for an evaluation of the merits of disposing of defense high-level waste in the same repository to be used for commercial spentfuel and high-level waste. Following a study by DOE (1985a), co-disposal of defense high-level waste with commercial spent fuel andhigh-level waste in a single repository was recommended, and thisis the current policy.

In 1985, EPA established the first environmental standards fordisposal of spent fuel and high-level waste in 40 CFR Part 191 (EPA,1985); these standards were revised in 1993 (EPA, 1993a). The EPAstandard was intended to apply to disposal of spent fuel and high-level waste at any site and using any technology.

NRC’s first licensing criteria that govern DOE activities at geologicrepositories were developed in 10 CFR Part 60 (NRC, 1983). Thisregulation applied to disposal of spent fuel and high-level waste onlyif a geologic repository were used, and it also applied to any otherradioactive waste that might be sent to a repository. NRC’s licensingcriteria for geologic repositories were intended to be compatible withEPA’s first environmental standards in 40 CFR Part 191 (EPA,1985).

In 1992, Congress directed EPA to issue a new environmentalstandard for disposal of spent fuel and high-level waste that wouldapply only to the candidate geologic repository at the Yucca Mountainsite in Nevada (NEPA, 1992). Thus, the existing EPA standards in


40 CFR Part 191 (EPA, 1993a), as well as NRC’s licensing criteriain 10 CFR Part 60 (NRC, 1983), no longer apply to this site. EPA’senvironmental standards for the Yucca Mountain site were estab-lished in 40 CFR Part 197 (EPA, 2001a) and NRC has issued itsfinal licensing criteria for the site in 10 CFR Part 63 (NRC, 2001).

Under authority of AEA, DOE has established policies for manage-ment and disposal of defense high-level waste and any other materi-als which, because of their highly radioactive nature, require similarhandling (DOE, 1988a; 1999a). In addition to specifying that disposalof these wastes in a geologic repository shall comply with require-ments of NWPA, general environmental standards developed byEPA, and NRC’s licensing criteria, DOE policies address (1) storageof high-level waste in doubly-contained and singly-contained tanksystems, principally at the Hanford, Washington, and SavannahRiver, South Carolina, sites, and (2) options for disposal of defensehigh-level waste that is not readily retrievable.

Current policies and requirements for disposal of spent fuel andhigh-level waste thus can be summarized as follows. A geologic repos-itory at the Yucca Mountain site in Nevada is the only candidatefacility for disposal of commercial spent fuel and high-level waste.A geologic repository is expected to provide a high degree of isolationof the waste from the biosphere, and the need for such a disposalsystem is based primarily on the high concentrations of long-lived,alpha-emitting radionuclides in spent fuel and high-level waste. TheYucca Mountain site will be developed and operated by DOE. Thefacility must comply with EPA’s environmental standards in 40 CFRPart 197 (EPA, 2001a), and it will be licensed by NRC in accordancewith 10 CFR Part 63 (NRC, 2001). Defense high-level waste will beco-disposed in the same repository with commercial spent fuel andhigh-level waste, and the two types of waste will be subject to thesame environmental standards and licensing criteria.

4.1.2.3.2 Transuranic waste. As described in Section 4.1.2.2.2,transuranic waste originally was defined by AEC as solid waste thatcontains long-lived, alpha-emitting transuranium radionuclides or233U in concentrations greater than 0.4 kBq g�1. Transuranic wasteso defined was not generally acceptable for shallow-land burial.

Statutory and Regulatory Definitions. The earliest statutory defini-tions of transuranic waste were contained in AEA (1954), theNational Security and Military Applications of Nuclear EnergyAuthorization Act (NSMA, 1980), and the Low-Level RadioactiveWaste Policy Act (LLRWPA, 1980). All of these laws defined transu-ranic waste in terms of concentrations of long-lived, alpha-emitting


transuranium radionuclides greater than 0.4 kBq g�1 as in AEC’soriginal definition.

In 1982, federal agencies concurred with a recommendation toincrease the lower limit on concentrations of long-lived, alpha-emitting transuranium radionuclides in transuranic waste from 0.4to 4 kBq g�1 (Steindler, 1982). This change in the definition of transu-ranic waste was made in response to difficulties in routinely measur-ing levels of alpha activity near 0.4 kBq g�1 in bulk solid waste andanalyses which indicated that risks to public health from shallow-land burial of transuranium radionuclides in concentrations up to4 kBq g�1 should be acceptable.

In 1985, EPA developed a regulatory definition of transuranicwaste in 40 CFR Part 191 (EPA, 1985) that incorporated the increasein the lower limit on concentrations of long-lived, alpha-emittingtransuranium radionuclides. This definition was retained when40 CFR Part 191 was repromulgated in 1993 (EPA, 1993a). EPA’sdefinition is the same as the current statutory definition describedbelow.

The current statutory definition of transuranic waste is containedin the WIPP Land Withdrawal Act of 1992 (WIPPLWA, 1992).Specifically:

Transuranic waste is waste that contains more than 4 kBq g�1

of alpha-emitting transuranium isotopes, with half-lives greaterthan 20 y, except for:1. high-level radioactive waste;2. waste that the Secretary of DOE has determined, with the

concurrence of the Administrator of EPA, does not need thedegree of isolation required by the disposal regulations in40 CFR Part 191 (EPA, 1985); or

3. waste that NRC has approved for disposal on a case-by-casebasis in accordance with 10 CFR Part 61 (NRC, 1982a).

In addition to specifying the lower limit on concentrations of alpha-emitting transuranium radionuclides, this definition specifies theirminimum half-life. In contrast to the earliest definition developedby AEC, this definition does not include waste that contains highconcentrations of long-lived, alpha-emitting non-transuraniumradionuclides (e.g., 233U).

The current statutory definition is explicit that transuranic wasteexcludes high-level waste, which also contains high concentrationsof long-lived, alpha-emitting transuranium radionuclides. The othertwo exceptions in the definition allow that some wastes, other thanhigh-level waste, that contain concentrations of long-lived, alpha-emitting transuranium radionuclides greater than 4 kBq g�1 may


be excluded from transuranic waste. The second exception refers towaste, particularly DOE’s defense waste that is not regulated byNRC, with concentrations of such radionuclides sufficiently low thatdisposal in a geologic repository would not be required to protectpublic health, and a less isolating disposal system could be used.The third exception could apply to commercial waste with concentra-tions of such radionuclides sufficiently low that shallow-land disposalwould provide adequate protection of public health (NRC, 1982b).

Thus, by definition, transuranic waste contains sufficient concen-trations of longer-lived, alpha-emitting transuranium radionuclides.These radionuclides usually are the most important in transuranicwaste. However, transuranic waste also may contain high concentra-tions of fission products (e.g., 137Cs) and long-lived, alpha-emittingnon-transuranium radionuclides (e.g., 232Th, 233U), and these constit-uents can determine the radiological properties of transuranic wasteand the risk it poses (DOE, 1997a).

Most transuranic waste has been generated in DOE’s atomicenergy defense activities (DOE, 1997a); this waste is not subjectto licensing by NRC. Prior to the current statutory definition inthe WIPP Land Withdrawal Act, DOE developed its own definitionof transuranic waste (DOE, 1988b). This definition included wastecontaminated with alpha-emitting transuranium radionuclideswith half-lives greater than 20 y and concentrations greater than4 kBq g�1 and it also specified that other alpha-contaminated wastescould be managed as transuranic waste. Based on this definition,DOE sites managed waste that contained high concentrations oflong-lived, alpha-emitting non-transuranium radionuclides (e.g.,233U) or high concentrations of alpha-emitting transuranium radionu-clides with half-lives less than 20 y (e.g., 244Cm and 252Cf) as transu-ranic waste (DOE, 1997a). However, in accordance with the currentstatutory and regulatory definition described above, these wastescannot be classified as transuranic waste unless they also containmore than 4 kBq g�1 of alpha-emitting transuranium radionuclideswith half-lives greater than 20 y; this definition has been adoptedby DOE (1999b). Such wastes that are not transuranic waste underthe current definition would be classified as low-level waste (seeSection 4.1.2.3.3), regardless of the concentrations of alpha-emittingradionuclides.

Defense transuranic waste is further categorized as ‘‘contact-handled’’ if it requires little or no shielding or ‘‘remotely-handled’’if it requires shielding or remote handling due to high levels of photonor neutron radiation (DOE, 1996a). An external dose-equivalent rateat the surface of a waste package of 2 mSv h�1 is used to distinguishthe two subclasses of transuranic waste. This subclassification is


based on requirements for protection of workers during waste opera-tions, rather than requirements for protection of public health andthe environment following permanent disposal.

NRC has not developed a definition of transuranic waste, primarilybecause only small amounts of transuranic waste subject to licensingby NRC are generated in the commercial sector. NRC regards com-mercial transuranic waste as a form of higher-activity low-levelwaste. However, NRC’s licensing criteria for near-surface disposalof radioactive waste in 10 CFR Part 61 (NRC, 1982a) acknowledgethe current statutory and regulatory definition of transuranic waste,because commercial waste that contains more than 4 kBq g�1 ofalpha-emitting transuranium radionuclides, with half-lives greaterthan 5 y, is not generally acceptable for near-surface disposal. Theminimum half-life of 5 y for alpha-emitting transuranium radionu-clides specified by NRC differs from the value of 20 y specified inthe WIPP Land Withdrawal Act and EPA’s 40 CFR Part 191 (EPA,1985); the minimum half-life of 20 y specified by EPA would applyin classifying any commercial waste as transuranic waste.

In summary, in accordance with current laws and regulations,transuranic waste essentially can be defined as follows:

Transuranic waste is waste, except for high-level waste, that con-tains alpha-emitting transuranium radionuclides, with half-livesgreater than 20 y, in concentrations greater than 4 kBq g�1.

Although this definition specifies a lower limit on the concentrationof particular radionuclides, it also depends on the qualitative, source-based definition of high-level waste and, thus, is not strictly quantita-tive. Alpha-emitting transuranium radionuclides with half-livesgreater than 20 y are expected to be the principal constituentsof most transuranic waste, but the definition does not specify anylimits on the concentrations of other radionuclides that may occurin transuranic waste, including fission products, alpha-emitting non-transuranium radionuclides, and alpha-emitting transuraniumradionuclides with half-lives less than 20 y.

Requirements for Disposal. The National Security and MilitaryApplications of Nuclear Energy Authorization Act (NSMA, 1980)established the current DOE program for disposal of defense transu-ranic waste at the WIPP facility in New Mexico. The Act specificallyauthorized test emplacements of waste for purposes of research anddevelopment. WIPPLWA (1992) then authorized permanent disposalof defense transuranic waste at this facility. The Act specifies thatthe WIPP facility may not be used for disposal of high-level waste,commercial transuranic waste, or any DOE non-defense transuranic


waste, which is transuranic waste generated in DOE activities notrelated to national security such as production of nuclear weapons.This facility also may not be used for disposal of commercial or DOElow-level waste.

Defense transuranic waste sent to the WIPP facility is emplacedin a bedded-salt formation located far below ground. Thus, the WIPPfacility is similar to a geologic repository for spent fuel and high-level waste in its expected waste-isolation capabilities.

Disposal of defense transuranic waste at the WIPP facility is notlicensed by NRC. However, the National Security and Military Appli-cations of Nuclear Energy Authorization Act (NSMA, 1980) createdthe Environmental Evaluation Group as an agency of the state ofNew Mexico to provide independent technical oversight of DOE activ-ities at the WIPP facility. Disposal of defense transuranic waste atWIPP must comply with EPA’s general environmental standards in40 CFR Part 191 (EPA, 1993a). EPA also developed criteria in40 CFR Part 194 (EPA, 1996b) that DOE must use in certifying thatthe WIPP facility complies with the disposal standards. In 1998,EPA ruled that disposal of DOE’s defense transuranic waste at WIPPcomplies with the standards in 40 CFR Part 191 (EPA, 1998a), andpermanent disposal of waste at the site has begun.

DOE (1988b; 1999b) has established policies for management anddisposal of its transuranic waste. In addition to activities associatedwith permanent disposal at the WIPP facility, these policies addresswaste storage at DOE sites and alternatives for long-term manage-ment of transuranic waste that was buried at DOE sites prior to1970. An unresolved issue at the present time is the developmentof facilities for permanent disposal of DOE waste that has beenmanaged as transuranic waste but is not currently classified astransuranic waste; these wastes contain relatively high concentra-tions of such radionuclides as 233U, 244Cm, or 252Cf but concentrationsof long-lived, alpha-emitting transuranium radionuclides less than4 kBq g�1. Wastes that are not classified as transuranic waste, asdefined in WIPPLWA (1992), are not acceptable for disposal at theWIPP facility (DOE, 1996a). Any non-defense transuranic wastegenerated by DOE also cannot be sent to WIPP. DOE has used so-called greater confinement disposal systems, which are intermediatein depth and waste-isolation capabilities between near-surface facili-ties and geologic repositories, for small volumes of selected transu-ranic waste (DOE, 1997b). However, greater confinement disposalhas not been developed to the extent needed to accept all transuranicwaste that cannot be sent to the WIPP facility.

There are no laws that explicitly address disposal of commercialtransuranic waste, again because little such waste has been gener-ated. At the present time, there are two alternatives for disposal of


commercial transuranic waste. The first is near-surface disposal, ona case-by-case basis, in accordance with NRC’s licensing require-ments in 10 CFR Part 61 (NRC, 1982a). This alternative could beappropriate for small volumes of waste with concentrations of longer-lived, alpha-emitting transuranium radionuclides only slightlygreater than 4 kBq g�1. The second alternative is disposal in thecandidate geologic repository for spent fuel and high-level waste atthe Yucca Mountain site in accordance with EPA’s environmentalstandards in 40 CFR Part 197 (EPA, 2001a) and NRC’s licensingcriteria in 10 CFR Part 63 (NRC, 2001).

4.1.2.3.3 Low-level waste. Low-level radioactive waste is pro-duced in many commercial and non-commercial activities, and thesewastes vary widely in radionuclide compositions and concentrations.

Statutory and Regulatory Definitions. Current statutory definitionsof low-level waste are contained in NWPA (1982) and the Low-LevelRadioactive Waste Policy Amendments Act (LLRWPAA, 1986). Inthe Nuclear Waste Policy Act, low-level waste is defined as radioac-tive waste that:

Clause (A): is not high-level waste, spent fuel, transuranic waste,or byproduct material as defined in Section 11(e)(2) ofAEA; and

Clause (B): NRC, consistent with existing law, classifies as low-level waste.

In Clause (A), the byproduct material defined in Section 11(e)(2)of AEA (1954) essentially is uranium or thorium mill tailings.LLRWPAA contains a similar definition, except transuranic wasteis not excluded. Thus, the two laws differ in regard to whethertransuranic waste is distinct from low-level waste.

The statutory definitions of low-level waste apply only to radioac-tive waste that arises from operations of the nuclear fuel cycle; i.e.,to waste that contains source, special nuclear, or byproduct materialas defined in AEA (see Section 4.1.2.1). This restriction, althoughnot explicit in the definitions, is indicated by the applicability ofNWPA and LLRWPAA to fuel-cycle waste only and by the referenceto NRC, which can only regulate fuel-cycle waste. Thus, low-levelwaste does not include NARM waste.

DOE has defined low-level waste as in Clause (A) above (DOE,1988c; 1999c). In the earlier definition (DOE, 1988c), test specimensof fissionable material irradiated for purposes of research and devel-opment could be classified as low-level waste, provided the concentra-tion of long-lived, alpha-emitting transuranium radionuclides was


less than 4 kBq g�1. However, the revised definition (DOE, 1999c)does not include this provision. Thus, DOE’s current definition is inaccordance with existing legal definitions. DOE’s current definitionalso specifies that low-level waste excludes NORM other than ura-nium or thorium mill tailings. As noted above, this provision isnot explicit in current legal definitions. Based on the current legaldefinitions, DOE waste that contains alpha-emitting radionuclidesthat has been managed as transuranic waste but cannot be classifiedas transuranic waste under current law (see Section 4.1.2.3.2) is aform of low-level waste. An important example is waste that containshigh concentrations of 233U.

EPA has not yet developed a regulatory definition of low-levelwaste. Such a definition presumably would be developed in the courseof establishing general environmental standards for land disposalof low-level waste.

NRC has developed licensing criteria for near-surface disposal ofwaste that contains source, special nuclear, or byproduct materialsin 10 CFR Part 61 (NRC, 1982a). These regulations are intended toapply primarily to disposal of commercial low-level waste. They donot include a definition of low-level waste but essentially defer tothe current statutory definition in the Low-Level Radioactive WastePolicy Amendments Act of 1985. Thus, low-level waste can includewastes with high concentrations of radionuclides that are not gener-ally acceptable for near-surface disposal in accordance with thelicensing criteria in 10 CFR Part 61 (NRC, 1982a).

In summary, in accordance with current laws and regulations,low-level waste is defined only by exclusion and essentially as follows:

Low-level waste is any radioactive waste that arises from opera-tions of the nuclear fuel cycle except for spent fuel, high-levelwaste, transuranic waste, and uranium or thorium mill tailings.

This definition clearly depends on the source-based definition of high-level waste.

Some definitions of low-level waste differ from the one summarizedabove. In particular, transuranic waste is not excluded in the defini-tion in the Low-Level Radioactive Waste Policy Amendments Act of1985, and transuranic waste thus is a form of low-level waste. How-ever, this inconsistency has little practical significance, because theAmendments Act governs disposal of commercial low-level wasteonly, unless DOE waste is sent to a commercial facility, and thereis very little commercial transuranic waste requiring disposal.

NRC has statutory authority to define radioactive materials aslow-level waste, consistent with existing law, but has not done so.Given that NRC can only regulate radioactive materials defined in


AEA, and given the current statutory definition of low-level wasteby exclusion and the applicability of this definition only to wastesregulated under AEA, it is not evident how NRC could develop anew definition of low-level waste that would be different from thecurrent exclusionary definition unless NRC first developed a newdefinition of high-level waste in accordance with Clause (B) of NWPA(see Section 4.1.2.3.1).

Because low-level waste is defined by exclusion of other types ofwaste, this waste class does not necessarily contain relatively lowconcentrations of radionuclides, in contrast to the earliest descrip-tions discussed in Section 4.1.2.2. In addition to very high concentra-tions of short-lived radionuclides, such as 60Co and short-lived fissionproducts, low-level waste can contain high concentrations of long-lived, non-transuranium radionuclides (e.g., 99Tc, 232Th) such thatthe risks posed by disposal of the waste are comparable to the risksposed by disposal of some high-level and transuranic wastes. Thedefinition does not describe the constituents or properties of low-level waste and, thus, is not related in any way to requirements forsafe handling and storage or disposal.

Requirements for Disposal. The Low-Level Radioactive Waste PolicyAct of 1980 (LLRWPA, 1980), as amended by the Policy AmendmentsAct (LLRWPAA, 1986), governs disposal of commercial low-levelwaste. A particular disposal technology is not specified, but shallow-land burial was presumed in accordance with contemporary prac-tices. The original Act (LLRWPA, 1980) directed NRC to identifyalternatives to shallow-land burial for commercial low-level wasteand to establish technical guidance and requirements for licensingof alternative disposal methods. NRC published technical studiesof alternative disposal technologies (Bennett, 1985; Bennett andWarriner, 1985; Bennett et al., 1984; Miller and Bennett, 1985;Warriner and Bennett, 1985), but specific licensing criteria for thesealternatives have not been established.

Near-surface disposal of commercial low-level radioactive wasteis licensed in accordance with criteria established by NRC in10 CFR Part 61 (NRC, 1982a) or compatible licensing requirementsestablished by Agreement States. These regulations do not apply todisposal of (1) specified wastes by individual licensees in accordancewith provisions in 10 CFR Part 20 (NRC, 1991), (2) high-level wastein a geologic repository in accordance with licensing criteria in10 CFR Part 60 (NRC, 1983) or 10 CFR Part 63 (NRC, 2001),(3) uranium or thorium mill tailings, or (4) waste not regulated underauthority of AEA (NARM waste). In addition, these regulations do


not require that materials must be classified as low-level waste tobe acceptable for near-surface disposal.

NRC’s licensing criteria in 10 CFR Part 61 (NRC, 1982a) andcompatible Agreement State requirements include a waste classifi-cation system that is intended mainly to provide protection of inad-vertent intruders at near-surface disposal sites. NRC’s wasteclassification system specifies limits on concentrations of radionu-clides that are generally acceptable for near-surface disposal underspecified conditions. These limits are based on: (1) assumed expo-sure scenarios for intrusion into disposal facilities at 100 to 500 yafter disposal; (2) assumed limits on radiation dose to intruders;(3) requirements on institutional controls, the waste form, and dis-posal methods; and (4) consideration of reported radionuclide con-centrations in commercial low-level waste. The following wasteclasses, with increasing limits on concentrations of radionuclides andincreasingly stringent requirements on the waste form and disposalmethods, are defined: (1) Class-A, -B and -C wastes that containradionuclides with half-lives less than about 30 y; and (2) Class-Aand -C wastes that contain longer-lived radionuclides. Waste withconcentrations of radionuclides greater than the Class-C limits isnot generally acceptable for near-surface disposal. However, near-surface disposal of greater-than-Class-C waste may be approved byNRC or an Agreement State on a case-by-case basis.

The further classification of low-level waste by NRC is indicatedat the bottom of Figure 4.2. It is important to note that NRC’swaste classification system in 10 CFR Part 61 (NRC, 1982a) doesnot constitute a definition of low-level waste. Rather, it is a subclassi-fication of waste developed primarily for purposes of facilitatingmanagement and disposal of commercial low-level waste in near-surface facilities.

Following establishment of NRC’s waste classification system fornear-surface disposal described above, the Low-Level RadioactiveWaste Policy Amendments Act (LLRWPAA, 1986) assigned responsi-bility for disposal of commercial greater-than-Class-C low-levelwaste to DOE, subject to licensing by NRC. A subsequent DOE studydid not resolve the issue of acceptable alternatives to near-surfacedisposal for the small volumes of these wastes (DOE, 1987a). Inaccordance with an amendment to NRC’s licensing criteria in 10 CFRPart 61 (NRC, 1989), disposal of commercial greater-than-Class-Cwaste in a geologic repository now is required, unless disposal else-where is approved by NRC on a case-by-case basis.

Current DOE policy also permits near-surface disposal of most ofits low-level waste (DOE, 1988c; 1999c). Disposal of DOE’s low-levelwaste is not licensed by NRC, unless the waste is sent to a licensed


facility intended primarily for disposal of commercial waste. DOEhas not adopted NRC’s waste classification system for near-surfacedisposal in 10 CFR Part 61 (NRC, 1982a) discussed earlier, exceptlow-level waste that would be classified as greater-than-Class-C inaccordance with 10 CFR Part 61 normally is handled as special cases.DOE has used greater confinement disposal (see Section 4.1.2.3.2) forsmall volumes of selected high-activity low-level waste, includingwaste that is not acceptable for near-surface disposal at the generat-ing site (DOE, 1997b).

In summary, current laws and regulations do not specify thatparticular disposal technologies must be used for low-level waste.Most low-level waste is intended for disposal in near-surface facili-ties, except the small volumes of commercial greater-than-Class-Cwaste, as defined by NRC, are intended for disposal in a geologicrepository. DOE’s low-level waste that would be classified as greater-than-Class-C and any other waste that is not acceptable for near-surface disposal at the generating site also require a disposal technol-ogy considerably more confining than a near-surface facility, eithera geologic repository or a greater confinement disposal system.

4.1.2.3.4 Uranium or thorium mill tailings. Mill tailings are theresidues resulting from extraction or concentration of uranium orthorium from any ore processed primarily for its source materialcontent. Mill tailings do not include residues from mining operations(e.g., uranium mine overburden) or other chemical extraction indus-tries, such as wastes from radium processing and phosphogypsumwaste piles. Mill tailings are a form of byproduct material as definedin Section 11(e)(2) of AEA (1954). The principal concern with milltailings is the relatively high concentrations of radium and emana-tion rates of radon.

Under current law, mill tailings are not a form of low-level waste(see Section 4.1.2.3.3), even though the concentrations of uranium,thorium, and radium generally are much less than the concentra-tions of long-lived, alpha-emitting radionuclides in high-level wasteand transuranic waste. Mill tailings are not included in low-levelwaste primarily because the very large volumes of these materials(DOE, 1997a) necessitate different approaches to management anddisposal from those used for low-level waste.

Management and disposal of most uranium or thorium mill tail-ings are governed by the Uranium Mill Tailings Radiation ControlAct of 1978 (UMTRCA, 1978). This Act is concerned with the controland stabilization of mill tailings for protection of public health andthe environment. It addresses (1) remedial actions at inactive ura-nium or thorium processing sites or on properties in the vicinity of


such sites, which are performed by DOE with the concurrence ofNRC, and (2) disposal of uranium or thorium mill tailings at activeprocessing sites. Regulations for management and disposal of ura-nium or thorium mill tailings have been established by EPA in40 CFR Part 192 (EPA, 1983; 1995b).

In contrast to requirements for disposal of low-level waste, theUranium Mill Tailings Radiation Control Act and its implementingregulations emphasize control and stabilization of mill tailings inplace. If removal of residual radioactive material from the vicinityof mill properties is required to protect public health and the environ-ment, the Act calls for permanent disposal and stabilization of thesematerials at or near processing sites. Thus, most mill tailings arenot intended for disposal in facilities for commercial or DOE low-level waste. Small volumes of DOE waste that contains uranium orthorium mill tailings have been managed as low-level waste(DOE, 1988d).

4.1.2.3.5 Characteristics of the system for classification and dis-posal of fuel-cycle waste. The current classification system for radio-active waste that arises from operations of the nuclear fuel cycle inthe United States and the current requirements for disposal of wastein the different classes have the important characteristics dis-cussed below.

Definitions of Different Classes of Fuel-Cycle Waste. The definitionsof the different classes of radioactive waste that arises from opera-tions of the nuclear fuel cycle in the United States may be summa-rized as in Table 4.1 (see Section 4.1.2.1). These definitions applyonly to waste regulated under AEA, i.e., to waste that containssource, special nuclear, or byproduct material.

The classification system for fuel-cycle waste in the United Stateshas the following important characteristics:

● Most of the definitions are not explicit in regard to the primaryconstituents of the waste or its radiological properties.

● The definitions of the different waste classes are not quantita-tive, i.e., expressed strictly in terms of limits on concentrationsof radionuclides or other waste properties.

● The definitions are not generally applicable to any fuel-cyclewaste, regardless of its source (i.e., how the waste is generated).

● The definitions are not based primarily on considerations of risk,particularly risks resulting from waste disposal.

These characteristics result from three factors: (1) the qualitativedefinition of high-level waste as waste from a particular source (fuel


reprocessing); (2) the origin of the definition of high-level waste inoperational requirements for safe management of liquid waste,rather than requirements for permanent disposal of solid waste; and(3) the dependence of the definitions of transuranic waste and low-level waste on the definition of high-level waste.

The classification system for fuel-cycle waste does not distinguishunambiguously between waste in different classes. For example,high-level waste, transuranic waste, and low-level waste can havesimilar radiological properties and require similar methods of safemanagement and disposal. Such similarities are a consequence ofdefinitions that depend on the source of the waste (high-level waste)or the presence of particular radionuclides (transuranic waste) anda definition by exclusion only (low-level waste). Low-level waste cancontain high concentrations of long-lived radionuclides (e.g., 14C,94Nb, 99Tc, and 233U) and can pose long-term risks similar to those ofhigh-level waste and transuranic waste that contains high concen-trations of long-lived, alpha-emitting transuranium radionuclides.As another example, low-level waste that contains mostly naturallyoccurring radionuclides (e.g., uranium) can resemble mill tailings.

The definition of low-level waste only by exclusion is particularlyproblematic. The term ‘‘low-level’’ gives the impression that wastein this class contains low concentrations of radionuclides or lowradiation levels compared, for example, with high-level waste. How-ever, this is not necessarily the case because low-level waste cancontain the highest concentrations of radionuclides of any waste,including high concentrations of radionuclides with half-lives ofabout 30 y or greater. Since waste in this class can range from innocu-ous to highly hazardous, the definition of low-level waste is notrelated in any way to its radiological properties or to requirementsfor safe management and disposal. The lack of a definition of whatlow-level waste is also has undesirable social and political ramifica-tions, in that it presents a barrier to public understanding and publicdiscourse on waste issues and, thus, may foster mistrust of wastemanagement and disposal activities (Wiltshire and Dow, 1995).

Requirements for Disposal and Their Relationship to Waste Classifi-cation. Under current laws and regulations, spent fuel, high-levelwaste, transuranic waste, and low-level waste generally do notrequire particular disposal systems. However, only certain typesof disposal systems are authorized for some types of waste (seeTable 4.1). In particular: (1) spent fuel, high-level waste, transuranicwaste, and greater-than-Class-C low-level waste normally areintended for disposal in a geologic repository, such as the proposedYucca Mountain facility and the Waste Isolation Pilot Plant; and


(2) low-level waste with concentrations of radionuclides less thanthe Class-C limits normally is intended for disposal in near-surfacefacilities, such as the currently operating commercial and DOE facili-ties. Thus, the current statutory and regulatory framework empha-sizes two options for disposal of radioactive waste. In addition, DOEhas utilized greater confinement disposal systems, which are inter-mediate in depth and waste-isolation capabilities between near-sur-face facilities and a geologic repository, for some of its transuranicwaste and low-level waste. However, this type of facility has not beenused for large volumes of DOE waste and has not been developed forcommercial waste.

In managing uranium or thorium mill tailings, current laws andregulations emphasize control and stabilization in place, rather thanshipment to dedicated disposal facilities. Although mill tailingsresemble some low-activity low-level wastes in their radiologicalproperties, the very large volumes of mill tailings necessitate a differ-ent approach to management and disposal of most of these wastescompared with the approaches used for low-level waste. However,small volumes of mill tailings may be disposed of in facilities intendedprimarily for low-level waste.

These considerations lead to an important conclusion regardingthe relationship between classification of fuel-cycle wastes andrequirements for their disposal—namely, that the selection of accept-able systems for disposal of fuel-cycle wastes does not depend on thedefinitions of waste classes. Rather, the types of disposal systemsthat are expected to provide adequate protection of public health(e.g., a near-surface facility or a geologic repository) are selectedbased on the radiological properties of waste, essentially withoutregard for how the waste is classified. Thus, general requirementsfor disposal are not affected by the qualitative, source-based, andambiguous definitions in the classification system for fuel-cyclewaste.

4.1.2.4 Naturally Occurring and Accelerator-Produced RadioactiveMaterial. NARM includes any radioactive material other thansource, special nuclear, or byproduct material as defined in AEA(1954). Thus, NARM refers to any radioactive material not associatedwith the nuclear fuel cycle.

NARM waste generally is divided into waste that contains NORMand waste produced in an accelerator (see Figure 4.2). These twocategories are not formally defined in federal law or regulations.Rather, they are based mainly on the different properties of the twotypes of waste and the fact that they would rarely, if ever, be gener-ated at the same site. NORM waste, especially waste that arises in


various mining and energy-related activities, often resembles ura-nium or thorium mill tailings in having large volumes but relativelylow concentrations of radionuclides, although some wastes can haverelatively small volumes but considerably higher concentrations ofradionuclides (e.g., waste from treatment of drinking water, radiumneedles used in cancer therapy, pipe scale from extraction of oil).Accelerator-produced waste, such as accelerator targets or wastesthat arise from production of certain medical isotopes, generallyoccurs only in small volumes, and it usually resembles forms oflow-level waste in which most of the activity is due to short-livedradionuclides and the concentrations of longer-lived radionuclidesare relatively low. However, because of the way that radioactivematerials are defined in AEA, diffuse NORM wastes are not a formof mill tailings, and the more concentrated NORM and accelerator-produced wastes are not forms of low-level waste.

At the present time, most commercial NARM waste is not subjectto federal regulation and, thus, is regulated only by the states. Anexception is phosphogypsum materials, which are regulated by EPAfor their radium content and radon emissions (EPA, 1992b) underthe Clean Air Act (CAA, 1963). States generally regulate commercialaccelerator-produced waste as low-level waste (Jacobi, 2000).11 Avariety of approaches have been taken in regulating commercialNORM waste, particularly waste produced in mining, energy exploi-tation, and other industrial activities (NAS/NRC, 1999a). Somestates do not currently regulate these forms of NORM waste asradioactive waste. States that do regulate NORM waste generallyspecify concentrations of radium below which materials are exemptfrom regulation as radioactive waste, but the concentrations ofradium that distinguish regulated and unregulated NORM wastevary from state to state. The distinction between regulated andunregulated (including exempt) waste is indicated by the two sub-classes of NORM waste shown in Figure 4.2.

DOE is responsible for management and disposal of all NARMwaste generated in any of its authorized activities, based on theprovision of AEA (1954) that requires DOE to protect public healthand safety in any such activity. DOE’s NORM waste usually is man-aged as uranium or thorium mill tailings, except small volumesmay be managed as low-level waste, and DOE’s accelerator-producedwaste generally is managed as low-level waste (DOE, 1988d; 1999c).

11 Jacobi, W. (2000). Personal communication (Colorado Department of Public Healthand Environment, Denver, Colorado) to Kocher, D.C. (Oak Ridge National Laboratory,Oak Ridge, Tennessee.)


EPA may develop general environmental standards for manage-ment and disposal of NARM waste under authority of TSCA (1976);e.g., see Cameron (1996). Such standards would subject commercialNARM waste to federal regulation, and they would also apply toDOE’s NARM waste. In principle, EPA also could regulate NARMwaste under RCRA (1976), because the exclusion of radioactive wastefrom regulation under RCRA applies only to waste that containssource, special nuclear, or byproduct material as defined in AEA(1954). However, NARM waste can be regulated under RCRA onlyif it is included in the definition of hazardous waste in 40 CFRPart 261 (EPA, 1980b). The current definition of hazardous waste(see Section 4.2.1) specifically excludes many important wastes frommining and energy exploitation activities that contain naturallyoccurring radionuclides substantially above average backgroundlevels.

4.1.2.5 Exempt Radioactive Waste. The classes of waste discussedin Sections 4.1.2.3 and 4.1.2.4 generally are presumed to requiredisposal in facilities dedicated to radioactive waste in order to protectpublic health and the environment. It has long been recognized,however, that there are materials containing such low amounts ofradioactivity that they could be managed in all respects as if theywere nonradioactive and still protect public health and the environ-ment. Such considerations have led to the concept of an exemptclass of radioactive waste. The primary advantage of establishingexemption levels for radioactive waste would be the considerablylower costs of waste disposal (e.g., in a municipal/industrial landfill)compared with the cost of disposal in a dedicated facility for radioac-tive waste.

This Section describes the concepts used in exempting waste thatcontains radioactive material and discusses efforts in the UnitedStates to establish exemption levels for radioactive waste.

4.1.2.5.1 Concepts and definitions. Two concepts are potentiallyuseful in establishing exemption levels for radioactive waste. Thefirst is the concept of a generally applicable negligible (de minimis)dose or risk, and the second is the concept of amounts of radionuclidesthat are exempt or below regulatory concern (BRC) for particularpractices or sources.

A negligible dose would be generally applicable to all man-madesources of radiation and would define a dose below which furthercontrol of sources by regulatory authorities is deemed to be unwar-ranted. If all doses were below a negligible level, no further reduc-tions in dose using the ALARA principle would be attempted (see


Section 3.3.1). A negligible dose is based on consideration of a negligi-ble risk from radiation exposure, without regard for whether sucha dose is reasonably achievable for any particular exposure situation.

In contrast to a generally applicable negligible dose based on con-sideration of a negligible risk, radioactive materials that are exemptor BRC represent doses judged by regulatory authorities to beALARA for specific practices at any site (e.g., waste disposal).Because doses that are ALARA may depend on the particular expo-sure situation, levels of radioactivity that are exempt or BRC mayvary from one practice to another. Exemption levels for specific prac-tices generally could be higher than levels corresponding to a gener-ally applicable negligible dose based, for example, on considerationsof cost-benefit in choosing among options for management and dis-posal of waste.

4.1.2.5.2 Exemption levels for radioactive waste. This Section dis-cusses exemption levels for radionuclides in waste materials thathave been established or were proposed by NRC. Exemption levelsfor radioactive waste have not been established by DOE or EPA.

Established Exemption Levels. NRC’s radiation protection standardsin 10 CFR Part 20 (NRC, 1991) include limits on concentrationsor annual releases of radionuclides for unrestricted discharge intosanitary sewer systems, except any excreta from individuals under-going medical treatment with radioactive material are exempt fromthe limits. These regulations also include an exemption for landdisposal of liquid scintillation materials and animal carcasses thatcontain 2 kBq g�1 (0.05 �Ci g�1) or less of 3H or 14C, although theexempted scintillation materials must be managed in accordancewith RCRA requirements due to the presence of toluene.

Current NRC regulations for source material in 10 CFR Part 40(AEC, 1961) and byproduct material in 10 CFR Part 30 (AEC, 1965a)specify conditions for exemption of many products or materials thatcontain small amounts of radioactive material (see also Schneideret al, 2001). These exemptions apply to commercial or specializedindustrial uses of radioactive materials, as well as their disposal,and they include many common consumer products (e.g., timepieces,smoke detectors, thorium gas mantles). These exemptions wereestablished based on judgments by AEC and NRC that the benefitsof exempt uses far outweighed the risks to public health.

NRC regulations described above represent a case-by-caseapproach to establishing exemption levels for radioactive material.Although the various exemption levels are expected to correspondto low doses from use and disposal of materials compared, for exam-ple, with dose limits in radiation protection standards for the public


or doses due to natural background radiation (AEC, 1965b; NCRP,1987c), many of the exemption levels are not clearly related to doseto the public. Furthermore, the doses associated with use and dis-posal of the different exempt materials vary widely (Schneideret al., 2001) and, in many cases, are well above doses that might beregarded as de minimis.

In addition to the exemptions established in regulations, NRCissued guidance on concentration limits for disposal of residual tho-rium or uranium from past operations with no restrictions on burialmethod (NRC, 1981). There would be no restrictions on burial methodif the concentrations were less than (1) 0.4 Bq g�1 for natural thoriumor uranium with its decay products present and in activity equilib-rium, (2) 1.3 Bq g�1 for depleted uranium, and (3) 1 Bq g�1 forenriched uranium. These concentration limits were intended to pro-vide criteria for remediation of contaminated sites to permit unrest-ricted use by the public, but they could be applied to waste disposalas well.

The exemption levels for residual thorium or uranium in NRCguidance described above are more than an order of magnitudegreater than average levels of naturally occurring thorium or ura-nium in surface soil (NCRP, 1984a). Since the average annual dosefrom exposure to naturally occurring thorium, uranium, and theirdecay products, including radon, is about 2 mSv (NCRP, 1987b) andis greater than the annual dose limit for continuous exposure ofmembers of the public to man-made sources of 1 mSv establishedby NRC (1991), the exemption levels for natural thorium or uraniumin particular clearly do not correspond to doses (and risks) that wouldbe widely regarded as de minimis.

Proposed Generic Policy on Below Regulatory Concern. To provide acommon risk basis for exempting specific practices or sources regu-lated by NRC, and to replace the present system of case-by-caseexemptions described above, NRC (1990) issued a proposed policyon doses from certain sources or practices by its licensees that wouldbe BRC. This policy was intended to apply, for example, to consumerproducts, recycle/reuse, and waste disposal. NRC proposed thatsources or practices would be BRC if (1) the annual dose to individualswould be 100 �Sv or less for practices affecting a limited number ofindividuals or 10 �Sv or less for practices affecting a large number ofindividuals and (2) the annual collective dose would be 10 person-Svor less, with annual doses to individuals less than 1 �Sv not needingconsideration in estimating collective dose. The exemption level of10 �Sv for practices affecting a large number of individuals is onepercent of the dose limit for members of the public (NRC, 1991) and


is about 0.3 percent of the average dose from natural backgroundradiation (NCRP, 1987b).

Following widespread public objection and in accordance with aprovision of the National Energy Policy Act (NEPA, 1992) revokingNRC’s authority under AEA to exempt broad classes of radioactivematerial from its licensing requirements, NRC withdrew its genericpolicy on BRC (NRC, 1993). The objections to the BRC policy werenot based on technical or scientific arguments, but were related tothe process used to develop the policy and the perception that thepolicy represented an abrogation of NRC’s responsibilities to protectpublic health and safety. NRC will continue to address requests forexemption from licensing requirements for radioactive material ona case-by-case basis using the criteria and guidance issued previously(AEC, 1965b).

4.1.2.5.3 NCRP recommendation on a negligible individualdose. NCRP’s current recommendations on radiation protection(NCRP, 1993a) include a recommendation that annual effectivedoses to individual members of the public of 10 �Sv or less from anypractice or source are negligible. The recommendation on a negligibleindividual dose was based on considerations of the magnitude of thedose and its associated risk, the difficulty in detecting and measuringdoses and associated responses at very low doses, and the estimatedrisk associated with the mean and variance of doses from naturalbackground radiation. For continuous exposure over a 70 y lifetime,the recommended negligible individual dose corresponds to a fatalcancer risk of about 4 � 10�5 (see Table 3.3).

The NCRP recommendation on a negligible individual dose couldbe used to establish exemption levels for radioactive waste. However,a negligible individual dose of 10 �Sv y�1 would be useful mainly inestablishing exemption levels for man-made radionuclides, becauseexposure to naturally occurring radionuclides (e.g., radium, thorium,and uranium) in their undisturbed state results in much higherdoses of about 2 mSv y�1 (NCRP, 1987b) when the contributionsfrom their radiologically significant shorter-lived decay products aretaken into account. Exemption levels for naturally occurring radio-nuclides could be established based on considerations other than anegligible individual dose as indicated, for example, by NRC’s guid-ance on unrestricted disposal of residual thorium or uranium frompast operations (NRC, 1981) discussed in the previous section.

4.1.2.5.4 Summary of exemptions for radioactive waste in theUnited States. At the present time, exemption levels for radionu-clides in waste materials, or materials intended for beneficial use,


have been established by NRC only on a case-by-case basis. Further-more, there is no clear relationship between the existing exemptionlevels and doses (risks) to the public from unrestricted use or disposalof exempt materials.

NCRP has developed a recommendation on a negligible individualdose that could be used to establish exemption levels for radioactivewaste for such purposes as disposal and recycle/reuse, but this recom-mendation has not been adopted by regulatory authorities. Indeed,NRC is prohibited by law from implementing a proposed genericpolicy on exemption of radioactive materials that was consistentwith the NCRP recommendation.

4.1.2.6 Proposals for Alternative Radioactive Waste ClassificationSystems. This Section discusses a number of proposals in theUnited States for developing alternative classification systems forradioactive waste. These proposals illustrate difficulties with theexisting classification system, and they indicate approaches thatcould be used to overcome these difficulties.

4.1.2.6.1 NRC discussion on definition of high-level waste. In1985, NRC described an approach to developing a quantitativedefinition of high-level waste in response to the definition inClause (B) of NWPA (1982) discussed in Section 4.1.2.3.1 (Fehringer,1985). In accordance with the Clause (B) definition, high-level wastewould be any waste that arises from operations of the nuclear fuelcycle, other than the primary waste from chemical reprocessing ofspent nuclear fuel, that is ‘‘highly radioactive’’ and ‘‘requires perma-nent isolation.’’

NRC’s primary concern in this study was to identify concentrationsof radionuclides that require permanent isolation, i.e., disposal in ageologic repository or equivalent. From an evaluation of radionuclideconcentrations in commercial and defense high-level waste from fuelreprocessing, NRC suggested that it might be appropriate to considerother waste with concentrations of radionuclides greater than 30times the Class-C limits for near-surface disposal, as specified in10 CFR Part 61 (NRC, 1982a) (see Section 4.1.2.3.3), to be high-level waste. However, NRC did not undertake further analyses toinvestigate the feasibility of this approach (NRC, 1988; 1989).

4.1.2.6.2 Generally applicable waste classification system proposedby Kocher and Croff. In response to the definition of high-levelwaste in Clause (B) of NWPA (1982) discussed in Section 4.1.2.3.1,Kocher and Croff (1987; 1988) developed a proposal for a quantita-tive, generally applicable, and risk-based radioactive waste classifi-cation system that addresses the definitions of high-level waste in


Clauses (A) and (B) of the Act, as well as the definitions of otherwaste classes. In this proposal, all radioactive waste would be placedinto one of three classes, which are defined conceptually as follows:

● High-level waste is any waste that is highly radioactive andrequires permanent isolation.

● Transuranic waste and equivalent is any waste that requirespermanent isolation but is not highly radioactive.

● Low-level waste is any waste that does not require permanentisolation, without regard for whether it is highly radioactive.

In these definitions, ‘‘highly radioactive’’ refers to high levels of decayheat and external radiation, due primarily to shorter-lived radionu-clides, and ‘‘requires permanent isolation’’ refers to high concentra-tions of long-lived radionuclides; i.e., these terms have the sameinterpretations as in the definitions of high-level waste in NWPA.

Kocher and Croff then suggested the following implementation ofthe conceptual definitions of the three waste classes given above,based on analyses of risks from waste management and disposal:

● ‘‘Highly radioactive’’ means a thermal power density (decay heat)in the waste greater than 50 W m�3 or an external dose-equivalentrate at a distance of 1 m from unshielded waste greater than1 Sv h�1.

● ‘‘Requires permanent isolation’’ means concentrations of radio-nuclides greater than those that would be generally acceptablefor near-surface disposal.

The concentration limits for near-surface disposal either are theClass-C limits specified in NRC’s 10 CFR Part 61 (NRC, 1982a) anddiscussed in Section 4.1.2.3.3 or, for other radionuclides, are theClass-C limits calculated using NRC’s risk analysis methodology fornear-surface disposal (Oztunali and Roles, 1986; Oztunali et al.,1986).

The generally applicable and risk-based radioactive waste clas-sification system proposed by Kocher and Croff would have thefollowing consequences. High-level waste would include most wastepresently classified as high-level waste because of its source (fuelreprocessing) as well as waste from any other source with similarproperties, such as greater-than-Class-C low-level waste with highlevels of decay heat or external radiation. Transuranic waste andequivalent would include most waste presently classified as transu-ranic waste as well as waste from any source with similar properties,such as greater-than-Class-C low-level waste with low levels of decayheat or external radiation. Low-level waste would include commer-cial Class-A, -B, or -C waste, most DOE waste presently classified


as low-level waste, and any other waste that is generally acceptablefor near-surface disposal, including most mill tailings and mostNARM waste. Thus, the proposed waste classification system notonly quantifies the definitions of waste classes based on risk, but italso associates waste classes with particular disposal technologies,either near-surface disposal systems for low-level waste or a consid-erably more isolating system for high-level waste or transuranicwaste and equivalent (e.g., a geologic repository).

4.1.2.6.3 Generally applicable waste classification system proposedby Smith and Cohen. Smith and Cohen (1989) developed a proposalfor a comprehensive and risk-based radioactive waste classificationsystem largely in response to the definition in Clause (B) of NWPA(1982) that high-level waste is ‘‘highly radioactive’’ and ‘‘requires per-manent isolation’’ (see Section 4.1.2.3.1). As in the proposal by Kocherand Croff discussed in the previous section, this proposal associateswaste classes with particular disposal technologies. Four waste classescontaining increasing levels of radioactivity and/or increasing duration(persistence) of the hazard from waste disposal were defined. Thesewaste classes are described as follows:

● BRC waste is waste with such low concentrations of radionu-clides that the waste can be managed according to its nonradio-logical characteristics.

● Low-level waste is waste with concentrations of radionuclidesless than the Class-C limits specified in NRC’s 10 CFR Part 61(NRC, 1982a) and, thus, is generally acceptable for near-sur-face disposal.

● Intermediate-level waste is waste with concentrations of radionu-clides greater than NRC’s Class-C limits but which does notpose a sufficient long-term hazard to justify disposal in a geologicrepository.

● High-level waste is waste with such high concentrations of long-lived radionuclides that disposal in a geologic repository orequivalent is required.

Smith and Cohen did not perform a detailed risk analysis to quan-tify the boundaries of the different waste classes. However, as anexample, if concentration is used as the measure of radioactivity inwaste, the following 239Pu-equivalent concentrations (concentrationsfor which the hazard would be equivalent to that of 239Pu) weresuggested for use in quantifying the different waste classes: (1) BRCwaste would be any waste which, after 10 y of decay, has an equiva-lent concentration less than 40 MBq m�3; (2) low-level waste wouldbe any non-BRC waste which, after 100 y of decay, has an equivalent


concentration less than 4 GBq m�3; (3) intermediate-level wastewould be any waste more radioactive than low-level waste which,after 1,000 y of decay, has an equivalent concentration less than0.4 TBq m�3; and, (4) high-level waste would be any waste which,after 1,000 y of decay, has an equivalent concentration greater than0.4 TBq m�3.

4.1.2.6.4 Waste classification system proposed by LeMone andJacobi. LeMone and Jacobi (1993) developed a proposed classifica-tion system for radioactive waste based primarily on a classificationsystem developed previously by IAEA (1981) (see Section 4.1.3.1).The proposed system includes four classes of radioactive waste,which are described as follows:

● BRC waste is waste with such low concentrations of radionu-clides that the waste would be unregulated with respect to itsradioactivity. BRC waste generally would correspond to verylow concentrations of low-level, short-lived waste and low-level,long-lived waste as defined by IAEA (1981).

● Low-level waste is waste with only low concentrations of interme-diate-level, short-lived waste or intermediate-level, long-livedwaste as defined by IAEA (1981). Low-level waste would besuitable for disposal in a municipal/industrial landfill that metcurrent EPA standards and would include relatively low-activityClass-A waste, as defined in NRC’s 10 CFR Part 61 (NRC,1982a).

● Intermediate-level waste is waste with high concentrations of inter-mediate-level, short-lived waste or intermediate-level, long-livedwaste (IAEA, 1981). Such waste would be suitable for disposal ina near-surface facility incorporating engineered barriers and wouldinclude higher-activity Class-B and Class-C waste, as defined inNRC’s 10 CFR Part 61 (NRC, 1982a).

● High-level waste is waste with high concentrations of long-livedradionuclides (IAEA, 1981). High-level waste would require adisposal system considerably more confining than a near-surfacefacility (e.g., a geologic repository).

This proposal differs from the others discussed previously in thatthe first three waste classes all would include waste that is generallyacceptable for disposal in a near-surface facility. However, thesethree classes differ in the extent to which engineered barriers wouldbe relied upon to inhibit migration of radionuclides and exposuresof inadvertent intruders.

LeMone and Jacobi also suggested that the proposed waste classi-fication system could be quantified by means of the following limits


on annual dose to maximally exposed individuals: BRC waste couldbe defined by a limit from unregulated disposal of 1 �Sv, low-levelwaste by a limit from disposal in a municipal landfill approved byEPA of 1 to 10 �Sv, and intermediate-level waste by a limit fromdisposal in a licensed facility using engineered barriers of 10 to250 �Sv. High-level waste would include any waste that could notmeet the dose limit for intermediate-level waste. LeMone and Jacobidid not implement their proposal to derive radionuclide-specific con-centration limits for the different waste classes.

4.1.3 IAEA Recommendations on Radioactive WasteClassification and Exemption Principles

IAEA has been developing recommendations on classification ofradioactive waste and principles for exempting radioactive wastefrom regulatory requirements for radioactive material for more than30 y. This Section briefly reviews these developments.

Not discussed in this Section are radioactive waste classificationsystems developed in other countries, particularly in Europe. Wasteclassification systems in European countries are discussed, for exam-ple, in a report of the Commission of the European Communities(CEC, 1990), and waste classification systems in a number of coun-tries have been reviewed by Numark et al. (1995). The waste classifi-cation systems developed in other countries often have been based,at least in part, on the source-based classification system in theUnited States or the various IAEA recommendations discussed inthis Section; they generally do not include any new concepts of wasteclassification.

4.1.3.1 Recommendations on Waste Classification. The earliestclassification systems proposed by IAEA (1970; 1981) placed radioac-tive waste into one of three classes, which were defined as follows:

● High-level waste is (1) the highly radioactive liquid, containingmainly fission products as well as some actinides, which is sepa-rated during chemical reprocessing of irradiated fuel; i.e., theaqueous waste from the first solvent extraction cycle and thosewaste streams combined with it; (2) any other waste with radio-activity levels intense enough to generate significant amountsof heat by the radioactive decay process; and (3) spent nuclearfuel, provided it is declared a waste.

● Intermediate-level waste is waste which, because of its radionu-clide content, requires shielding but needs little provision forheat dissipation during handling and transport.


● Low-level waste is waste which, because of its low radionuclidecontent, does not require shielding during normal handling andtransportation.

Within the low-level and intermediate-level waste classes, a fur-ther distinction was made between short- and long-lived waste, aswell as alpha-bearing waste (IAEA, 1981). Short-lived waste referredto waste that would decay to low activity levels during the timeperiod of perhaps a few centuries when administrative control overthe waste can be expected to last, and long-lived waste referred towaste that would not decay to low levels during an administrativecontrol period. Alpha-bearing waste referred to waste that containsone or more alpha-emitting radionuclides in amounts above accept-able limits established by national authorities.

Although the waste classification system described above was use-ful for general purposes, it had several limitations. First, the classifi-cation system was not clearly linked to safety aspects of radioactivewaste management, particularly disposal. Second, it was not con-sistent with definitions of radioactive wastes developed in somecountries, particularly when waste was classified according to thefacilities in which the waste is generated or by the processes thatgenerate the waste. Third, it lacked quantitative boundaries betweenclasses. Fourth, it lacked recognition of a class of waste that containsso little radioactive material that it may be exempted from controlas radioactive waste. Finally, it lacked recognition of wastes, suchas those from mining and milling of uranium ore, that contain lowlevels of naturally occurring radionuclides but occur in very largevolumes.

To address the limitations of the waste classification systemdescribed above, new recommendations on waste classification weredeveloped (IAEA, 1994). A particular aim of the new system was toassociate waste classes with intended disposal technologies (options),at least to some degree. The recommended classification systemincludes the following three major classes of waste: exempt waste,low- and intermediate-level waste, and high-level waste. These wasteclasses and the associated disposal options are summarized inTable 4.2 and described as follows.

Exempt waste would be defined as waste that contains such lowconcentrations of radionuclides that it could be exempted from regu-latory control as radioactive material because the radiological haz-ards associated with disposal of the waste would be negligible. Thebasis for defining exempt radioactive waste recommended by IAEAis a limit on annual dose to individuals from waste disposal of 10 �Sv(see Section 4.1.3.2).


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In recommending a single waste class called low- and intermediate-level waste, IAEA recognized that the previous distinction betweenlow-level and intermediate-level waste is of secondary importancein developing a waste classification system that is closely linked tosafety aspects of waste disposal. In the past, low-level waste oftenwas defined as radioactive waste that does not require shieldingduring normal handling and transportation, whereas radioactivewaste that required shielding but needed little or no provision forheat dissipation was classified as intermediate-level waste. An exter-nal dose rate at the surface of the waste of 2 mSv h�1 often wasused to distinguish between the two classes. For purposes of wastedisposal, however, classification should be related to amounts ofindividual radionuclides taking into account the exposure routes(e.g., ingestion) of greatest importance in post-closure scenarios forreleases from a disposal facility.

In IAEA’s new recommendations, low- and intermediate-levelwaste thus contains concentrations of radionuclides above those forexempt waste but still sufficiently low that heat dissipation is nota concern in ensuring safe disposal. IAEA recommends that thethermal power density for this class of waste be restricted to about2 kW m�3. This class would cover a wide range of radionuclide concen-trations, and a variety of disposal methods may be appropriatedepending on the radiological properties of the waste.

IAEA continues to recommend that low- and intermediate-levelwaste be further classified as short-lived or long-lived. Short-livedwaste could contain high concentrations of shorter-lived radionu-clides, with half-lives less than about 30 y, subject to the restrictionon thermal power density of about 2 kW m�3. However, concentra-tions of long-lived, alpha-emitting radionuclides in short-lived wasteshould be limited to 4 kBq g�1 in individual waste containers andto an average of 0.4 kBq g�1 in all containers in a disposal facility.Short-lived low- and intermediate-level waste thus would containmainly short-lived radionuclides that decay appreciably during theperiod of institutional control over the disposal facility. Short-livedwaste often should be acceptable for disposal in a near-surface facil-ity; but disposal in a geologic repository could be considered basedon the results of safety analyses or if co-disposal of short-lived andlong-lived wastes is anticipated. Long-lived waste would containconcentrations of long-lived radionuclides greater than the restric-tions on short-lived waste; such waste normally would require dis-posal in a geologic repository.

Finally, high-level waste would include any waste with (1) a ther-mal power density greater than about 2 kW m�3, due mainly to highconcentrations of short-lived radionuclides, and (2) concentrations


of long-lived radionuclides greater than the restrictions on short-lived low- and intermediate-level waste described above. High-levelwaste would require disposal in a geologic repository. The first partof this description takes into account that provisions for heat dissipa-tion would be needed in the design of waste disposal facilities forhigh-level waste, and the second part takes into account the signifi-cant long-term radiological hazard.

IAEA also has given some consideration to waste that containslong-lived, naturally occurring radionuclides (uranium, thorium, orradium) that may be generated by mining and milling of ores ordecommissioning of nuclear facilities. Although these wastes containlong-lived radionuclides, and decommissioning waste may containman-made radionuclides as well, they are not expected to requiredisposal in a geologic repository. In some cases, the radionuclideconcentrations may be sufficiently low that the waste can beexempted; in other cases, disposal options similar to those for short-lived low- and intermediate-level waste may be considered, depend-ing on the results of safety assessments. However, waste thatcontains long-lived, naturally occurring radionuclides is not consid-ered to be part of the basic waste classification system consisting ofexempt waste, low- and intermediate-level waste, and high-levelwaste.

In its recommendations, IAEA emphasizes that waste classifica-tion, even if it focuses on waste disposal, does not provide an adequatesubstitute for site-specific safety assessments of particular disposalsystems to ensure the acceptability of waste disposal. IAEA alsorecognizes the role of national authorities in implementing wasteclassification systems and ensuring the safety of waste disposal, andthat different countries may choose to classify waste in differentways depending on their particular situations. However, IAEAbelieves that, if for no other reason than to facilitate communication,it would be desirable to achieve some level of uniformity of wasteclassification systems in different countries. IAEA recommends thatit is particularly important to obtain an international consensus onthe boundary for determining unconditionally exempt material thatmay be transferred from one country to another, especially for pur-poses of recycle/reuse.

4.1.3.2 Recommendations on Exemption Principles. IAEA hasdeveloped recommendations on general principles for exemption ofradioactive material from regulatory control (IAEA, 1988). Any prac-tice or source could be exempted from regulatory control if (1) theannual dose to individuals would be less than 10 �Sv and (2) theannual collective dose from an unregulated practice would be less


than 1 person-Sv. The negligible dose to individuals is the same asthe value recommended by NCRP (1993a) and the value for practicesaffecting a large number of individuals contained in NRC’s proposedgeneric policy on BRC (NRC, 1990), and the negligible dose to exposedpopulations is an order of magnitude less than the value containedin NRC’s generic policy (see Sections 4.1.2.5.2 and 4.1.2.5.3).

Based on the negligible annual dose to individuals of 10 �Sv andassumed scenarios for unrestricted disposal of waste, IAEA hasdeveloped recommendations on exemption levels for radionuclidesin solid waste (IAEA, 1995); the recommended exempt concentra-tions have values in the range of about 0.1 to 104 Bq g�1 dependingon the radionuclide. IAEA also has issued recommendations on totalactivities and activity concentrations of radionuclides that could beexempted from any requirements for notification, registration, orlicensing of sources or practices, based on the same exemption princi-ples and assumed scenarios for exposure of the public (IAEA, 1996).The recommended exemption levels for naturally occurring radionu-clides are limited to their incorporation in consumer products, useas a radioactive source, or use for their elemental properties.

4.1.4 Comparison of the United States and IAEA RadioactiveWaste Classification Systems

The present classification system for radioactive waste in theUnited States summarized in Table 4.1 (see Section 4.1.2.1) andIAEA’s recommended classification system summarized in Table 4.2have certain similarities, but they also have important differences.These are briefly summarized below.

The radioactive waste classification systems developed in theUnited States and by IAEA are similar in some of their practicalimplications. In the United States system, different classes of wastehave the following characteristics: most (but not all) low-level wastecontains relatively low concentrations of radionuclides; transuranicwaste contains relatively high concentrations of long-lived, alpha-emitting transuranium radionuclides and usually (but not always)contains relatively low concentrations beta/gamma-emitting radio-nuclides; and, high-level waste contains relatively high concentra-tions of beta/gamma-emitting fission products and long-lived, alpha-emitting radionuclides. Thus, most waste classified in the UnitedStates as low-level or transuranic waste would be similar to wasteclassified by IAEA as low- and intermediate-level waste, and wasteclassified as high-level waste in the United States would be similarto waste classified as high-level waste by IAEA. Furthermore, the


intended disposal systems for most of these wastes are the same inthe two classification systems.

However, the two radioactive waste classification systems differin several important respects. First, in the United States, NARMwaste is not included in the classification system for waste thatarises from operations of the nuclear fuel cycle. In IAEA’s classifica-tion system, fuel-cycle and NARM wastes are included in the sameclassification system.

Second, high-level waste as defined in the United States includesonly the primary waste from reprocessing of spent nuclear fuel. InIAEA’s classification system, any waste from sources other than fuelreprocessing with similar radiological properties would be includedin high-level waste.

Third, low-level waste as defined in the United States can containvery high concentrations of shorter-lived beta/gamma-emittingradionuclides, resulting in high levels of thermal power density(decay heat) and external dose rate. In IAEA’s classification system,concentrations of shorter-lived radionuclides in low-level wastewould be limited by imposing a limit on thermal power density. Low-level waste as defined in the United States also can contain highconcentrations of long-lived, beta/gamma-emitting radionuclides(e.g., 14C, 94Nb, and 99Tc) such that the risk from waste disposalwould be comparable to or greater than the risk corresponding tothe concentration limits for long-lived, alpha-emitting radionuclidesin low- and intermediate-level waste as defined by IAEA. Waste thatcontains high concentrations of long-lived, beta/gamma-emittingradionuclides is not accounted for explicitly in IAEA’s classificationsystem. Similarly, low-level waste as defined in the United Statescan contain high concentrations of long-lived, alpha-emitting non-transuranium radionuclides (e.g., 233U), but the concentrations ofthese radionuclides are limited in low- and intermediate-level wasteas defined by IAEA.

Fourth, the definitions of waste classes in the United States are notrelated to requirements for disposal. In IAEA’s waste classificationsystem, there is some linkage between the definitions of waste classesand the types of disposal technologies that would be required, partic-ularly for high-level waste. However, not all waste classes in IAEA’ssystem are linked to required disposal technologies, because low-and intermediate-level waste could be acceptable for near-surfacedisposal or could require disposal in a geologic repository depending,for example, on the concentrations of long-lived radionuclides.

Finally, recommendations on principles that could be used toexempt radioactive waste from regulatory control as radioactivematerial have been developed both in the United States (NCRP,

4.2 CLASSIFICATION AND DISPOSAL OF CHEMICAL WASTE / 211

1993a) and by IAEA (1988), and IAEA has implemented its recom-mendations by developing concentration limits of radionuclides inexempt waste (IAEA, 1995). In the United States, however, NRC isforbidden by law from implementing generally applicable exemptionprinciples, and radioactive waste presently can be exempted only ona case-by-case basis.

4.2 Classification and Disposal of HazardousChemical Waste

This Section discusses the current system for classification anddisposal of hazardous chemical waste in the United States. Mosthazardous chemical waste is managed under RCRA (1976) and itsimplementing regulations established by EPA, and the system forclassification and disposal of hazardous chemical waste establishedunder RCRA is discussed in this Section. However, some hazardouschemical wastes are regulated under other environmental laws.Examples include wastes that contain dioxins, PCBs, or asbestos,which are regulated by EPA under TSCA (1976), and sewage sludge,which is regulated by EPA under the Clean Water Act (CWA, 1972).

4.2.1 Classification System for Hazardous Chemical WasteUnder the Resource Conservation and Recovery Act

This Section discusses the definitions of hazardous chemical wastedeveloped under RCRA (1976) and its implementing regulations.RCRA was not preceded by substantial federal involvement in themanagement of hazardous chemical waste (EPA, 1978). Thus, formalsystems for defining and classifying such waste had not been developedpreviously. RCRA and its implementing regulations were establishedbased, in part, on a Congressional finding that disposal of hazardouschemical waste in or on the land without careful planning and manage-ment had endangered human health and the environment.

The classification system for hazardous chemical waste was devel-oped independently of the radioactive waste classification systemdiscussed in Section 4.1.2. In contrast to the classification system forradioactive waste, the classification system for hazardous chemicalwaste was not developed in recognition of the unavoidable risksfrom exposure to naturally occurring hazardous chemicals in theenvironment (see Section 6.3.1.2.1).


4.2.1.1 Description of EPA’s Hazardous Waste Classification Sys-tem. RCRA (1976) and its implementing regulations (EPA, 1980b)have defined ‘‘solid waste’’ and ‘‘hazardous waste.’’ As defined inSection 1004(27) of RCRA, a solid waste is any garbage, refuse, orsludge from a waste treatment plant, water supply treatment plant,or air pollution control facility and other discarded material, includ-ing solid, liquid, semisolid, or contained gaseous material resultingfrom industrial, commercial, mining, and agricultural operationsand from community activities. Solid waste does not include solidor dissolved material in domestic sewage, or solid or dissolved mate-rial in irrigation return flows or industrial discharges that are pointsources subject to permits under the Clean Water Act (CWA, 1972).Also specifically excluded from solid waste are source, specialnuclear, and byproduct materials as defined in AEA (1954). A moredetailed definition of ‘‘solid waste’’ and description of materialsexcluded from solid waste are given in the implementing regulations(EPA, 1980b). The exclusion of radioactive materials defined in AEAfrom regulation under RCRA leads to the concept of ‘‘mixed waste’’that contains radioactive and hazardous chemical waste. Manage-ment and disposal of mixed waste is discussed in Section 4.3.

Given the definition of ‘‘solid waste’’ described above, Section 1004(5)of RCRA (1976) then defines ‘‘hazardous waste’’ as follows:

Hazardous waste is a solid waste, or combination of solid wastes,which because of its quantity, concentration, or physical, chemi-cal or infectious character may:

Clause (A): cause, or significantly contribute to, an increase inmortality or an increase in serious irreversible, orincapacitating reversible, illness; or

Clause (B): pose a substantial present or potential hazard tohuman health or the environment when improperlytreated, stored, transported, or disposed of, or other-wise managed.

This definition is further amplified by Section 3001(a) of RCRA(1976), which specifies that EPA shall develop and promulgate crite-ria for identifying the characteristics of hazardous waste and forlisting hazardous waste, taking into account its toxicity, persistence,and degradability in nature, potential for accumulation in tissue,and other related factors such as flammability, corrosiveness, andother hazardous characteristics.

Based on the legal definition of ‘‘hazardous waste’’ given above,EPA developed a definition of hazardous chemical waste in 40 CFRPart 261 (EPA, 1980b). Hazardous chemical waste is defined as a


solid waste that meets at least one of several criteria: it exhibits thecharacteristic of ignitability, corrosivity, reactivity, or toxicity; or itis a specifically listed waste. Biohazardous waste is not includedbecause such waste normally is rendered nonhazardous before dis-posal according to EPA guidelines (EPA, 1986a). The various typesof hazardous chemical waste defined by EPA under RCRA aredescribed below.

An ignitable waste is a solid waste that meets one of the followingcriteria: (1) it is a liquid, other than an aqueous solution containingless than 24 percent alcohol, that has a flash point less than 60 °C;(2) it is not a liquid and is capable, under standard temperature andpressure, of causing fire through friction, absorption of moisture orspontaneous chemical changes and, when ignited, burns so vigor-ously and persistently that it creates a hazard; (3) it is an ignitablecompressed gas; or (4) it is a specified oxidizing agent.

A corrosive waste is a solid waste that meets one of the followingcriteria: (1) it is an aqueous waste with a pH of less than or equalto two or greater than or equal to 12.5; or (2) it corrodes SAE 1020steel at a rate greater than 6.35 mm y�1 at 55 °C.

A reactive waste is a solid waste that meets one of the followingcriteria: (1) it is normally unstable and readily undergoes violentchange without detonating; (2) it reacts violently with water; (3) itforms potentially explosive mixtures with water; (4) it generatestoxic gases, vapors or fumes in a quantity sufficient to pose a dangerto human health or the environment when mixed with water; (5) itis a cyanide or sulfide that releases toxic gases, vapors or fumeswhen in contact with materials at a pH between 2 and 12.5; (6) itis capable of detonation or explosive reaction when subjected to astrong initiating source or heated under confinement; (7) it is readilycapable of detonation or explosive decomposition at standard temper-ature and pressure; or (8) it is a forbidden, Class-A, or Class-B explo-sive as defined by the U.S. Department of Transportation.

A toxic waste is a solid waste which, when tested using the toxicitycharacteristic leaching procedure (EPA, 1980b, Appendix II), yieldsan extract containing any of 40 contaminants (heavy metals andorganic compounds) at or above specified concentrations (EPA,1980b, Table 1). The contaminants considered by EPA in definingthe toxicity characteristic were mainly (but not exclusively) thoseincluded in drinking water standards (EPA, 1975) developed underthe Safe Drinking Water Act (SDWA, 1974). Thus, toxic waste, asdefined under RCRA, is waste which, when placed in a landfill,could result in contamination of groundwater at a nearby well abovedrinking water standards, based on modeling of transport of leachedcontaminants to the well. However, the toxicity characteristic does


not take into account the presence of many hazardous chemicals inwaste materials that could be leached into groundwater in significantamounts. EPA originally intended that the toxicity characteristicleaching procedure would be used to identify hazardous chemicalwaste that contains substances other than those included in drinkingwater standards, but EPA was unable to do so because concentra-tions of concern in drinking water had not been established for manyother substances (EPA, 1980b).

In addition to chemical waste that may be classified as hazardousbased on one or more of the characteristics described above, a chemi-cal waste may be classified as hazardous if it is specifically listed(EPA, 1980b). Chemical wastes are listed based on their source or thepresence of specific hazardous substances. Listed hazardous wastesinclude wastes from nonspecific sources (the so-called ‘‘F’’ list),wastes from specific sources (‘‘K’’ list), acutely toxic hazardous wastefrom any source (‘‘P’’ list), and toxic (other than acute) waste fromany source (‘‘U’’ list).

When a hazardous chemical waste is mixed with a nonhazardouswaste, the entire waste is considered hazardous if the initial hazard-ous waste is a listed waste or if the final waste exhibits a hazardouscharacteristic (the so-called ‘‘mixture rule’’) (EPA, 1980b; 1992c;2001b). Mixing of a hazardous chemical waste with a nonhazardouswaste can result in a waste that is nonhazardous only if the initialhazardous waste is not a listed waste and mixing eliminates anyhazardous characteristics in the initial hazardous waste. Any wastederived from processing of a listed waste also is a listed waste,without regard for the amounts of listed substances, until it isdelisted, which allows the waste to be managed as a nonhazardoussolid waste (the so-called ‘‘derived-from rule’’) (EPA, 1992c; 1993b;2001b).

Based on the descriptions of hazardous chemical waste givenabove, a waste is either hazardous or it is not, and there is no furtherclassification of hazardous waste with respect to degree of hazard.Some states have defined a category of extremely hazardous waste(see Section 4.2.1.3), and extremely hazardous substances are speci-fied by EPA (1987b) under the Emergency Response and CommunityRight-to-Know Act, which is a free-standing title of CERCLA (1980).However, these designations have not affected how hazardous wasteis classified, managed, and disposed of under RCRA.

4.2.1.2 Discussion of EPA’s Hazardous Waste Classification Sys-tem. Waste that is hazardous because it is ignitable, corrosive, orreactive, as defined above, must be treated to remove these character-istics prior to disposal. Examples of appropriate treatment methods


include neutralization of acidic or basic corrosive waste, chemicalreaction of a reactive waste to render it nonreactive, or incinerationof an ignitable waste. A waste that is hazardous only because it isignitable, corrosive, or reactive is no longer considered hazardousafter treatment to remove these characteristics. However, ignitable,corrosive, or reactive waste may still be considered hazardous aftertreatment to remove these characteristics if, prior to and followingtreatment, the waste exhibits the toxicity characteristic or it containsa listed substance.

Waste that is hazardous because it exhibits the toxicity character-istic also must be treated to remove this characteristic prior to dis-posal. Techniques to remove the toxicity characteristic include, forexample, destruction of organic compounds by incineration or incor-poration of the waste in an immobilizing waste form (e.g., grout).However, in contrast to ignitable, corrosive, or reactive waste, aproperly treated toxic waste may still be considered hazardous insome cases, even if it is not characteristically hazardous after treat-ment and does not contain any listed substances. For example, awaste that is toxic because it contains high levels of heavy metalscould be treated to reduce the leachability of the metals to acceptablelevels by incorporation in an appropriate waste form, but the treatedwaste may still be considered hazardous when the toxic substancesof concern are not destroyed by treatment and the possibility existsthat their leachability from the waste form could increase substan-tially after disposal.

As noted previously, a listed hazardous waste cannot be renderednonhazardous by treatment or by dilution or mixing with nonhazardousmaterial. EPA has issued proposals related to establishing exemptionlevels for listed hazardous waste that contains small amounts of listedsubstances (EPA, 1992d; 1995c; 1999c), but exemption provisions forlisted hazardous waste have not yet been established.

At the present time, specific hazardous chemical wastes can beexempted from RCRA requirements in one of two ways. First, awaste generator may petition EPA to ‘‘delist’’ a listed hazardouswaste, and the waste would be exempted from RCRA requirementsif the petition were approved (EPA, 1993b). Thus, listed wastes aredelisted on a case-by-case basis. Second, EPA has exempted certainmaterials from requirements for regulation as hazardous wasteunder RCRA and other laws, including the Clean Water Act (CWA,1972) and Clean Air Act (CAA, 1963), to permit their beneficialuse. Examples of such materials include ash and sludge from coal-burning power plants used in construction materials and in cementand concrete products (EPA, 2000b), sewage sludge used as fertilizer(EPA, 1999d), and phosphogypsum used in outdoor agriculture (EPA,


1992b). Coal ash and sewage sludge contain elevated levels of heavymetals and naturally occurring radionuclides, and phosphogypsumcontains elevated levels of radium that result in elevated levels ofradon in air.

Of the various ways of designating a solid waste as hazardousdescribed above, only the toxicity characteristic is based on a quanti-tative assessment of potential risks resulting from waste disposal.The specifications of ignitable, corrosive, and reactive waste arebased on qualitative considerations of risk, in that the presence ofmaterials with these characteristics in a disposal facility clearlyconstitutes a hazard that could compromise the ability of the facilityto protect public health. The specifications of listed hazardous wastesare based on risk in the sense that the listed substances have beenidentified as potentially hazardous to human health. However,requirements for treatment and disposal of listed waste discussedin Section 4.2.2 do not distinguish between different wastes basedon considerations of risk from disposal.

Although EPA’s hazardous waste classification system developedunder RCRA applies to a great many chemical wastes, the systemis not comprehensive because it does not apply to all potentiallyhazardous wastes. Some wastes are not regulated under RCRAbecause they are regulated under other environmental laws, includ-ing TSCA (1976) and the Clean Water Act (CWA, 1972). In addition,many potentially important wastes containing hazardous chemicalsthat are not regulated under other environmental laws are specifi-cally excluded from the definition of hazardous waste in 40 CFRPart 261 (EPA, 1980b) and, thus, may not be regulated under RCRA.Examples include: certain wastes from combustion of coal or otherfossil fuels; drilling fluids and other wastes associated with the explo-ration, development, or production of crude oil, natural gas, orgeothermal energy; and a variety of wastes from the extraction,beneficiation, and processing of ores and minerals. These wastes cancontain concentrations of heavy metals well above average back-ground levels in soil and rock, as well as hazardous substancesintroduced in processing of materials. The exclusions from hazardouswaste specified in 40 CFR Part 261 (EPA, 1980b) generally are basedon the source of the waste rather than the risk associated withmanagement or disposal.

4.2.1.3 State Programs. Many states have been authorized toadminister their own hazardous waste management programs. Eachof these states uses the definition of hazardous waste described above(Lathrop, 1992a; 1992b). Details of selected state programs are givenin a report of the Office of Technology Assessment (OTA, 1981).


The states of Washington and California have considered a classi-fication of hazardous chemical waste based on risk and have devel-oped a category of extremely hazardous waste (California, 1999;Mehlhaff et al., 1979; NAS/NRC, 1999b). However, the requirementsfor treatment and disposal of extremely hazardous waste differ littlefrom those applied to other hazardous waste. Thus, the designationof a class of extremely hazardous waste based on relative hazardhas had little effect on waste management and disposal.

4.2.2 Treatment and Disposition of Hazardous Chemical Waste

There generally are three available dispositions for hazardouschemical waste: incineration, deep-well injection, and disposal in anear-surface facility. Incineration involves high-temperature burn-ing to destroy the hazardous constituents of the waste. This leavesonly a small residual ash that is typically nonhazardous, unless itcontains high levels of heavy metals; in some cases, incinerator ashmay be rendered nonhazardous by use of a waste form (e.g., grout)or by delisting of the waste, depending on the nature of the hazard.Incineration is most commonly used to destroy liquid hazardouswaste, although certain kinds of solid hazardous waste also may beincinerated. Deep-well injection involves drilling a borehole into apermeable geologic formation below all potential sources of ground-water of usable quality and quantity. Waste then is pumped intothe formation and the borehole is sealed. This method is most com-monly used for large quantities of contaminated wastewater but notfor solidified hazardous waste. Incineration and deep-well injectionare not considered further in this Report, because they are intendedfor the destruction or disposal of a narrow range of hazardous chemi-cal wastes.

All waste classified as hazardous under RCRA, including wastethat has been treated to remove the toxicity characteristic butis still considered hazardous, is managed under Subtitle C ofRCRA and implementing regulations established by EPA in 40 CFRParts 260–268 (EPA, 1986b). Emplacement in a near-surface burialsite (hazardous waste disposal facility) is the usual disposition ofsolidified hazardous chemical waste. Burial sites must meet strin-gent location requirements with respect to seismic considerations,floodplains, salt domes and beds, underground mines and caves, andpotential sources of drinking water. Each disposal facility must beconstructed with an appropriate liner system, leachate collectionand removal system, and leak detection system. The requirementson the design, construction, and operation of a disposal facility reflect


the importance of preventing contamination of groundwater. A burialsite is to be actively maintained for 30 y after closure, at which timethe operator can cease active maintenance and rely on passive safetyfeatures if EPA so approves. Institutional control generally must bemaintained over disposal sites for as long as the waste remainshazardous. However, the basis for judging that waste at a disposalsite is no longer hazardous has not been established, and no RCRAburial sites have gone through this process.

Certain restrictions are imposed on hazardous wastes that maybe placed in a disposal facility or in the same disposal cell at a site.The applicable regulations in 40 CFR Part 268 (EPA, 1986b) containan extensive catalog of hazardous chemical wastes for which disposalis prohibited, called land disposal restrictions (LDRs). LDRs requiretreatment of these wastes before disposal. The required treatmentmay be incineration, stabilization, extraction of metals, or otherappropriate technologies, depending on the nature of the waste.Ignitable, corrosive, or reactive waste generally requires treatmentto remove the hazardous characteristic prior to disposal. Incompati-ble wastes, or incompatible wastes and materials, cannot be placedin the same cell at a disposal site. Certain listed wastes (F020, F021,F022, F023, F026, and F027), which may contain 2,3,7,8-tetrachloro-dibenzodioxin, may not be buried unless EPA has approved a disposalplan (EPA, 1986b). From a practical viewpoint, an operator of ahazardous chemical waste disposal site may segregate the wastereceived in any way seen fit to best utilize the facility, as long asthe conditions of the operating permit are not violated.

The requirements on treatment and disposal of hazardous chemi-cal waste established by EPA under RCRA, especially LDRs specifiedin 40 CFR Part 268 (EPA, 1986b) and the intention to limit contami-nation of groundwater, are based on a desire to limit risks to publichealth and the environment. However, these requirements are notbased on long-term projections of health risks to the public beyondthe site boundary, nor is any consideration given to potential risksto individuals who might inadvertently intrude onto the disposalsite after institutional control ceases. Rather, in addition to thedetailed technical requirements on waste treatment and disposalthat apply at any site, the approach to protection of public healthand the environment under RCRA relies on monitoring of releasesof hazardous substances, especially releases into groundwater, cor-rective actions if releases exceed specified standards, and an inten-tion to maintain institutional control over disposal sites for as longas the waste remains hazardous.

Based on the foregoing discussions, management of hazardouschemical waste is essentially not risk-based for the following reasons:

4.3 REGULATION OF MIXED WASTE / 219

● The same management system is applied to all hazardous chemi-cal waste, regardless of the degree of hazard or potential risk.

● Essentially all solidified hazardous chemical waste is intendedfor disposal in near-surface facilities, with prescribed actionsto prevent unacceptable releases of hazardous material (e.g.,leachate collection and treatment). However, these facilitieshave been developed and operated essentially without consider-ation of the potential long-term risks posed by the waste in theabsence of active monitoring and maintenance, including potentialrisks to future inadvertent intruders, or the requirements on siteclosure and release from institutional control that would ensurelong-term protection of public health and the environment.

● Exclusion or exemption of waste that contains hazardous sub-stances from the management system for hazardous chemicalwaste has been based primarily on the source of the waste ratherthan the risk that it poses.

Section 3019 of RCRA (1976) calls for risk-based analyses to pro-vide justification for closing hazardous waste disposal sites, andEPA’s implementing regulations in 40 CFR Part 268 (EPA, 1986b)incorporate risk-based groundwater protection standards. However,these types of risk analyses and groundwater protection standardsgenerally have been applied at hazardous waste disposal sites onlyon a real-time or near-term basis. They have not been applied pro-spectively over long time periods in the future.

RCRA (1976) also addresses nonhazardous waste, and disposal ofnonhazardous waste in sanitary (municipal/industrial) landfills isgoverned under Subtitle D. This type of waste includes householdtrash, various industrial wastes, and characteristically hazardouswaste that has been treated and is no longer considered hazardous.In current EPA regulations implementing Subtitle D in 40 CFR Part 258(EPA, 1991b), requirements on siting, design, operation, and closureof landfills are similar to the requirements that apply to hazardouswaste disposal facilities regulated under Subtitle C of RCRA. There-fore, management and disposal of hazardous and nonhazardouswastes differ mainly in regard to requirements on waste generatorsand the storage and treatment of waste, as well as requirements oninstitutional control after closure of a disposal facility.

4.3 Regulation of Mixed Radioactive and HazardousChemical Waste

Mixed waste is waste that contains both radionuclides and hazard-ous chemicals; it is not a separate class of waste. This Section dis-cusses mixed waste issues as they relate to waste classification and


waste management and disposal. Particular emphasis is given tothe historical development of current approaches to regulating mixedwaste. Rather than waste classification per se, the issues discussedin this Section are largely concerned with difficulties that have beenencountered in the management of mixed waste.

4.3.1 Introduction

In 1992, Congress enacted the Federal Facility Compliance Act(FFCA, 1992), which defined mixed waste as waste that containsboth hazardous chemicals regulated under RCRA (1976) and source,special nuclear, or byproduct material regulated under AEA (1954).Although the primary purpose of the Federal Facility ComplianceAct was to waive sovereign immunity under RCRA for federal facili-ties, the Act contains special provisions for mixed waste that addressa range of legal and institutional issues associated with mixed wastegenerated by the federal government. Also, states may have defini-tions of hazardous chemical waste, and thus mixed waste, that varyfrom the federal definitions. For example, several states currentlyinclude PCBs in their RCRA statutes concerning hazardous chemicalwastes, whereas PCBs are hazardous under federal regulations onlyas a result of provisions of TSCA (1976). Federal facilities that generatemixed waste in these states need to accommodate such differences.

Most mixed waste in the United States is the responsibility of DOE,which has about 525,000 m3 currently in storage and is expected togenerate approximately 30,000 m3 in the next 35 y (DOE, 1997a).DOE’s mixed waste is about 71 percent high-level waste, 14 percentlow-level waste, and 15 percent transuranic waste, and it consists ofaqueous liquids, inorganic sludges and particulates, assorted debris,and a variety of other forms (Bloom and Berry, 1994). The volumesof mixed high-level waste and transuranic waste have remainedfairly constant over time, but there has been a sharp decline inthe amount of mixed low-level waste. Non-DOE (commercial) mixedwaste is generated at a rate of about 4,000 m3 y�1; since commercialmixed waste is more amenable to treatment using existing capabili-ties, only about 2,100 m3 is currently in storage.

Because mixed waste contains radionuclides and hazardous chem-icals, it is subject to dual regulation under AEA and RCRA. Mixedwaste generated in commercial activities is regulated under EPA orstate RCRA requirements and NRC or comparable Agreement Staterequirements under AEA. Mixed waste generated by DOE is regu-lated by DOE under AEA and by EPA or a state under RCRA. Thisdual regulatory framework has created difficulties for mixed waste


managers in attempting to understand and comply with require-ments under the two laws. In addition, uncertainties in the volumeand characteristics of mixed low-level waste have been a significantbarrier to development of treatment and disposal facilities formixed waste.

4.3.2 Establishing Dual Regulation of Mixed Waste

Prior to the mid-1980s, most mixed waste was managed and dis-posed of as radioactive waste. Although the issue of mixed wastesurfaced during development of NRC’s licensing requirements forland disposal of low-level waste in 10 CFR Part 61 (NRC, 1982a),until 1985 mixed waste was regulated principally for its radiologicalcharacteristics. Many mixed waste managers believed that the exclu-sion of source, special nuclear, and byproduct material from thedefinition of solid waste at Section 1004(27) of RCRA (1976) excludedmixed waste from the requirements of RCRA. In addition, manybelieved that there were inconsistencies in the requirements of AEAand RCRA and that Section 1006(a) of RCRA12 would result in mixedwaste not being subject to RCRA. Finally, it was commonly believedthat, in many cases, mixed waste could be managed according to thepredominant hazard associated with the waste, which usually wasdetermined to be the radioactive material. During the mid-1980s,several significant actions changed the way in which mixed wastewas managed.

In 1984, the U.S. District Court for the Eastern District of Tennes-see established that hazardous chemical waste generated at DOE’sY-12 Plant in Oak Ridge, Tennessee, was subject to the requirementsof RCRA (LEAF, 1984). The plaintiffs in this case, the Legal Environ-mental Assistance Foundation (LEAF), charged that DOE was inviolation of RCRA and the Clean Water Act (CWA, 1972). Inresponse, DOE argued that application of RCRA to the Y-12 Plantwas inconsistent with AEA because (1) AEA precluded state regula-tion of DOE activities but RCRA subjected federal facilities to stateregulation, (2) AEA gave the authority to set waste disposal stan-dards to DOE while RCRA gave this authority to EPA and to stateand local authorities, and (3) AEA restricted the dissemination of

12 Section 1006(a) of RCRA (1976) states that ‘‘Nothing in this Act shall be construedto apply to (or authorize any State, interstate, or local authority to regulate) anyactivity or substance which is subject to . . . the Atomic Energy Act of 1954 (42 U.S.C.2011 and following) except to the extent that such application (or regulation) is notinconsistent with the requirements of such Acts.’’


data pertaining to nuclear weapons while RCRA would require thepublic disclosure of this information.

The Court found in favor of LEAF by concluding that applicationof RCRA to the Y-12 Plant was not inconsistent with AEA. The Courtalso concluded that the most reasonable reconciliation of the twostatutes was that facilities regulated under AEA are subject toRCRA, except RCRA does not apply to those materials that areexpressly regulated under AEA (i.e., source, special nuclear, andbyproduct materials).

A second action occurred as an outcome of legislative debates on theLow-Level Radioactive Waste Policy Amendments Act (LLRWPAA,1986). During these debates, various proposals were made to assignjurisdiction for regulation of mixed low-level waste to a single agency,in order to provide relief to mixed waste generators and ease theburdens associated with dual regulation of mixed waste. There alsowas a concern that delays in issuing permits for mixed waste treat-ment and disposal facilities would undermine state efforts to complywith schedules for developing new disposal capacity for low-levelradioactive waste. Because of the complexity of the issue and thedesire to avoid creating a special class of hazardous chemical wastethat would not be regulated under RCRA, Congress encouraged NRCand EPA to resolve mixed waste issues administratively by usingthe flexibility in the regulatory programs under both statutes. Thus,consideration of sole agency jurisdiction over mixed waste was sus-pended. This ensured that commercial mixed waste would remainsubject to dual regulation by NRC and EPA.

In 1985, DOE published a notice of proposed rulemaking (theByproduct Material Rule) that would have established an interpre-tive rule clarifying RCRA’s applicability to DOE’s radioactive waste(DOE, 1985b). This rule would have established a distinctionbetween ‘‘direct process’’ waste, which is waste directly yielded inor necessary to the process of producing and utilizing special nuclearmaterial, and other radioactive waste. Direct process waste, evenwhen it contained hazardous chemical waste, would have been regu-lated solely as byproduct material. Any non-direct process wastethat contained hazardous chemical waste would have been managedas mixed waste. Based on additional operational experience andcomments on the proposed rule, DOE chose to abandon an attemptto distinguish direct process waste from other radioactive waste andadopted a narrower interpretation of the definition of byproductmaterial, which limited the authority of AEA to the radionuclidesalone (DOE, 1987b).

In 1986, EPA published a notice that a state must have the author-ity to regulate mixed waste in order for that state to obtain and


maintain authorization to administer and enforce a hazardous chem-ical waste program pursuant to Subtitle C of RCRA (EPA, 1986c).As part of this notice, EPA discussed the applicability of RCRA tomixed waste, concluding that waste containing hazardous chemicalsand radionuclides is subject to RCRA regulations. Prior to this notice,EPA had not established that mixed waste also is subject to therequirements of RCRA, and states with RCRA authorization werenot required to obtain the authority to regulate the hazardous chemi-cal portion of mixed waste under their RCRA programs. EPA alsoconcluded that states that had previously obtained RCRA authoriza-tion did not have authorization to regulate mixed waste until theyobtained specific authorization to do so. This notice essentially recog-nized that mixed waste contains both a radioactive component sub-ject to AEA and a hazardous chemical component subject to RCRA,and it specified that states would need to recognize this dual regula-tory framework in order to maintain their RCRA programs.

DOE acknowledged the dual regulatory framework for mixedwaste in 1987 with a notice clarifying the definition of byproductmaterial (DOE, 1987b). In this notice, DOE issued a final interpretiverule establishing that the exclusion of byproduct material atSection 1004(27) of RCRA applied only to the radionuclides in mixedwaste and that the nonradioactive portion of the waste was subjectto RCRA. In addition, in 1987, DOE recognized that RCRA LDRs (seeSection 4.2.2) and other RCRA requirements applied to transuranicwaste intended for disposal at the Waste Isolation Pilot Plant (seeSection 4.1.2.3.2).

Thus, by the late 1980s, it was firmly established that mixed wastewas subject to dual regulation under AEA and RCRA, and thatgenerators of mixed waste would be required to comply with regula-tions for the control of radioactive material and hazardous chemicalwaste under both statutes. Because compliance with two sets ofregulations would be difficult, many generators believed that relieffrom dual regulation might be obtained by identifying inconsisten-cies in requirements of AEA and RCRA. Throughout the late 1980s,however, NRC and EPA repeatedly concluded that no inconsistenciesexisted in the regulatory programs established under the two stat-utes that could not be resolved through the existing flexibility withineach agency’s programs. The agencies also advanced the positionthat if such inconsistencies existed, relief from RCRA requirementswould be limited to those specific requirements that were inconsis-tent with those of AEA. In the face of such regulatory uncertainties,private companies have been largely unwilling to take the financialrisk to design, build, and operate disposal facilities that would acceptall mixed low-level waste. A near-surface disposal facility in Clive,


Utah, which has a radioactive waste disposal license and a RCRApermit from the state, accepts large volumes of mixed waste contain-ing low concentrations of radionuclides that resembles uranium orthorium mill tailings.

4.3.3 Facilitating Compliance with Dual Regulation of MixedLow-Level Waste

In 1981, during the development of licensing requirements fornear-surface disposal of radioactive waste, principally low-levelwaste, in 10 CFR Part 61, NRC stated that the standards for radioac-tive waste were adequate for hazardous chemical waste as well, andthat NRC would work with EPA to ensure that mixed low-level wastewas disposed of in a manner that met both agencies’ requirements(NRC, 1982a). In response to NRC’s request for comments onthe proposed low-level waste disposal requirements, severalcommenters suggested that NRC’s waste classification system (theClass-A, -B, and -C limits and associated disposal requirements; seeSection 4.1.2.3.3) should incorporate a ‘‘total hazard’’ approach thatwould consider both the radiological hazard and the chemical hazardof the waste. NRC stated publicly that if it were technically feasibleto classify waste by total hazard, then it would make ‘‘eminentlygood sense’’ to do so. NRC also stated that although it was not awareof a scheme for such classification, it appeared that DOE intendedto support research into the development of a classification systemfor hazardous chemical waste that might be compatible with10 CFR Part 61 (NRC, 1982a). In the final disposal standards, NRCestablished that waste posing nonradiological hazards be treated,to the maximum extent practicable, to eliminate or minimize suchhazards, although NRC does not regulate these constituents.

After Congress declined to address mixed low-level waste in theLow-Level Radioactive Waste Policy Amendments Act (LLRWPAA,1986), NRC and EPA identified several issues that required resolu-tion through the development of joint guidance documents, includingthe issues of what the two agencies considered to be mixed wasteand what environmental and design criteria should be consideredin developing mixed waste disposal facilities. The agencies’ objectivein developing these guidance documents was to facilitate compliancewith the dual regulatory framework and to remove institutionalbarriers to the development of commercial low-level radioactive wastedisposal facilities in accordance with the schedules in LLRWPAA.In addition, mixed low-level waste generators identified the need forguidance on the testing and storage of mixed waste.


To address these issues, EPA and NRC developed and issued guid-ance documents on:

● the definition and identification of mixed low-level waste (EPA/NRC, 1989)

● siting of mixed low-level waste disposal facilities (EPA/NRC,1987a)

● the design of mixed low-level waste disposal facilities (EPA/NRC, 1987b)

● testing of mixed low-level waste (NRC, 1992)● the elements of a mixed waste minimization program (NRC,

1994a)● storage of mixed waste (NRC, 1995)

By developing these joint guidance documents, NRC and EPAattempted to provide assistance to mixed low-level waste managersfaced with the complicated task of understanding and complyingwith the requirements of the two agencies. While these guidancedocuments were developed for commercial mixed waste, the conceptsalso apply to management of mixed low-level waste generated atDOE facilities.

In 1990, EPA published the first treatment standards for mixedwaste, including LDRs, as part of the ‘‘Third Thirds’’ rule (EPA,1990b).13 The remaining parts of this rule were issued later in40 CFR Part 268 (EPA, 2001b). Under LDRs, untreated hazardouschemical waste may not be disposed of in a land disposal facility,except under limited conditions such as demonstrating that therewill be no migration of the waste from the disposal unit for as longas the waste remains hazardous. LDRs prescribe treatment stan-dards for hazardous chemical waste as either concentrations of spe-cific hazardous substances in the treated waste (or waste extract)or as a specified technology [best demonstrated available technology(BDAT)]. Examples of these mixed wastes and their BDAT treatmentstandards include vitrification of high-level radioactive waste gener-ated during the reprocessing of spent nuclear fuel, macroencapsula-tion of lead solids, amalgamation of radioactively contaminatedelemental mercury, and incineration of hydraulic oil contaminatedwith mercury.

In addition to promulgating the treatment standards for mostmixed waste in 1990, EPA established a 2 y National Capacity Vari-ance for mixed waste for which there was no available treatment or

13 LDRs specified in RCRA required EPA to develop treatment standards for hazard-ous chemical waste and established deadlines for EPA to develop treatment standardsfor those wastes for which treatment standards did not exist. Congress divided LDRhazardous waste into several categories: solvents and dioxins; California listed wastes;first, second, and third listed wastes; and characteristically hazardous wastes.


disposal capacity. Under this variance, which expired in May 1992,mixed waste generators were not required to treat their mixed wastesfor which treatment capacity was not available, provided the wastewas disposed of in a facility that met EPA’s minimum technologystandards. However, in the absence of disposal facilities having RCRApermits, such disposal did not occur during the capacity variance.

In 1991, because of a lack of adequate treatment capacity for mostmixed waste, EPA adopted a policy that assigned a lower enforcementpriority to violations of storage prohibitions under Section 3004(j) ofRCRA (1976), provided certain conditions were met (EPA, 1991c).RCRA requirements prohibit storage of LDR-restricted hazardouswaste unless such storage is solely for the purpose of accumulatingsufficient quantities to allow proper recovery, treatment, or disposal.EPA policy on enforcement was limited to facilities that generated lessthan 28 m3 y�1 of LDR-restricted mixed waste, was limited in duration,and initially expired on December 31, 1993. EPA has extended itsenforcement policy at 2 y intervals (EPA, 1994c; 1996c; 1998b), becausethe limited availability of treatment and disposal capacity had notchanged substantially.

In order for facilities to be afforded the lower enforcement priority,they must demonstrate to EPA that they are managing their mixedwaste in an environmentally sound manner. EPA indicated that itwould consider a variety of indicators of environmentally responsiblemanagement in determining the civil enforcement priority at individ-ual facilities. These indicators include, but are not limited to,whether the facility has:

● conducted an inventory of its mixed waste storage areas to assessand assure compliance with all other applicable RCRA storagefacility standards;

● identified and kept records of its mixed waste, including sources,waste codes, generation rates, and volumes in storage;

● developed a mixed waste minimization plan, or can demonstratethat waste minimization is not technically feasible for itswaste; and

● documented periodically that it has made good faith efforts toascertain the availability of treatment capacity for its mixedwaste.

EPA policy concerning relaxed enforcement of prohibitions on stor-age of mixed waste does not extend to Executive Branch federalfacilities, including DOE facilities. In 1992, Congress amended Sec-tion 6001 of RCRA (1976) through the Federal Facility ComplianceAct (FFCA, 1992) to clarify that federal facilities are subject toadministrative orders and civil and administrative penalties and


fines related to management and disposal of hazardous chemicalwaste. The Federal Facility Compliance Act allows EPA to takeadministrative enforcement action pursuant to the enforcementauthority in RCRA. The Act requires that DOE submit to EPA andthe governor of each state in which DOE stores or generates mixedwaste a report containing a national inventory of mixed waste anda report containing a national inventory of mixed waste treatmentcapacities and technologies. DOE also must develop a plan for estab-lishing treatment capacities and technologies to treat mixed wasteat each of its facilities. This plan must contain a schedule of theactions necessary to accomplish the treatment and disposal of mixedwaste or, for mixed waste for which treatment capacity does notexist, a schedule of actions necessary to initiate research and develop-ment of this technology.

In 1992, NRC and EPA issued the National Profile on commerciallygenerated mixed low-level waste (Klein et al., 1992) in response toa request from the Host State Technical Coordinating Committee(Alvarado, 1990). In its request, the committee stated that the infor-mation was needed by states, compact officials, private developers,and federal agencies to plan and develop treatment and disposalfacilities for commercial mixed low-level waste. The National Profilewas based on a survey of over 1,300 licensed nuclear facilities.

The National Profile indicated that commercial nuclear facilitieslicensed by NRC or an Agreement State generated about 4,000 m3

of mixed low-level waste in 1990. Industrial facilities produced thelargest amount (1,400 m3) and nuclear utilities the least (400 m3).Liquid scintillation fluids comprised the largest portion of commer-cial mixed low-level waste in 1990, about 71 percent. In addition,2,100 m3 of commercial mixed low-level waste was in storage asof the end of 1990. The National Profile indicated that adequatetreatment capacity existed for much of the commercial mixed low-level waste, although additional treatment capacity was required totreat all the waste generated or in storage as of the end of 1990.Wastes that contain chlorinated fluorocarbons, lead, or mercury wereidentified as needing additional treatment capacity. Since 1990,additional treatment capacity using vitrification, steam reforming,and molten metal technology has been developed to address someof this capacity shortfall (Kirner et al., 1995).

Significant quantities of mixed low-level waste have been gener-ated and are in storage at more than 40 DOE sites, including nationallaboratories and naval shipyards. These wastes contain materialslisted as hazardous or having hazardous characteristics under RCRAand wastes that are considered hazardous under TSCA (1976). Atthe end of 1994, DOE sites had in storage 91,000 metric tons of


RCRA-contaminated mixed low-level waste and 15,000 metric tonsof TSCA-contaminated mixed low-level waste. The volumes of thesewastes are about 138,000 m3 and 24,000 m3, respectively. The largestcomponent of these wastes is inorganic solids and contaminatedsoils. DOE’s Integrated Data Base Report (DOE, 1997a) containsmore information on the locations, characteristics, and projectedgeneration rates of DOE’s mixed low-level waste. Future cleanupsat DOE sites may greatly increase the volumes of contaminated soilsthat need to be treated as mixed waste. Treatment capabilities forboth DOE and commercial mixed low-level wastes are described ina report by DOE’s National Low-Level Waste Management Program(DOE, 1996b).

As indicated above, the volumes of commercial mixed low-levelwaste are much less than the volumes at DOE sites. Therefore, inthe interest of cost-effective waste management, consideration wasgiven to the possibility of DOE accepting the commercial waste fortreatment and disposal (Owens et al., 1993). However, given thedifficulties that DOE has experienced in managing its own mixedlow-level waste in accordance with legal and regulatory require-ments under RCRA and the Federal Facility Compliance Act andthe additional complications that would arise in accepting responsi-bility for commercial mixed waste, this possibility has not been pur-sued to any significant extent.

In 1993, the U.S. Court of Appeals for the District of ColumbiaCircuit denied a petition filed by the Edison Electric Institute andother plaintiffs concerning EPA’s prohibition at Section 3004(j) ofRCRA (1976) on indefinite storage of mixed waste for which treat-ment and disposal capacity did not exist (EEI, 1993). EPA’s interpre-tation of the statute was that it was unlawful to store waste forindefinite periods of time pending the development of adequate treat-ment or disposal capacity. The plaintiffs contended that this inter-pretation was inconsistent with the statute and unreasonable as itapplied to generators of mixed waste. The Court denied the petitionand ruled that EPA’s interpretation was permissible and, in fact,mandated by the statute. In its opinion, the Court stated the follow-ing: ‘‘Thus, we conclude that . . . Section 3004(j) clearly proscribesthe indefinite storage of wastes pending the development of treat-ment and disposal capacity. We wish to emphasize that we are notunsympathetic to the hardships that this decision implies for mixedwaste generators. They find themselves in the unenviable positionof having no choice but to violate the law. Nevertheless, the possibil-ity that such hardships will occur is inherent in statutes such asRCRA that are expressly designed to force technology by threateningextreme sanctions. Moreover, the fact that technology may not keep


up with time-tables established by Congress does not mean thatCourts are at liberty to ignore them, however burdensome the result-ing enforcement. Accordingly, if the petitioners are to obtain reliefit must come from Congress.’’

In 2001, EPA issued a new regulation that provides increasedflexibility to facilities that manage mixed low-level waste by reducingthe burden of dual regulation under AEA and RCRA (EPA, 2001c).This regulation specifies conditions under which storage and treat-ment of commercial mixed low-level waste at the generating site isexempt from RCRA requirements when the generator is licensed byNRC or an Agreement State, and conditions under which commercialmixed low-level waste is exempt from RCRA requirements on wastemanifests, transportation, and disposal, except LDR treatment stan-dards under RCRA (see Section 4.2.2) remain in effect. The provisionsof this regulation also apply to chemically hazardous waste thatcontains NARM waste, provided the radioactive material is regu-lated by a state (see Section 4.3.5). The relaxation of requirementsfor storage, treatment, transportation, and disposal of mixed wastedoes not apply to any DOE mixed waste.

The specified exemptions from RCRA requirements for manage-ment and disposal of mixed low-level waste were prompted by severalconcerns (EPA, 1999e; 2001c):

● the burdensome, duplicative, and costly requirements of dualregulation that do not provide greater protection of humanhealth and the environment than achieved under a single regula-tory regime;

● increased radiation exposure of workers at storage and treat-ment facilities;

● the lack of availability of disposal facilities that can accept cer-tain kinds of commercial mixed low-level waste and the verylow possibility of siting a new disposal facility that could acceptall commercial mixed low-level waste;

● the continued limited capacity for treatment of commercialmixed low-level waste and the unwillingness of treatment facili-ties to accept waste for which there is no viable disposal option;

● the continued limited capacity for treatment and disposal ofmixed low-level waste at DOE sites; and

● the continual need for EPA to extend its policy on lower enforce-ment priority of the prohibition on storage of mixed waste atSection 3004(j) of RCRA (1976), due to the lack of adequatetreatment and disposal capacity.

The exemptions reflect EPA’s assessment that NRC and AgreementState regulations for storage and disposal of low-level radioactive


waste under AEA would not compromise human health and environ-mental protection from chemical risks when the specified conditionson management and disposal of mixed low-level waste are met (EPA,1999e; 2001c).

4.3.4 Dual Regulation of Other Fuel-Cycle Wastes

The previous section mainly considered the considerable impactsof dual regulation of mixed waste on management and disposal ofmixed low-level waste. High-level waste, transuranic waste, anduranium or thorium mill tailings also may be subject to dual regula-tion under AEA (1954) and RCRA (1976). This Section briefly consid-ers the impacts of dual regulation on these wastes.

4.3.4.1 High-Level Waste. DOE currently manages its high-levelradioactive waste produced in chemical reprocessing of spent fuelas mixed waste (DOE, 1999a). Liquid waste from fuel reprocessingand sludges resulting from settling or further processing of the liquidwaste are classified as hazardous under RCRA because they exhibitsome of the characteristics of hazardous waste, including corrosivityor toxicity, and they contain high concentrations of toxic heavy met-als that cannot be removed by waste treatment.

Dual regulation of high-level waste presents a number of chal-lenges. First, long-term storage of reprocessing wastes in under-ground tanks at various DOE sites would appear to be in violationof the RCRA prohibition on indefinite storage of mixed waste dis-cussed in the previous section. DOE has begun the process of convert-ing waste liquids and sludges in the storage tanks to a vitrified wasteform (borosilicate glass) suitable for permanent disposal, but it willbe many years before treatment of DOE’s high-level waste currentlyin storage is completed and the waste is ready for disposal.

Second, solidified forms of high-level waste intended for perma-nent disposal are subject to LDRs for hazardous waste (EPA, 1986b;1990b) discussed in the previous section (see also Section 4.2.2).LDRs specify that vitrified high-level waste is an acceptable wasteform under RCRA, but there are as yet no such provisions for otherforms of high-level waste that might be intended for disposal.

Finally, if high-level waste is considered to be hazardous wasteunder RCRA, requirements on construction, operation, and closure ofa disposal facility, including the provision of a liner system, leachatecollection and removal system, and leak detection system (seeSection 4.2.2), would need to be addressed. Such requirements areimpractical at a geologic repository for disposal of high-level waste


and could have an adverse impact on the long-term performance ofa facility. However, as noted in Section 4.3.3, LDRs allow waiver ofmany requirements on the waste form and disposal facility if EPAfinds that there will be no significant migration of waste from thefacility for as long as the waste remains hazardous. Obtaining sucha ‘‘no-migration’’ variance is one option for essentially exemptinga geologic repository from requirements for disposal of hazardouschemical waste under RCRA. Exemption from RCRA requirementsalso could be based on a finding that all forms of solidified high-levelwaste and spent fuel are acceptable waste forms under RCRA, ora finding that compliance with EPA standards for the radioactivecomponent of the waste (see Section 4.1.2.3.1) fulfills the RCRArequirement concerning no significant migration of waste (see follow-ing section).

4.3.4.2 Transuranic Waste. Much of DOE’s transuranic radioac-tive waste is classified as hazardous waste under RCRA and is man-aged as mixed waste (DOE, 1999b). Many transuranic wastes arehazardous due to the presence of toxic heavy metals or organic chemi-cals introduced into the waste during processing of plutonium.

In contrast to high-level waste, much of the existing transuranicwaste is loose trash and is not prepared for disposal using standardwaste forms (DOE, 1997a). The variety of transuranic wastes pre-sents a challenge in meeting RCRA requirements on waste character-ization, in that sampling and analysis of hazardous waste as calledfor in EPA regulations (EPA, 1986b) can lead to increased radiationexposures of workers and, thus, a potential conflict with require-ments to maintain exposures ALARA established under AEA (DOE,1993a; NRC, 1991).

Given the variety of transuranic wastes requiring disposal, thereare no specifications in RCRA regulations for exempting mixed trans-uranic waste from LDRs (EPA, 1986b). As in the case of high-levelwaste disposal in a geologic repository, it is impractical to applysome RCRA requirements on construction, operation, and closure ofa hazardous waste disposal facility to WIPP, which will be used fordisposal of DOE’s defense transuranic waste (see Section 4.1.2.3.2).These concerns were addressed in a 1996 amendment to WIPPLWA(1992). This amendment exempted the WIPP facility from LDRs forhazardous chemical waste, thus allowing disposal of mixed transu-ranic waste to proceed if the state of New Mexico approves plans formanaging the hazardous component prior to termination of institu-tional control. This exemption was based on a finding by EPA (1998a)that the WIPP facility complies with standards for disposal of theradioactive component of transuranic waste in 40 CFR Part 191,


which apply for 10,000 y (EPA, 1993a). It was the sense of Congressthat the LDRs for the hazardous component of the waste are redun-dant with the standards for radionuclides; i.e., that compliancewith the standards for radionuclides satisfies the conditions for a‘‘no-migration’’ variance from LDRs for the hazardous component(EPA, 1996d).

4.3.4.3 Uranium or Thorium Mill Tailings. In contrast to high-level waste, transuranic waste, and low-level waste, regulations formanagement and disposal of uranium or thorium mill tailings weredeveloped in recognition that these radioactive wastes also are chem-ically hazardous, due mainly to the presence of toxic heavy metals.EPA regulations established under AEA (EPA, 1983) specifythat operations and closure at mill tailings sites must conform toRCRA requirements for hazardous chemical waste. The regulationsdo not require liner systems but they include requirements on(1) caps to control infiltration of water, as well as releases of radon,(2) monitoring of groundwater, and (3) mitigation of releases ofradionuclides and hazardous chemicals to groundwater if standardsfor groundwater protection, which are consistent with drinkingwater standards in 40 CFR Part 141 (EPA, 1975), are exceeded.

4.3.5 Dual Regulation of Naturally Occurring and Accelerator-Produced Radioactive Material Waste

Issues of dual regulation also arise in management and disposal ofwaste that contains NARM and waste classified as hazardous underRCRA. This type of waste is subject to dual regulation essentiallybecause the definition of hazardous waste developed by EPA underRCRA (EPA, 1980b) does not include NARM waste (Section 4.2.1.2).Waste that contains NARM can be regulated under RCRA only if it isspecifically included in the definition of hazardous waste, even thoughthe exemption of radioactive materials defined in AEA from regulationunder RCRA does not apply to NARM.

Although NARM is not a radioactive material defined in AEA,DOE is responsible for management and disposal of NARM wastegenerated by any of its authorized activities, based on the provisionof AEA that all DOE activities must be protective of public healthand safety (AEA, 1954). Current DOE policy specifies that NARMwaste is to be managed as mixed waste under AEA and RCRA orTSCA (1976) if the waste is hazardous under either of the latter twolaws (DOE, 1999c). Thus, all issues that arise in management anddisposal of DOE’s mixed low-level waste (see Section 4.3.3) also applyto DOE’s mixed NARM waste.


In the commercial sector, issues of dual regulation of mixed NARMwaste arise whenever a state regulates NARM as radioactive mate-rial and the waste also is hazardous under RCRA or TSCA. Statesgenerally regulate accelerator-produced waste as low-level waste,but not all states regulate waste that contains elevated levels ofNORM as radioactive material and the states that do regulate NORMwaste have taken a variety of approaches (see Section 4.1.2.4). EPA(2001c) has exempted mixed NARM waste in the commercial sectorfrom certain RCRA requirements if the radioactive component of thewaste is regulated by a state and specified conditions are met (seeSection 4.3.3). In particular, the radioactive component of mixedNARM waste would need to be acceptable for disposal in a facilityfor low-level radioactive waste licensed by NRC or an AgreementState under 10 CFR Part 61 (NRC, 1982a).

4.3.6 Summary of Mixed Waste Issues

In managing and disposing of waste that contains radionuclidesand hazardous chemicals, it is now well established that AEA appliesonly to the radionuclides in the waste (i.e., to source, special nuclear,and byproduct materials), and that all other hazardous constituentsof the waste are subject to regulation under other laws, principallyRCRA but also TSCA for waste that contains, for example, dioxins,PCBs, or asbestos. The concept of mixed waste also extends to wastethat contains NARM and hazardous chemicals when the radioactivecomponent of the waste is regulated by DOE or a state. Requirementsfor management and disposal of hazardous chemical waste underRCRA or TSCA were developed largely independently of require-ments for radioactive waste developed under AEA. Dual regulationhas led to costly and inefficient approaches to management anddisposal of mixed wastes, including substantial delays in their treat-ment and disposal, without commensurate improvements in protec-tion of public health and the environment.

Dual regulation of mixed waste is an important concern for radio-active waste that arises from operations of the nuclear fuel cycle,because until the 1980’s policies and regulations for managementand disposal of high-level waste, transuranic waste, and low-levelwaste were developed under AEA largely without regard for thepossible presence of hazardous chemicals and without taking intoaccount requirements for management and disposal of hazardouschemical waste that were being developed under RCRA or TSCA.Only uranium and thorium mill tailings were regulated taking into


account the presence of hazardous chemicals and requirements formanaging hazardous chemical waste.

Dual regulation of mixed waste has no effect on classification ofthe radioactive component of the waste, and classification of waste aschemically hazardous is not affected by the presence of radionuclides.Rather, dual regulation mainly affects requirements on managementand disposal of waste that had previously been managed as if it wereradioactive but not chemically hazardous.

In essence, the approach to protecting public health and the envi-ronment under AEA has been based on numerical standards thatspecify acceptable overall performance of a radioactive waste man-agement or disposal system. The standards for acceptable systemperformance are in the form of limits on radiation dose to membersof the public or other related criteria. Radioactive waste generatorsand radioactive waste management and disposal facilities areafforded considerable flexibility in meeting these standards, andthere are few prescriptive technical requirements that apply to allfacilities.

The approach to management and disposal of hazardous chemicalwaste under RCRA is quite different. As does AEA, RCRA requiresthat public health be protected in management and disposal of haz-ardous chemical waste. RCRA and its implementing regulations par-ticularly emphasize protection of groundwater in accordance withdrinking water standards in meeting this requirement. However,the approach to protection of public health under RCRA is basedlargely on detailed and highly prescriptive technical standards forwaste generators and waste treatment, storage, and disposal facili-ties. Each type of facility must meet the same technical standards,largely without regard for the nature of the hazardous wastes orlocal environmental conditions. Thus, in contrast to facilities regu-lated under AEA, facilities regulated under RCRA have little flexi-bility in meeting the overall objective of protecting public health.

In spite of differences in the regulatory approaches under AEAand RCRA, dual regulation of mixed waste does not present anyinsurmountable technical obstacles to waste management and dis-posal. Rather, the need for generators of radioactive waste and facili-ties for management, storage, and disposal of radioactive waste,which had previously been regulated only under AEA, to complywith RCRA requirements when the waste also is chemically hazard-ous has been the main impediment to successful management anddisposal of mixed waste. Regulation of hazardous waste based ondetailed and prescriptive technical requirements that apply at allstages from generation to disposal and to any facility, as well asthe sometimes complex and difficult procedural requirements for

4.4 NCRP RECOMMENDATIONS / 235

obtaining operating permits under RCRA or waivers of RCRArequirements (e.g., delisting of hazardous waste or a ‘‘no-migration’’variance for disposal of hazardous waste), represent a regulatoryapproach for which the radioactive waste management communitywas initially unprepared. Similar considerations apply to mixedwaste regulated under AEA and TSCA. Legal and regulatory actionsin recent years have succeeded in easing the burdens of dual regula-tion of mixed waste, especially mixed high-level waste, transuranicwaste, and commercial low-level waste. However, relief has not beenextended to many important mixed wastes, such as DOE’s mixedlow-level waste.

4.4 NCRP Recommendations Relevant toWaste Classification

This Section briefly reviews previous recommendations of NCRPthat are potentially relevant to the development of a risk-basedwaste classification system. The topics discussed include NCRP’srecommendations on radiation protection of the public and the com-parative hazards of ionizing radiation and chemicals.

4.4.1 Recommendations on Radiation Protection of the Public

An important function of NCRP is to develop basic recommenda-tions on radiation protection; NCRP’s current recommendations arecontained in Report No. 116 (NCRP, 1993a). With regard to radiationprotection of the public, two recommendations are potentially rele-vant to the development of a risk-based waste classification system.These recommendations involve limits on radiation dose and a negli-gible individual dose.

4.4.1.1 Radiation Dose Limits. For routine exposure of individualmembers of the public to all man-made sources of radiation combined(i.e., excluding exposures due to natural background, indoor radon,and deliberate medical practices), NCRP currently recommends thatthe annual effective dose should not exceed 1 mSv for continuous orfrequent exposure or 5 mSv for infrequent exposure. The quantity‘‘effective dose’’ is a weighted sum of equivalent doses to specifiedorgans and tissues (ICRP, 1991), which is intended to be proportionalto the probability of a stochastic response for any uniform or nonuni-form irradiations of the body (see Section 3.2.2.3.3).


The recommended dose limits for the public define limits on theprobability of stochastic responses that are regarded as necessaryfor protection of public health. Doses above the limits are regardedas intolerable and normally must be reduced regardless of cost orother circumstances, except in the case of accidents or emergencies(see Section 3.3.1). For continuous exposure over a 70 y lifetime, andassuming a nominal probability coefficient for fatal cancers (i.e., theprobability of a fatal cancer per unit effective dose) of 5 � 10�2 Sv�1

(ICRP, 1991; NCRP, 1993a), the dose limit for continuous exposurecorresponds to an estimated lifetime fatal cancer risk of about4 � 10�3. However, meeting the dose limits is not sufficient to ensurethat routine exposures of the public to man-made sources would beacceptable.

In addition to requiring that doses to individuals should not exceedspecified limits, an important principle of radiation protection is thatall doses should be maintained ALARA, economic and other societalconcerns being taken into account (ICRP, 1991; NCRP, 1993a). Inthe past, application of the ALARA principle emphasized considera-tions of cost-benefit in optimizing collective doses to affected popula-tions [e.g., see 10 CFR Part 50, Appendix I (NRC, 1977), and DOE(1991)]. However, in controlling exposures of the public, the ALARAprinciple has increasingly been implemented in part by means ofstandards for specific practices or sources of exposure, called sourceconstraints (ICRP, 1991), that limit the dose from each practice orsource to a fraction of the dose limits for all man-made sourcescombined. Source constraints often represent judgments by regula-tory authorities about doses that are reasonably achievable for spe-cific practices or sources at any site, and they provide a practicalbasis for ensuring that the dose limits for all man-made sourcescombined will be met (Kocher, 1988).

NCRP (1993a) also has emphasized the importance of source con-straints in radiation protection of the public. NCRP has reaffirmeda previous recommendation (NCRP, 1984b; 1987a) that wheneverthe potential exists for routine exposure of an individual member ofthe public to exceed 25 percent of the limit on annual effective doseas a result of irradiation attributable to a single site, the site operatorshould ensure that the annual effective dose to the maximallyexposed individual from all man-made sources combined does notexceed 1 mSv on a continuous basis. Alternatively, if such an assess-ment is not conducted, no single source or set of sources under onecontrol should result in an individual receiving an annual effectivedose of more than 0.25 mSv.

The recommended limit on annual effective dose of 0.25 mSv persource corresponds to an estimated lifetime fatal cancer risk of about

4.4 NCRP RECOMMENDATIONS / 237

1 � 10�3. Annual effective doses in the range of 0.25 to 1 mSv fromall man-made sources combined are acceptable if they are ALARA.However, doses toward the upper end of this range are regarded asonly barely tolerable (ICRP, 1991), and doses below this range areexpected to be justifiable and achievable in most cases, based onsite-specific application of the ALARA principle. Therefore, lifetimerisks from routine exposure to all man-made sources combined usu-ally should not exceed about 1 � 10�3.

4.4.1.2 Negligible Individual Dose. The second NCRP recommen-dation that is potentially relevant to developing a risk-based wasteclassification system is concerned with a negligible individual dose(NCRP, 1993a). A negligible dose is based on the concept of a negligi-ble (de minimis) risk, and it defines a level below which furtherreductions in dose using the ALARA principle generally would notbe warranted.

NCRP has recommended that annual effective doses to individualsfrom any practice or source of 10 �Sv or less are negligible (seeSection 4.1.2.5.3). This dose is one percent of the dose limit forcontinuous exposure to all man-made sources combined discussedin the previous section, and it also is about one percent of the dosefrom natural background radiation, excluding radon (NCRP, 1987b).The recommended negligible individual dose corresponds to an esti-mated lifetime fatal cancer risk of about 4 � 10�5.

4.4.1.3 Application of NCRP Recommendations to Waste Classifi-cation. NCRP’s recommendations on dose limits and a negligibledose for individual members of the public, and their associated cancerrisks, could be used in developing a risk-based waste classificationsystem. Specifically, the dose limits applicable to all man-madesources of exposure combined could be used in establishing concen-tration limits of radionuclides or hazardous chemicals in dedicatedhazardous waste disposal facilities based on assumed scenarios forexposure of the public. Similarly, the negligible individual dose couldbe used in establishing concentration limits of radionuclides in dis-posal facilities for nonhazardous waste. These applications arediscussed in Sections 6.2 and 6.3 where NCRP’s recommendationson risk-based waste classification are presented.

4.4.2 Comparative Carcinogenicity of Ionizing Radiationand Chemicals

NCRP has published an evaluation of the extent to which princi-ples and methods that have been developed for use in assessing


cancer risks from exposure to ionizing radiation are applicable tochemical carcinogens (NCRP, 1989). Some of the conclusions of thisstudy are summarized below.

1. The carcinogenic effects of certain chemicals in man and labo-ratory animals are similar to those of ionizing radiation.

2. Cancers induced by ionizing radiation and chemicals are indi-vidually indistinguishable from those of the same type inducedby other causes. Thus, in either case, induction of cancers canonly be inferred from statistical analyses of a dose-dependentincrease in their frequency in exposed populations.

3. From studies of human populations exposed to certain chemi-cals, available data are sufficient to characterize the dose-incidence relationships for some types of cancer at high doselevels. However, as in the case of ionizing radiation, the dataare not sufficient to define the dose-incidence relationshipsprecisely for any form of cancer over a wide range of dosesand dose rates. Therefore, the probability of cancer inductionthat may be associated with low doses of chemicals that wouldbe of primary concern in protection of public health can beestimated only by interpolation and extrapolation of data athigher doses and dose rates, based on assumptions about thedose-incidence relationships and mechanisms of toxicity. Forthe few chemicals for which incidence data are available overa range of doses, the dose-incidence relationship is not incon-sistent with linearity, but this result does not constitute proofof linearity.

4. Few chemicals identified as carcinogens in laboratory animalsare known to cause cancer in humans, and the dose to affectedtissues for these chemicals usually is not known well enoughto define the dose-response relationship except in a generalway. In this respect, the carcinogenic effects of most chemicalsin humans are far less well known than are those of ioniz-ing radiation.

5. Dosimetry (i.e., the dose delivered to target tissues per unitintake of material) generally is more uncertain for chemicalsthan for radionuclides, because the dose of a chemical at itsbiological site of action may depend on a number of metabolic andpharmacokinetic factors that are not relevant for radionuclides.

6. Because chemicals differ widely in molecular structure, biolog-ical activity, and mode of action and because the relationshipsamong these properties are poorly understood, the toxic effectsof one chemical usually cannot be predicted with confidencefrom those of another.

4.5 SUMMARY / 239

7. An understanding of chemical toxicity is complicated by thelarge differences in potency among chemicals and in the stagesof the carcinogenic process at which they act. For example,some chemicals predominantly affect late stages of carcinogen-esis, whereas others affect earlier stages.

8. The interactive responses of two or more carcinogens may beindependent, synergistic, or antagonistic, but few such interac-tions are well characterized. Furthermore, the combined effectsof exposure to complex mixtures of chemicals, such as are typi-cally encountered in human life, are virtually unexplored.

9. In spite of the differences among carcinogens, the principlesof dose-response assessment that have proven to be useful forionizing radiation appear to be applicable, within limits, tochemicals, particularly those chemicals that resemble radia-tion in genotoxicity, cytotoxicity, and in the stages of carcino-genesis that are affected.

10. For a given exposure situation, the choice of a dose-incidencemodel for risk assessment is a matter of scientific judgmentwhich must be based on consideration of all pertinent epidemi-ological and experimental data, including the results of short-term tests where applicable.

11. Dose-response assessments for chemical carcinogens gener-ally are more uncertain than dose-response assessments forionizing radiation.

In developing a risk-based waste classification system, the primaryemphasis would be on risk management, rather than estimation ofrisk for actual exposure situations. However, differences in the stateof knowledge of the carcinogenicity of ionizing radiation and chemi-cals could be taken into account in establishing limits on allowabledoses (hypothetical risks) for radionuclides and chemicals to be usedin classifying waste.

4.5 Summary

Classification systems for radioactive waste and requirements fordisposal of different classes of radioactive waste have been developedlargely independently of classification systems and disposal require-ments for hazardous chemical waste. This Section has discussed theclassification systems for radioactive and hazardous chemical wastesand the relationships between waste classification and requirementsfor disposal. Impacts of the different systems for waste classificationand disposal on management and disposal of waste that contains


mixtures of radionuclides and hazardous chemicals also has beendiscussed.

The current definitions of the different classes of radioactive wasteand the intended disposal system (technology) for waste in each classin the United States are summarized in Table 4.1 (see Section 4.1.2.1).The system for classification and disposal of radioactive waste inthe United States has several important characteristics. First, theclassification system is not comprehensive, because NARM waste isnot included in the classification system for waste that arises fromoperations of the nuclear fuel cycle. This distinction is based solelyon the source of the waste, rather than its radiological properties orrequirements for safe management and disposal. Second, the classi-fication system for fuel-cycle waste is qualitative (i.e., the differentwaste classes are not defined solely in terms of limits on concentra-tions of radionuclides or other properties of the waste), it is source-based (i.e., the different classes of waste are distinguished mainlyon the basis of how the waste is generated, rather than its properties),and it is not based on considerations of risk, especially risks resultingfrom waste disposal. Third, waste classes are not defined in relationto the type of disposal system that is expected to be acceptable (e.g.,a near-surface facility or geologic repository). As a result of the secondand third characteristics, the different classes of fuel-cycle wasteare not defined unambiguously, and waste in different classes canhave similar radiological properties and require similar approachesto waste management and disposal.

A number of alternatives to the qualitative and source-based clas-sification system for radioactive waste in the United States havebeen proposed. The alternative waste classification systems havethree important features in common. First, they are comprehensive,in that NARM waste and nuclear fuel-cycle waste are included inthe same classification system. Second, they are based on the conceptthat waste classes should be defined primarily on the basis of risk,particularly the risk resulting from waste disposal. Finally, to somedegree, they associate waste classes with particular disposal systemsthat are expected to be generally acceptable. None of these featuresis embodied in the radioactive waste classification system in theUnited States. In addition, some proposed classification systemsinclude an exempt class of radioactive waste that contains negligiblysmall amounts of radionuclides. Waste in this class would be regu-lated in all respects as if it were nonhazardous. A general class ofexempt waste is not included in the radioactive waste classificationsystem in the United States.

Classification and disposal of hazardous chemical waste is basedmainly on EPA regulations and guidance developed under RCRA,

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although individual states have supplemented EPA policies in somecases. Some hazardous chemical wastes, such as sewage sludge orwaste that contains dioxins, PCBs, or asbestos, are classified andmanaged under other laws, including TSCA or the Clean Water Act.

Hazardous chemical waste is defined in RCRA regulations as asolid waste that exhibits the characteristic of ignitability, corrosivity,reactivity, or toxicity, or is a specifically listed waste. The definitionof hazardous waste specifically excludes radioactive material (source,special nuclear, or byproduct material) defined in AEA.

Under current EPA regulations, a chemical waste is either hazard-ous or it is not, and there is no further classification of hazardouschemical waste with respect to the degree of hazard. Some stateshave defined classes of hazardous chemical waste (e.g., extremelyhazardous waste) but, in practice, the requirements on managementand disposal of all hazardous wastes have resulted in essentially thesame approaches being used regardless of hazard. When a hazardouschemical waste is mixed with a nonhazardous solid waste, the entirewaste is classified as hazardous unless the former is a characteristi-cally hazardous waste that does not contain any listed waste andmixing with the nonhazardous waste removes the hazardous charac-teristic.

Emplacement in a near-surface disposal facility is the commondisposition of solidified hazardous chemical waste, regardless of thehazard posed by the waste. Disposal sites must meet location require-ments, and they must be provided with appropriate liner, leachatecollection and removal, and leak detection systems.

The system for classification and disposal of hazardous chemicalwaste under RCRA is neither comprehensive nor is it based strictlyon considerations of risks posed by waste. As noted above, all hazard-ous chemical wastes are managed alike, regardless of hazard, andthis policy extends to waste that contains only minuscule amountsof listed hazardous substances. In addition, many potentially impor-tant wastes that contain hazardous chemicals are excluded from thedefinition of hazardous waste and, thus, are not presently regulatedunder RCRA. The distinction between regulated and unregulatedwastes is based primarily on the source of the waste, rather thanits hazard.

The term ‘‘mixed waste’’ refers mainly to waste that contains radio-nuclides regulated under AEA and hazardous chemical waste regu-lated under RCRA. Mixed waste is subject to dual regulation as aresult of the exclusion of radioactive materials defined in AEA fromregulation under RCRA. Dual regulation of mixed waste also extendsto waste that contains NARM and hazardous chemicals, since NARMwaste is not defined as a hazardous waste under RCRA, and to


radioactive waste that contains hazardous chemicals that areregulated under other laws (e.g., TSCA). By the early 1990s, dualregulation of mixed waste was firmly established as a result of courtdecisions and federal law.

Dual regulation of mixed waste does not affect classification of itsradioactive and hazardous chemical constituents. However, success-ful management and disposal of mixed waste has been a formidablechallenge. Difficulties in managing mixed waste have not resultedfrom technical obstacles, such as fundamental differences in (1) thenature of radioactive and hazardous chemical wastes and how theyshould be managed or (2) requirements on waste management anddisposal related to protection of public health and the environment.Rather, the main impediment to successful management and dis-posal of mixed waste has been the difficulties in obtaining operatingpermits for waste treatment, storage, and disposal facilities underRCRA and in obtaining waivers from RCRA requirements. Thesedifficulties are due mainly to the significant differences between thedetailed and prescriptive technical requirements in RCRA regula-tions and the less prescriptive, outcome-based requirementsnormally imposed on facilities that manage or dispose of radio-active waste under AEA. The result has been costly and inefficientapproaches to management and disposal of mixed wastes that havenot led to improvements in protection of public health and theenvironment.

5. Desirable Attributes of aWaste ClassificationSystem

Previous sections have presented technical and historical informa-tion on radiation and chemical risk assessment and on classificationof radioactive and hazardous chemical wastes. This information pro-vides important perspectives for establishing the foundations of anew hazardous waste classification system. Before establishing thesefoundations, it is useful to specify the attributes that an ideal wasteclassification system should possess. The following sections identifythe desirable attributes of a waste classification system includingthat the system should be risk-based, it should allow for exemptionof waste, and it should be comprehensive, consistent, intrinsic, com-prehensible, quantitative, compatible with existing systems, andflexible. These attributes should be recognized as goals that are notall likely to be fully realized in a practical waste classification system.

For many years, the hazardous waste classification systems thatexisted at a particular time performed adequately, although thereclearly were inconsistencies and occasional difficulties. In recentyears, however, increased interest in protecting and cleaning up theenvironment has resulted in a proliferation of waste classificationsystems (generic and situation-specific) and application of these inways that have increased the undesirable legal, sociopolitical, andeconomic ramifications of existing waste classification systems. Asa consequence, difficulties that were previously minor have assumedmajor proportions. The following sections also summarize some ofthese difficulties for the purpose of illustrating the need for a consoli-dated approach to waste classification and identifying some of themajor issues that must be addressed by a new system.

5.1 Risk-Based

Society desires that waste disposition activities be conducted in amanner that provides long-term protection of human health. Manymeasures of ‘‘long-term protection’’ have been developed over the

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244 / 5. DESIRABLE ATTRIBUTES

years to aid in this endeavor. However, the most fundamental mea-sure of the extent to which protection is provided is the incrementalhealth risk to the public associated with disposition of waste. Thus,a waste classification system should be based on risk (Adam andRogers, 1978; Waters et al., 1993), in order to facilitate dispositionof hazardous wastes by means that are expected to limit risks to thepublic to levels deemed acceptable.

There are two possible alternatives to using risk directly as thebasis for waste classification: non-risk-based systems and surrogatesystems. Non-risk-based systems could use any conceivable attributeof hazardous waste as a basis for classification, including its source(see Sections 4.1 and 4.2 for examples) or the date it was produced.These bases are at best somewhat related to risk and at worst aretotally unrelated. Because of this variable relationship, the use ofnon-risk-based approaches to waste classification could result in anunacceptable risk if the waste is managed in a way that does notprovide adequate long-term protection, or an inappropriate alloca-tion of resources if relatively innocuous wastes are managed in thesame way as much more hazardous wastes.

Surrogate systems attempt to compensate for the shortcomings ofnon-risk-based systems by using a subset of the attributes of wastethat determine risk. One of the more common measures is the toxicityof hazardous substances (the probability of a response per unit dose).Less frequently, such other parameters as radioactivity, persistence,or mobility are employed. While these represent an improvementover non-risk-based approaches to waste classification, they are stillinadequate. Examples of shortcomings include that highly toxicmaterials may not persist long enough to pose a significant risk orthat persistent materials may be so immobile that human exposuresare virtually nil.

Based on these considerations, classification of waste based onrisk is the preferred approach. However, it is not without shortcom-ings. By far the most important is that estimation of risks to humanhealth is often a complicated, multi-step process involving manyassumptions and attendant uncertainties. It is this shortcoming thatspawned the use of surrogates. However, in applications relatedto waste classification, risk assessment is significantly simplifiedbecause it is by definition not site-specific. While this results insome uncertainties related to classification of wastes, these can beaccommodated by introducing prudent degrees of conservatism ingeneric risk assessments. Additionally, the magnitude of uncertain-ties and their potential impact are taken into account in site-specificrisk assessments performed in the process of licensing particularwaste disposal facilities.

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Existing waste classification systems have a highly variable rela-tionship to the risk posed by waste. In some cases, waste classifica-tion is based on the source of the waste rather than risk. Examplesinclude high-level radioactive waste (see Section 4.1) and some of thehazardous chemical wastes listed in, or excluded from, regulationsimplementing RCRA (see Section 4.2). Some high-level radioactivewastes have decayed and been chemically processed to the point thatthey could be managed as low-level waste under NRC regulations,and other high-level wastes are similar to defense transuranic waste(Kocher and Croff, 1987). All listed chemical wastes must be man-aged as hazardous waste without regard for the amounts of listedsubstances present, and many potentially important wastes areexcluded from the definition of hazardous waste based on their sourcerather than their properties. A risk-based waste classification systemwould result in these wastes being more appropriately grouped,which should result in more cost-effective management commensu-rate with the risks posed by the wastes.

Some existing waste classification systems are risk-based to asubstantial degree. For example, the present system for subclassify-ing commercial low-level radioactive waste (NRC, 1982a) identifieswaste that is generally acceptable for near-surface disposal basedon a set of dose limits for a hypothetical inadvertent intruder thatprovide a surrogate for risk. However, the relationship of the doselimits to the radionuclide concentrations that quantify the classifi-cation system is not always evident or consistent, although theremay be valid reasons for this (Kocher and Croff, 1987). In addition,if unanticipated low-level wastes are generated by future processes,including them in NRC’s classification system on a consistent basismay be difficult. For example, NRC did not anticipate that commer-cial low-level waste might include large volumes of waste that con-tains elevated levels of long-lived, naturally occurring radionuclides(NRC, 1994b). Most importantly, because the general class of low-level waste is defined only by exclusion (see Section 4.1.2.3.3) andthe definitions of the excluded waste classes are not risk-based, thedefinition of low-level waste is not risk-based.

For hazardous chemical waste, there is no federal classificationsystem other than a specification that the waste is hazardous or thatit can be managed as if it were nonhazardous because it has beenshown not to be characteristically hazardous or has been delistedor specifically excluded.14 Hazardous chemical waste that is not

14 A few states have developed a category of high-hazard chemical waste called‘‘extremely hazardous’’ or similar term (California, 1999; Mehlhaff et al., 1979; NAS/NRC, 1999b; OTA, 1981) (see Section 4.2.1.3). However, extremely hazardous wastesappear to be managed in much the same way as other hazardous wastes, primarilybecause of the increasingly stringent regulations being applied to the latter.


destroyed by incineration normally is disposed of in dedicated near-surface facilities. However, there presumably are hazardous chemi-cal wastes that would pose an unacceptable risk from near-surfacedisposal in the absence of perpetual institutional control (Okrentand Xing, 1993). Such waste could require a disposal technologymore isolating than a near-surface facility. This subset of hazardouschemical waste is not yet identified or managed according to itslong-term risk, primarily because risk assessments for disposal ofhazardous chemical waste tend to have a relatively short time hori-zon (Doty and Travis, 1989).

Limits on allowable risk established by regulatory authorities havebeen used to derive maximum acceptable concentrations or invento-ries of waste constituents in particular disposal situations. However,risk has been expressed in a variety of ways. For example, is therisk to an individual (NRC, 1982a) or population (EPA, 1993a)?Maximum or average? Annual or lifetime? Critical organ or weightedresponses in multiple organs? To the public, workers, or inadvertentintruders? Adults or children? Humans, animals, or plants? Severalof these approaches to expressing risk have been used in practice,which means that existing waste classification systems ostensiblybased on risk may differ substantially. A properly constituted risk-based waste classification system will address such issues.

5.2 Exemption

Ideally, the incremental costs of waste treatment and disposalshould not exceed the resulting benefits in health risks averted(Waters et al., 1993), given that the resources that can be allocatedto reducing risks are finite. Use of acceptable methods for disposalof nonhazardous waste is one option that should be considered toappropriately match the costs and benefits of waste disposal. Forexample, municipal solid waste is sent to an approved municipal/industrial landfill, and other wastes that pose no greater risk to thepublic should be acceptable for disposal in a similar facility, evenwhen these wastes contain small amounts of hazardous chemicalsor radionuclides. Similarly, consideration of reuse of slightly contam-inated materials should be allowed if the benefits clearly outweighthe costs, including the incremental health risks. Viewed morebroadly, resources spent on waste treatment and disposal are notavailable for use in achieving other desirable outcomes, such ashealth care or other types of environmental interventions, so thetradeoff ultimately is reduction of risks in one area at the expenseof neglecting risks in other areas.

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Allowing for exemption of waste materials that contain sufficientlysmall amounts of hazardous substances is a potentially importantmeans of balancing the resources required to manage waste and thebenefits in health risks averted. As a consequence of the discussionin Section 5.1, it is desirable that the definition of waste that canbe exempted and, thus, managed as if it were nonhazardous shouldbe risk-based. Furthermore, waste should be exempted based onthe consideration that the associated risks should not exceed levelsgenerally regarded as negligible.

Existing waste classification systems in the United States do notinclude general principles for exempting waste that contains smallamounts of hazardous substances from requirements for manage-ment and disposal as hazardous waste. Regulations governing haz-ardous chemical waste include provisions allowing delisting of listedhazardous waste on a case-by-case basis (EPA, 1993b), but the regu-lations do not provide a clear indication of conditions under which ahazardous waste might be delisted and the process can be difficult. Afew chemically hazardous wastes have been exempted to allow theirbeneficial use. Exemptions for radioactive waste or beneficial uses ofradioactive materials also have been established only on a case-by-casebasis (see Section 4.1.2.5.2), and the existing exemptions correspond topotential doses to the public that vary widely (Kennedy et al., 1992;Schneider et al., 2001).

The lack of general exemption principles for radioactive and haz-ardous chemical wastes has important consequences for the costs ofwaste treatment and disposal. The ubiquitous nature of radioactivityin combination with the sensitivity of modern analytical techniquesoften makes it impossible to determine whether the radioactivity ina material is naturally occurring or resulted from some operation.This leads to expensive undertakings such as the common DOEpractice of assuming that any material that has been in a radiationarea is contaminated and must be managed as radioactive material.The amounts of such material are expected to increase greatly inthe future as aging nuclear facilities (DOE and commercial) aredecontaminated and dismantled. A risk-based waste classificationsystem would address such issues by specifying a risk below whicha material would be exempt from regulation as hazardous waste,thus alleviating a significant expense in the hazardous waste man-agement system while still protecting public health.

5.3 Comprehensive

A risk-based waste classification system should apply to all wastesthat contain hazardous substances. That is, it should apply irrespec-tive of the nature of the hazardous substances in the waste (chemical,


radioactive, or both), the process generating the waste, or the organi-zation generating the waste. This attribute is a necessary ramifica-tion of the waste classification system being based on risk: if theclassification system is not comprehensive, it will differentiateamong wastes based on attributes other than risk.

The existing waste classification systems for radioactive and haz-ardous chemical wastes clearly are not comprehensive. At a funda-mental level, entirely separate and quite different classificationsystems have been developed for the two types of hazardous waste.In addition, each classification system is not comprehensive in thecontext of the general type of waste to which each system applies.In the existing radioactive waste classification system, waste thatarises from operations of the nuclear fuel cycle is classified separatelyfrom NARM waste. The existing classification system for hazardouschemical waste excludes many potentially important wastes thatcontain hazardous chemicals.

5.4 Consistent

Another desirable attribute of a waste classification system thatis a corollary of the system being risk-based is that it treat wastesthat pose similar health risks consistently. A chemically hazardouswaste estimated to pose a certain risk should be in the same wasteclass as a radioactive waste that poses an equivalent risk, and simi-larly for mixed waste. Consistency also implies that wastes posingsimilar risks could be disposed of using essentially the same technol-ogy (municipal/industrial landfill, licensed near-surface facility forhazardous waste, or geologic repository).

Differences in the approaches to classification of radioactive andhazardous chemical wastes have resulted in inconsistent waste man-agement practices. Radioactive waste classification is more complex,with categories for mainly short-lived waste, which is commonlycalled low-level waste, and long-lived waste, such as defense transu-ranic waste (among others). Shorter-lived waste that contains suffi-ciently small amounts of long-lived radionuclides is typically suitablefor disposal in near-surface facilities. Long-lived waste is destinedfor disposal in deeper facilities (e.g., geologic repositories) that areexpected to provide substantially greater waste isolation than near-surface facilities. Chemical wastes are deemed hazardous or not underthe federal classification system, and hazardous chemical waste usu-ally is sent to a near-surface facility. Thus, disposal of a long-livedchemical waste, such as a heavy metal or degradation-resistant pesti-cide, uses the same technology as disposal of a short-lived (e.g.,

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readily degradable) waste, rather than a technology similar to thatused for its conceptual radioactive counterpart (transuranic waste).The inconsistency inherent in emplacing long-lived chemical wastein near-surface facilities could lead to unacceptably high risks(Okrent and Xing, 1993) in the absence of perpetual institutionalcontrol.

Waste classification can impede waste management when materialsare classified under multiple systems that contain incompatible pro-visions concerning waste management. As discussed in Section 4.3,management of solidified mixed waste is clearly impeded at the pres-ent time, as evidenced by the following:

● Substantial amounts of mixed low-level waste continue to bestored and inventories are increasing. Little of this waste isbeing sent to facilities intended for permanent disposal becausefew exist and those that are operating have restrictive wasteacceptance criteria.

● The capacity for treating mixed low-level waste to meet applica-ble (primarily RCRA) regulations is inadequate to the point thatEPA has had to extend its policy concerning relaxed enforcementof requirements that limit the time that mixed waste can bestored without treatment (EPA, 1994c). However, the decisionto continue this policy is not binding on the many states thatare authorized to regulate mixed waste. This situation mainlyimpacts DOE’s mixed low-level waste, which comprises the bulkof such waste requiring treatment and disposal.

● Under terms of the Federal Facility Compliance Act (FFCA,1992), DOE was subject to RCRA requirements beginning inOctober 1995, including individual states’ variations thereof.DOE was expected to have mixed low-level waste treatmentplans for each site approved by the host state, but adequatetreatment capacity for some mixed waste will not be availableuntil far beyond the time when its storage becomes non-compli-ant. As a result, solidified mixed low-level waste is and willcontinue to be stored in a noncompliant manner for times longerthan those allowed under RCRA. This practice continues to betolerated by EPA for responsible generators because of the lackof practical alternatives, but only until such time as appropriatetreatment or disposal capacity becomes available.

● DOE intends to dispose of its mixed defense transuranic wasteat WIPP. This facility is located hundreds of meters undergroundin a bedded salt formation, which clearly is much more isolatingand protective than the near-surface facilities in which mostRCRA hazardous waste is currently emplaced. Nonetheless, the


need to obtain a waiver from certain RCRA requirements onwaste treatment and the construction, operation, and closureof the WIPP facility constituted a time-consuming and costlyimpediment to obtaining the necessary permits for disposal ofmixed waste. Although the issue of disposing of mixed transu-ranic waste at the WIPP facility was resolved successfully (EPA,1996d; 1998a), the effort expended did little, if anything, toimprove the capability of the facility to protect public healthand the environment.

● The extent to which differences in waste classification andapproaches to waste management may impede the disposal ofhigh-level radioactive waste and spent nuclear fuel is not yetclear because of uncertainties in the final waste forms intendedfor disposal and the fact that siting and licensing of a repositoryis still in the investigative phase.

The above examples of waste management actions that have beenimpeded represent the manifest symptoms of the wastes being classi-fied and regulated under separate and inconsistent frameworks. Theroot cause of these impediments is the separate waste classificationsunder AEA (low-level waste, transuranic waste, high-level waste)and RCRA (chemically hazardous waste) and the different require-ments for waste management and disposal under the two laws. EPAhas recognized these difficulties and has begun to address them(EPA, 2001c), but relief from requirements imposed by dual regula-tion is not yet established for many mixed wastes.

5.5 Intrinsic

A risk-based waste classification system should do what its nameimplies—namely, classify waste so that some value of risk deemedacceptable in a given context would not be exceeded. The purposeof the system is not to classify waste containers, disposal sites con-taining waste, or processes that generate waste. The logical ramifi-cation of this is that the classification system must be based onintrinsic properties of waste. Additionally, it is desirable that theseproperties be readily measurable, especially in bulk solid waste. Thisnecessity is reinforced by the arguments in Section 2.1.2, whichpoint out that waste often must be classified before the method ofpackaging or the intended disposal site is known.

Basing waste classification on intrinsic properties of waste pro-vides significant advantages because in a mature system the genera-tor would be able to manage waste in the interim with confidence

5.6 COMPREHENSIBLE / 251

that one or more disposal options suitable for waste in a particularclass would be available. Similarly, waste disposal sites would nothave to be concerned about the source of waste when it is prop-erly classified.

Significant parts of the existing waste classification systems arebased on intrinsic properties of waste. The system for subclassifyinglow-level waste in 10 CFR Part 61 (NRC, 1982a) and the determina-tion of whether a chemical waste is characteristically hazardous(see Section 4.2.1.1) are examples of waste classification based onintrinsic properties.

However, some important waste classifications are based on attri-butes not intrinsic to the waste. A prime example is waste that isclassified based on its source (process or organization), such as high-level radioactive waste and many listed hazardous chemical wastes.The undesirable effects of this are summarized previously and arenot repeated here.

5.6 Comprehensible

A classification system can be risk-based and comprehensive andstill not be very useful if it is complex and difficult to use. At aminimum, the waste classification system must be comprehensible tothose involved in managing and regulating the disposition of waste.Otherwise, waste may not be properly classified. It also is desirablethat the waste classification system be transparent to nonexpertsoutside the waste management system, ranging from potentiallyimpacted individuals to the general population (Wiltshire and Dow,1995). Transparency would greatly enhance acceptability of the clas-sification system and the associated waste management system.Comprehensibility would be greatly facilitated if the waste classifi-cation system applies to all waste (i.e., there is only one system) andis simple in its concepts and applications.

Unfortunately, however, it is difficult for anyone to fully compre-hend the existing classification systems for radioactive and hazard-ous chemical wastes. These systems are not based on clearly statedprinciples from which a logical and transparent classification systemmight follow, and the two systems approach classification and dis-posal of hazardous waste in different ways. The systems intermixlegal and technical considerations in ways that sometimes defy logic.A few examples of the incongruities in the waste classification sys-tems that result in a lack of transparency and difficulties in compre-hension are described below.


● In regulations implementing RCRA, a ‘‘solid waste’’ is defined toinclude solid, liquid, semisolid, and contained gaseous materials.While there is a reasonable rationale for such a definition, itruns counter to common understanding of the meaning of ‘‘solid,’’and it immediately signals that the system for classifying andmanaging hazardous chemical waste will be quite complex anddifficult for operators, as well as regulators and the public, tounderstand.

● In many respects, the system for classifying and managing haz-ardous chemical waste under RCRA makes no distinctionbetween highly hazardous waste and virtually innocuous wastethat contains very low levels of hazardous substances. Further-more, many wastes that contain hazardous chemicals, as wellas radionuclides not regulated under AEA, are excluded fromthe definition of hazardous waste based on the source of thewaste, even though the excluded wastes can be just as hazardousas other wastes that are deemed hazardous under RCRA.

● Some high-level radioactive waste can be less hazardous thanhigh-activity (Class-C or greater-than-Class-C) low-level wastein regard to the levels of radioactivity due to shorter-lived radio-nuclides and the long-term risks that arise from disposal dueto long-lived radionuclides.

● Some low-level radioactive wastes can be more radioactive thanany other type of waste (e.g., high-activity 60Co sources, 90Sr or137Cs capsules).

● Surplus nuclear materials that consist almost entirely of 233Uby mass and also contain high activity concentrations of232U and its short-lived, photon-emitting decay products, ifdeclared to be waste, would be classified as low-level waste,rather than transuranic or high-level waste (Bereolos et al.,1998a; 1998b).

● Wastes that contain high concentrations of long-lived radionu-clides are destined for highly isolating disposal facilities, suchas a geologic repository, whereas similar chemical wastes aredestined for near-surface disposal.

Such incongruities only serve to confuse the public (Wiltshire andDow, 1995), and this confusion leads to mistrust that is manifest inunwarranted obstruction of the facilities and activities required tomanage hazardous wastes. A straightforward, consistent systembased on a few simple principles would serve to make waste classifi-cation and approaches to waste management more transparent andunderstandable.

A further impediment to comprehension of waste classificationand waste management systems is the practice of assigning different

5.7 QUANTITATIVE / 253

meanings to important nomenclature. An important example forpurposes of this Report is the differing meanings attached to ‘‘accept-able’’ and ‘‘unacceptable’’ levels of risk in the chemical and radiationparadigms for risk management (see Section 3.3).

5.7 Quantitative

It is highly desirable that a waste classification system be ex-pressed in quantitative terms. More specifically, the intrinsicwaste characteristics that define the boundaries between wasteclasses should be stated numerically. Qualitative definitions ofwaste classes, such as the definition of high-level radioactive wastediscussed in Section 4.1.2.3.1, simply defer the issue of waste classi-fication to a subsequent definition of the qualitative terms or to case-by-case determinations that typically occur after waste is generated.

Quantitative definitions of waste classes make the classificationsystem relatively unambiguous and also enhance the comprehensi-bility of the system. Exceptions to the rule can be handled by allowingregulatory authorities to include wastes that exceed a boundary orexclude wastes within a boundary on a case-by-case basis.

Some existing waste classification systems are quantitative. Forexample, the concentrations of radionuclides defining the different sub-classes of low-level radioactive waste that is generally acceptable fornear-surface disposal are clearly stated in the regulations (NRC,1982a), as are the quantitative conditions defining ignitable, corrosive,reactive, and toxic hazardous chemical wastes (see Section 4.2.1.1).

However, there are significant instances of nonquantitative defi-nitions. Important examples include the qualitative definitions ofthe different classes of radioactive waste that arises from operationsof the nuclear fuel cycle and the definitions of listed hazardous chemi-cal wastes. If a waste classification system is not quantitative, theinevitable result is uncertainty or inappropriate classification, thetypical manifestations of which are paralysis (e.g., storage of increas-ing amounts of waste) and legal challenges to proposed or ongoingactivities. Waste classification systems that are based on the sourceof the waste, including some listed chemical wastes and essentiallyall radioactive wastes, yield the undesirable effects discussed above.Even definitions of waste classes that contain specific qualitativelanguage are inadequate because the interpretation of importantterms is ambiguous. An example is the legal definition of high-levelradioactive waste as material that is ‘‘highly radioactive’’ and‘‘requires permanent isolation.’’ The absence of quantification makes


these terms largely meaningless (see Section 4.1.2.3.1), resulting inperpetuation of the traditional source-based definition of thiswaste class.

5.8 Compatible

The waste classification system should be developed in recognitionof the types of information that are available and likely to be obtain-able, and it should be specified to maximize compatibility with avail-able information consistent with maintaining the fundamentalintegrity of the system. Establishment of a risk-based waste classifi-cation system must begin with the existing classification systemsand associated databases (e.g., toxicity of hazardous substances).These would be expanded and refined as needed. However, if thefoundations of a risk-based waste classification system or its imple-mentation involve radically new concepts or call for data that cannotfeasibly be obtained, the effort will be for naught. A realistic wasteclassification system must use the existing base of concepts and datato achieve the desired result.

Existing waste classification systems are generally compatiblewith available data. Compatibility is the result of the databaseshaving been acquired to meet the needs of the waste classificationsystems.

5.9 Flexible

The waste classification system must be flexible so that it canaccommodate special circumstances without need of a continuingseries of separate classifications or ad hoc solutions. Commoninstances where flexibility is required include (1) taking the presenceor absence of engineered waste forms into account, (2) providingfor classification of small amounts of highly hazardous materials,(3) dealing with situations where the cost of disposing of a leg-acy waste to meet acceptable risk values is prohibitive, and(4) classifying new hazardous substances or types of waste.

It is highly desirable, however, that flexibility be applied in areasonably consistent manner to different special circumstances.This means that the rationale for exceptions should be developedusing the precepts of the risk-based waste classification system,and that decisions should be documented in a form that is readilyavailable so as to constitute a body of precedent.

5.9 FLEXIBLE / 255

Essentially all existing waste classifications systems include someprovisions to incorporate flexibility. However, such provisions oftenare established by ad hoc decisions of regulatory authorities. Litiga-tion intended to either engender or challenge flexibility is ofteninvolved. The results typically are exceptions to the rule and arecodified in separate sections of the regulations covering waste classi-fication, instead of being related to and justified in the context ofthe framework for classification.

6. Principles andFramework for aComprehensive and Risk-Based Hazardous WasteClassification System

This Section develops NCRP’s recommendations on the principlesand framework for a comprehensive and risk-based hazardous wasteclassification system. Implementation of the system also is discussed.These recommendations focus on classification of waste thatcontains hazardous substances for purposes of permanentdisposal. The proposed waste classification system was developedto address deficiencies in the existing waste classification systemsdiscussed in Sections 2, 4 and 5.

The basic framework for the waste classification system developedin this Report is depicted in Figure 6.1. Starting with the objectivesthat the classification system should apply to any waste that containsradionuclides or hazardous chemicals and that all such waste shouldbe classified based on risks to the public posed by its hazardousconstituents, the fundamental principle of the proposed system isthat hazardous waste should be classified in relation to disposalsystems (technologies) that are expected to be generally acceptablein protecting public health. This principle leads to the definitions ofthree classes of waste, and to quantification of the boundaries of thedifferent waste classes based on considerations of risks that arisefrom different methods of disposal. The boundaries normally wouldbe specified in terms of limits on concentrations of hazardous sub-stances. At the present time, nearly all hazardous and nonhazardouswastes are intended for disposal in a near-surface facility or a geo-logic repository, and these are the two types of disposal systemsassumed in classifying waste. The three waste classes and theirrelationship to acceptable disposal systems are described in moredetail in Section 6.2.

Given the assumed types of disposal systems (near-surface facili-ties or geologic repositories), waste would be classified as exempt,low-hazard, or high-hazard based on the magnitude of its risk index,

256

6. PRINCIPLES AND FRAMEWORK / 257

Fig. 6.1. Basic framework for proposed hazardous waste classificationsystem.

a quantity which is introduced in Section 6.2 and further developedin Sections 6.3 and 6.4. For any hazardous substance, the risk indexessentially is the ratio of a calculated risk based on a postulatedexposure scenario for an assumed type of disposal system to a speci-fied limit on allowable risk for that disposal system. If the risk indexis less than a specified value (e.g., unity), the risk posed by the wasteis within acceptable bounds for the assumed type of disposal system

258 / 6. PRINCIPLES AND FRAMEWORK

and the waste would be classified accordingly. If the risk index isnot less than the specified value, the waste would be assigned to aclass containing more hazardous waste, except in the case of high-hazard waste that contains the most hazardous materials.

NCRP reiterates that the risk-based waste classification systemdeveloped in this Report does not, and cannot, obviate the need toestablish waste acceptance criteria at each hazardous waste disposalsite based on the characteristics of the site, the particular disposaltechnology, and characteristics of the wastes that are intended fordisposal at the site. NCRP expects that most waste that would beassigned to a particular class will be acceptable for disposal usingthe associated type of disposal technology indicated in Figure 6.1.However, the disposal capabilities of particular sites and engineeredsystems can vary substantially and can depend on the waste charac-teristics. The primary function of any waste classification systemis to facilitate development of cost-effective approaches to wastemanagement and disposal and effective communication on wastematters (see Section 2.1.2).

6.1 Issues of Risk Assessment and Risk Management

Previous sections of this Report have discussed concepts, prece-dents, and technical information that are important to developmentof NCRP’s recommendations on a comprehensive and risk-basedhazardous waste classification system. This Section discussesselected aspects of this background information that are criticalto establishing the principles and framework for the recommendedhazardous waste classification system. The topics discussed involvetechnical aspects of risk assessment and issues of risk management.

6.1.1 Measures of Response from Exposure toHazardous Substances

Development of a comprehensive and risk-based hazardous wasteclassification system requires assumptions about the measure ormeasures of response (adverse health effects) from exposure to radio-nuclides and hazardous chemicals that should be used in classifyingwaste. Possible measures of response discussed in Section 3.2.3include fatalities, incidence, or some combination of the two, suchas total detriment (ICRP, 1991). The following sections discuss themeasures of response from exposure to hazardous subtances that

6.1 ISSUES OF RISK ASSESSMENT AND RISK MANAGEMENT / 259

have been used in health protection and NCRP’s recommendationson the measure of response that should be used in classifying waste.

6.1.1.1 Measures of Response for Substances Causing DeterministicResponses. For purposes of health protection in routine exposuresituations, incidence has been the primary measure of deterministicresponse for both radionuclides and hazardous chemicals. Fatalitiesalso are of concern for substances that cause deterministic responses,but only at doses substantially above the thresholds for nonfatalresponses. Given that the objective of standards for health protectionis to prevent the occurrence of deterministic responses, incidence isnot modified by any subjective factors that take into account, forexample, the relative severity of different nonfatal responses withrespect to a diminished quality of life. Judgments about the impor-tance of deterministic responses are applied only in deciding whichresponses are sufficiently adverse to warrant consideration in settingprotection standards.

For the purpose of developing a risk-based hazardous waste classi-fication system, prevention of deterministic responses should be ofconcern only for hazardous chemicals, but not for radionuclides.Deterministic responses from exposure to radionuclides can beignored because radiation dose limits for the public intended to limitthe occurrence of stochastic responses are sufficiently low that thedoses in any organ or tissue would be well below the thresholds fordeterministic responses (see Section 3.2.2.1).

6.1.1.2 Measures of Response for Substances Causing StochasticResponses. Stochastic responses from exposure to both radionu-clides and hazardous chemicals must be taken into account in devel-oping a comprehensive and risk-based waste classification system.Such responses are assumed to occur with some probability at anydose and the responses of concern (primarily cancers) often are fatal.Therefore, consideration must be given to the question of whetherfatalities, incidence, or some combination of the two is the mostappropriate measure of response for substances causing stochasticresponses. The following sections discuss the advantages and disad-vantages of the three options.

6.1.1.2.1 Incidence. In the first option, the common measure ofstochastic response from exposure to radionuclides and hazardouschemicals would be incidence, without any modifications to accountfor such factors as differences in lethality fractions for responses indifferent organs or tissues or expected years of life lost per fatality.Such modifications are intended to represent differences in the sever-ity of different stochastic responses.


This option offers a number of advantages. First, incidence is themeasure of response that generally has been used for chemicals thatinduce stochastic responses, mainly because dose-response data formost chemicals obtained from studies in animals are reported onlyin terms of incidence. Second, incidence is the only measure ofresponse that is generally applicable to any hazardous subtancesbecause many deterministic responses of concern are nonfatal.Therefore, with this option, the same measure of response would beused for all hazardous substances in classifying waste, regardlessof the associated type of response. Third, incidence is a simple mea-sure of response that is easily understood by the public. It mayprovide the best representation of societal concerns about hazardoussubstances, although cancer perhaps would not be so fearful if itwere not often fatal. Fourth, incidence does not depend greatly onthe availability and intensity of medical care. In general, if incidencewere used as the common measure of response for any hazardoussubstance, waste classification essentially could be based on thesimple notion of avoidance of harm.

However, this option presents some difficulties for radionuclides,because studies of radiation effects in human populations havefocused on cancer fatalities as the measure of response and probabil-ity coefficients for radiation-induced cancer incidence have not yetbeen developed by ICRP or NCRP for use in radiation protection.Probabilities of cancer incidence in the Japanese atomic-bomb survi-vors have been obtained in recent studies (see Section 3.2.3.2), butprobability coefficients for cancer incidence appropriate for use inradiation protection would need to take into account available dataon cancer incidence rates from all causes in human populations ofconcern, which may not be as reliable as data on cancer fatalities.Thus, in effect, if incidence were used as the measure of stochasticresponse for radionuclides, the most technically defensible databaseon radiation effects in human populations available at the presenttime (the data on fatalities in the Japanese atomic-bomb survivors)would be given less weight in classifying waste.

Another possible disadvantage of using incidence is that stochasticresponses that are rarely fatal (e.g., skin and thyroid cancers) wouldbe given the same weight as responses that are almost always fatal,even though the latter presumably are of greater concern. Althoughany effort to weight the severity of nonfatal responses relative tofatal responses necessarily involves subjective judgment, this pointis addressed by ICRP (1991) in its recommendation that nonfatalresponses should be weighted by the lethality fraction in assessingtotal detriment from radiation exposure.


6.1.1.2.2 Fatalities. In the second option, the common measureof stochastic response from exposure to radionuclides and hazardouschemicals would be fatalities, without any modifications to accountfor such factors as differences in lethality fractions for responses indifferent organs or tissues or expected years of life lost per fatality.This option is particularly advantageous for radionuclides, becausefatalities is the measure of response provided by the most scientifi-cally defensible database on stochastic radiation effects in humans.Fatalities is the measure of response normally emphasized in radia-tion risk assessments.

A disadvantage of using fatalities is that it does not take intoaccount the effects of nonfatal cancers on the quality of life. Althoughcancer is fearful mainly because it is often fatal, treatments even incases that are rarely fatal (e.g., thyroid and skin cancer) generallyare not welcomed, and some successful cures can significantly affectan individual’s quality of life. The use of fatalities also does not takeinto account that cures for cancer depend greatly on the availabilityand intensity of medical care, which in turn depends on socio-economic conditions.

Another disadvantage of this option is that it might be difficult toimplement for chemicals that cause stochastic responses. At thepresent time, probability coefficients for nearly all substances thatcause stochastic responses are based on data on incidence only, andthe data are obtained mostly from studies in animals. This difficultycould be addressed if the observed responses in animals were thesame as known responses in humans. In such cases, the lethalityfraction (k) for cancers in different organs or tissues (see Table 3.2)could be used to convert cancer incidence to fatalities. In most cases,however, the organs or tissues in which cancers are induced in studyanimals are not the same as the cancer sites in humans, or theestimates of cancer incidence in animals are based on pooledresponses at all sites. In other cases, the observed responses inanimals are not known to occur in humans. In either of these situa-tions, judgment would be needed in applying a lethality fraction tothe data on cancer incidence obtained from studies in animals. Anassumption of a lethality fraction of 0.6 to 0.7, which is the averagefor all types of cancers in many organs, should be reasonable in thesecases, because the actual value could not be underestimated by morethan 30 to 40 percent. Difficulties in converting data on cancer inci-dence to fatalities would not occur when the data on cancer incidenceare obtained from studies in humans. However, human data areavailable for only a few chemicals that induce stochastic responses.

6.1.1.2.3 ICRP’s total detriment. In the third option, the commonmeasure of stochastic response from exposure to radionuclides and


hazardous chemicals would be total detriment, as developed by theICRP (1991) for use in radiation protection (see Section 3.2.2.3.2).Total detriment is calculated from probability coefficients for fatalresponses by applying modifying factors that take into account non-fatal responses, which are weighted by the lethality fraction forresponses in each organ or tissue, and the expected years of life lostfrom fatal responses in each organ or tissue relative to the expectedyears of life lost from all fatal responses. The basic assumptionused in calculating total detriment is that fatalities are the primarystochastic responses of concern, but that nonfatal responses alsowarrant consideration in health protection.

This option does not appear to be advantageous for either radionu-clides or chemicals that cause stochastic responses. In radiationprotection, total detriment is used mainly to develop the tissueweighting factors in the effective dose (see Section 3.2.2.3.3), butICRP and NCRP have continued to emphasize fatal responses asthe primary health effect of concern in radiation protection andradiation risk assessments. Since total detriment is based on anassumption that fatalities are the primary health effect of concern,the same difficulties described in the previous section would occurif this measure of response were used for chemicals that inducestochastic responses. Other disadvantages of using total detrimentinclude that detriment is not a health-effect endpoint experiencedby an exposed individual and the approach to weighting nonfatalresponses in relation to fatalities is somewhat arbitrary. Further-more, total detriment is not as simple and straightforward to under-stand as either incidence or fatalities.

6.1.1.3 Recommendations on Selection of a Measure of Response.For purposes of waste classification, NCRP believes that it wouldbe desirable, in principle, to use the same measure of response forall hazardous substances, essentially because this approach wouldhelp give equal weight to all such substances in classifying waste.

Incidence is the common measure of response for all substancesthat cause a deterministic effect, including radionuclides, used inroutine health protection, and there is no evident reason to changethis. As indicated by the discussions in the previous sections, argu-ments can be advanced in favor of using either incidence or fatalitiesas the common measure of stochastic response. Use of ICRP’s totaldetriment appears to be disadvantageous, compared with either inci-dence or fatalities, and is not considered further.

In classifying waste based on risk, incidence appears to be the mostlogical common measure of response for all hazardous substances,primarily because incidence is the only measure that is generally


applicable to all subtances that cause stochastic or deterministicresponses. Furthermore, this approach would involve the fewest sub-jective judgments about the relative importance of fatal and nonfatalresponses, although it clearly incorporates a judgment for substancesthat cause stochastic responses that the incidence of any type ofresponse would be of equal concern, regardless of the likelihood thatthe response would be fatal.

However, given the current state of knowledge and methods ofdose-response assessment for substances that cause stochasticresponses, there appear to be important technical and institutionalimpediments to the use of either incidence or fatalities exclusively.Data on radiation-induced cancer incidence and chemical-inducedcancer fatalities for use at the low doses and dose rates relevant tohealth protection are not readily available, and current regulatoryguidance calls for calculation of cancer incidence for hazardous chem-icals. Since use of a common measure of response for all substancesthat cause stochastic responses may not be practical in the nearterm, both measures (fatalities for radionuclides and incidence forhazardous chemicals) could be used in the interest of expediency.The primary advantage of this approach is that the measures ofstochastic response for radionuclides and hazardous chemicals wouldbe based on the best available information from studies in humansand animals, and it would involve the fewest subjective modifyingfactors. This approach also would be the easiest to implement.

The approach of using fatalities for radionuclides but incidence forhazardous chemicals clearly would not result in a consistent measureof stochastic response for all substances of concern. However, cancerincidence and fatalities do not differ by more than a factor of two tothree in most organs or tissues except the thyroid and skin (seeTable 3.2). Thus, the difference between incidence and fatalitieswould not be large for most substances that cause stochasticresponses, particularly compared with uncertainties in the data onwhich the estimated probabilities of stochastic responses are based,uncertainties in extrapolating data from animals to humans, anduncertainties in extrapolating from data at high doses and dose ratesto the low doses and dose rates of concern in routine exposuresof the public. Furthermore, the difference between incidence andfatalities would be insignificant in most cases compared with differ-ences discussed in the following section that result from the differentapproaches to establishing probability coefficients for stochasticresponses from exposure to radionuclides and hazardous chemicals.

6.1.2 Dose-Response RelationshipsDevelopment of a comprehensive and risk-based hazardous waste

classification system requires assumptions about thresholds in the


dose-response relationships for hazardous chemicals that cause deter-ministic effects and probability coefficients for induction of stochasticresponses from exposure to radionuclides and hazardous chemicals.Establishment of dose-response relationships for radionuclides andhazardous chemicals is discussed in Section 3.2. The following sec-tions discuss NCRP’s recommendations on suitable approaches toaddressing these issues for purposes of waste classification.

6.1.2.1 Deterministic Responses. At the low levels of exposure ofconcern to waste classification, deterministic responses generally areimportant only for hazardous chemicals, but not for radionuclides.

For chemicals that cause deterministic effects, NCRP believesthat threshold doses in humans should be estimated using the bench-mark dose method; the benchmark dose is a dose that correspondsto a 10 percent increase in the number of responses and is obtainedby statistical fitting of a dose-response model to the dose-responsedata in the region where the number of responses is increased (seeSection 3.2.1.2.7). Specifically, NCRP believes that a suitable repre-sentation of the threshold in the dose-response relationship in virtu-ally all humans is a dose that is a factor of 10 lower than the lowerconfidence limit of the benchmark dose obtained in a high-qualityhuman study or a dose that is a factor of 100 lower than the lowerconfidence limit of the benchmark dose obtained in a high-qualityanimal study. The reduction by a factor of 10 when the benchmarkdose is obtained in a human study takes into account the need toprotect sensitive population groups (e.g., infants and children, theelderly and infirm). This reduction is consistent with the approachused in radiation protection of the public, where deterministic doselimits are set at a factor of 10 lower than nominal thresholds fordeterministic radiation effects in adults. The further reduction bya factor of 10 when the benchmark dose is obtained in an animalstudy takes into account that the animals may be less sensitive thanhumans. The recommended approach to estimating threshold dosesof chemicals that induce deterministic effects in humans acknowl-edges the considerable uncertainty in estimating the highest doseat which no significant effects would be observed. However, theapproach should not be unduly conservative and, thus, should notgive disproportionate weight to chemicals that induce deterministiceffects, compared with radionuclides and chemicals that cause sto-chastic effects, in classifying waste. Characteristics of high-qualitystudies in animals are discussed in Section 3.1.4.1.2.

As an alternative to using the benchmark dose method, the moretraditional approach of estimating threshold doses of substancesthat cause deterministic effects based on NOAELs could be used. In


most high-quality studies, the two methods are largely equivalentbecause NOAEL is approximately the same as the lower confidencelimit of the benchmark dose that corresponds to a 10 percent increasein response. Thus, the nominal threshold in humans could be set ata factor of 10 or 100 lower than NOAEL obtained in a high-qualityhuman or animal study, respectively. However, NCRP prefers thebenchmark dose method, mainly because the method makes use ofthe full range of data on dose-response, rather than a single datapoint (NOAEL). The benchmark dose method also can address diffi-culties that arise when NOAEL is not obtained in a high-qualitystudy or is not included in a data set.

Estimated thresholds for deterministic responses in virtually allhumans based on lower confidence limits of benchmark doses orNOAELs, as described above, would be used as points of departure inestablishing allowable doses of chemicals that induce deterministicresponses for purposes of waste classification. NCRP’s recommenda-tions on the magnitude of safety and uncertainty factors that shouldbe applied to benchmark doses or NOAELs in classifying waste aredescribed in Section 6.3.1.1.

6.1.2.2 Stochastic Responses. Consideration of the dose-responserelationships and the nominal probability coefficients for inductionof stochastic responses at low doses is important for both radionu-clides and hazardous chemicals.

For radionuclides, NCRP reaffirms use of a best estimate (MLE)of the response probability obtained from a linear or linear-quadraticmodel as derived from data in humans, principally the Japaneseatomic-bomb survivors. This model essentially is linear at the lowdoses of concern to waste classification. Specifically, for purposes ofhealth protection of the public, NCRP reaffirms use of a probabilitycoefficient for fatal cancers (probability per unit effective dose) of0.05 Sv�1 (ICRP, 1991; NCRP, 1993a). Although this probabilitycoefficient is less rigorous for intakes of some long-lived radionuclidesthat are tenaciously retained in the body than for other exposuresituations, such as external exposure or intakes of short-lived radio-nuclides (Eckerman et al., 1999), it is adequate for the purpose ofgenerally classifying waste, especially when the lack of data on can-cer risks in humans for most chemicals is considered.

For chemicals that cause stochastic responses, NCRP believes thata linearized multi-stage model should be used to estimate risks atlow doses based on data at high doses in humans or animals. Further-more, NCRP believes that best estimates (MLEs) of response proba-bilities obtained from that model should be used for purposes of risk-based waste classification, rather than UCLs that are used nearly


universally in health risk assessments for chemicals that cause sto-chastic responses. The recommended approach should provide esti-mates of probability coefficients for chemicals that are reasonablyconsistent with the value for radionuclides given above. Thus, compa-rable weight would be given to the two types of substances in classify-ing waste.

NCRP believes that the use of MLEs of probability coefficients forradionuclides and chemicals that cause stochastic effects can bejustified on the grounds that the types of exposure scenarios thatwould be assumed for purposes of classifying waste (see Section 6.1.3below) are likely to provide considerable overestimates of actualexposures at waste disposal sites. However, uncertainties in proba-bility coefficients should not be ignored in classifying waste. Whenrisk is calculated using MLEs of probability coefficients, judgmentsabout allowable risk that are required in classifying waste based onrisk should take uncertainties in probability coefficients into account,along with such other factors as judgments about the quality of thedata on dose-response, desired margins of safety in protecting thepublic, and the cost-benefit of different choices. This approach wouldprovide a clear separation between risk assessment and risk manage-ment aspects of waste classification. Risk assessment would focuson central estimates of risk for assumed exposure scenarios, andrisk management decisions based on judgments about allowable risk,which can be substance-specific, could incorporate any desireddegrees of conservatism in protecting the public beyond those embod-ied in the assumed scenarios.

6.1.3 Exposure Scenarios for Waste Classification

Assumptions about exposure scenarios are required in developinga risk-based waste classification system. These scenarios would beused to calculate potential risks posed by hazardous wastes for pur-poses of waste classification. An exposure scenario essentially is aset of assumptions about events and processes that could result inexposure of humans.

In principle, for any type of disposal system that could be assumedfor purposes of classifying waste, such as a near-surface disposalfacility for hazardous wastes, a multitude of exposure scenariosmight be considered. However, NCRP believes that only a singletype of exposure scenario should be considered in classifying waste.Specifically, NCRP believes that the concept of a hypothetical inad-vertent intruder at waste disposal sites provides a suitable basis for


defining exposure scenarios relevant to risk-based waste classifica-tion. Scenarios for inadvertent intrusion are appropriate, in part,because they often do not depend greatly on the characteristics of aparticular disposal site and facility.

Given the concept of a hypothetical inadvertent intruder at wastedisposal sites, a multitude of scenarios for exposure of such individu-als could be considered. NCRP believes that the types of scenarioscommonly used in risk assessments at near-surface disposal sitesfor low-level radioactive waste (NRC, 1982b) or cleanup of sites con-taminated with radionuclides or hazardous chemicals (EPA, 1989;Kennedy and Strenge, 1992) would be appropriate. These scenariosgenerally assume permanent residence at a disposal site at anytime after loss of institutional control and, thus, should provideconservative overestimates of risks to inadvertent intruders that arereasonably likely to occur at any site, including risks associated withscenarios involving short-term exposure that might occur during theperiod of institutional control. Although other scenarios could beenvisioned that might result in higher estimates of exposure, suchscenarios would not be appropriate for the purpose of classifyingwaste if they were not reasonably likely to occur for the typesof disposal systems of concern. NCRP does not believe that risk-based waste classification should be based on implausible, worst-case assumptions.

NCRP also recognizes that potential exposures of members of thepublic beyond the boundaries of disposal sites generally are of con-cern in determining acceptable disposal practices at any site. How-ever, at most disposal sites, off-site releases of many hazardoussubstances and resulting exposures of the public are determinedprimarily by the movement of water and, thus, are expected to behighly site-specific. The dependence of exposure scenarios involvingoff-site release of contaminants on site-specific characteristics makesthese types of scenarios inappropriate for use in classifying wastebased on generic assumptions about disposal systems.

The general concern about limiting off-site releases of hazardoussubstances is the primary reason why classification of waste basedon risks to hypothetical inadvertent intruders at waste disposal sitesdoes not provide a substitute for site-specific risk assessments whendetermining acceptable disposal practices. Nonetheless, experiencewith risk assessments at near-surface disposal sites for low-levelradioactive waste has indicated that, for most radionuclides, disposallimits that provide adequate protection of future inadvertent intrud-ers should provide adequate protection of the public and the environ-ment at off-site locations as well. Exceptions are expected to occuronly in unusual cases of long-lived and highly mobile radionuclides.


Similar conclusions should apply to hazardous chemical waste, espe-cially waste that contains heavy metals. Thus, basing waste classifi-cation on scenarios for inadvertent intrusion should not result inlarge quantities of waste that would not be acceptable for disposalusing the intended disposal technology for the particular waste classat well chosen sites.

The types of scenarios for inadvertent intrusion that could be usedin classifying waste are discussed further in Section 6.3.2.

6.1.4 Approaches to Risk Management

Classification of hazardous waste based on risk requires assump-tions about allowable risks from exposure to hazardous substances.Therefore, an understanding of current approaches to risk manage-ment for radionuclides and hazardous chemicals, especially theirdifferences and how they can be reconciled, is important in classify-ing waste.

Two different approaches to risk management, referred to as theradiation and chemical paradigms, are embodied in current lawsand regulations addressing hazardous substances in the environ-ment (see Section 3.3). The radiation paradigm involves establishinglimits on acceptable risk and requiring reductions in risk below thelimits to levels that are ALARA, economic and social factors beingtaken into account. In contrast, the chemical paradigm involvesestablishing goals for acceptable risk and allowing relaxations inrisks above the goals based primarily on considerations of technicalfeasibility and cost. As a result of this difference, the two paradigmsattach different meanings to the terms ‘‘acceptable’’ and ‘‘unaccept-able’’ commonly used to describe risk (see Table 3.5).

NCRP believes that a single paradigm for risk management shouldbe applied in developing a risk-based classification system for wastethat contains radionuclides or hazardous chemicals, and NCRP rec-ommends use of the radiation paradigm for this purpose. In makingthis recommendation, NCRP recognizes that there is the appearanceof allowing higher stochastic risks than might be permitted if thechemical paradigm were used in classifying waste. However, NCRPemphasizes that this is not necessarily the case because (1) stochasticrisks regarded as ‘‘unacceptable’’ in the chemical paradigm (i.e.,excess lifetime cancer risks above negligible levels in the range ofabout 10�4 to 10�6) often are permitted, based essentially on the sameapplication of the ALARA principle as in the radiation paradigm, and(2) application of the ALARA principle in the radiation paradigmusually reduces stochastic risks to levels well below those that are


regarded as intolerable (i.e., well below lifetime risks in the rangeof about 10�1 to 10�3). Indeed, in the radiation and chemical para-digms, the ALARA principle is the primary basis for most decisionsabout stochastic risk management, without regard for any limits inthe radiation paradigm or goals in the chemical paradigm (Kocher,1999; NAS/NRC, 1999a).

The ALARA principle also has been used in decisions about riskmanagement for chemicals that cause deterministic effects. RfDsoften are used to define acceptable exposures to such substances.However, given the large safety and uncertainty factors often usedin deriving RfDs from a NOAEL or LOAEL (see Section 3.2.1.2.4),RfDs generally correspond to doses considered negligible, and dosesabove an RfD may be permitted in particular situations if RfD isnot achievable at a reasonable cost (see Section 3.3.2).

In recommending use of the radiation paradigm in classifyingwaste, NCRP does not mean to imply that the chemical paradigm isinappropriate for use in risk management. Indeed, NCRP recognizesthat the chemical paradigm is a valid approach to risk managementif the risk goals are properly interpreted as defining negligible risks.However, NCRP believes that the radiation paradigm offers threeimportant advantages compared with the chemical paradigm: (1) aclear concept of risks that are intolerable and normally must bereduced regardless of cost or other circumstances, (2) explicit recog-nition of the importance of the ALARA principle in reducing risksbelow levels regarded as barely tolerable, and (3) a clear distinctionbetween risks that are negligible and higher risks that are acceptableprovided they are ALARA. For purposes of waste classification, theradiation paradigm, which defines risks that are unacceptable (intol-erable), acceptable if ALARA, and negligible, permits the use ofdifferent limits on allowable risk to define different waste classes.Specifically, as indicated in Figure 6.1, a negligible risk can be usedto distinguish between exempt and low-hazard waste, and a substan-tially higher acceptable (barely tolerable) risk can be used to distin-guish between low-hazard and high-hazard waste.

6.1.5 Legal and Regulatory Constraints

NCRP recognizes that if the waste classification system describedin this Report is to gain acceptance, it must be broadly compatiblewith current approaches to management and disposal of hazardouswastes. However, NCRP believes that development of a new wasteclassification system to address deficiencies in the existing systemsshould not be constrained by provisions of current laws or regulations


that might discourage such a system. Examples of such constraintsinclude: the distinction between radioactive waste that arises fromoperations of the nuclear fuel-cycle and other (NARM) waste (seeSection 4.1.2.1); the provision of the National Energy Policy Act thatprohibits NRC from establishing dose criteria that could be usedto develop exemption levels for radionuclides in any waste (seeSection 4.1.2.5.2); and the ‘‘mixture’’ and ‘‘derived-from’’ rules forlisted hazardous chemical wastes under RCRA (Section 4.2.1.1).NCRP believes that its recommendations should focus on the techni-cal basis for risk-based hazardous waste classification withoutregard for any legal or regulatory constraints that are largely unre-lated to risks posed by waste.

6.2 Framework for Risk-Based Waste Classification

This Section presents NCRP’s recommendations on a frameworkfor a comprehensive and risk-based hazardous waste classificationsystem. These recommendations focus primarily on the conceptsand principles embodied in the new system. Approaches to imple-menting the waste classification system by specifying quantitativeboundaries of different waste classes in the form of limits on concen-trations of hazardous substances are discussed in this Section and inSection 6.4.5, and numerical examples are developed in Section 7.1.However, NCRP believes that the task of specifying such boundariesis properly the role of regulatory authorities, and specific recommen-dations on limits on concentrations of hazardous substances in differ-ent waste classes are not presented.

6.2.1 Framework of the Proposed Waste Classification System

The framework for the comprehensive and risk-based waste classi-fication system developed in this Report is depicted in Figure 6.1 atthe beginning of Section 6. Classification of waste is based on specificobjectives and the fundamental principle of defining waste classesin relation to acceptable disposal systems, and these lead to thedefinitions of three basic waste classes.

The remainder of this Section through Section 6.5 discusses theframework for the hazardous waste classification system depictedin Figure 6.1 and its expected consequences in more detail. Themerits of this general framework in the context of radioactive wasteclassification and its basis in the three distinct types of disposalsystems have been recognized for many years (Adam and Rogers,

6.2 FRAMEWORK FOR RISK-BASED WASTE CLASSIFICATION / 271

1978; King and Cohen, 1977). The possibility of defining subclassesof the three basic waste classes is discussed in Section 6.6.

6.2.2 Framework for Waste Classification

The basic element of the recommended framework for a compre-hensive and risk-based waste classification system is the assumptionthat any waste that contains sufficiently small amounts of radionu-clides or hazardous chemicals should be classified as exempt, oressentially nonhazardous. Waste that contains greater amounts ofhazardous substances then would be classified as nonexempt, andfurther classification of nonexempt wastes, based also on theamounts of hazardous substances present, would be appropriate.

Limits on amounts of hazardous substances in each waste classwould be calculated based on values of the so-called risk index foreach hazardous substance in the waste and the composite risk indexfor mixtures of hazardous substances. For the purpose of describingthe recommended framework for a risk-based hazardous waste clas-sification system, the risk index is generally defined as:

RI � F(risk from disposal)

(allowable risk), (6.1)

where F is a modifying factor described below (F � 0). RI essentiallyis the ratio of a calculated risk that arises from disposal of a givenwaste using a particular type of disposal system (technology) to aspecified limit on allowable risk for that type of disposal system. Thecalculated risk in the numerator would be based on an assumedexposure scenario that is appropriate to the assumed type of disposalsystem. If the modifying factor is omitted, the risk index is in theform of a hazard quotient, which often is used to describe exposuresto chemicals that induce deterministic effects (EPA, 1989).

The modifying factor in the risk index represents any considera-tions of importance to waste classification other than those that aredirectly incorporated in the calculated risk from disposal and thespecified allowable risk. The modifying factor can take into account,for example, the probability of occurrence of assumed exposure sce-narios used in classifying waste, uncertainties in the assessment ofrisk from disposal and in the data required to evaluate the riskindex, levels of naturally occurring hazardous substances in surfacesoil and their associated health risks to the public, and the costsand benefits of different means of waste disposal. The modifyingfactor is discussed further in Section 6.3.3.


The basic definitions of exempt, low-hazard, and high-hazardwaste shown in Figure 6.1 are considered in the following sections.Recommendations on approaches to calculating the risk from wastedisposal in the numerator of the risk index and recommendationson specifying allowable risks in the denominator of the risk indexfor the purpose of classifying waste are discussed in Section 6.3.

6.2.2.1 Exempt Waste. The class of exempt waste embodies theconcept that there are amounts of hazardous substances in wastewhich are so low that the associated risks to the public for anymethod of disposal generally would not be of concern. Thus, if wastethat contains radionuclides were classified as exempt, the wastecould be disposed of as if it were nonradioactive, and similarly forwaste that contains hazardous chemicals or mixtures of the two.Further, mixed wastes that contain exempt amounts of radionuclidescould be managed based on their hazardous chemical content, andvice versa.

NCRP believes that different classes of waste should be definedin relation to general types of disposal systems that presently existor are likely to be developed in the future. In accordance with currentwaste disposal practices, the exempt class of waste (essentially non-hazardous) thus is defined as any waste containing sufficiently smallamounts of hazardous substances that the waste would be generallyacceptable for disposal in a municipal/industrial landfill (or equiva-lent) for nonhazardous materials. This type of disposal facility isregulated under Subtitle D of RCRA (1976).

Because disposal of exempt waste would be unregulated in regardto its hazardous constituents, NCRP recommends that limits onconcentrations of radionuclides and hazardous chemicals in exemptwaste should be defined on the basis of a negligible risk to hypotheti-cal inadvertent intruders at near-surface waste disposal sites. Thatis, the allowable risk in the denominator of Equation 6.1 shouldcorrespond to a negligible risk. Waste would be classified as exemptif the risk index calculated in this way were less than unity, butotherwise would be nonexempt.15 NCRP again notes that, in accor-dance with the radiation paradigm for risk management, a negligiblerisk is distinct from, and considerably less than, an acceptable(barely tolerable) risk.

15 Expressing the decision rule for waste classification in terms of a risk index lessthan unity conforms to the approach used by NRC in its classification system fornear-surface disposal of radioactive waste in 10 CFR Part 61 (NRC, 1982a).

6.2 FRAMEWORK FOR RISK-BASED WASTE CLASSIFICATION / 273

6.2.2.2 Nonexempt Waste. NCRP recommends that nonexemptwaste—i.e., waste that contains amounts of radionuclides, hazard-ous chemicals, or both greater than the allowable amounts in exemptwaste—be placed in one of two classes, called low-hazard waste andhigh-hazard waste.

6.2.2.2.1 Low-hazard waste. NCRP recommends that low-hazardwaste be defined as any nonexempt waste that is generally acceptablefor disposal in a dedicated near-surface facility for hazardous wastes.Examples of such facilities include licensed or permitted for disposalof low-level radioactive waste under AEA (1954) or disposal of haz-ardous chemical waste under Subtitle C of RCRA (1976).

Because nonexempt waste would be carefully regulated withrespect to its hazardous constituents, NCRP recommends that limitson concentrations of radionuclides and hazardous chemicals in low-hazard waste should be defined on the basis of an acceptable (barelytolerable) risk to hypothetical inadvertent intruders at near-surfacewaste disposal sites. That is, the allowable risk in the denominatorof Equation 6.1 should correspond to an acceptable risk. NCRP reiter-ates that, in accordance with the radiation paradigm for risk man-agement, an acceptable risk is distinct from, and considerablygreater than, a negligible risk that would be used as a basis forclassifying exempt waste. As a result, limits on concentrations ofhazardous substances in low-hazard waste generally would be sub-stantially higher than in exempt waste. Waste would be classifiedas low-hazard if the risk index calculated in this way were less thanunity. Otherwise, the waste would be classified as high-hazard.

The use of an acceptable (barely tolerable) risk to classify nonex-empt waste can be justified, in part, on the following grounds. Dis-posal facilities for exempt and low-hazard waste both are locatednear the ground surface, and many scenarios for inadvertent intru-sion into municipal/industrial landfills for nonhazardous waste alsowould be credible occurrences at disposal sites for low-hazard waste.However, these types of scenarios should be less likely to occur athazardous waste sites, compared with sites for disposal of nonhaz-ardous waste, given the intention to maintain institutional controland records of past disposal activities for a considerable period oftime after closure of hazardous waste sites and the possibility thatsocietal memory of disposal activities will be retained long afterinstitutional control is relinquished. Thus, the risk to future inadver-tent intruders at dedicated hazardous waste disposal sites, takinginto account the probability that exposures according to postulatedscenarios would actually occur, should be comparable to the risk atdisposal sites for nonhazardous waste.


6.2.2.2.2 High-hazard waste. NCRP recommends that high-hazard waste be defined to include any nonexempt waste with con-centrations of hazardous substances greater than those that aregenerally acceptable for disposal in a dedicated near-surface facilityfor hazardous wastes. High-hazard waste would require a disposalsystem considerably more isolating than a near-surface facility forhazardous waste.

An essential feature of acceptable disposal systems for high-hazardwaste is that the waste would need to be isolated so that inadvertentintrusion into the waste as a result of normal human activities, suchas drilling or excavation, would be highly unlikely. Therefore, ananalysis of the potential consequences to an inadvertent intruder,given an assumption that a highly unlikely intrusion scenario wouldactually occur, does not provide a reasonable basis for determiningdisposal systems that should be generally acceptable for highly haz-ardous wastes (NAS/NRC, 1995a). Disposal systems for high-hazardwaste also need to be capable of limiting environmental releases ofhighly concentrated wastes to acceptable levels for long periods oftime using natural and engineered barriers. In this regard, the conse-quences of an unlikely intrusion event, such as drilling, for increasingpotential releases to the biosphere by the groundwater pathway area legitimate concern in licensing disposal facilities for high-hazardwaste (EPA, 2001a; NAS/NRC, 1995a). However, these considera-tions are site-specific, and they do not provide an appropriate basisfor classifying waste in general terms.

At the present time, a geologic repository is the intended disposalsystem for most radioactive waste that is not acceptable for near-surface disposal. Alternatives to near-surface disposal facilities havenot been considered for hazardous chemical waste that containsunusually high concentrations of persistent substances (e.g.,heavy metals).

6.3 Development of the Risk Index for IndividualHazardous Substances

For the purpose of developing a comprehensive and risk-basedhazardous waste classification system, a simple method of calculat-ing the risk posed by mixtures of radionuclides and hazardous chemi-cals in waste is needed. The method should properly account for thepresence of subtances in waste that cause deterministic or stochasticresponses. As a first step in accomplishing this, it is necessary tospecify a method for calculating the risk posed by individual sub-stances with associated deterministic or stochastic responses.

6.3 DEVELOPMENT OF THE RISK INDEX / 275

NCRP assumes that the risk from disposal of any hazardous sub-stance in waste can be described by means of a dimensionless riskindex. The risk index for the ith hazardous substance is defined asthe calculated risk from disposal of that substance, based on anassumed exposure scenario, relative to a specified allowable risk forthe assumed type of disposal system. Based on this definition, therisk index is written as:

RIi � Fi(risk from disposal) i

(allowable risk) i, (6.2)

where F is a modifying factor introduced with Equation 6.1 anddiscussed in more detail in Section 6.3.3. The modifying factor candepend on the particular hazardous substance.

The risk index in Equation 6.2 is expressed in terms of risk (i.e., theprobability that an adverse response will occur during an individual’slifetime). This definition is consistent with the fundamental objectiveof developing a risk-based hazardous waste classification system.However, the use of health risk per se in calculating the risk indexpresents some difficulties because risk is not proportional to dosefor substances that cause deterministic effects. For this type of sub-stance, the risk is presumed to be zero at any dose below a nominalthreshold. Since the allowable dose should always be less than thethreshold in order to prevent the occurrence of adverse responses,expressing the risk index in terms of risk would result in an indeter-minate value and, more importantly, a lack of distinction betweendoses near the nominal thresholds and lower doses of much lessconcern. For any hazardous substance, including carcinogens forwhich risk is assumed to be proportional to dose without threshold,it is generally useful to express the risk index as the ratio of acalculated dose [e.g., sieverts, mg (kg d)�1] to an allowable dose thatcorresponds to an allowable risk:

RIi � Fi(dose from disposal) i

(allowable dose) i, (6.3)

NCRP believes that use of a risk index expressed in terms of doseis acceptable and desirable as long as (1) the units of the numeratorand denominator are consistent at a conceptual level, (2) the assump-tions embodied in the proportionality constants between dose andresponse for substances that cause stochastic responses are clearlystated, and (3) the allowable doses are adjusted when the proportion-ality constants between dose and response for substances that causestochastic responses or the thresholds for substances that causedeterministic responses change significantly.


6.3.1 Establishing Allowable Risks or Doses ofIndividual Substances

Establishing allowable risks or doses in the denominator of therisk index involves two major steps. First, societal judgments aboutan allowable risk for the general types of disposal systems assumedin classifying waste are required. Second, when the risk index isexpressed in terms of dose, an allowable risk must be related to dose.For chemicals that cause deterministic effects, these steps involveestablishing an appropriate threshold in the dose-response relation-ship and applying judgments about safety and uncertainty factorsthat should be used in setting allowable doses at levels below thethreshold. For radionuclides and chemicals that cause stochasticeffects, the second step involves an assumption about the probabilityof a response per unit dose. These probability coefficients, whenapplied to an assumed allowable risk, determine allowable doses.

NCRP’s recommendations on specifying an allowable risk or dosein the denominator of the risk index for the purpose of classifyingwaste are discussed in the following two sections. A general discus-sion of dose-response assessment for hazardous chemicals and radio-nuclides is presented in Section 3.2.

6.3.1.1 Establishing Allowable Doses of Substances That CauseDeterministic Responses. The risk index for substances that causedeterministic responses normally should be expressed in terms ofdose, rather than risk, given the assumption of a threshold dose-response relationship. The allowable dose of substances that causedeterministic responses in the denominator in Equation 6.3 shouldbe related to thresholds for induction of deterministic responses indifferent organs or tissues.

NCRP recommends continued use of the current approach to con-trolling exposures to substances that cause deterministic responsesso that the probability of a response is essentially zero. Thus, dosescorresponding to an allowable risk should be set so that averageindividuals in the most sensitive population subgroups (e.g., infantsand children) would not be expected to experience a significant deter-ministic response. As noted previously, the issue of establishingallowable doses of substances that induce deterministic effects for thepurpose of classifying waste should be of concern only for hazardouschemicals. NCRP’s recommendations on establishing negligible andacceptable doses of chemicals that cause deterministic responses aredescribed in the following two sections.

6.3.1.1.1 Dose corresponding to a negligible risk. For the purposeof classifying waste as exempt, NCRP believes that a negligible dose


of a chemical that causes a deterministic effect should be set at asmall fraction (e.g., 10 percent) of the nominal threshold in virtuallyall humans estimated using the benchmark dose method as describedin Section 6.1.2.1. Furthermore, under the presumption that thedata on dose-response from studies in humans or animals are ofsufficiently high quality that the nominal threshold can be estimatedwith considerable confidence, the negligible dose should be set atthe same fraction of the nominal threshold for all chemicals thatcause deterministic effects. The recommended approach to establish-ing negligible doses of chemicals that induce deterministic effectsshould ensure that such effects would be precluded in almost allindividuals with a substantial margin of safety.

RfDs established by EPA also could be used to define negligibledoses of noncarcinogenic hazardous chemicals because RfDs areintended to be well below thresholds for deterministic responses inhumans. However, NCRP believes that RfDs should not be usedwithout presenting NOAELs or LOAELs used to derive the values.In addition, when RfDs for important waste constituents are derivedusing large safety and uncertainty factors, thus indicating that thequality of the data is poor, NCRP believes that further studies shouldbe undertaken to reduce uncertainties in the nominal threshold inhumans, to avoid introducing undue levels of conservatism in classi-fying waste. To promote consistency in waste classification, NCRPbelieves that it would be desirable to define negligible doses of allsubstances that cause deterministic responses at approximately thesame fraction of the nominal thresholds in humans.

6.3.1.1.2 Dose corresponding to an acceptable risk. For the pur-pose of classifying waste as low-hazard or high-hazard, NCRPbelieves that an acceptable (barely tolerable) dose of a chemical thatcauses deterministic effects should be set at the nominal thresholdin virtually all humans estimated as described in Section 6.1.2.1, orperhaps slightly below the nominal threshold (e.g., within a factorof two to three) if an additional margin of safety is warranted. Whenthe data on dose-response are of sufficiently high quality that thenominal threshold can be estimated with considerable confidence,the recommended approach to establishing acceptable (barely tolera-ble) doses of chemicals that induce deterministic effects shouldensure that average individuals in the most sensitive populationgroups would be adequately protected.

The use of multiples of RfDs established by EPA to define accept-able (barely tolerable) doses of chemicals that cause deterministicresponses also could be considered because RfDs normally areintended to be well below nominal thresholds for deterministic


responses in humans. However, the cautions about using RfDs dis-cussed in the previous section also apply in establishing acceptabledoses, especially when the quality of the data on dose-response ispoor. As in establishing negligible doses of substances that causedeterministic effects, NCRP prefers an approach in which acceptabledoses are based directly on nominal thresholds in humans and appli-cation of small safety factors, as appropriate, to promote transpar-ency and consistency in waste classification.

6.3.1.2 Establishing Allowable Risks or Doses of Substances ThatCause Stochastic Responses. Given the assumption of a linear dose-response relationship for substances that cause stochastic responseswithout threshold, either risk or dose may be used to calculate therisk index. The following two sections discuss suitable approachesto establishing negligible and acceptable risks or doses of substancesthat cause stochastic responses.

6.3.1.2.1 Establishing a negligible risk or dose. There are a num-ber of precedents for establishing a negligible risk or dose of sub-stances that cause stochastic responses for the purpose of classifyingexempt waste. For radionuclides, NCRP has recommended that anannual effective dose to individuals of 0.01 mSv generally can beconsidered negligible (see Section 4.4.1.2). Assuming a probabilitycoefficient of 0.05 Sv�1, the recommended negligible dose correspondsto an estimated lifetime fatal cancer risk of about 4 � 10�5. Anannual dose of 0.01 mSv also has been used by IAEA to define anexempt class of radioactive waste and, more generally, to defineradioactive material that can be exempted from regulatory control(see Section 4.1.3). Similarly, a negligible lifetime risk of about1 � 10�5 was proposed, but not implemented, by EPA (1992d) forthe purpose of exempting waste that contains chemicals that causestochastic responses from requirements of RCRA.

A negligible risk or dose of substances that cause stochasticresponses consistent with the precedents described above can besupported by considering the unavoidable risks due to natural back-ground radiation and naturally occurring chemicals. The averageannual dose due to natural background radiation, including indoorradon, is about 3 mSv (NCRP, 1987b). This dose corresponds to anestimated lifetime fatal cancer risk of about 10�2. The risk fromexposure to naturally occurring chemicals (e.g., carcinogenic heavymetals such as arsenic, natural pesticides in food, and organic com-pounds in soil) is not as well characterized but, based on availableinformation, the estimated lifetime cancer risk also is about 10�2

(Ames and Gold, 1995; Travis and Hester, 1990). Given the magni-tude of the unavoidable cancer risks from exposure to the natural


background of radiation and chemicals, a negligible risk from expo-sure to man-made sources reasonably could be set at a small fraction(e.g., one percent) of the average background risk. Such a risk (e.g.,an excess lifetime cancer risk of about 10�4) should be less than thevariability in the background risk at any location due to differencesin living habits. The negligible individual dose of 0.01 mSv discussedabove corresponds to about one percent of the annual dose due tonatural background radiation, excluding radon.

Exemption of waste that contains naturally occurring substancesthat cause stochastic responses warrants further considerationbecause the negligible cancer risks or doses described above maycorrespond to exemption levels that are less than background levelsin soil or rock. For example, average background levels of radiumand thorium in soil (NCRP, 1984a; 1987b) correspond to annualeffective doses well above 0.01 mSv (NCRP, 1999a) and estimatedlifetime cancer risks well above 10�4. Similarly, average backgroundlevels of arsenic in soil (Buonicore, 1995) correspond to estimatedlifetime cancer risks well above 10�5 (EPA, 1996e). As a consequence,application of these exemption criteria to all wastes could precludeexemption of virtually any waste derived from earthen materials,even when the concentrations of naturally occurring hazardous sub-stances are not enhanced by human activities. In order to providea practical system for exempting such wastes, NCRP believes thatexemption levels for naturally occurring substances that cause sto-chastic responses should be based on considerations of backgroundlevels in surface soil and their associated health risks to the public,in addition to the negligible risks that would be used to establishexemption levels for man-made substances. Similar considerationscould apply to naturally occurring substances that cause determinis-tic responses for which normal intakes are a substantial fraction ofthe nominal threshold in humans.

6.3.1.2.2 Establishing an acceptable risk or dose. There also area number of precedents for establishing an acceptable (barely tolera-ble) risk or dose of substances that cause stochastic responses forthe purpose of classifying waste as low-hazard or high-hazard. Forradionuclides, the annual dose limit for the public of 1 mSv currentlyrecommended by ICRP (1991) and NCRP (1993a) and contained incurrent radiation protection standards (DOE, 1990; NRC, 1991)could be applied to hypothetical inadvertent intruders at licensednear-surface disposal facilities for low-hazard waste. This dose cor-responds to an estimated lifetime fatal cancer risk of about 4 � 10�3.Alternatively, the limits on concentrations of radionuclides in radio-active waste that is generally acceptable for near-surface disposal,


as established by NRC in 10 CFR Part 61 (NRC, 1982a), could beapplied directly to waste classification, because these concentrationlimits were based on scenarios for inadvertent intrusion followingan assumed loss of institutional control at 100 y after disposal (seeSection 4.1.2.3.3). An acceptable (barely tolerable) risk from exposureto chemicals that cause stochastic responses has not formally beenconsidered by EPA. However, based on an analysis of case-by-caseregulatory decisions prior to 1985 discussed in Sections 3.3.2 and3.3.3, EPA generally acted to reduce risks in the range of about 10�2

to 10�3, the particular value depending in part on the size of theexposed population. Thus, there are precedents for setting an accept-able risk from exposure to chemicals that cause stochastic responsesat about the same level as the value for radionuclides.

An acceptable risk or dose of substances that cause stochasticresponses consistent with the precedents described above can besupported by available information on cancer risks from exposureto natural background radiation and naturally occurring chemicals.As noted in the previous section, the estimated lifetime cancer risksdue to the background of radiation and chemicals each are about10�2. An acceptable risk could be set at a value that correspondsapproximately to the geographical variability in the background risk,because people normally do not consider this variability in decidingwhere to live. For example, excluding indoor radon, the standarddeviation of the geographical distribution of the dose to an averageindividual due to natural background radiation is a few tens ofpercent of the mean dose (NCRP, 1987b).

6.3.2 Developing Exposure Scenarios for Purposes ofWaste Classification

In general, calculation of the risk or dose from waste disposal inthe numerator of the risk index in Equation 6.2 or 6.3 involves therisk assessment process discussed in Section 3.1.5.1. As summarizedin Section 6.1.3, NCRP recommends that generic scenarios for expo-sure of hypothetical inadvertent intruders at waste disposal sitesshould be used in calculating risk or dose for purposes of wasteclassification. Implementation of models describing exposure scenar-ios for inadvertent intruders at waste disposal sites and their associ-ated exposure pathways generally results in estimates of risk ordose per unit concentration of hazardous substances in waste. Theseresults then are combined with the assumptions about allowable riskdiscussed in the previous section to obtain limits on concentrations ofhazardous substances in exempt or low-hazard waste.


6.3.2.1 Exposure Scenarios for Classifying Exempt Waste. Basedon the definition of exempt waste as any waste that would be gener-ally acceptable for disposal in a municipal/industrial landfill for non-hazardous waste, scenarios for inadvertent intrusion appropriate tothis type of facility should be used in determining whether a wastewould be classified as exempt.

A municipal/industrial landfill for nonhazardous waste normallyis constructed without engineered barriers that would deter inadver-tent intrusion into the waste during normal human activities on theground surface. Furthermore, the waste itself often is in a readilypenetrable physical form. Therefore, scenarios for inadvertent intru-sion involving permanent occupancy of disposal sites and normalhuman activities that could access waste, such as excavation of wastein the construction of homes and residence on top of exposed waste,would be appropriate. These types of scenarios have been used inevaluating inadvertent intrusion at near-surface disposal facilitiesfor radioactive waste (NRC, 1982b; Oztunali and Roles, 1986;Oztunali et al., 1986) and hazardous chemical waste (Okrent andXing, 1993), and they are used in risk assessments of contaminatedsites subject to remediation under CERCLA (EPA, 1989) or AEA(Kennedy and Strenge, 1992). Since institutional control is notexpected to be maintained for a substantial period of time afterclosure of a landfill for nonhazardous waste, intrusion scenariosinvolving permanent occupancy of a site could be assumed to occuressentially at the time of facility closure.

The assumptions about exposure scenarios described above wouldapply to any allowable means of disposal of exempt waste on or nearthe ground surface. An assumption that exempt waste would besent to a disposal facility for nonhazardous waste permitted underSubtitle D of RCRA (1976) is not required.

6.3.2.2 Exposure Scenarios for Classifying Low-Hazard Waste.The assumed technologies for disposal of low-hazard and exemptwaste are similar in that both involve near-surface facilities. Thus,scenarios for inadvertent intrusion used in classifying low-hazardwaste could be the same in many respects as those used in classifyingexempt waste. However, there are important differences that shouldbe taken into account in developing intrusion scenarios at near-surface facilities for low-hazard waste. This type of disposal facilityfrequently includes engineered barriers, impenetrable waste forms,or deliberate placement of more hazardous wastes at relatively inac-cessible locations (e.g., at greater depths or beneath other wastes).All of these features are intended to deter inadvertent intrusion forsome time after disposal or to make it less likely that waste would


be accessed by normal human activities after occupancy of the sitecould occur. Furthermore, given the intention to maintain institu-tional control over hazardous waste disposal sites for a considerableperiod of time after facility closure, substantial decay of many radio-nuclides, as well as significant chemical transformations of otherhazardous substances to less hazardous forms, could occur prior tothe time that permanent occupancy of disposal sites by the publicwould be possible. All of these factors should be taken into accountin developing generic scenarios for inadvertent intrusion, and theirassociated exposure pathways, for the purpose of classifying low-hazard waste.

An additional consideration in classifying low-hazard waste basedon scenarios for inadvertent intrusion at a near-surface facility isthat some forms of intrusion would be credible even during theinstitutional control period. However, appropriate intrusion scenar-ios during the institutional control period would differ from scenariosinvolving permanent occupancy of disposal sites after loss of institu-tional control in regard to the credible exposure pathways and theduration of exposures. For example, exposure pathways involvingconsumption of contaminated foodstuffs obtained from the disposalsite, which are credible when permanent occupancy of a site couldoccur, would not be credible during the institutional control period,and exposures reasonably could occur for only a small fraction ofthe time during a year.

The role of institutional control over near-surface hazardous wastedisposal sites is particularly important in cases of very large volumesof waste, such as uranium mill tailings and wastes from miningand milling of ores to extract nonradioactive materials, that containconcentrations of naturally occurring hazardous substances, such asradium and heavy metals, far above background levels in Earth’scrust (see Section 7.1.5). For such wastes, the risk to an inadvertentintruder often would be well above any level that could be consideredacceptable if permanent occupancy of near-surface disposal sitescould occur. In the case of the large volumes of uranium mill tailings,however, disposal in facilities located well below the ground surfaceis not considered practical at the present time (EPA, 1982). There-fore, the intention is to maintain perpetual institutional control overnear-surface disposal sites for these wastes to prevent scenarios forinadvertent intrusion involving permanent site occupancy. Similarconsiderations could apply to large volumes of other mining andmilling wastes.

6.3.2.3 Classification as High-Hazard Waste. Waste that wouldnot be generally acceptable for near-surface disposal in dedicated


facilities for hazardous wastes, based on assessments of risk or dosefor the types of scenarios for inadvertent intrusion described in theprevious section, would be classified as high-hazard waste. Wastein this class generally would be intended for disposal in a facilitythat provides substantially greater isolation than a near-surfacefacility for low-hazard waste (e.g., a geologic repository). As discussedin Section 6.2.2.2.2, assessments of risk or dose to hypothetical inad-vertent intruders do not provide a reasonable basis for determiningacceptable disposals of waste in facilities located far below the groundsurface, and there would be no limits on concentrations of hazardoussubstances in high-hazard waste because more confining waste dis-posal concepts are not evident.

6.3.3 Application of the Modifying Factor in Risk Index

The modifying factor in the risk index in Equation 6.2 or 6.3represents any factors deemed important in classifying waste otherthan those explicitly accounted for in the calculated risk or dosefrom waste disposal in the numerator or the allowable risk or dosefor the waste class of concern in the denominator. The modifyingfactor generally can be substance- or waste-specific.

The inclusion of a modifying factor in the risk index is intendedto represent the essential role of judgment in classifying waste, eventhough the objective is to develop a classification system based onsound science. For example, generic scenarios for exposure of hypo-thetical inadvertent intruders at waste disposal sites could take intoaccount not only the calculated exposure of an individual, given thata postulated scenario occurs, but the probability that the assumedscenario might occur as well. This probability could be included inthe modifying factor and could depend, for example, on assumptionsabout the ability of an engineered disposal facility or the intendedplacement of waste in the facility to deter inadvertent intrusion intothe waste. The modifying factor also could take into account, forexample, the quality of the data underlying the assumed dose-response relationships, background levels of naturally occurring haz-ardous substances of concern, considerations of cost-benefit in wastedisposal, and societal concerns about particular wastes. In principle,the modifying factor could assume any value. A value less than unitywould represent factors judged to mitigate risk, a value of unitywould mean that there are no significant factors that were notalready taken into account in calculating risk, and a value greaterthan unity would represent factors judged to enhance risk.


The use of the modifying factor in the risk index is illustrated byassumptions used by NRC in developing the concentration limits ofradionuclides in Class-C low-level radioactive waste in 10 CFR Part61 (NRC, 1982a; 1982b) (see Section 4.1.2.3.3). The concentrationlimits for the relatively small volumes of Class-C waste incorporateseveral assumptions that were not used in developing the concentra-tion limits for the much larger volumes of Class-A waste. Specifically,the Class-C limits incorporate assumptions that the use of engi-neered barriers and waste forms or the selective emplacement ofClass-C waste at greater depths than other waste would delay accessto the waste for a period of 500 y after facility closure, and that suchmeasures would also reduce the probability of exposure to Class-Cwaste by a factor of 10. In addition, the Class-C limit for 137Cs wasfurther increased based on knowledge of the volumes of waste thatcontains high concentrations of this radionuclide and considerationsof the balance of costs and benefits of different options for managingand disposing of that waste.

The modifying factor also can represent more qualitative consider-ations. An example is provided by current federal policies regardingdisposal of uranium and thorium mill tailings. As noted inSection 6.3.2.2, these materials contain such high concentrationsof radium that disposal in a near-surface facility would result inintolerable risks to an inadvertent intruder if unrestricted access totailings piles were allowed. Nonetheless, the decision was made todispose of most uranium and thorium mill tailings on or near theground surface (EPA, 1982; UMTRCA, 1978), based primarily onthe consideration that disposal of the very large volumes of thesewastes far below ground appeared to be impractical and might notprovide adequate protection of the environment, especially ground-water. Thus, an intention to maintain perpetual institutional controlover tailings piles to prevent intrusion scenarios involving long-termexposures is a crucial consideration in this decision. In this case,the modifying factor essentially represents an assumption that sce-narios for inadvertent intrusion should be restricted to those involv-ing short-term access to a tailings pile, rather than permanentresidence on a disposal site.

NCRP notes that the modifying factor in the risk index should beapplied independently of the requirement to achieve a negligible riskor dose for exempt waste or an acceptable (barely tolerable) risk ordose for nonexempt waste, in order to provide regulatory flexibilityin classifying particular wastes. NCRP believes that such flexibilityis highly desirable to promote cost-effective management and dis-posal of waste, provided it is applied in a transparent manner.

6.4 DEVELOPMENT OF THE COMPOSITE RISK INDEX / 285

6.4 Development of the Composite Risk Index forMultiple Substances

The risk index defined in Equation 6.1 (see Section 6.2.1) isintended to provide a measure of the potential risk that arises fromdisposal of any waste that contains hazardous substances. In Section6.3, the general definition of the risk index is elaborated and recom-mendations on suitable approaches to calculating the risk indexfor individual hazardous substances are presented. For purposesof developing a comprehensive and risk-based waste classificationsystem, a simple method of calculating the risk from disposal ofmixtures of hazardous substances is needed. The method must takeinto account that the allowable concentrations of particular hazard-ous substances in waste of a given class generally will be lower whenmultiple substances are present than when only a single substanceis present. Such a method is presented and discussed in this Section.

NCRP believes that a conceptually simple approach to calculatingthe risk from disposal of mixtures of hazardous substances can bedeveloped which, although it may not be scientifically rigorous inestimating health risks from exposure to multiple hazardous sub-stances, is adequate for the purpose of classifying waste. Becausethe dose-response relationships for substances that cause stochasticor deterministic responses are fundamentally different (see Section3.2), care must be taken in developing a risk index for multiplehazardous substances. In recognition of this difference, separate riskindexes are developed for mixtures of either type of substance inSection 6.4.1. These risk indexes are combined in Section 6.4.2 toyield a composite risk index for waste that contains mixtures of anyhazardous substances. Development of the risk index is completedin Section 6.4.3 by reiterating the need to specify risk indexes basedon both negligible and acceptable risks for the purpose of classifyingwaste as exempt or low-hazard, respectively. Examples of how thecomposite risk index for mixtures of hazardous substances is calcu-lated using hypothetical data are presented in Section 6.4.4. Theprocess by which boundaries between waste classes might be estab-lished is discussed in Section 6.4.5. Finally, potential shortcomingsand advantages of the risk index are described in Section 6.4.6.

6.4.1 Risk Indexes for Mixtures of Hazardous Substances

Hazardous wastes can contain mixtures of substances that causestochastic or deterministic responses, or a single substance can causeboth types of responses (e.g., arsenic, uranium). The two types of


hazardous substances differ in important ways (see Section 3.2).Specifically, a threshold dose-response relationship is assumed forsubstances that cause deterministic responses, whereas a linear,nonthreshold dose-response relationship is assumed for substancesthat cause stochastic responses. As a consequence, the objective ofrisk management for substances that cause deterministic responsesis to keep the dose sufficiently below the threshold in any organ ortissue that the occurrence of adverse responses is prevented in themost sensitive population groups, whereas the objective of risk man-agement for substances that cause stochastic effects is to limit thefrequency of occurrence of adverse responses, taking into accountresponses in all organs or tissues.

The implication of the difference described above is that the mathe-matical form of the risk index for the two types of hazardous sub-stances must be different. Thus, while NCRP believes that it isappropriate to develop a single risk index that accounts for mixturesof substances that cause stochastic or deterministic responses, sepa-rate risk indexes for these two types of substances are formulatedfirst.

6.4.1.1 Risk Index for Multiple Substances That Cause StochasticResponses. The risk index for mixtures of substances that causestochastic responses (radionuclides and chemicals) is based on anassumption of a linear, nonthreshold dose-response relationship.This risk index takes into account the stochastic risk in all organsor tissues, and it assumes that the risk in any organ is independentof risks in any other organs. Based on these conditions, and express-ing the risk index for a single hazardous substance in terms of dose(see Equation 6.3), the risk index for mixtures of substances thatcause stochastic responses, denoted by RIs, can be expressed as:

RI sj � �

i�

r�

T

Fi(dose from disposal) s

i, j, r,T

(allowable dose) si, j, r,T

, (6.4)

where j is an index indicating whether the denominator in the riskindex represents a negligible or an acceptable (barely tolerable) dose(i.e., whether the waste is being evaluated for classification as exemptor low-hazard), the index i again denotes the particular hazardoussubstance, T denotes each organ or tissue at risk, and r denotes thedifferent stochastic responses of concern (e.g., cancers and severehereditary effects). The index j is included in the numerator, as wellas the denominator, to indicate that exposure scenarios used to calcu-late risk can be different for disposal of exempt waste in municipal/industrial landfills (or equivalent means of disposal) compared withdisposal of low-hazard waste in dedicated near-surface facilities (see


Section 6.3.2). Equation 6.4 is expressed in terms of dose, ratherthan risk, mainly because this form is consistent with the preferredform of the risk index for mixtures of substances that cause determin-istic responses presented in the following section. Both ways ofexpressing the risk index are equivalent for substances that causestochastic responses when a linear, nonthreshold dose-response rela-tionship is assumed.

The order in which the various summations in Equation 6.4 areexecuted is arbitrary, because the probability of a stochastic responsein a particular organ or tissue from exposure to any such substanceis assumed to be independent of any other responses caused by thatsubstance and independent of exposures to any other substancesthat cause stochastic or deterministic responses. The method of cal-culating the risk index for mixtures of substances that cause stochas-tic responses also ignores possible synergistic or antagonistic effects.Information on any such responses, which is rarely available, couldbe taken into account for particular wastes either in establishingthe allowable dose of each substance or in evaluating the modifyingfactor which can be substance-specific.

In practice, Equation 6.4 would be greatly simplified. For radionu-clides, doses in all organs or tissues (T) and the different responsesof concern (r) are incorporated in the effective dose (see Section3.2.2.3.3). Thus, calculation of the risk index for mixtures of radionu-clides is reduced to a single summation over all radionuclides of theratio of a calculated effective dose from exposure to each radionuclideto the allowable effective dose for the particular waste class of con-cern. Furthermore, the denominator normally would be the same forall radionuclides in a given waste, and any differences in judgmentsabout an allowable effective dose for different wastes in the sameclass could be included in the modifying factor. Similarly for chemi-cals that cause stochastic responses, information on responses insingle or multiple organs or tissues at risk is incorporated in sub-stance-specific probability coefficients, and the summations over allorgans and tissues (T) and responses (r) thus are reduced to a singleratio of a calculated dose to an allowable dose. The simplified formof the risk index for mixtures of chemicals that cause stochasticresponses also could be expressed in terms of risk.

Therefore, for waste that contains mixtures of substances thatcause stochastic responses, the risk index in Equation 6.4 generallycan be reduced to a single summation over all such substances (i)of the ratio of a calculated dose to an allowable dose. The risk indexfor such mixtures of substances thus is in the form of a simple sum-of-fractions rule. An example calculation is described in Section 6.4.4.


The modifying factor (F ) in Equation 6.4 generally can besubstance-specific. However, in a given waste, its value often wouldbe the same for all substances that cause stochastic responses.

6.4.1.2 Risk Index for Multiple Substances That Cause Determinis-tic Responses. The risk index for mixtures of substances that causedeterministic responses should be expressed in terms of dose, ratherthan risk, because risk is not proportional to dose and the goal ofrisk management is to limit doses to less than the threshold in thedose-response relationship (see discussion of Equation 6.2 in Section6.3). As noted previously, deterministic responses from exposure toradionuclides should not be of concern in classifying waste, in whichcase only the risk index for chemicals that induce deterministicresponses needs to be considered.

Formulation of the risk index for mixtures of substances that causedeterministic effects is considerably more complex than in the caseof substances that cause stochastic effects discussed in the previoussection. The added complexity arises from the threshold dose-response relationship for these substances and the need to keeptrack of the dose in each organ or tissue at risk in evaluating whetherthe dose in each organ is less than the allowable dose in that organ.For substances that cause deterministic responses, the index T canrefer not only to a specific organ or tissue (e.g., the liver or skin) butalso to a body system that may be affected by a particular chemical,such as the immune or central nervous system.

Taking into account the threshold dose-response relationship, therisk index for mixtures of substances that cause deterministicresponses is based on the following assumptions. First, the doses inany organ due to multiple substances are assumed to be additive,even though the deterministic responses induced in that organ maynot be the same for each substance. Second, the threshold doses fordeterministic responses in any organ caused by any substance areassumed to be independent of doses in all other organs due to anysubstance that cause deterministic (or stochastic) responses andindependent of doses in the same organ due to all other substanceswith deterministic (or stochastic responses). Based on these assump-tions, the risk index for mixtures of substances that cause determin-istic responses, denoted by RId, can be represented as:

RIdj � INTEGER �MAXT �

i�

r

Fi(dose from disposal) d

i, j, r,T

(allowable dose) di, j, r,T

� , (6.5)

where MAX is a function yielding the maximum value of a set ofnumbers and INTEGER is a function yielding the truncated integer


value of a number. As in Equation 6.4, the index i denotes the partic-ular hazardous substance, j is an index indicating whether the wasteis being evaluated for classification as exempt or low-hazard, Tdenotes each organ or tissue at risk, and r denotes the differentresponses of concern.

In essence, the procedure for evaluating Equation 6.5 involvescalculation of a separate risk index for each organ at risk and acomparison of the results. The various steps in the procedure aredescribed as follows:

● For each substance, the organ or organs (including tissues orbody systems) in which deterministic responses can be inducedare identified. If a substance can induce responses in more thanone organ, all such organs are included in calculating the riskindex.

● For each substance, the ratio of a calculated dose in each organat risk to an allowable dose in that organ is obtained, basedon an assumed exposure scenario for the waste class (disposalsystem) of concern (i.e., exempt or low-hazard). If a substancecan induce responses in more than one organ, the allowabledose can depend on the particular organ, because the thresholdgenerally will not be the same in all organs. The result of thisstep is a set of substance-specific and organ-specific ratios ofcalculated doses to allowable doses.

● For each organ at risk, the substance-specific ratios of calculateddoses in that organ to the corresponding allowable doses aresummed over all substances, without regard for any differencesin the deterministic responses induced by the different sub-stances. This calculation is based on an assumption that dosesin any organ due to multiple substances that cause deterministicresponses are additive, even though the responses induced inthat organ may not be the same for each substance. The resultof this step is a set of organ-specific ratios of calculated dosesto allowable doses in which the particular substances in thewaste that cause deterministic responses and their associatedresponses are no longer distinguished.

● By examination of the organ-specific ratios of calculated dosesto allowable doses obtained in the previous step, the maximumvalue of these ratios is selected. Application of the MAX functionto these organ-specific ratios is based on an assumption thatinduction of deterministic responses in any organ is independentof doses in any other organs or, equivalently, that the thresholdin the dose-response relationship for any substance that causesdeterministic responses is not affected by exposure to multiple


substances that cause deterministic (or stochastic) responses.The result of this step is a single number representing the riskindex for the organ at greatest risk (the critical organ), takinginto account all substances in the waste that cause determin-istic responses and all organs at risk from exposure to thosesubstances.

● The highest deterministic risk index in any organ selected inthe previous step is truncated using the INTEGER function.Truncation of the highest risk index in any organ, rather thanrounding to the nearest integer value, is based on an assumptionthat the probability of a deterministic response is zero if thedoses in all organs are less than the corresponding allowabledoses, but is unity otherwise.

The order in which the summations over the responses (r) andsubstances (i) of concern are executed in the second and third stepsabove is arbitrary. However, these steps must be executed beforethe MAX and INTEGER functions are applied to the result. If therisk index for substances causing deterministic responses were basedon calculations of health risk per se, rather than dose, the INTEGERfunction in Equation 6.5 would not be necessary, because the riskwould be zero whenever a dose is below the threshold. Again, how-ever, evaluation of the risk index for substances that cause determin-istic responses based on dose is recommended when the dose-response relationship is assumed to have a threshold. The use of doseis supported by the observation that the dose-response relationshipabove the threshold generally is nonlinear.

The method of calculating the risk index for mixtures of substancesthat cause deterministic effects described above assumes that alldeterministic responses that are taken into account in determiningallowable doses in any organ are equally undesirable. The methodalso ignores possible synergistic or antagonistic effects of exposureto multiple substances. While it would be desirable to take sucheffects into account, there are few data that could be used to supportparticular assumptions. The possibility of synergistic or antagonisticeffects could be taken into account, if so desired, in the safety anduncertainty factors applied to the assumed threshold doses in establish-ing the allowable dose of each substance for the waste class of concernor in the substance-specific modifying factor used to determine therisk index.

It must be emphasized that the sum-of-fractions rule for substancesthat cause stochastic responses (see Equation 6.4) generally does notapply in calculating the risk index for mixtures of substances thatcause deterministic responses. That is, based on an assumption of a


threshold dose-response relationship for substances that cause deter-ministic responses, it generally is inappropriate to simply sum theratios of calculated doses in each organ to the corresponding allowabledoses without regard for the particular organs at risk. For example,consider a case of exposure to two substances that cause deterministicresponses, each of which affects only a single organ, and suppose thatthe ratio of the calculated dose in the critical organ to the allowabledose is 0.6 for each substance. If the critical organ is the same for bothsubstances, the risk index for the mixture of the two before applyingthe INTEGER function would be 0.6 � 0.6 � 1.2, based on the assump-tion that doses in any organ are additive without regard for any differ-ences in the responses induced in that organ by the differentsubstances. However, if the critical organ is not the same for the twosubstances, the dose in each organ would be less than the correspondingallowable dose and the risk index for the mixture of the two wouldbe zero, based on the assumption that the induction of deterministicresponses in any organ is independent of doses in other organs. Thus,the risk of a deterministic response due to multiple substances gener-ally depends on the particular organs at risk for each substance causingdeterministic effects, and the sum-of-fractions rule does not apply whenmultiple organs are at risk. Additional examples of calculating therisk index for mixtures of substances causing deterministic responses,which illustrate the need to keep track of the different organs andtissues at risk, are given in Section 6.4.4.

As in the case of the risk index for mixtures of substances thatcause stochastic responses discussed in the previous section, themodifying factor (F) in Equation 6.5 generally can be substance-specific, but its value often would be the same for all substances ina given waste that cause deterministic responses.

6.4.2 Composite Risk Index for All Hazardous Substances

Hazardous waste generally can contain mixtures of substancesthat cause stochastic or deterministic responses. The composite riskindex for any mixture of hazardous substances in a given waste canbe represented as the sum of risk indexes for multiple substancesthat cause stochastic or deterministic responses given in Equations6.4 and 6.5:

RIj � RI sj � RI d

j . (6.6)

Again, the index j denotes whether the waste is being investigatedfor classification as exempt or low-hazard. It is included because


both the numerator (the calculated risk) and the denominator (allow-able risk) in Equations 6.4 and 6.5 depend the assumed disposaltechnology and the waste class of concern.

Given the form of the deterministic risk index in Equation 6.5,which results in zero or integer values, the composite risk index forall hazardous substances in Equation 6.6 also can be expressed asthe maximum of the separate risk indexes for multiple substancescausing stochastic or deterministic responses:

RIj � MAX [RI sj, RI d

j ]. (6.7)

If the risk index for all substances that cause deterministic responsesin the waste (RId) in Equation 6.5 is zero (i.e., the doses of all sub-stances that cause deterministic responses are less than the allow-able values), classification is determined solely by the risk index forall substances that cause stochastic responses (RIs) in Equation 6.4;the latter must be nonzero based on the assumption of a linear,nonthreshold dose-response relationship. On the other hand, if therisk index for all substances that cause deterministic responses isunity or greater, the calculated risk exceeds the allowable risk forthe waste class of concern without the need to consider the riskposed by substances that cause stochastic effects. The only advantageof the form of the composite risk index in Equation 6.6 is that itindicates more explicitly that the total risk posed by a given waste isthe sum of the risks posed by the two types of hazardous constituents,however approximate that representation may be.

6.4.3 Implications of the Framework for Calculating theRisk Index

The risk-based waste classification system described in Section 6.2.2contains two boundaries: one between exempt waste and low-hazardwaste, based on a negligible risk, and one between low-hazard andhigh-hazard waste, based on an acceptable (barely tolerable) risk.Consequently, there are two separate sets of Equations 6.4, 6.5 and6.6, which are distinguished by different meanings of the index j.One set of equations is used to evaluate the general acceptability ofdisposal in a municipal/industrial landfill for nonhazardous waste,and the other is used to evaluate the general acceptability of disposalof nonexempt waste in a dedicated near-surface facility for hazardouswaste. As discussed in Section 6.2.2, the exposure scenario usedin classifying exempt waste generally can differ from the exposurescenario used in classifying low-hazard waste, even when disposalin a near-surface facility is assumed in both cases.


6.4.4 Example Calculations of the Risk Index

This Section provides example calculations of the composite riskindex for a simple, hypothetical waste that contains a mixture ofsubstances that cause stochastic or deterministic effects. Applicationof the risk index in classifying real wastes is considered in Section 7.1.

For the purpose of illustrating how the composite risk index inEquation 6.6 would be used to classify a hypothetical waste, it ishelpful to simplify Equations 6.4 and 6.5. This is done by assumingthat the summation over all responses (index r) has been calculated,that only one waste classification boundary represented by theindex j is being considered (i.e., the boundary between exempt andlow-hazard waste, based on a negligible risk, or the boundarybetween low-hazard and high-hazard waste, based on an acceptablerisk), and that the modifying factor (F) is unity. Further, the calcu-lated dose in the numerator of the risk index is denoted by D andthe allowable dose in the denominator is denoted by L. Then, thecomposite risk index for all hazardous substances in the waste,expressed in the form of Equation 6.6, can be written as:

RIj � INTEGER �MAXT �i

D di,T

L di,T� � �

i�

T

D si,T

L si,T

. (6.8)

Calculation of the composite risk index for the purpose of wasteclassification based on the simplified Equation 6.8 is illustrated usingthe hypothetical data given in Table 6.1. Consistent with the formof the risk index in Equations 6.3 and 6.8, risk indexes for individualhazardous substances in Table 6.1 are expressed as the ratio of a

TABLE 6.1—Hypothetical values of risk indexes for individualsubstances and organs in first example calculation of a composite

risk index.

(Dd/Ld)a (Ds/Ls)b

Substance (i) Organ A Organ B Organ A Organ B

1 0.4 — 0.2 0.32 0.8 — 0.1 0.23 — 1.6 — —

a Ratio of calculated dose (D) to allowable dose (L) of a substance thatcauses deterministic responses (d).

b Ratio of calculated dose (D) to allowable dose (L) of a substance thatcauses stochastic responses (s).


calculated dose based on an assumed exposure scenario to an allow-able dose for the waste class of concern (i.e., the risk index is notexpressed in terms of risk per se). We then suppose that the wastewould be placed in so-called Class 1 if the composite risk index wereless than unity, but would be placed in Class 2 otherwise.

Based on the information given above, the composite risk indexfor the waste can be calculated and the resulting waste classificationobtained. Substituting the values in Table 6.1 into Equation 6.8results in the following:

RI � INTEGER {MAX [(0.4�0.8), 1.6]}� [(0.2�0.1) � (0.3�0.2)]

(6.9)RI � INTEGER (1.6) � 0.8RI � 1 � 0.8 � 1.8

Substances that cause deterministic responses (the first term inEquation 6.8) contribute a value of one to the composite risk indexof 1.8, and substances that cause stochastic responses account forthe remaining 0.8. Thus, the presence of the substances that causedeterministic responses alone would be sufficient to place this wastein Class 2. This result also would be indicated if the alternative formof the composite risk index in Equation 6.7 were used.

As another example, suppose that each risk index for the individ-ual hazardous substances given in Table 6.1 were a factor of twolower. Then, by the procedure described above, the composite riskindex for the waste would be determined as follows:

RI � INTEGER {MAX [(0.2�0.4), 0.8]}� [(0.1�0.05) � (0.15�0.1)]

(6.10)RI � INTEGER (0.8) � 0.4RI � 0 � 0.4 � 0.4

The maximum deterministic risk index for any organ (0.8 forOrgan B) is less than unity, which means that the doses due to allsubstances that cause deterministic responses are below the allow-able doses in each organ at risk. Thus, the composite risk index is0.4, due solely to substances that cause stochastic responses, andsubstances that cause deterministic responses do not contribute.This result also would be indicated if the alternative form of thecomposite risk index in Equation 6.7 were used. In this example,the waste would be placed in Class 1.

The second example illustrates the importance of identifying andkeeping track of the specific organs at risk from exposure to sub-stances that cause deterministic responses. If the deterministic riskindexes for each substance were simply summed without regardfor the organs at risk, the risk index for all substances that cause


deterministic responses in this example would be 1.2, thus indicatingthat the dose due to all such substances would exceed the allowabledose. However, this is not the proper interpretation when the dosein each organ is less than the corresponding allowable dose anddeterministic responses in any organ are assumed to be independentof doses in other organs. In this case, the proper interpretation is thatthe risk of a deterministic response in any organ is essentially zero.

6.4.5 Establishing a Waste Classification System Based on theFramework and Risk Index

The foregoing development of the foundations and framework for acomprehensive and risk-based hazardous waste classification systembegan with a discussion of fundamental principles of waste classifi-cation and the basic definitions of waste classes, and eventuallyarrived at detailed conceptual equations to be used in classifyingwaste. However, establishment of the proposed waste classificationsystem probably would not involve waste generators undertakingthe calculations implied by the formulas constituting the risk index.The more likely approach to implementation would be undertakenby regulatory authorities, and this approach is outlined in Section6.4.5.1. Following this discussion, the questions of when a wasteshould be classified and the time frame following disposal over whicha risk assessment should be performed for the purpose of classifyingwaste are addressed in Sections 6.4.5.2 and 6.4.5.3, respectively.Finally, implementation of the proposed waste classification systemover time, to replace the existing classification systems for radioac-tive and hazardous chemical wastes, is discussed in Section 6.4.5.4.

6.4.5.1 Process of Implementing the Waste Classification System.Taken together, the framework for waste classification discussed inSection 6.2 and the risk index developed in Section 6.3 and thisSection constitute the foundations of a comprehensive and risk-basedhazardous waste classification system. Such a waste classificationsystem could be established by regulatory authorities using the fol-lowing general process:

● For substances that cause stochastic effects (radionuclides andhazardous chemicals), specify negligible and acceptable (barelytolerable) risks to be used in classifying waste. Then, establishthe corresponding negligible and acceptable dose of each sub-stance of concern based on an assumed probability coefficient(risk per unit dose).


● For each substance of concern that causes deterministic effects(hazardous chemicals only), establish nominal thresholds forinduction of deterministic responses in humans, taking intoaccount all organs and tissues at risk. Then, establish organ-specific negligible and acceptable doses of each substance byapplying appropriate safety and uncertainty factors to theassumed thresholds.

● Establish generic exposure scenarios for inadvertent humanintrusion into a municipal/industrial landfill for disposal ofexempt waste and intrusion into a dedicated near-surface facilityfor disposal of low-hazard waste.

● For each generic exposure scenario to be used in classifyingwaste, and taking into account all relevant exposure pathwaysin each scenario, calculate the dose per unit concentration ofeach hazardous substance in the waste. These doses generallywould be the highest values calculated over an assumed timeframe for the risk assessment (see Section 6.4.5.3), taking intoaccount the time-dependence of the concentrations of hazardoussubstances in the waste. For example, the quantity calculatedfor radionuclides would be the annual effective dose (sievert)per unit activity concentration (Bq m�3), and the quantity calcu-lated for hazardous chemicals would be the dose (intake,mg kg�1 d�1) per unit concentration (kg m�3).

● Divide the negligible and acceptable doses of each hazardoussubstance by the corresponding doses per unit concentration,resulting in limits on the concentrations of each hazardous sub-stance in exempt and low-hazard waste, respectively. Waste thatcontains concentrations of hazardous substances greater thanthe limits in low-hazard waste would be classified as high-hazard.

● Specify rules for applying the concentration limits, such as thesum-of-fractions rule for waste that contains mixtures of sub-stances that cause stochastic effects (NRC, 1982a) and the rulesfor combining risk indexes for mixtures of substances that causedeterministic effects taking into account the substance-specificorgans or tissues at risk.

● Promulgate tables of concentration limits of hazardous sub-stances that cause stochastic or deterministic effects in exemptand low-hazard waste and the rules for using the tables. Forexample, concentration limits of substances that cause deter-ministic effects should include an identification of the organ ororgans at risk from exposure to each substance, so that the riskindex for multiple substances that cause deterministic effectscan be evaluated properly.


For substances that cause stochastic effects, the risk index can beexpressed in terms of risk, rather than dose. In this case, the riskper unit dose would be incorporated in the calculated risk in thenumerator, based on the assumed exposure scenario, rather than inthe denominator. However, the effective dose provides a convenientsurrogate for risk for radionuclides, because all organs at risk andall stochastic responses of concern are taken into account, and theuse of dose for all substances that cause stochastic effects is consis-tent with the form of the risk index for substances that cause deter-ministic effects, which generally should be expressed in terms of dosebased on the assumption of a threshold dose-response relationship.

In effect, using the risk index to establish a risk-based wasteclassification system involves performing the risk assessment pro-cess described in Section 3.1 in reverse order by beginning with anassumed allowable risk and ending with the calculation of concentra-tions of hazardous substances in waste that are generically equiva-lent to that risk. This process is essentially the same as that usedby NRC to define subclasses of low-level radioactive waste that aregenerally acceptable for near-surface disposal in licensed facilities(NRC, 1982a; 1982b) (see Section 4.1.2.3.3).

Use of the risk index in classifying waste requires that adequatedata be available to allow estimation of dose-response relationshipsfor substances that induce stochastic or deterministic responses. Theavailability of suitable data is a potential problem only for hazardouschemicals. If suitable data are not available for particular hazardoussubstances, there is no satisfactory approach that could be used toinclude these substances in classifying waste. However, this wouldbe an important deficiency only if substances with inadequate dataon dose-response posed an important hazard in the waste. NCRPdoes not expect that the most important hazardous substances inwaste in regard to potential risks would be lacking information onthe dose-response relationship.

6.4.5.2 Time When Waste Should be Classified. Time is an impor-tant issue in waste classification in two respects. The first issuediscussed in this Section is the question of when waste should beclassified. The second issue, which is discussed in the following sec-tion, is the question of the time frame following disposal of wasteover which risk assessments should be carried out for the purposeof classifying waste.

Even after acknowledging that waste must often be classified wellbefore a specific method of disposal is known (see Section 2.1.2), thetime at which such classification occurs is an important issue in arisk-based waste classification system because the hazard of many


wastes changes with time. The hazard of many wastes declines asa result of radioactive decay or chemical degradation, but the hazardfrom some wastes increases because the decay or degradation prod-ucts are more hazardous than their parents.

In general, material deemed to be waste should be classified whenit is so declared or is ready for disposal. Many materials are declaredto be waste at the time of generation, and classification of waste atthat time is appropriate. However, some materials, such as spentnuclear fuel and surplus special nuclear materials, have economicvalue and often are not declared to be waste until long after theyare generated. In cases where the predominant hazard is due tohazardous substances with relatively high decay or degradationrates (e.g., half-lives of a few years or less), a strategy of storage toallow decay or degradation to occur would be appropriate if it wouldresult in the waste being in a lower class.

6.4.5.3 Time Frame for Risk Assessment in Classifying Waste. Inclassifying waste based on assessments of potential risks from dis-posal, the time frame over which the assumed exposure scenariosshould be evaluated is an important consideration. There are twoissues involving this time frame. The first is the earliest time afterdisposal at which exposures could occur according to assumed scenar-ios. The second is the time following the assumed onset of exposuresover which risk assessments should be carried out for the purposeof determining the highest potential dose or risk at any time. It isthis dose or risk that normally would be used in classifying waste.

The earliest time at which exposures could occur according toassumed scenarios is important because the concentrations of manyhazardous substances, such as shorter-lived radionuclides and biode-gradable organic chemicals, decrease significantly over time. There-fore, the concentration limits of these types of substances in exemptand low-hazard waste will depend on assumptions about the earliesttimes at which exposure to waste in the different classes could occur.For example, based on expectations embodied in current laws andregulations, the period of institutional control over waste disposalsites following closure may range from essentially zero at landfillsfor nonhazardous waste to about 30 to 100 y at dedicated near-surface facilities for hazardous wastes. This difference by itself wouldresult in substantially higher limits on concentrations of shorter-lived radionuclides in low-hazard waste compared with exemptwaste.

The issue of the time following the assumed onset of exposuresover which risk assessments should be carried out arises becausethe risk posed by some hazardous substances increases substantially


over time. For example, the risk posed by depleted uranium, whichconsists primarily of 238U, increases for about 106 y, due to very longhalf-life of the uranium and buildup of the activity of its decayproducts 234U, 230Th, and 226Ra over that time. Long-term buildup ofradiologically significant decay products also is potentially importantfor other long-lived radionuclides (e.g., 237Np, 233U). Similarly, it ispossible that some persistent hazardous chemicals (e.g., heavy met-als) could be transformed into more hazardous chemical compoundsover time. Therefore, consideration needs to be given to the maxi-mum time after disposal over which risks should be assessed for thepurpose of waste classification.

The question of appropriate time frames for risk assessments tobe used in classifying waste is discussed, in part, in Section 6.3.2,where the types of exposure scenarios that could be used in classify-ing exempt and low-hazard waste are described. These discussionsand other considerations by NCRP may be summarized as follows:

● In assessing risks based on scenarios for exposure of hypotheticalinadvertent intruders at municipal/industrial landfills for non-hazardous waste (i.e., in determining whether a waste would beclassified as exempt or nonexempt), scenarios involving perma-nent occupancy of a disposal site should be assumed to occurbeginning at the time of facility closure, based on the expectationthat institutional control will not be maintained over this typeof facility for a significant period of time after closure.

● In assessing risks based on scenarios for exposure of hypotheticalinadvertent intruders at dedicated near-surface disposal facili-ties for hazardous wastes (i.e., in determining whether a wastewould be classified as low-hazard or high-hazard), scenariosinvolving permanent occupancy of disposal sites should beassumed to occur beginning at the time after facility closurewhen institutional control over a disposal site is assumed tocease. In addition, credible scenarios involving temporary (short-term) access to a disposal site during the institutional controlperiod and exposure pathways appropriate to such short-termevents should be considered. The more restrictive of the twotypes of scenarios should be used to establish limits on concentra-tions of hazardous substances in low-hazard waste.

● The maximum time after disposal over which generic exposurescenarios should be evaluated for the purpose of classifyingwaste should not be any longer than the maximum time overwhich potential exposures of members of the public are evalu-ated for the purpose of determining the acceptability of specificwastes for disposal at specific sites. This time could range from


hundreds to thousands of years, especially when classifying non-exempt waste.

In general, the question of the time frame for risk assessments tobe used in classifying waste is a matter of judgment to be addressedby regulatory authorities.

6.4.5.4 Implementation of the Waste Classification System Over Time.NCRP recognizes that developing and promulgating a comprehen-sive and risk-based hazardous waste classification system to replaceexisting classification systems for radioactive and hazardous chemi-cal wastes probably cannot happen quickly or in one step, owing tothe continuing generation of hazardous wastes and the need to man-age them safely without interruption. NCRP recommends that arisk-based waste classification system be established by using thefoundations and framework provided in this Report as a road mapto the ultimate objective, and that existing waste classification sys-tems be addressed one at a time under these unifying principles.Such a process could take many years.

One likely outcome of this approach is that during the transitionperiod there will be a need for risk-based waste classifications toacknowledge and be integrated with existing classification systems,which are not risk-based, to sustain interim operations. Any provi-sions that are deemed necessary to facilitate the transition shouldbe explicitly identified so that they can be readily eliminated whenno longer needed.

6.4.6 Shortcomings and Advantages of the Risk Index

Risk indexes for mixtures of substances that induce stochastic ordeterministic responses given in Equations 6.4 and 6.5, respectively,have at least one possible shortcoming. Specifically, both assumethat the responses in a given target organ or tissue due to all suchhazardous substances can be added, even when the nature of theresponses and mechanisms of action in that organ or tissue mightbe different. This implies that all responses in a particular organare equally important, and that the hazardous substances andresponses are not synergistic or antagonistic. Thus, the approach isnot scientifically rigorous to the extent that these assumptions areinvalid. However, this shortcoming is not normally addressed inrisk assessments for radionuclides or hazardous chemicals and isgenerally ignored in setting health protection standards for workersand the public, due to a lack of information on how different sub-stances interact in causing responses when exposure to multiple

6.5 EXPECTED CLASSIFICATION OF EXISTING WASTES / 301

substances occurs. Therefore, in this regard, the recommended approachto waste classification is consistent with current approaches to healthprotection.

In addition, the reliance on generic scenarios for inadvertent intru-sion in classifying waste cannot, by definition, represent site-specificrisks. However, this is not a serious shortcoming because such sce-narios have been used in establishing subclasses of low-level radioac-tive waste for disposal in near-surface facilities (NRC, 1982a).Furthermore, as emphasized in this Report, establishment of a risk-based waste classification system using particular exposure scenar-ios does not obviate the need to perform site-specific risk assessmentsfor the purpose of establishing waste acceptance criteria at eachdisposal site.

NCRP also believes that the recommended approach to waste clas-sification has two important benefits. First, the approach is conceptu-ally simple and transparent. Specifically, it is based on conceptuallysimple definitions of waste classes in relation to disposal technologiesthat are expected to be acceptable in protecting the public, it isimplemented using a conceptually simple risk index to describe expo-sure to any hazardous substance, and risk indexes for substancesthat induce deterministic or stochastic responses are clearly relatedto the assumed form of the dose-response relationship in each case.Second, the risk index for either type of substance includes a substan-tial margin of safety in protecting the public if prudently conservative(pessimistic) assumptions are used in selecting and evaluating expo-sure scenarios. For example, an assumption that exposures of indi-viduals would occur continuously over a lifetime in accordance withscenarios for inadvertent intrusion involving permanent occupancyof waste disposal sites should overestimate exposures of nearly allindividuals who might actually access a disposal site at some timein the future.

Finally, NCRP emphasizes that calculated risk indexes for sub-stances that induce deterministic or stochastic responses are notintended to be used as predictors of the probability of a response forany actual or hypothetical exposure situation. The risk index isnothing more than a simple, dimensionless representation of therisk posed by hazardous substances in waste to be used for purposesof waste classification.

6.5 Expected Classification of Existing Wastes

There is sufficient information available to allow NCRP to antici-pate the classification of a number of existing wastes that would


result from implementation of the risk-based waste classificationsystem proposed in this Report. This evaluation assumes that prece-dents concerning suitable values of negligible and acceptable risksor doses to be used in classifying waste discussed in Section 6.3.1.2would prevail. The discussion in this Section should be taken as avery preliminary indication of how risk-based waste classificationmight impact current waste classifications and management. Theevaluation of different wastes in the following sections is based, inpart, on analyses of specific wastes presented in Section 7.1.

6.5.1 Wastes Expected to be Classified as Exempt

In contrast to the cases of low-hazard and high-hazard waste dis-cussed in the following two sections, NCRP did not investigate in anydetail the kinds and quantities of radioactive or hazardous chemicalwastes that might be classified as exempt, based on the definitionthat disposal of waste in this class in a municipal/industrial landfillfor nonhazardous waste would pose no more than a negligible riskto a hypothetical inadvertent intruder. However, based on the sub-stantial number of case-by-case exemptions for radioactive materialsin NRC regulations (see Section 4.1.2.5), assessments of risk fromdisposal of waste that contains low levels of radionuclides (EPRI,1989; Schneider et al., 2001), and studies in support of proposedregulations to establish exemption levels for listed hazardous chemi-cal wastes (EPA, 1992d; 1995c; 1999c), NCRP expects that sub-stantial quantities of waste currently managed as radioactive orchemically hazardous waste could be classified as exempt for pur-poses of disposal.

6.5.2 Wastes Expected to be Classified as Low-Hazard

By defining low-hazard waste as any radioactive or hazardouschemical waste that is generally acceptable for disposal in a dedi-cated near-surface facility for hazardous waste, NCRP expects thatthis class would include most radioactive waste presently classifiedin the United States as low-level waste. Because low-hazard wastewould include radioactive waste from any source, this class alsoshould include most NARM waste.

Most uranium or thorium mill tailings presumably could be classi-fied as low-hazard waste, but only under conditions of perpetualinstitutional control over near-surface disposal sites. In the absence

6.5 EXPECTED CLASSIFICATION OF EXISTING WASTES / 303

of institutional control, however, most mill tailings would be classi-fied as high-hazard waste. Regardless of how mill tailings are classi-fied, NCRP recognizes that a distinction between mill tailings andother low-hazard or high-hazard wastes would be reasonable,because the much larger volumes of mill tailings have necessitateddifferent approaches to waste management and disposal from thosethat have been used for other hazardous wastes. A distinctionbetween mill tailings and other hazardous wastes would not be partof the basic waste classification system developed in this Report, butit could be taken into account in developing subclassifications ofbasic waste classes (see Section 6.6). Similar considerations couldapply to other wastes with large volumes that are produced in miningand milling of ores to obtain nonradioactive materials and containelevated levels of NORM or heavy metals.

NCRP also expects that many hazardous chemical wastes pres-ently generated in the United States would be classified as low-hazard waste. Indeed, for hazardous chemical waste, low-hazardwaste essentially would correspond to the one waste class presentlydefined in the United States—namely, solid hazardous waste (seeSection 4.2.1).

Although most hazardous chemical waste generated in the UnitedStates is considered acceptable for disposal in a regulated near-surface facility, NCRP emphasizes that the concept of permanentdisposal—i.e., placement in a facility based on the results of long-term performance assessments with no intent to retrieve the wasteor maintain perpetual institutional control even though the wastemay remain hazardous for a very long time—generally has not beenapplied to hazardous chemical waste. In addition, the concept of ahypothetical inadvertent intruder at waste disposal sites, whichwould be the basis for defining low-hazard chemical waste, has notbeen used to determine the acceptability of hazardous chemicalwaste for disposal in a regulated near-surface facility. Potential risksto inadvertent intruders apparently could be of concern for somewastes that contain heavy metals (Okrent and Xing, 1993). Thus,when disposal facilities for hazardous chemical waste are consideredfor closure after the active management period (assumed to be 30 y),the available options would appear to be removal of waste that posesunacceptable risks to inadvertent intruders or continuation of activesite management essentially in perpetuity.

Perpetual institutional control over near-surface disposal sitesalso is envisioned for uranium mill tailings, on account of the unac-ceptably high risks that could result if tailings piles were releasedfrom control and the view that disposal of the very large volumes ofthese wastes in underground facilities is not feasible (EPA, 1982;


1983). However, there is an important difference between mill tail-ings and hazardous chemical wastes. In the case of mill tailings, theneed for permanent institutional control was recognized during thedevelopment of regulations, not only to protect against inadvertentintrusion but also to limit releases of radon to the air and releasesof radionuclides and heavy metals to groundwater. In the case ofhazardous chemical waste, however, the potential long-term implica-tions of near-surface disposal, including potential impacts on inad-vertent intruders, were not considered in any detail in developingregulations governing the design, operation, and closure of dis-posal facilities.

6.5.3 Wastes Expected to be Classified as High-Hazard

By defining high-hazard waste as any radioactive or hazardouschemical waste that generally would require a disposal system con-siderably more isolating than a near-surface facility, NCRP expectsthat this waste class would include most radioactive waste presentlyclassified in the United States as high-level waste (including spentnuclear fuel when it is declared to be waste), transuranic waste,and any other radioactive waste with similar properties, such asgreater-than-Class-C low-level waste as defined by NRC (1982a). Inaccordance with current practices in the United States, most high-hazard radioactive waste would be intended for disposal in a geologicrepository. However, greater confinement disposal systems, whichhave depths intermediate between a near-surface facility and a geo-logic repository, also could be appropriate for some high-hazardwaste. Again, for purposes of waste classification, a critical featureof an acceptable disposal system for high-hazard waste is that inad-vertent intrusion into the waste as a result of expected human activi-ties should be highly unlikely.

A number of dispositions could be acceptable for high-hazard chem-ical waste, including destruction (e.g., incineration), treatment toreduce the hazard to levels that would be acceptable for near-surfacedisposal, or disposal using a technology considerably more isolatingthan a near-surface facility. At the present time, there are no plannedalternatives to near-surface facilities for disposal of high-hazardchemical wastes in the United States.16 However, there do not appear

16 Facilities located at a considerable depth below the ground surface, such as minedcavities, are used in some countries (e.g., Germany) for disposal of hazardous chemicalwastes, as well as low-level radioactive waste. However, the selection of a deep disposalsystem often is based on general land-use policies that prohibit disposal of hazardouswastes on or near the land surface, as well as a desire to protect public health andthe environment, and no distinction is made between wastes that pose a lesser orgreater hazard in selecting such disposal systems and in developing site-specific wasteacceptance criteria.

6.6 SUBCLASSIFICATION OF BASIC WASTE CLASSES / 305

to be any substantive technical reasons why high-hazard radioactiveand chemical wastes could not be placed in a similar disposal facility,such as a geologic repository as presently intended for most high-hazard radioactive waste.

The types and amounts of chemical waste that might be classifiedas high-hazard cannot be estimated until a quantitative definitionof low-hazard chemical waste is developed and implemented. NCRPnotes that there are at least two precedents for defining a classof high-hazard chemical waste. The first is a classification systemdeveloped for the state of Washington (Mehlhaff et al., 1979), whichdesignates the most toxic chemical wastes as ‘‘extremely hazardouswaste.’’ The second is regulations of the state of California, whichinclude a definition of ‘‘extremely hazardous waste’’ (Pilorin, 1994),17

and a proposed revision of these regulations (California, 1999; NAS/NRC, 1999b). However, in neither case is the distinction betweenextremely hazardous waste and less hazardous waste based onassessments of risks from waste disposal, nor is a separate and moreisolating system for disposal of extremely hazardous chemical wasteidentified. Indeed, the state of Washington currently sends extremelyhazardous waste to the same disposal site as all other less toxicchemical wastes.

6.6 Subclassification of Basic Waste Classes

The proposed framework for risk-based classification of all radioac-tive and hazardous chemical wastes developed in Section 6.2.2 repre-sents waste classification in its broadest, most general terms. Thus,this classification system can be viewed as the highest level of apossible hierarchy of hazardous waste classifications (e.g., seeFigure 4.2). Further subclassification of these broadly defined wasteclasses may be desirable for such purposes as protection of workersduring waste operations, protection of public health and the environ-ment following waste disposal, and development of efficient methodsof waste management taking into account the characteristics ofactual wastes.

Subclassifications of broadly defined waste classes are common-place in the existing classification systems for radioactive and haz-ardous chemical wastes in the United States (see Sections 4.1 and 4.2

17 Pilorin, R. (1994). Personal communication from Pilorin, R. (California Environ-mental Protection Agency, Sacramento, California) to Croff, A.G. (Oak Ridge NationalLaboratory, Oak Ridge, Tennessee).


and Figure 4.2). For example: low-level radioactive waste is furtherclassified as Class-A, -B, -C, or greater-than-Class-C, depending onthe concentrations of particular radionuclides (NRC, 1982a); transu-ranic waste is further classified as contact-handled or remotely-handled, depending on the levels of external photon and neutronradiation (DOE, 1996a); listed hazardous chemical waste is dividedinto four subclasses depending on the source of the waste, the pres-ence of particular substances, or the nature of toxic effects causedby a substance; and, highly hazardous chemical waste sometimes isdistinguished from other chemical waste, depending primarily onthe intrinsic toxicity of its hazardous constituents (Mehlhaff et al.,1979). In addition, the radioactive waste classification system recom-mended by IAEA (see Section 4.1.3.1) includes a subclassification ofthe class of low- and intermediate-level waste, which is based on theconcentrations of long-lived, alpha-emitting radionuclides.

Recommendations on subclassifications of the basic classes ofexempt, low-hazard, and high-hazard waste defined in Section 6.2.2are not developed in this Report. However, NCRP acknowledgesthat subclassifications of basic waste classes would be reasonable,particularly in the case of low-hazard and high-hazard wastes. NCRPbelieves that any such subclassifications should be consistent withthe physical, chemical, radiological, and toxicological properties ofwaste, and with requirements for safe management and disposal.NCRP believes that extrinsic and non-risk-related factors, such asthe source of a waste, should not be used in subclassifying risk-basedwaste classifications.

As indicated by the current subclassifications of existing wasteclasses summarized above, a variety of waste properties could beused to develop meaningful subclassifications of broadly definedwaste classes. These properties include, for example, waste volumes,levels of decay heat and external radiation, and the long-term persis-tence of the hazard posed by waste constituents. Subclassificationsof waste classes also could be based on the presence of particularhazardous substances. However, if the broadly defined waste classesare based on risk, as in the classification system proposed in thisReport, the intrinsic toxicity of hazardous substances normallywould not provide a basis for subclassification, because this propertyalready is accounted for in determining the basic classification ofany waste. Examples of possible approaches to subclassifying thebasic waste classes are discussed in the following paragraphs.

Volumes of particular wastes are relevant to subclassification ofbasic waste classes, especially when the very large volumes of somewastes in a particular class necessitate different approaches to man-agement and disposal than the much smaller volumes of other wastes

6.6 SUBCLASSIFICATION OF BASIC WASTE CLASSES / 307

in the same class. An example of such a subclassification is thepresent distinction between low-level radioactive waste and mosturanium and thorium mill tailings; this distinction also could applyto large volumes of other wastes from mining and processing of ores(see Section 6.5.2).

A waste class could be subclassified based on the levels of decayheat and external radiation, in order to distinguish between wastesthat require special protection systems for workers during wastehandling and storage, such as active or passive cooling systems andextensive shielding, and wastes that do not require such protectionsystems, even when both types of waste would require essentiallythe same type of disposal system. Decay heat also could be consideredin subclassifying a waste class for purposes of disposal, becauseemplacement of waste in a disposal facility should take into accounttemperature increases in the host environment and their effects onthe waste isolation capabilities of a site. An example of a subclassifi-cation based on considerations of external exposure of workers isthe present distinction in the United States between contact-handledand remotely-handled transuranic waste. An example of a subclassi-fication based on considerations of the effects of decay heat on wastedisposal is the distinction between high-level waste and long-lived,intermediate-level waste in the radioactive waste classification sys-tem recommended by IAEA (see Section 4.1.3.1).

Long-term persistence of the hazard posed by waste constituentscould be used to subclassify basic waste classes. For example, differ-ent radioactive wastes that presumably would be classified as high-hazard in accordance with NCRP’s recommendations, such as spentfuel and high-level waste, transuranic waste, and greater-than-Class-C low-level waste, often have substantially different concen-trations of long-lived radionuclides. Many transuranic wastes con-tain much lower concentrations of long-lived alpha-emittingradionuclides than spent fuel or high-level waste, and some greater-than-Class-C low-level waste consists mostly of radionuclides withhalf-lives of about 30 y or less. Even though all of these wastes wouldrequire disposal well below the ground surface, disposal systemsless confining and less costly than a geologic repository could beacceptable for waste containing relatively low concentrations of long-lived radionuclides. Similar considerations could apply in distin-guishing waste that contains degradable hazardous chemicals fromwaste that contains nondegradable (persistent) substances, such asheavy metals.

A waste class could be subclassified based primarily on its particu-lar hazardous constituents, even though essentially the same dis-posal system might be used for all waste in that class. For example,


since the presence of organic materials can enhance the mobilityof radionuclides and heavy metals in the environment, it could bereasonable to distinguish between wastes that contain mainlyorganic hazardous chemicals and wastes that contain mainly inor-ganic hazardous materials, based on considerations of risks fromwaste disposal. Given the present legal and regulatory distinctionbetween radioactive and hazardous chemical wastes, it also mightbe reasonable to consider subclassifications of low-hazard and high-hazard waste that distinguish between radionuclides and hazardouschemicals, in order to facilitate management of waste in these broadclasses under some aspects of current laws and regulations thatdo not conflict with the precepts of risk-based waste classification.However, such subclassifications should be based on considerationsof the risk posed by all hazardous substances in a particular waste.

Finally, a waste class could be subclassified based on multiplefactors. For example, NRC’s classification system for near-surfacedisposal of low-level radioactive waste in 10 CFR Part 61 (NRC,1982a), which includes concentration limits for Class-A, -B, and -Cwaste and separate requirements for disposal of waste in each classwithin the same facility, takes into account, among several factors,the long-term persistence of the hazard posed by its constituentsand the expected volumes of waste in each class.

As indicated in Section 6.5 and discussed further in Section 7.1,NCRP believes that its proposed waste classification system wouldnot have serious adverse impacts on existing classification systemsfor radioactive and hazardous chemical wastes. Indeed, there gener-ally is a clear and logical correspondence between existing classesof radioactive and hazardous chemical wastes and the waste classifi-cation system proposed in this Report, even though the definitionsof existing and proposed waste classes have quite different bases.Therefore, the various subclassifications of existing classes of radio-active and hazardous chemical wastes should be compatible with theproposed waste classes. For example, given the proposed definition oflow-hazard waste as waste that is generally acceptable for disposalin a dedicated near-surface facility for hazardous wastes, NRC’sclassification system for near-surface disposal of low-level radioac-tive waste (NRC, 1982a) and current disposal practices for suchwaste should be unaffected.

6.7 Future Development Needs for Risk-BasedWaste Classification

Previous discussions have indicated that a number of technical,social, legal, and regulatory issues would need to be addressed and

6.7 FUTURE DEVELOPMENT NEEDS / 309

resolved in establishing the waste classification system proposed inthis Report. These issues are discussed further in this Section.

6.7.1 Standardization of Nomenclature

In developing the waste classification system presented in thisReport, NCRP found that differences in the meanings of commonterms used by practitioners in the fields of radioactive and hazardouschemical materials, including materials management and riskassessment, constituted a significant initial impediment to progress.Even among experts in the different fields, a substantial amount oftime was required to develop an understanding of the differences interms and to establish a mutually comprehensible nomenclature.NCRP believes that making meaningful and efficient progresstoward establishing a comprehensive and risk-based hazardouswaste classification system would be helped by an initial effort tostandardize the relevant nomenclature.

The most important difference in nomenclature is the different mean-ings attached to ‘‘acceptable’’ and ‘‘unacceptable’’ risks in the radiationand chemical paradigms for risk management (see Sections 3.3.3and 3.3.4). Reconciling the two risk management paradigms dependscritically on developing a common understanding of the meaningsof these two terms (Kocher, 1999). This understanding is importantif a consistent approach to risk management is to be applied to allhazardous substances in developing a risk-based waste classificationsystem. In particular, a common understanding that an ‘‘acceptable’’risk is not necessarily negligible and that risks above negligiblelevels are not necessarily ‘‘unacceptable’’ is an essential aspect ofthe waste classification system developed in this Report. This issueis discussed further in Section 6.7.5.

6.7.2 Approaches to Estimating Dose-Response Relationships forRadionuclides and Hazardous Chemicals

An important technical issue that requires resolution in develop-ing a comprehensive and risk-based waste classification system con-cerns the approaches that should be used to estimate health risksfrom a given exposure to radionuclides and hazardous chemicals.NCRP believes that reasonably consistent approaches should be usedfor all hazardous substances. Otherwise, some hazardous substances


could be assigned disproportionate risks and the classification sys-tem would not provide a meaningful correspondence with protectionof the public, which is the fundamental objective of a risk-basedsystem.

6.7.2.1 Approaches to Estimating Dose-Response Relationships forSubstances That Cause Stochastic Responses. The approach to esti-mating the probability of a response from a given dose (probabilitycoefficient) is of concern in classifying waste that contains substancesthat induce stochastic effects. This Report has discussed severaldifferences in current approaches to estimating probability coeffi-cients for radionuclides and chemicals that induce stochastic effectsthat complicate the development of a reasonably consistent approachto risk assessment. These differences include: (1) the use of fatalitiesas the primary measure of response for radionuclides but incidenceas the measure of response for chemicals that induce stochasticresponses, (2) the use of best estimates (MLEs) of probability coeffi-cients for radionuclides but upper-bound estimates (UCLs) for chemi-cals that induce stochastic responses, (3) general acceptance of asingle risk-extrapolation model for radionuclides but at least occa-sional use of a variety of extrapolation models for chemicals thatinduce stochastic responses that can give very different estimatesof risks at the low doses of concern to waste classification, and (4) anaccounting of stochastic responses in essentially all organs or tissuesfor radionuclides but estimation of responses for most chemicals thatcause stochastic responses based on observed responses in a singleorgan in laboratory animals.

The use of MLEs of probability coefficients for radionuclides butUCLs for chemicals that induce stochastic responses is the mostimportant issue that would need to be resolved to achieve a consistentapproach to estimating risks for the purpose of waste classification.For some chemicals, the difference between MLE and UCL can bea factor of 100 or more. The difference between using fatalities orincidence as the measure of response is unlikely to be important.Use of the linearized, multistage model to extrapolate the dose-response relationship for chemicals that induce stochastic effects,as recommended by NCRP, should be reasonably consistent withestimates of the dose-response relationship for radionuclides, andthis model has been used widely in estimating probability coefficientsin chemical risk assessments. The difference in the number of organsor tissues that are taken into account, although it cannot be recon-ciled at the present time, should be unimportant.


NCRP’s recommendation that MLEs of probability coefficientsshould be used in classifying waste based on risk, rather than UCLs(see Section 6.1.2.2) is based on an assumption that point estimatesof probability coefficients, as well as point estimates of parametersused to calculate exposure and dose in an assumed scenario, wouldbe used to calculate risk. This recommendation can be justified,in part, on the grounds that the assumed exposure scenarios forhypothetical inadvertent intruders at waste disposal sites should bepessimistic compared with actual exposure scenarios at future times.The use of conservative exposure scenarios helps compensate for thepossibility that MLEs of probability coefficients may underestimateactual responses from a given exposure. However, the use of UCLsof probability coefficients in conjunction with point estimates of dosebased on conservative exposure scenarios could result in estimatesof risk that are unreasonably biased for purposes of cost-effectiverisk management. As noted in Section 6.1.2.2, NCRP believes thatrisk assessments used in classifying waste should focus on centralestimates of risk for assumed exposure scenarios. Any desired degreeof conservatism in protecting the public, beyond those embodied inthe assumed scenarios, should be incorporated in risk managementdecisions made by regulatory authorities, which are represented bythe allowable risk and modifying factor in the risk index.

Ideally, risk assessments used in classifying waste, or for anyother purpose, should take into account the full range of uncertaintyin probability coefficients (e.g., the 90 percent confidence interval),not just point estimates of MLEs or UCLs. Furthermore, confidenceintervals should be incorporated in all other aspects of a risk assess-ment, including the definitions of exposure scenarios and the esti-mates of exposure and dose for those scenarios. Some of theseestimates would be highly subjective, such as the confidence intervalto be assigned to the probability of occurrence of defined exposurescenarios. Nonetheless, only in this way can the full weight of informa-tion about potential risks and their uncertainties be brought to bearin making transparent and cost-effective risk management decisions.

6.7.2.2 Approaches to Estimating Dose-Response Relationships forSubstances That Cause Deterministic Responses. Most of the fac-tors that must be considered in developing reasonably consistentapproaches to estimating risk for radionuclides and chemicals thatinduce stochastic responses discussed in the previous section donot apply to substances that induce deterministic responses. Forpurposes of health protection, incidence generally is the appropriatemeasure of response for substances that cause deterministicresponses. Furthermore, an accounting of deterministic responses


in multiple organs or tissues, which occur for some substances (e.g.,heavy metals), can be based on observed dose-response relationshipsin each organ or tissue at risk. Finally, the development of risk-extrapolation models to predict responses at low doses is not animportant concern for substances that induce deterministic effects,due to the threshold nature of the dose-response relationship.

In classifying waste, deterministic responses generally should be ofconcern only for hazardous chemicals (see Section 3.2.2.1). Therefore,the only important issue for risk assessment is the most appropriateapproach to estimating thresholds for induction of responses inhumans. The primary concern here is that consistent approachesshould be used for all substances that induce deterministic effects.NCRP’s recommendation that nominal thresholds in humans shouldbe estimated using the benchmark dose method and a safety factorof 10 or 100, depending on whether the data were obtained in astudy in humans or animals (see Section 6.1.2.1), is intended toprovide consistency in estimating thresholds for all substances thatcause deterministic effects.

For most chemicals that induce deterministic effects, the nominalthreshold in humans or animals has been estimated based onNOAELs or LOAELs. However, the benchmark dose method shouldprovide more reliable estimates of thresholds (see Section 3.2.1.2.7).Therefore, whenever the nominal threshold in humans for an impor-tant chemical in waste that induces deterministic effects has beenestimated based on NOAELs or LOAELs, NCRP believes that thedata should be re-evaluated using the benchmark dose method topromote greater consistency in classifying waste. As in the case ofchemicals that induce stochastic effects discussed in the previoussection, NCRP believes that uncertainties in the data beyond thoseincorporated in the benchmark dose method should be taken intoaccount, if need be, in setting allowable exposures, rather than inan estimate of the nominal threshold.

6.7.3 Allowable Risks from Exposure to Substances That CauseStochastic or Deterministic Effects

The risk-based waste classification system developed in thisReport is based fundamentally on the concepts of negligible (de mini-mis) and acceptable (barely tolerable) risks from exposure to radionu-clides and hazardous chemicals, with the crucial distinction thatacceptable risks generally can be considerably higher than negligiblerisks. Therefore, in implementing the waste classification system,decisions would need to be made by regulatory authorities about


appropriate values of negligible and acceptable risks or doses ofsubstances that cause stochastic or deterministic responses.

For substances that induce stochastic effects, decisions about neg-ligible and acceptable risks reflect societal judgments as well astechnical information on dose-response relationships at high dosesand appropriate methods of extrapolating observed effects to the lowdoses of concern to waste classification. Precedents for establishingnegligible and acceptable stochastic risks for purposes of risk-basedwaste classification are discussed in Section 6.3.1.2. NCRP particu-larly emphasizes that any such risks should be reasonably consistentwith those used in other risk management activities for substancesthat induce stochastic effects, such as control of routine releasesfrom operating facilities.

In establishing negligible and acceptable doses of hazardous chem-icals that induce deterministic effects, decisions are needed about themagnitude of safety and uncertainty factors that should be applied toestimates of nominal threshold doses in humans. NCRP’s recommen-dations are discussed in Section 6.3.1.1. NCRP particularly empha-sizes that RfDs should be used with caution, especially when theyincorporate large safety and uncertainty factors, even though RfDsare widely used in health protection of the public. When a substancethat causes deterministic effects is important in classifying wastebut the quality of the data is poor, NCRP believes that it would bepreferable to undertake additional studies in an effort to reduceuncertainties in the data rather than to simply incorporate largesafety and uncertainty factors in a risk assessment. NCRP believesthat the goal should be to use reasonably consistent safety and uncer-tainty factors in defining negligible and acceptable doses of all sub-stances that induce deterministic effects, in order to give about thesame weight to all such substances in classifying waste.

6.7.4 Selection of Exposure Scenarios

In implementing the risk-based waste classification system devel-oped in this Report, the selection of exposure scenarios appropriateto waste disposal is an important technical issue that must beaddressed. NCRP believes that scenarios for inadvertent intrusioninto near-surface disposal facilities are appropriate in classifyingwaste for purposes of disposal and, further, that scenarios involvingpermanent occupancy of disposal sites after loss of institutional con-trol would be appropriate (see Section 6.1.3); such scenarios arecommonly used in regulating near-surface disposal of low-level radio-active waste and in risk assessments at hazardous waste sites subjectto remediation under CERCLA.


However, alternatives to exposure scenarios involving permanentoccupancy of disposal sites also should be considered in the case ofdedicated near-surface facilities that are intended for disposal oflow-hazard waste. Some form of institutional control, such as fences,warning signs, and occasional surveillance, presumably will beestablished over dedicated hazardous waste disposal sites with theintention to maintain such control for a considerable period of timeafter facility closure (e.g., 30 to 100 y). Nonetheless, it is reasonableto assume that some type of accidental inadvertent intrusion wouldoccur during that time, because institutional control should not beassumed to be completely effective in deterring unwanted intrusiononto a disposal site. Thus, for the purpose of classifying waste as low-hazard or high-hazard, it would be appropriate to develop exposurescenarios involving activities of short duration at disposal sites dur-ing the institutional control period (see Section 7.1 for examples). Themore restrictive of scenarios involving short-term exposure during aninstitutional control period and chronic exposure after the institu-tional control period then could be used to classify nonexempt waste.NCRP also emphasizes that credible scenarios, rather than implausi-ble, worst-case assumptions, should be used in classifying waste,because the probability that exposures of inadvertent intruders willoccur according to postulated scenarios is less than unity.

6.7.5 Legal and Regulatory Development Needs

Development and implementation of the comprehensive and risk-based hazardous waste classification system presented in this Reportwould be facilitated by changes in the current legal and regulatoryframework for managing radioactive and hazardous chemical wastesin the United States. A number of examples have been discussedpreviously in this Report and are summarized below.

The present distinction between radioactive waste that arises fromoperations of the nuclear fuel cycle and NARM waste provides anunnecessary impediment to development of a classification systemthat applies to all radioactive wastes. This distinction is not basedon considerations of protection of public health but is based only onthe source of the waste. NCRP notes that EPA’s proposed guidanceon radiation protection of the public (EPA, 1994d) encourages elimi-nation of this legal distinction, because the guidance specifies thatdose limits for all sources of radiation exposure combined and author-ized limits for individual sources or practices should be applied toessentially all controllable sources, excluding indoor radon, not justto sources associated with the nuclear fuel cycle.


The provision of the National Energy Policy Act (NEPA, 1992)that prohibits NRC from establishing dose criteria that could beused to exempt radioactive wastes from licensing requirements fordisposal clearly is an impediment to development of generally appli-cable exemption levels for radioactive waste. An exempt class ofradioactive and hazardous chemical waste is the cornerstone of therisk-based waste classification system developed in this Report, andany legal and regulatory impediments to establishment of generallyapplicable exemption levels would need to be removed.

Development of a comprehensive hazardous waste classificationsystem based on the distinct concepts of negligible and acceptablerisks would be facilitated by an appropriate reconciliation of theradiation and chemical paradigms for control of exposures to radio-nuclides and chemicals that induce stochastic effects (Section 3.3).Such a reconciliation can be based on the recognition that differencesin the approaches to management of stochastic risks under the twoparadigms are more a matter of perception and differences in nomen-claure than reality (Kocher, 1999; NAS/NRC, 1999a). Risks achievedusing the radiation paradigm often are well below the limit on accept-able (barely tolerable) risk, and risks achieved using the chemicalparadigm often are well above the risk goals. In reality, risk manage-ment decisions using either paradigm are based primarily on theprinciple that exposures should be ALARA, largely without regardfor the regulatory limits or goals in either case.

The radiation and chemical paradigms for management of stochas-tic risks can be reconciled in the following way. First, the conceptof an intolerable (de manifestis) risk—i.e., a risk so high that itnormally must be reduced regardless of cost or other circumstances—should be incorporated in the chemical paradigm. Current laws andregulations that apply the chemical paradigm do not distinguishbetween risks so high that action to reduce risk normally should berequired and lower risks that only warrant consideration of whetherrisk reduction is feasible (e.g., cost-effective) and should be under-taken on that basis. Such a distinction would emphasize that goalsfor acceptable risk in the chemical paradigm essentially define negli-gible risks, rather than limits on acceptable (barely tolerable) risk.Second, the ALARA principle could be incorporated more explicitlyin the chemical paradigm to emphasize, as noted above, that applica-tion of the ALARA principle is the basis for almost all risk manage-ment decisions. Finally, the concept of a generally applicableexemption level, such as a negligible individual dose, should be incor-porated explicitly in the radiation paradigm (NCRP, 1993a). At thepresent time, radiation practices or sources are exempted only on acase-by-case basis.


In managing and disposing of mixed radioactive and hazardouschemical wastes under AEA and RCRA (see Section 4.3), there areno irreconcilable technical inconsistencies between the two laws.However, there are differences in the approaches to regulating wastedisposal under the two laws that could impede efforts to develop acomprehensive waste classification system if they are not reconciled.First, the goal of zero release to the environment under RCRA, whichis based on the chemical paradigm for risk management, clearly isdifferent from the concept of limits on allowable releases under AEA,which is based on the radiation paradigm. Second, the RCRA require-ment for monitoring and maintenance of disposal facilities for 30 yafter facility closure if hazardous waste remains at the site, whichessentially calls for perpetual care when the waste does not degradechemically, differs from the concept of permanent disposal underAEA, in which it is assumed that disposal sites will be abandonedafter an institutional control period with no intent to retrieve thewaste even though substantial amounts of hazardous substancesmay remain at the site. Third, acceptable disposals of radioactivewaste at any site are based in part on long-term projections of theperformance of disposal facilities in limiting releases of hazardoussubstances and potential exposures of the public, but no such projec-tions have been used in determining acceptable disposals of hazard-ous chemical waste at specific sites. Finally, the issuance of permitsfor hazardous waste disposal sites under RCRA does not yet takeinto account the concept of a hypothetical inadvertent intruder afterinstitutional control is ended. The concept of inadvertent intrusionprovides an important basis for developing acceptance criteria fordisposal of radioactive waste in near-surface facilities.

An additional constraint under RCRA that would need to beaddressed in implementing the waste classification system presentedin this Report involves solid waste that is identified as hazardousby listing (see Section 4.2.1). At the present time, any solid wastethat is hazardous by listing cannot be rendered nonhazardous bytreatment. Rather, in accordance with the ‘‘mixture’’ and ‘‘derived-from’’ rules in 40 CFR Part 261 (EPA, 1980b; 1992c; 2001b), anylisted waste is considered to be hazardous regardless of the concen-trations of listed hazardous substances, unless the waste is specifi-cally ‘‘delisted.’’ The waste classification system developed in thisReport, which includes an exempt class of waste as an essentialelement, could be implemented only if these rules were revised toallow establishment of exemption levels for listed hazardous chemi-cal wastes.

6.8 SUMMARY OF THE PROPOSED RISK-BASED SYSTEM / 317

6.8 Summary of the Proposed Risk-Based WasteClassification System

The hazardous waste classification system recommended by NCRPis depicted in Figure 6.1 at the beginning of Section 6. This proposalwas developed with two fundamental objectives in mind. First, allwastes that contain radionuclides, hazardous chemicals, or mixturesof the two should be included in the same classification system.A comprehensive hazardous waste classification system should bedeveloped to replace the separate, and quite different, classificationsystems for radioactive and hazardous chemical wastes, as well asthe separate classification systems for radioactive waste that arisesfrom operations of the nuclear fuel cycle and NARM waste. Second,all hazardous wastes should be classified based on considerations ofrisks to the public that arise from disposition of the material. In thisReport, permanent disposal in a permitted facility for hazardous ornonhazardous waste is the assumed disposition of waste containinghazardous substances that has no further use to its present custo-dian. An important consequence of these two objectives is that thesame rules should apply in classifying any waste that contains haz-ardous substances.

Based on these objectives, the fundamental principle embodied inthe proposed classification system is that waste should be classifiedin relation to disposal systems (technologies) that are expected tobe generally acceptable in protecting public health. The types ofdisposal systems assumed in classifying waste should represent cur-rent or planned practices for radioactive or hazardous chemicalwastes.

Based on the principle that hazardous waste should be classifiedin relation to disposal systems (technologies) that are expected tobe generally acceptable in protecting the public, three basic classesof hazardous waste are defined: (1) exempt waste is any waste thatwould be generally acceptable for disposal in a municipal/industriallandfill for nonhazardous waste; (2) low-hazard waste is any non-exempt waste that would be generally acceptable for disposal in adedicated near-surface facility for hazardous waste; and (3) high-hazard waste is any waste that requires a disposal system consider-ably more isolating than a near-surface hazardous waste facility (e.g.,a geologic repository). In a general way, these qualitative definitionsclearly relate waste classification to risks that arise from disposalof waste. Based on the waste isolation capabilities of the three typesof disposal systems specified in the definitions, exempt waste wouldcontain the lowest concentrations of hazardous substances and high-hazard waste the highest.


Given the qualitative definitions of the three waste classes, theboundaries of the waste classes would be quantified based on explicitdescriptions of how the definitions are related to risk. The boundarieswould be expressed in terms of limits on amounts (concentrations)of individual hazardous substances, with specified rules for how toclassify waste that contains mixtures of hazardous substances, suchas the sum-of-fractions rule for mixtures of substances that inducestochastic effects. Specifically, waste would be classified as exemptif the risk that arises from disposal in a municipal/industrial landfillfor nonhazardous waste does not exceed negligible (de minimis)levels. Use of a negligible risk to quantify limits on concentrationsof hazardous substances in exempt waste is appropriate because thewaste would be managed in all respects as if it were nonhazardous.Nonexempt waste would be classified as low-hazard if the risk thatarises from disposal in a dedicated near-surface facility for hazardouswastes does not exceed acceptable (barely tolerable) levels. Anessential condition of the definitions of exempt and low-hazard wasteis that an acceptable (barely tolerable) risk must be substantiallygreater than a negligible risk. Waste would be classified as high-hazard if it would pose an unacceptable (de manifestis) risk whenplaced in a dedicated near-surface facility for hazardous wastes.

The boundaries between different waste classes would be quanti-fied in terms of limits on concentrations of hazardous substancesusing a quantity called the risk index, which is defined in Equation6.1. The risk index essentially is the ratio of a calculated risk thatarises from waste disposal to an allowable risk (a negligible or accept-able risk) appropriate to the waste class (disposal system) of concern.The risk index is developed taking into account the two types ofhazardous substances of concern: substances that cause stochasticresponses and have a linear, nonthreshold dose-response relation-ship, and substances that cause deterministic responses and havea threshold dose-response relationship. The risk index for any sub-stance can be expressed directly in terms of risk, but it is moreconvenient to use dose instead, especially in the case of substancesthat cause determinstic responses for which risk is a nonlinear func-tion of dose and the risk at any dose below a nominal threshold ispresumed to be zero. The risk index for mixtures of substances thatcause stochastic or deterministic responses are given in Equations 6.4and 6.5, respectively, and the simple rule for combining the two toobtain a composite risk index for all hazardous substances in waste isgiven in Equation 6.6 or 6.7 and illustrated in Equation 6.8. The risk(dose) that arises from waste disposal in the numerator of the risk indexis calculated based on assumed scenarios for exposure of hypothetical


inadvertent intruders at landfills for nonhazardous waste or at dedi-cated near-surface facilities for hazardous wastes.

Use of the risk index in classifying waste is illustrated inFigure 6.2. Classification of waste essentially is a two-step process.The first step involves a determination of whether a waste can beclassified as exempt, based on an assumed negligible risk and anexposure scenario for inadvertent intruders appropriate to disposalof waste in a municipal/industrial landfill for nonhazardous waste.If the waste is not exempt, the second step involves a determinationof whether a waste can be classified as low-hazard, based on anassumed acceptable (barely tolerable) risk and an exposure scenariofor inadvertent intruders appropriate to disposal in a dedicated near-surface facility for hazardous wastes.

An important issue in developing a risk-based hazardous wasteclassification system is the degree of conservatism in protecting pub-lic health that should be embodied in the foundations and frameworkof the system and its implementation. The specific issues are, first,the extent to which calculations of risk in the numerator of the riskindex should deliberately overestimate expected risks that arise fromdisposal of hazardous waste and, second, the extent to which the

Fig. 6.2. Decision diagram for classification of hazardous waste usingthe risk index (RI).


assumed allowable (negligible or acceptable) risks in the denomina-tor of the risk index should incorporate margins of safety.

In many respects, the foundations and framework of the proposedrisk-based hazardous waste classification system and the recom-mended approaches to implementation are intended to be neutralin regard to the degree of conservatism in protecting public health.With respect to calculations of risk or dose in the numerator of therisk index, important examples include (1) the recommendation thatbest estimates (MLEs) of probability coefficients for stochasticresponses should be used for all substances that cause stochasticresponses in classifying waste, rather than upper bounds (UCLs) asnormally used in risk assessments for chemicals that induce stochas-tic effects, and (2) the recommended approach to estimating thresh-old doses of substances that induce deterministic effects in humansbased on lower confidence limits of benchmark doses obtained fromstudies in humans or animals. Similarly, NCRP believes that theallowable (negligible or acceptable) risks or doses in the denominatorof the risk index should be consistent with values used in healthprotection of the public in other routine exposure situations. NCRPdoes not believe that the allowable risks or doses assumed for pur-poses of waste classification should include margins of safety thatare not applied in other situations.

NCRP also recognizes that it would be reasonable to incorporatesignificant degrees of conservatism in implementing the proposedwaste classification system. An important example involves theselection of exposure scenarios to be used in calculating risk in thenumerator of the risk index. Assuming that disposal in a near-surfacefacility is the intended disposition of exempt and low-hazard waste,NCRP believes that it would be reasonable to assume for purposesof waste classification that inadvertent intrusion into waste at adisposal site would occur immediately after an assumed loss of insti-tutional control and that an intruder would permanently occupy thesite over a normal (70 y) lifetime while engaging in activities typicalof a self-sufficient homesteader. These assumptions should be pessi-mistic compared with exposure conditions that are likely to occur atwaste disposal sites in the future, thus providing a margin of safetyin classifying waste. For this reason, NCRP does not believe thatimplausible, worst-case assumptions should be used in developingand implementing models of relevant exposure pathways in theselected exposure scenarios.

In general, degrees of conservatism could be incorporated in arisk-based waste classification system to account for such factors asuncertainties in assumptions, models, and data, as well as the needto protect sensitive population groups (e.g., infants and children). If


regulatory authorities choose to incorporate intentionally conserva-tive assumptions in implementing the waste classification system,NCRP recommends that these assumptions be explicitly identifiedand justified based, for example, on a quantitative uncertainty analy-sis. An open approach would foster consistency and transparency indeveloping the system and in applying it to the wide variety ofhazardous wastes. Furthermore, NCRP believes that conservativeassumptions beyond those incorporated in assumed exposure scenar-ios should be applied to the risk management aspect of waste classi-fication (establishment of allowable risks), rather than in calculatingrisk based on assumed exposure scenarios. Ideally, risk-based wasteclassification, and any other activities in health protection or riskassessment, should be based on a full accounting of uncertainties inall the supporting information and assumptions.

7. Implications of theRecommended Risk-Based WasteClassification System

NCRP’s recommendations on the principles and framework for acomprehensive and risk-based hazardous waste classification systemare given in Section 6. In this system, waste would be classifiedbased in large part on the value of the risk index, which essentiallyis the ratio of the calculated risk that arises from an assumed disposi-tion to a specified allowable risk for that disposition. Simple exam-ples of how the composite risk index for waste that contains mixturesof hazardous substances would be calculated using hypothetical dataare provided in Section 6.4.4. However, to explore the implicationsof the recommended principles and framework for classifying hazard-ous wastes, it is necessary to apply the risk index using typicalcompositions of existing wastes. Example waste classifications arepresented in Section 7.1. The recommendations in Section 6 alsohave implications for the existing legal and regulatory frameworkfor classifying hazardous wastes. These are discussed in Section 7.2.

7.1 Example Applications of the Risk-Based WasteClassification System

This Section provides example applications of the recommendedrisk-based waste classification system to a variety of hazardouswastes to illustrate its implementation and potential ramifications.Disposal is the only disposition of waste considered in these exam-ples. In Section 7.1.1, a general set of assumptions for assessing theappropriate classification of hazardous wastes is developed, includ-ing a variety of assumed exposure scenarios for inadvertent intrudersat waste disposal sites and assumed negligible and acceptable risksor doses from exposure to radionuclides and hazardous chemicals.Subsequent sections apply the methodology to several examplewastes.

322

7.1 EXAMPLE APPLICATIONS OF THE RISK-BASED SYSTEM / 323

It is not NCRP’s intent to recommend specific boundaries betweenwaste classes. Rather, the examples illustrate that the recommendedframework has the potential to be practical and to result in animplementable waste classification system when a variety of plausi-ble assumptions are used. Many assumptions are made in developingthe examples. NCRP endorsement or disapproval should notbe construed from the use or absence of specific assumptionsabout exposure scenarios and allowable doses or risks. It isthe responsibility of the appropriate regulatory authorities todevelop and guide implementation of any waste classificationsystem.

7.1.1 General Approach to the Example Applications

The approach to waste classification presented in this Reportinvolves assessments of risk to hypothetical inadvertent intrudersat generic waste disposal sites. When implementing NCRPs recom-mendations on classification of hazardous wastes, it should be recog-nized that maintaining the operational integrity of a disposal facilityand meeting requirements for protection of the public and the envi-ronment are an integral part of the design and operation of specificfacilities. These considerations are of limited importance to wasteclassification because they require foreknowledge of site-specificcharacteristics. Irrespective of how a waste is classified, appropriatedesigns and controls based on site-specific considerations areassumed to be in place to ensure that applicable standards for protec-tion of the public and the environment will be met. Waste classifica-tion is necessarily based on non-site-specific factors, includingassumptions about an appropriate level of protection for a hypotheti-cal inadvertent intruder. There are two primary considerations inselecting the appropriate level of protection for intruders: the diffi-culty of an intruder accessing disposed waste and the extent of expo-sures and health effects resulting from such access.

Classification of a given waste is based on an evaluation of therisk index specified in Equations 6.4, 6.5 and 6.6 for assumed typesof disposal systems. If the risk index is less than unity, the wasteis acceptable for inclusion in the associated waste class; otherwise,the waste generally requires a more protective disposal system andwould be placed in a class for more hazardous wastes. The appro-priate classification depends on the level of protection requiredwhich, in turn, depends on the characteristics of the waste relativeto the capabilities of assumed disposal technologies. This concept isa fundamental part of the risk index. General assumptions about

324 / 7. IMPLICATIONS OF THE RECOMMENDED SYSTEM

disposal technologies, exposure scenarios, and allowable doses orrisks used in the example waste classifications based on the riskindex are summarized in Table 7.1 and discussed below. Otherassumptions about exposure scenarios not listed in this table areincluded in some of the example waste classifications to illustratethe importance of plausible alternatives.

7.1.1.1 Exempt Waste. When wastes that contain small amountsof radionuclides are considered for disposal in a landfill for nonhaz-ardous waste, the allowable dose may be the same as that specified

TABLE 7.1—Summary of assumptions used in example hazardouswaste classifications.

Scenario and AllowableWaste Disposal Period of Dose or

Classification Technology Exposurea Riskb Comments

Exempt Disposal in a Residential 0.02 mSv y�1 Long-termnear-surface (with garden- occupancy offacility suit- ing) or commer- 10�5 lifetime disposal site isable for cial risk assumed to benonhazardous possiblewastes 25 or 30 y RfD

Low-hazard Disposal in a Intruder drills 20 mSv per Duration ofnear-surface into waste event intrusion isfacility resulting in limited bysuitable for 1,000 h of 10�3 lifetime occasionalhazardous exposure once risk per event surveillancewastes during a and recog-

lifetime 10 � RfD nition of wasteby intruder

High-hazard Disposal in a Intrusion Not applicable Mostgeologic considered to waste protectivefacility unlikely classification disposalsuitable for practice; anyhighly waste nothazardous acceptable inwastes other classes is

included inthis class

a Other assumptions about exposure scenarios are included in discussions of examplewaste classifications.

b These assumptions are not necessarily recommendations of NCRP but representpossible alternatives that are reasonable. Decisions regarding allowable risks or dosesto be used in defining the boundaries between waste classes are the responsibilityof regulatory authorities. Other examples of allowable risks or doses are discussedin considering classification of specific wastes.


in standards for protection of the public that apply to a single sourceof radiation exposure (e.g., a small fraction of the annual dose limitof 1 mSv, corresponding to a lifetime risk of about 10�5 for an assumedexposure time of 30 y). In developing exposure scenarios at a disposalfacility for nonhazardous waste, fewer access restrictions shouldbe assumed than in the case of disposal in a regulated facility forradioactive waste, and the ultimate use of the site could be residen-tial, commercial or resident-farmer. Selection of the end use shouldbe based on plausibly conservative but realistic assumptions aboutfuture land uses.

Similar considerations should apply to waste that contains smallamounts of hazardous chemicals that might be sent to a disposalfacility for nonhazardous waste. Allowable doses could correspondto a negligible lifetime risk of about 10�5 in the case of substancesthat induce stochastic effects or an intake at an RfD (Section 3.2.1.2)in the case of substances that induce deterministic effects. The con-siderations of exposure scenarios should be the same as in the caseof radioactive wastes.

7.1.1.2 Low-Hazard Waste. In a scenario involving disposal ofhazardous waste in a licensed facility, the allowable risk or dose canbe higher than in cases of disposal in a facility for nonhazardouswaste, based on the reduced duration of exposure that probablywould result from occasional surveillance of the site or recognitionof waste by an intruder. In addition, it is not likely that the publicwill gain long-term access to hazardous waste disposal sites for theforeseeable future. Therefore, in the examples that follow, a higher(less restrictive) allowable risk or dose is assumed to be appropriatewhen evaluating wastes for classification as low-hazard (e.g., a radia-tion dose of 20 mSv for a one-time exposure of 1,000 h, correspondingto a risk of about 10�3).

In the case of near-surface disposal at hazardous waste sites, itis assumed in the following examples that an intruder gains accessto the disposal site, drills into the waste, thus bringing some wasteto the surface, and is subsequently exposed to the waste constituentsfor one-half of a working year (1,000 h). This assumption is believedto be conservative because (1) the postulated occasional surveillanceat licensed facilities makes it unlikely that an activity of the magni-tude required to intrude into disposed waste could actually continuefor half a year and (2) the discovery of waste drums or other unusualbarriers or features of the disposal site would alert the intruder andmeasures likely would be taken to minimize the exposure time. Itis assumed that the drilling activity results in mixing of the wastewith clean soil or cover material, thereby diluting the waste.


7.1.1.3 High-Hazard Waste. No scenarios are developed for inad-vertent intrusion into a disposal facility for high-hazard waste,essentially because intrusion into suitable facilities for these wastesshould be highly unlikely (see Section 6.2.2.2.2) and evaluationsof highly unlikely scenarios do not provide a reasonable basis fordetermining the general acceptability of waste disposal systems(NAS/NRC, 1995a). Any waste that is not generally acceptable fornear-surface disposal in a licensed facility for hazardous waste wouldbe relegated to the high-hazard waste class.

7.1.1.4 Development of Examples. The example classifications ofexisting wastes presented in Sections 7.1.2 through 7.1.8 use therisk-based approach generally developed in Section 6 and theassumptions shown in Table 7.1 and summarized earlier. Alternativeassumptions about exposure scenarios are used in some cases. Sev-eral risk/dose estimation procedures are used to determine the appro-priate classification for the waste compositions used in the examplesto illustrate possible approaches to implementation. The goal is todemonstrate the feasibility of the concept and various procedures foremploying it. The procedures range from simple screening methodsrequiring a minimum of effort (e.g., use of conservative assumptionsfor the contaminants that are the principle contributors to risk) tomore detailed risk or dose assessments for all hazardous materialsin the waste.

In presenting the examples, qualitative considerations that areimportant in applying the recommended risk-based waste classifica-tion system also are discussed. While the classification system isbased on risk assessment, its establishment involves major risk man-agement aspects. Risk assessment as manifested in the risk indexis a tool in making good risk management decisions; however, ifused alone without clear recognition of the implications of the riskestimates, including their limitations and uncertainties, risk assess-ment can be just as likely to result in a poor decision as a good one.

7.1.2 Consideration of Exempt Wastes

NCRP did not undertake a detailed investigation into the kindsand quantities of radioactive or hazardous chemical wastes contain-ing low levels of hazardous substances that might be classified asexempt, based on the consideration that allowable dispositionsshould pose no more than a negligible risk or dose. Rather, publishedstudies are cited to indicate that substantial quantities of wastecurrently managed as radioactive or chemically hazardous waste


are potential candidates for exemption, especially for purposes ofdisposal.

7.1.2.1 Radioactive Wastes. In 1989, the Electric Power ResearchInstitute (EPRI) issued a study of wastes that arise from operationsof nuclear power plants that potentially would be below regulatoryconcern (BRC) and, thus, would be acceptable for disposal in a landfillfor nonhazardous waste (EPRI, 1989). NRC’s proposed criteria fordefining BRC waste are discussed in Section 4.1.2.5.2; they are con-sistent with the precedents for defining a negligible dose discussedin Section 6.3.1.2.1 and the assumption about an allowable risk givenin Table 7.1. Based on assumptions used in EPRI study that (1) thetotal activity concentration of all photon-emitting radionuclides inthe waste would not exceed 40 Bq g�1, (2) each reactor station wouldproduce 180 metric tons (about 100 m3) of potentially exemptiblewaste annually, and (3) the waste from each station would be mixedwith 54,000 metric tons of nonradioactive waste prior to disposal,the estimated annual dose to a resident homesteader at a disposalsite after closure of the site was about 3 �Sv. This estimate is abouta factor of 30 less than NRC’s BRC criterion of 100 �Sv for practicesaffecting only a few individuals. The wastes considered in this studyincluded most of the so-called dry-active waste generated at nuclearpower plants. This type of waste includes rags, paper, plastic floorcovering or bags, protective clothing, contaminated tools, wood, anddiscarded plant equipment and hardware. Other wastes consideredin the analysis included contaminated soil, secondary ion-exchangeresins, grit blast material, and sludge from water treatment.

NRC has issued an assessment of potential doses to the publicassociated with the distribution, use, and disposal of exempt prod-ucts or materials containing low levels of source or byproduct mate-rial (Schneider et al., 2001) (see Section 4.1.2.5.2). In a case involvingdisposal of large volumes of zircon sand produced in processing ofzirconium-bearing minerals, the estimated annual dose to a futureon-site resident at a disposal site was 100 �Sv, due to the elevatedlevels of thorium and uranium. In all other cases, however, theestimated annual dose was substantially less than 10 �Sv. Since thevolumes of exempt material were large in many cases, this analysisindicates that substantial volumes of waste that contains low levelsof radionuclides are potentially exemptible.

A noteworthy result of NRC’s analysis of its present exemptionsis that doses to individual members of the public during use of exemptproducts or materials generally are higher than doses that arise fromdisposal (Schneider et al., 2001). This is due in part to differences inthe assumed exposure scenarios for use and disposal; the dilution


of radionuclide concentrations by mixing with uncontaminated mate-rials prior to closure of a disposal facility also is an important factor.This result indicates that materials that would be exempt for purposesof disposal would not necessarily be exempt for any other purpose.

7.1.2.2 Hazardous Chemical Wastes. NCRP has not consideredstudies of particular wastes containing low levels of hazardous chem-icals that are potential candidates for exemption. However, studiesin support of proposed regulations to establish exemption levels forlisted hazardous wastes (EPA, 1992d; 1995c; 1999c) indicate that sub-stantial quantities of waste currently managed as chemically hazard-ous waste could be classified as exempt for purposes of disposal.

7.1.3 DOE Low-Level Radioactive Waste

DOE currently operates a number of disposal facilities at its sitesthat receive low-level radioactive wastes from DOE’s weapons com-plex and energy research facilities. One of the largest disposal facili-ties is on the Hanford site in the state of Washington. The wasteinventory for 1990 (DOE, 1993b) was used in this example to deter-mine if this waste is acceptable for near-surface disposal under theexample assumptions given in Table 7.1. The concentrations of radio-nuclides in this waste clearly exceed those that would be permittedfor disposal in a landfill for nonhazardous waste, and the waste wouldnot be exempt. Therefore, potential doses were evaluated assumingtemporary intrusion by unknowing individuals working in the dis-posal area.

The risk index normally is determined by computing the risk orby using dose as a surrogate for risk. In the example in Section7.1.3.1, the calculated dose associated with intrusion into the wasteis divided by the assumed maximum allowable dose to estimate therisk index. In the example in Section 7.1.3.2, limits on acceptableconcentrations are developed as surrogates for the allowable risk.The concentrations in the waste are then divided by these allowableconcentrations to determine the risk index. The same approach isused in the example in Section 7.1.3.3, except the allowable concen-trations are lower because a less protective disposal option is evalu-ated. The consequences of alternative assumptions about intrusionscenarios on classification of the Hanford waste are considered inSection 7.1.3.4.

7.1.3.1 Classification by Calculation of Total Dose. Exposurepathways considered in this analysis involve external exposure,ingestion of waste materials, and inhalation. Doses were estimated


based on the concentrations of radionuclides shown in Table 7.2using the RESRAD18 code (Yu et al., 1993). It was assumed thatthe intrusion event occurs at 100 y after disposal of the waste. Theassumed time of intrusion takes into account the intention thatactive institutional controls (e.g., fences and guards) will be maintainedfor at least 100 y after closure of the facility (DOE, 1988c; 1999c).

For the purposes of this example, it was assumed that the wastewas placed 4 m deep and covered with a cap and soil that was atleast 3 m thick. As a consequence, the assumed scenario was an on-site drilling event. The dose analysis assumes a two-fold volumeincrease (50 percent dilution) of the drill tailings by uncontaminatedmaterial. The mixture of waste and uncontaminated cover materialis spread on the surface of the site, and individuals working in thearea are exposed to the tailings for 1,000 h. The thickness of thelayer of contaminated drill tailings is assumed to be about 5 cm andthe area to be about 3.3 m2. Using dose as a surrogate for risk,analysis of this scenario yields a dose of 0.002 mSv from all radionu-clides. Since the assumed allowable dose is 20 mSv (see Table 7.1),the risk index would be 0.002/20 � 10�4, which is well below thevalue of unity, and the waste would be classified as low-hazard.

7.1.3.2 Classification Using Pre-Established Limiting Concentra-tions. Another approach to classifying the Hanford low-level waste

TABLE 7.2—Hanford low-level waste radionuclide contents and therisk index for drilling scenario.

Radionuclide Content in WasteRisk-Based

Activity Concentration Concentration RiskNuclides (TBq) (Bq g�1) Limit (Bq g�1) Index

Co-60 3.6 � 103 1.7 � 103 5.9 � 108 2.9 � 10�6

Cs-137 8.9 4.1 4.8 � 104 8.7 � 10�5

Ni-63 3.5 � 103 1.6 � 103 2.1 � 109 7.8 � 10�7

Plutonium 2.2 � 10�2 0.01 1.1 � 104 9.3 � 10�7

Th-232 2.0 � 10�4 9.6 � 10�5 3.7 � 102 2.6 � 10�7

Uranium (depleted) 0.28 0.13 3.7 � 102 1.1 � 10�4

Uranium (enriched) 0.037 0.17 7.5 � 103 2.2 � 10�6

All others — — — 1.7 � 10�7

Total risk index 2.0 � 10�4

18 The RESRAD code was used to illustrate implementation of the proposed wasteclassification system. NCRP did not evaluate the code or its underlying assumptionsand database, and its use should not be construed to constitute endorsement.


is to establish radionuclide-specific concentration limits obtainedfrom the RESRAD calculations described in Section 7.1.3.1 as asurrogate for the allowable risk. Table 7.2 lists the concentrationlimits resulting from this calculation. This approach is generallyconservative because some of the concentration limits for radionu-clides having very long half-lives exceed the maximum theoreticalconcentrations based on the specific activity of the radionuclidesthemselves. As shown in Table 7.2, the risk index calculated usingthis approach is 2 � 10�4 and, thus, the waste is again classified aslow-hazard. The benefit of this approach is that it avoids the needto model the potential doses or risks directly if concentration limitsfor different waste classes have been established. Classification ofwaste by using pre-established limiting concentrations would be thetypical approach (NRC, 1982a).

7.1.3.3 Classification Using Pre-Established Limiting Concentra-tions and Enhanced Access. The same process as in the previouscalculation was used to screen this example waste under conditionswhere intruder access to the waste would be enhanced. For example,if the cover for the waste is less than 2 m, it would be appropriateto consider that an intruder would remove sufficient cover material toexpose the waste. The maximum allowable concentrations of selectedradionuclides for this scenario calculated using the RESRAD codeare listed in Table 7.3. The analysis assumes that the intruder isexposed via external exposure, ingestion, and inhalation for a periodof 1,000 h. Using the sum of the ratios of the radionuclide concentra-tions in the waste to the maximum allowable concentrations as a

TABLE 7.3—Hanford low-level waste radionuclide contents and therisk index for enhanced-access intrusion scenario.


Concentration Concentration RiskNuclides Activity (TBq) (Bq g�1) Limit (Bq g�1) Index

Am-241 0.041 0.018 1.7 � 103 1.1 � 10�5

Co-60 3.6 � 103 1.7 � 103 2.4 � 107 6.8 � 10�5

Cs-137 8.9 4.1 2.3 � 103 1.8 � 10�3

Gd-153 2.1 � 10�5 1.0 � 10�5 0.78 1.3 � 10�5

Ni-63 3.5 � 103 1.6 � 103 1.0 � 108 1.6 � 10�5

Plutonium 2.1 � 10�2 1.0 � 10�2 1.0 � 103 1.0 � 10�5

Ra-226 3.5 � 10�3 1.6 � 10�3 8.1 � 101 2.0 � 10�5

Uranium (depleted) 0.29 0.13 2.7 � 103 4.9 � 10�5



surrogate for risk, the risk index was estimated to be 0.002. Thisresult is still much less than unity but is significantly greater thanin the example assuming deeper burial of the waste. This exampleassumes some dilution of the uncovered waste because (1) wasteemplacement procedures generally include the use of clean fillbetween containers (about a factor of two dilution) and (2) excavationof the cover material will likely dilute the waste (about a factor of2 to 10). However, because the screening analysis indicated a riskindex nearly three orders of magnitude less than unity, a moredetailed analysis of this scenario is not warranted.

7.1.3.4 Alternative Exposure Scenarios. In the scenarios for inad-vertent intrusion at a radioactive waste disposal facility consideredin Sections 7.1.3.1 through 7.1.3.3, intrusion is assumed to be a one-time event occurring at 100 y after disposal. This Section considersalternative scenarios and their impacts on classification of theHanford waste.

The assumption that an intrusion event would not occur until100 y after disposal is based on an intention to maintain institutionalcontrol over the disposal site for at least this long (DOE, 1988c;1999c). However, even during the period of institutional control,inadvertent and unnoticed access to a disposal site cannot be ruledout completely. The consequences of inadvertent intrusion during theinstitutional control period can be bounded by assuming that a drillingor limited excavation event would occur essentially at the time offacility closure (i.e., 100 y earlier) and that the amount of waste broughtto the surface is the same as the amount assumed in the drillingscenario at 100 y analyzed in Sections 7.1.3.1 and 7.1.3.2. A large-scaleexcavation, such as assumed in the analysis in Section 7.1.3.3, wouldnot to be credible during the institutional control period. The exposuretime is assumed to be 1,000 h, as in the previous examples.

The risk index for the assumed scenario at the time of facilityclosure can be obtained from the results in Table 7.2 by adjustingfor 100 y of radioactive decay. For example, for 137Cs, which has ahalf-life of 30 y, the risk index in Table 7.2 would be increased by afactor of 10 because the concentration at the time of facility closurewould be a factor of 10 higher than at 100 y after closure. Takinginto account similar increases in the risk index for 60Co (5.3 y) and63Ni (100 y), the resulting risk index for exposure at the time offacility closure would be 1.5, due almost entirely to 60Co. However,since drilling or limited excavation at the time of facility closure andexposure for 1,000 h to exhumed waste at that time are unlikely tooccur (i.e., the assumed scenario is conservative), the estimated dosefor any reasonably likely scenario during the institutional control


period should be considerably less. Thus, this result supports theconclusion that the waste would be classified as low-hazard.

Another credible assumption is that permanent access to the sitecould occur at the end of the 100 y period of institutional control.This assumption has been used in establishing waste acceptancecriteria at all DOE low-level waste disposal sites (DOE, 1988c;1999c), including the Hanford site, based on an acceptable dose fromchronic exposure of an inadvertent intruder of 1 mSv y�1. Therefore,the waste acceptance criteria for the Hanford site already take intoaccount an acceptable dose to an inadvertent intruder from perma-nent site occupancy, so the waste is acceptable for near-surface dis-posal as low-hazard waste according to this scenario without theneed for further analysis.

7.1.4 Average Commercial Low-Level Radioactive Waste

Another example involves classification of average commerciallow-level radioactive waste. The total activities of the dominantradionuclides in the waste, as obtained from data for all Class-A,-B and -C low-level waste emplaced in near-surface disposal facilitiesin the United States in 1990, are given in Table 7.4 (DOE, 1993b).These estimates do not account for decay that would occur duringthe 100 y institutional control period. That is, inadvertent intrusionis assumed to occur at the time of facility closure. Radionuclideconcentrations are based on the estimated volume of waste and an

TABLE 7.4—Average commercial low-level waste radionuclideconcentrations and the risk index for drilling intrusion scenario.


Concentration Concentration RiskNuclides Activity (TBq) (Bq g�1)a Limit (Bq g�1) Index

Am-241 1.2 � 102 2.4 1.7 � 102 1.5 � 10�2

Cs-137 3.5 � 105 7.4 � 103 4.3 � 104 1.5 � 10�2

Th-232 1.3 � 104 2.6 � 103 3.7 � 102 7.1 � 10�1

U-238 7.4 � 103 1.5 � 102 1.2 � 103 1.2 � 10�1

Uranium (depleted) 4.1 � 103 8.1 � 101 1.2 � 103 6.7 � 10�2

All others — — — 4.1 � 10�3


a Based on volume of 32,400 m3 and assumed density of 1.5 g cm�3.


assumed density of 1.5 g cm�3. The limiting radionuclide concentra-tions calculated using the RESRAD code, assuming a drilling intru-sion scenario as described in Section 7.1.3.1, are given in Table 7.4.Summing risk indexes for individual radionuclides yields a compos-ite risk index of 0.93 for this waste, indicating that it is a low-hazardwaste that is generally acceptable for near-surface disposal.

Because the risk index calculated using the concentration-basedscreening approach is very close to unity, additional review wasconducted to ensure that the approach was acceptable and to illus-trate how additional factors could be taken into account for wastesnear boundaries. As noted in Section 7.1.3.4, an assumption thatdrilling intrusion would occur at the time of facility closure shouldbe conservative. For example, as a result of the relatively short half-life of 137Cs, the concentration will be halved every 30 y, which wouldreduce the risk index for 137Cs from 0.015 to 0.002 after an assumedinstitutional control period of 100 y (NRC, 1982a; 1982b). Addition-ally, review of the amounts of 232Th assumed to be in the wasteindicate that the inventory may be overestimated by as much as afactor of 10 (DOE, 1993b). Since NRC does not specify limits onconcentrations of 232Th that are generally acceptable for near-surfacedisposal (NRC, 1982a) and the presence of this radionuclide doesnot have a significant effect on predicted doses to off-site individuals,there is little incentive for generators to report more realistic invento-ries. These two factors alone would reduce the estimated the riskindex to less than 0.3. Also, as noted in Section 7.1.3.2, use of theconcentration-limit approach may be somewhat conservative (e.g., afactor of two in the earlier example case).

A scenario involving permanent occupancy of a disposal site followingloss of institutional control at 100 y after disposal also could be consid-ered in classifying commercial low-level waste (see Section 7.1.3.4). Asin the case of DOE waste discussed above, the limits on concentrationsof radionuclides in commercial Class-A, -B, and -C waste that maybe sent to a near-surface facility, as established by NRC in 10 CFRPart 61, are based largely on analyses of this type of scenario (NRC,1982a; 1982b). Thus, average commercial low-level waste would beclassified as low-hazard waste according to this scenario without theneed for further analysis.

7.1.5 Typical Uranium Mill Tailings

Current environmental standards for uranium mill tailings (EPA,1983) permit surface disposal of these wastes with appropriate engi-neered controls. The controls include use of a cap that typically


consists of about 1 m or more of clay covered by a layer of rip-rapand soil. Mill tailings disposal sites will ultimately be owned by thefederal government or the host states.

Uranium mill tailings contain a variety of naturally occurringradionuclides and toxic heavy metals. The most significant of theseare 226Ra and its decay products. Concentrations of 226Ra in uraniummill tailings produced in the United States are in the range of about2 to 37 Bq g�1 (EPA, 1982). These concentrations are at least twoorders of magnitude higher than the average concentration in surfacesoil of about 0.02 Bq g�1 (NCRP, 1984a). As is apparent in the previ-ous examples, it is not always necessary to consider all hazardoussubstances in a waste, because a few constituents often dominatethe risk. This example considers the risk due only to 226Ra.

The average concentration of 226Ra in surface soil results in anaverage dose from external exposure of about 0.07 mSv y�1 (NAS/NRC, 1999a). Since the concentration of 226Ra in mill tailings is atleast a factor of 100 higher than the average concentration in surfacesoil, as noted above, it is obvious without further analysis thatunrestricted release of mill tailings disposal sites would result innon-negligible doses and risks to individuals who might reside on asite. Therefore, mill tailings could not possibly be classified as exemptwaste. Furthermore, since the dose due only to external exposureduring permanent residence on a tailings pile would exceed thecurrent dose limit for the public of 1 mSv y�1 (DOE, 1990; NRC,1991) by about an order of magnitude or more, mill tailings could beclassified as low-hazard waste only if perpetual institutional controlwere maintained over disposal sites to prevent long-term occupancyby the public. An assumption of institutional control is used in thefollowing example.

In this analysis, it was conservatively assumed that an individualcould enter a disposal site and remove a portion of the protectivesurface cover (about 200 m2) and expose on-site workers to the tail-ings.19 It was further assumed that such an unlikely scenario wouldoccur only once in an individual’s lifetime, and that exposure wouldoccur over a period of 1,000 h. Exposure pathways considered in thisanalysis involve external exposure, inhalation of outdoor radon andits short-lived decay products, inhalation of particulates, and inges-tion of waste. The activity concentrations of 226Ra and its decay prod-ucts in the waste were conservatively assumed to be 37 Bq g�1, a

19 Such an action is mitigated by two considerations. First, it is expected that evenwith closed disposal facilities, annual visual inspections, if not caretaker activities,will occur. Furthermore, the recommended disposal cell design (including rip-rap andclay cover) will discourage the assumed activity. However, no credit was given forthese controls in this assessment.


value which is at the upper end of the range of concentrations indomestic mill tailings (EPA, 1982).

Based on the assumptions described above, the risk calculatedusing the RESRAD code (Yu et al., 1993) and EPA slope factors(HEAST, 1991; 1995) is a probability of cancer incidence of about7 � 10�4. The assumed acceptable risk for low-hazard waste is 10�3

(see Table 7.1), resulting in a risk index of 0.7. Hence, domesticuranium mill tailings could be classified as low-hazard waste accept-able for licensed near-surface disposal under conditions of perpetualinstitutional control over disposal sites.

An alternate analysis was performed using dose as a surrogatefor risk. Using the same assumptions given above and the RESRADcode, the dose to an intruder was estimated to be 15 mSv. Assumingan allowable dose of 20 mSv (see Table 7.1), the risk index wasestimated to be 0.8. Therefore, as expected, either approach yieldsabout the same result.

The example analysis used to classify uranium mill tailings aslow-hazard waste depends critically on the assumption of perpetualinstitutional control over disposal sites. An alternative approach toclassification of mill tailings could be considered in which permanentresidence on disposal sites is assumed to be plausible at some timein the future, as in classifying low-level radioactive waste in previousexamples. As noted above, the doses and risks based on such ascenario clearly would be intolerable and mill tailings would be clas-sified as high-hazard waste. However, disposal of the very largevolumes of mill tailings far below the ground surface, as intended forother high-hazard radioactive wastes, is considered to be impractical(EPA, 1982; 1983). Instead of taking the high risk to an inadvertentintruder at an uncontrolled near-surface disposal site into accountby assuming perpetual control in classifying mill tailings, regulatoryauthorities could grant an exception to disposal requirements forthis type of high-hazard waste. The net result would be the same ineither approach—namely, the need to maintain institutional controlover tailings piles essentially forever. This alternative also could beconsidered for other hazardous wastes that contain highly elevatedlevels of naturally occurring hazardous substances and occur in verylarge volumes, such as wastes from mining and processing of variousmineral ores to extract nonradioactive materials.

7.1.6 Residues from Processing of High-Grade Uranium Ore

During the early years of the nuclear energy and weapons develop-ment programs, ores containing unusually high concentrations of


uranium from nondomestic sources were processed at various loca-tions in the United States. Some of these ores contained as much as65 percent uranium oxide. Processing of these ores and concentratesproduced about 11,000 m3 of residues with 226Ra concentrations thatrange from tens of Bq g�1 to more than 18,000 Bq g�1 (NAS/NRC,1995b).

The exposure scenario described in the previous example of domes-tic uranium mill tailings was used to classify the high-radium resi-dues. The risk and dose assessments indicated a probability ofradiation-induced cancer incidence of about 0.6, potential doses inexcess of 10 Sv, and a risk index between 50 and 100. Thus, theseresidues would be classified as high-hazard waste, even under condi-tions of perpetual institutional control over near-surface disposalsites, and they would require some form of greater confinementdisposal well below the ground surface. This conclusion is consistentwith recommendations for disposition of these residues (NAS/NRC, 1995b).

7.1.7 Mixed Waste: Electric Arc Furnace Dust

Electric arc furnace dust is a listed hazardous chemical waste.This material is deemed hazardous because it contains relativelyhigh concentrations of heavy metals. The waste consists of the emis-sion control dust or sludge collected from electric arc furnaces duringthe manufacture of iron and steel. The principle chemicals of concernand their concentrations are listed in Table 7.5 (EPA, 1988).

The first part of the following analysis considers the toxic metalsin this waste and evaluates the waste for near-surface disposal. Ina second part of this example, the waste is presumed to be contami-nated by 137Cs from sources inadvertently included in scrap metalsthat are recycled into the manufacturing process.

7.1.7.1 Introduction to Analysis. All previous examples involvedwaste in which radionuclides were assumed to be the only hazardoussubstances. However, the contaminants of concern in electric arcfurnace dust include chemicals that induce stochastic and determin-istic effects. Furthermore, the deterministic chemicals affect differ-ent organs, and some affect more than one organ.

The composite risk index for mixtures of substances that causestochastic or deterministic effects is shown in its most general formin Equations 6.4, 6.5 and 6.6 (see Sections 6.4.1 and 6.4.2), and isrestated in a simpler and more convenient form in Equation 6.7 (seeSection 6.4.4). In calculating the risk index for mixtures of substances


TABLE 7.5—Concentrations of hazardous chemicals in untreated,high-zinc electric arc furnace dust waste.

Range of MeasuredConcentrations

ArithmeticLow High Mean

Chemical (ppm) (ppm) (ppm)a

Antimony 52 290 170Arsenic 40 400 220Barium 24 400 210Beryllium 0.15 1.5 0.82Cadmium 12 5,000 2,500Chromium 400 12,000 6,300Lead 500 140,000 70,000Mercury 0.0002 41 21Nickel 10 6,900 3,400Selenium 5 20 13Silver 2.5 59 31Vanadium 24 140 81Zinc 4,400 320,000 160,000

aAverage of low and high values.

that induce deterministic effects, it is necessary to identify the organor organs potentially affected by exposure to each such substance,taking into account that a given substance may induce deterministicresponses in more than one organ, and the allowable dose for eachcombination of organ and substance. For this example, RfD for eachchemical that induces a deterministic effect given in Table 7.6(ATSDR, 1987; HEAST, 1995; IRIS, 1988) was assumed to be theallowable dose; RfD generally depends on the route of intake. Forchemicals that induce stochastic effects, the slope factors (probabilitycoefficients for cancer incidence) given in Table 7.7 were used toobtain the allowable doses based on an assumed allowable risk.

As discussed in Sections 3.2.3, 3.3, and 6.1, RfDs and slope factorsare intended to provide conservative estimates of risk. Therefore,they are most suitable for use in establishing a negligible dose, i.e.,in determining whether a waste could be classified as exempt. Toreduce the amount of conservatism to a degree appropriate to estab-lishing an acceptable risk for the purpose of evaluating whether aparticular waste would be classified as low-hazard or high-hazard,RfDs are multiplied by a factor of 10 and the slope factors are dividedby a factor of 10.


TABLE 7.6—Organ-specific RfDs for toxic metals in electric arcfurnace dust.

Route of Intake andChemical Critical Organ RfDa [mg (kg d)�1]

Antimony Oral-cardiovascular 4.0 � 10�4

Arsenic Oral-dermal/ocular 3.0 � 10�4

Barium Oral-cardiovascular 7.0 � 10�2

Inhalation-multiple 1.4 � 10�4

Beryllium Oral-multiple 2.0 � 10�3

Inhalation-respiratory 5.7 � 10�6

Cadmium Oral-renal 1.0 � 10�3

Chromiumb Oral-multiple 3.0 � 10�3

Inhalation-respiratory 3.0 � 10�5

Leadc Oral-multiple 4.0 � 10�4

Mercury Oral-renal 3.0 � 10�4

Inhalation-renal 8.6 � 10�5

Nickel Oral-weight decrease 2.0 � 10�2

Selenium Oral-hepatic 5.0 � 10�3

Silver Oral-dermal/ocular 5.0 � 10�3

Vanadium Oral-renal 7.0 � 10�3

Zinc Oral-weight decrease 3.0 � 10�1

a Based on data given in EPA’s Integrated Risk Information System(www.epa.gov/iris), current as of March 1999, except as noted.

bAssumed to be hexavalent form.cRfD is based on drinking water standard (ATSDR, 1987).

TABLE 7.7—Slope factors for metals that induce stochastic effectsin electric arc furnace dust.

Ingestion slope factora Inhalation slope factora

Chemical [mg (kg d)�1] [mg (kg d)�1]

Arsenic 1.5 15Beryllium 8.4Cadmium 6.3Chromium (VI) 42Nickel 0.84b

a Based on data given in EPA’s Integrated Risk Information System(www.epa.gov/iris), current as of March 1999, except as noted.

bValue obtained from HEAST (1995).


7.1.7.2 Evaluation as Exempt Waste. The subject waste wascursorily evaluated against soil screening criteria developed byEPA (1996e) to ascertain whether it might qualify as exempt waste.The concentrations of antimony, arsenic, beryllium, cadmium, chro-mium, and lead all were orders-of-magnitude above the screeningcriteria. The maximum concentrations of most of the other elementslisted in Table 7.6 also exceed the screening values. The purpose ofthe screening criteria is to determine if further evaluation is needed.While these screening criteria are conservative and do not in them-selves indicate that a material is unacceptable for disposal as non-hazardous waste, the magnitude of the differences is sufficient toindicate the need for classification as at least a low-hazard waste.

7.1.7.3 Approach to Example Analysis. Similar to the previousexamples involving radioactive wastes, these residues were assumedto be placed in a typical near-surface disposal facility having a RCRASubtitle C permit. In this example, it is assumed that an inadvertentintruder excavates an area of the disposal site of approximately200 m2. This excavation is sufficient to reach the waste, and theexposure pathways considered involve inhalation of resuspendedwaste, ingestion of waste, and dermal absorption. The intrusion isidentified and halted prior to any structures being constructed onthe disposal site and before any farming activity can be developed.As in the similar scenarios used in the radioactive waste examples,exposure is assumed to continue for 1,000 h.

The analysis of exposures to hazardous chemicals for this examplewas in accordance with EPA guidance on evaluating human healthrisks from exposure to chemicals in soil. The calculations were per-formed using the RESRAD-CHEM code (Cheng and Yu, 1993), whichis similar to the RESRAD code for estimating doses and risks fromexposure to radionuclides (Yu et al., 1993). Intake rates for individ-ual pathways were calculated for each chemical (element) of interestin the waste, assuming a unit concentration of 1 mg kg�1 (i.e., ppm).The estimated intake rates per ppm in waste by dust inhalation andsoil ingestion were 6.2 � 10�9 and 3.3 � 10�8 mg (kg d)�1, respectively,for all chemicals. For dermal absorption, the intake rate depends onthe particular chemical element and ranged from 3.3 � 10�9 and1 � 10�9 mg (kg d)�1 for cadmium and lead, respectively, to about1 � 10�10 mg (kg d)�1 or less for all other elements. Thus, the totalintake rate by all routes of exposure is determined primarily by soilingestion, and is about 4 � 10�8 mg (kg d)�1 per ppm in the waste forall chemicals.

7.1.7.4 Deterministic Risk Index for Hazardous Chemical Constit-uents. In accordance with Equation 6.5, the risk index for all sub-stances in the waste that induce deterministic responses is


calculated taking into account all potentially affected organs and allresponses of concern. A risk index for each organ at risk is calculated,taking into account all substances affecting a given organ, the maxi-mum risk index for any organ is selected, and the result is truncatedto the nearest integer value to obtain the risk index for all substancesin the waste that cause deterministic responses. This calculationmust take into account that a given chemical can affect more thanone organ.

As indicated in Table 7.6, all hazardous chemicals in electric arcfurnace dust are assumed to induce deterministic responses. Thepossible responses include renal toxicity, effects on the cardiovascu-lar system, dermal or ocular effects, decrease in body weight, hepatictoxicity, and respiratory toxicity. Decrease in body weight is not aresponse in a particular organ but is assumed to be a health effectof concern. All deterministic responses are assumed to be inducedby more than one chemical in the waste. Furthermore, some of thechemicals (barium, beryllium, chromium, and lead) are assumed toinduce all responses.

Results of the calculations of organ- and endpoint-specific riskindexes for the substances that cause deterministic responses in thewaste are shown in Table 7.8. For each substance and organ orendpoint of concern, the calculated dose in the numerator of the riskindex is the product of the arithmetic mean concentration in thewaste in Table 7.5 and the intake rate per unit concentration in thewaste of about 4 � 10�8 mg (kg d)�1 per ppm obtained as describedin the previous section. The acceptable dose of each deterministicsubstance assumed for the purpose of classifying the waste as low-hazard or high-hazard is a factor of 10 higher than RfD given inTable 7.6 (see Section 7.1.7.1 and Table 7.1). When a hazardoussubstance affects more than one organ, the same RfD is used tocalculate the risk index for all such organs or endpoints. Thisassumption is conservative when RfD is based on the lowest thresh-old for deterministic responses in any organ and the thresholds forresponses in other organs are higher.

The results in Table 7.8 indicate that the organ- and endpoint-specific risk indexes are about 0.7 to 0.8 in all cases, due mainly tointakes of lead. The maximum risk index for any organ or endpointis about 0.8. Truncating this result using the INTEGER function,as indicated in Equation 6.5, gives a risk index for all deterministichazardous chemicals in the waste of zero. This result means thatthe calculated dose in all organs and for all endpoints due to exposureto all deterministic substances that cause deterministic responsesin the waste is less than the assumed acceptable dose of 10 timesRfDs. Therefore, based only on consideration of substances that


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cause deterministic responses and the assumptions about intakesof hazardous chemicals and acceptable doses, this waste could beclassified as low-hazard.

7.1.7.5 Stochastic Risk Index for Hazardous Chemical Constit-uents. Calculation of the risk index for all hazardous chemicals inthe waste that cause stochastic effects is performed in the samemanner as in the previous examples for radioactive wastes. Thecalculated risk for each such substance, based on the assumed expo-sure scenario, is summed and then divided by the acceptable lifetimerisk of 10�3 for classification as low-hazard waste (see Table 7.1).The risk for each chemical is calculated by multiplying the arithmeticmean of the concentration in the waste given in Table 7.5 by theintake rate from ingestion, inhalation, or dermal absorption per unitconcentration discussed in Section 7.1.7.3 and 10 percent of theappropriate slope factor in Table 7.7 (see Section 7.1.7.1) adjustedfor the exposure time. Since the slope factors assume chronic lifetimeexposure, they must be reduced by a factor of 70 based on the assump-tion that the exposure scenario at the hazardous waste site occursonly once over an individual’s lifetime. In addition, a simplifyingassumption is made that whenever more than one slope factor isgiven for a hazardous substance in Table 7.7, the higher value wasapplied to the total intake rate by all routes of exposure of about4 � 10�8 mg (kg d)�1 per ppm. This assumption should be conservative.

Based on the assumptions described above, the results of the calcu-lation of stochastic risk for all hazardous chemicals in the waste areshown in Table 7.9. From the calculated lifetime risk of 1.7 � 10�5

and the assumed acceptable risk of 10�3, the risk index for all hazard-ous chemicals that cause stochastic effects is (1.5 � 10�5)/10�3, orabout 0.02. Thus, based only on consideration of these substances,the waste would be classified as low-hazard.

TABLE 7.9—Stochastic risk for toxic metals in electric arcfurnace waste.

Chemical Slope Factora [mg (kg d)�1] Stochastic Risk

Arsenic 1.5 1.8 � 10�7

Beryllium 0.84 3.8 � 10�10

Cadmium 0.63 9.5 � 10�7

Chromium (VI) 4.2 1.4 � 10�5

Nickel 0.084 1.6 � 10�7

Total stochastic risk 1.5 � 10�5

aValues taken from Table 7.7 and multiplied by 0.1 (see Section 7.1.7.1).


7.1.7.6 Stochastic Risk Index for Radionuclides. NRC (1996b) hasproposed that certain wastes that contain incidental amounts ofradionuclides be allowed for disposal in facilities for hazardous chem-ical wastes permitted under Subtitle C of RCRA. The wastes includedin the NRC proposal are those related to emission control dust con-taminated with 137Cs as a result of inadvertent melting of a 137Cssource in scrap metal processed in an electric arc furnace or at afoundry. NRC proposed that packaged wastes that contain up to4.8 Bq g�1 of 137Cs or bulk waste that contains up to 3.7 Bq g�1 of137Cs may be sent to Subtitle C disposal facilities. NRC or an Agree-ment State would be required to monitor such disposition to ensurethat no Subtitle C facility receives more than 37 GBq of 137Cs fromall such sources.

To classify this mixed radioactive and hazardous chemical wastebased on risk, a composite risk index giving the sum of the riskindexes for chemicals that cause deterministic effects, chemicals thatcause stochastic effects, and radionuclides must be evaluated. Thefirst two elements of the composite risk index were evaluated inSections 7.1.7.4 and 7.1.7.5, respectively. In this Section, the stochas-tic risk index for 137Cs in the waste is computed.

A relatively simple calculation can be used to bound the risk fromexposure to 137Cs in the electric arc furnace waste. The disposedwaste is assumed to contain 4.8 Bq g�1 of 137Cs, which is the maximumallowable concentration in packaged waste. This concentration isconservative because it assumes that exposure would occur at thetime of disposal and it does not take into account the uncontaminatedmaterial that would be mixed with the waste after disposal. Whena radionuclide emits high-energy photons, which is the case for 137Csand its short-lived decay product 137mBa in activity equilibrium, exter-nal exposure is by far the most important and intakes by inhalationand soil ingestion are negligible; this assumption was verified bycalculations using the RESRAD code.

The external dose to an inadvertent intruder who is assumed tobe exposed to uncovered waste for a period of 1,000 h at the time offacility closure can be estimated as follows. For a 137Cs sourceassumed to be uniformly distributed in surface soil with its decayproduct 137mBa in activity equilibrium, and taking into account thedecay branching fraction of 0.946 (Kocher, 1981), the external doserate per unit concentration is 2.9 � 10�11 Sv s�1 per Bq g�1 (Eckermanand Ryman, 1993). Multiplying this external dose coefficient by theassumed concentration of 137Cs (4.8 Bq g�1) and exposure time (1,000 h)gives a total dose for the assumed scenario of 5 � 10�4 Sv.


The stochastic risk corresponding to the estimated dose is obtainedusing the nominal probability coefficient for fatal cancers of 0.05 Sv�1

(ICRP, 1991; NCRP, 1993a). Since this coefficient is intended torepresent a best estimate, rather than a conservative upper bound,the value is not increased by a factor of 10, as in the adjustment ofthe slope factors for chemicals that induce stochastic effects (Section7.1.7.5). Therefore, the calculated stochastic risk due to 137Cs in thewaste is (5 � 10�4 Sv)(5 � 10�2 Sv�1) � 2.5 � 10�5.

Given the assumption that an acceptable stochastic risk from dis-posal in a hazardous waste facility is about 10�3 (see Table 7.1), thestochastic risk index due to the presence of radionuclides in theelectric arc furnace waste is (2.5 � 10�5)/10�3 � 0.025. Since thisresult is much less than unity, the waste clearly would be classifiedas low-hazard due only to the presence of 137Cs, and there is no needto perform a less conservative analysis.

7.1.7.7 Calculation of the Composite Risk Index. The compositerisk index for the chemical and radioactive components of electricarc furnace waste is given by:

RI � RId (chemicals) � RIs (chemicals)� RIs (radionuclides).

(7.1)

Using the separate risk indexes for chemicals that cause determinis-tic effects, chemicals that cause stochastic effects, and radionuclidesobtained in Sections 7.1.7.4 to 7.1.7.6, the composite risk index forall hazardous substances in the waste is given by:

RI � 0 � 0.02 � 0.025 � 0.045. (7.2)

This result is much less than unity. Therefore, based on the assump-tions used in this analysis, the electric arc furnace waste would beclassified as low-hazard.

7.1.7.8 Consideration of Alternative Assumptions. Two aspects ofthe example analysis for electric arc furnace waste warrant furtherconsideration. The first is the assumption that an acceptable doseof each chemical that induces deterministic effects is 10 times itsRfD. The second is the assumption that exposures would occur onlyonce over a lifetime and for a period of 1,000 h.

The analysis for chemicals that induce deterministic effects pre-sented in Section 7.1.7.4 and summarized in Table 7.8 indicates thatlead is the most important such constituent. Furthermore, the riskindex for lead of about 0.7 is only marginally below the value ofunity used to define the boundary between low-hazard and high-hazard waste. Therefore, the assumption that an acceptable dose of


lead would be 10 times its RfD is important in classifying this waste.This assumption may not be warranted given (1) the possibility thatthe assumed RfD in Table 7.6 is based on outdated information onthe dose-response relationship, (2) the small safety and uncertaintyfactor typically used in deriving an RfD from a NOAEL or LOAELwhen the data are obtained from studies in humans (see Sections3.2.1.2.4 and 3.2.1.2.5), and (3) the heightened interest by regulatorsand the public in reducing levels of lead in children, as evidencedby EPA’s current assumption that there is essentially no thresholdfor induction of adverse effects in the young (IRIS, 1988). For exam-ple, if an acceptable dose of lead were assumed to be three times itsRfD, the waste would be classified as high-hazard based on this anal-ysis.

As an alternative to the assumption of a one-time exposure for1,000 h at the time of facility closure, permanent occupancy of adisposal site following loss of institutional control could be assumed(see Section 7.1.3.4). The assumption of chronic lifetime exposurewould affect the analysis for hazardous chemicals that induce deter-ministic effects only if estimated intakes due to additional pathways,such as consumption of contaminated vegetables or other foodstuffsproduced on the site, were significant. Based on the results for leadin Table 7.8, an intake rate from additional pathways of about 50percent of the assumed intake rate by soil ingestion, inhalation, anddermal absorption would be sufficient to increase the deterministicrisk index above unity. The importance of additional pathways wasnot investigated in this analysis, but they clearly would warrantconsideration. The increase in exposure time during permanent occu-pancy does not otherwise affect the analysis for chemicals that inducedeterministic effects, provided RfDs are appropriate for chronic expo-sure, because chronic RfDs incorporate an assumption that the levelsof contaminants in body organs relative to the intake rate (dose) areat steady state.

For substances that induce stochastic effects, an assumption ofpermanent site occupancy would increase the lifetime risk in propor-tion to the increase in exposure time. The risk indexes for chemicalsthat induce stochastic effects and radionuclides assuming 1,000 hof exposure obtained previously are 0.02 and 0.025, respectively. Ifpermanent occupancy of a disposal site were assumed to occur after100 y of institutional control, the risk index for radionuclides wouldbe reduced by a factor of 10, due to the half-life of 137Cs. Then, ifexposure were assumed to occur for 4,000 h y�1 (about half of thetime during a year) for a period of 30 y (see Table 7.1), the estimatedstochastic risk index would be about 0.02 � (4,000/1,000) � 30 �2.4, due mainly to the chemicals that induce stochastic effects. The


risk index would be higher if other exposure pathways, such as con-sumption of contaminated vegetables, were included. This resultindicates that the waste would be classified as high-hazard underthe assumed conditions of unrestricted release of a disposal site.

Based on the example analysis for electric arc furnace waste, theuse of different assumptions about exposure scenarios or allowabledoses of chemicals that induce deterministic effects could result ina difference in the resulting classification of the waste. This examplethus illustrates the importance of judgment in classifying waste.

7.1.8 Hazardous Chemical Waste

Okrent and Xing (1993) analyzed the cancer risk resulting frominadvertent intrusion into a RCRA facility for hazardous chemicalwaste. The facility was assumed to contain waste from productionof veterinary pharmaceuticals and other wastes that resulted inconcentrations of 1,000 mg kg�1 of arsenic and 100 mg kg�1 of beryl-lium, cadmium, chromium, and nickel. A scenario for inadvertentintrusion involving permanent site occupancy similar to the scenarioused by NRC to develop the Class-A, -B, and -C limits for near-surfacedisposal of radioactive waste (NRC, 1982b) was used to estimate thehuman health consequences of the postulated intrusion.

Okrent and Xing (1993) estimated the lifetime cancer risk to afuture resident at a hazardous waste disposal site after loss of institu-tional control. The assumed exposure pathways involve consumptionof contaminated fruits and vegetables, ingestion of contaminatedsoil, and dermal absorption. The slope factors for each chemicalthat induces stochastic effects were obtained from the IRIS (1988)database and, thus, represent upper bounds (UCLs). The exposureduration was assumed to be 70 y. Based on these assumptions, theestimated lifetime cancer risk was 0.3, due almost entirely to arsenic.If the risk were reduced by a factor of 10, based on the assumptionthat UCLs of slope factors for chemicals that induce stochastic effectsshould be reduced by this amount in evaluating waste for classifica-tion as low-hazard (see Section 7.1.7.1), the estimated risk would bereduced to 0.03. Either of these results is greater than the assumedlimit on acceptable risk of 10�3 (see Table 7.1). Thus, based on thisanalysis, the waste would be classified as high-hazard in the absenceof perpetual institutional control to preclude permanent occupancyof a disposal site.

Many of the examples presented in Sections 7.1.3 through 7.1.7incorporate an assumption that exposures of inadvertent intruderswould occur only for a period of 1,000 h during a single year. This


assumption is based on the possibility that an intruder would recog-nize the nature of the waste or that occasional monitoring and sur-veillance of the site would effectively preclude permanent occupancy.The scenario involving a one-time exposure over 1,000 h does notinclude the exposure pathway involving consumption of contami-nated vegetables and fruits. The analysis by Okrent and Xing (1993)can be adjusted to represent this scenario in the following way.The estimated risk due only to exposure by dermal contact and soilingestion over 70 y obtained by Okrent and Xing is 0.12. Reducingthis estimate by a factor of 70 to account for the assumption ofexposure during a single year and by a factor of 1,000/8,760 toaccount for the fraction of the year during which exposure occursgives an estimated risk of 2 � 10�4. This estimate would be reducedby a factor of 10 if the slope factor were adjusted to represent a bestestimate, rather than a UCL. Either of these estimates is less thanthe assumed limit on acceptable risk of 10�3.

As in the example of electric arc furnace waste in the previoussection, this result for a hazardous waste that contains heavy metalsindicates the importance of an intention to maintain perpetual insti-tutional control over hazardous waste disposal sites in allowing thewaste to be classified as low-hazard.

7.1.9 Discussion of Example Analyses

The example analyses for electric arc furnace dust in Section 7.1.7and a hazardous chemical waste in Section 7.1.8 lead to an importantconclusion about these particular wastes. The concentrations ofheavy metals, especially lead, in the electric arc furnace waste clearlyare sufficiently high that long-term exposure to the waste by aninadvertent intruder may need to be precluded in order to ensurethat deterministic effects would not occur. In addition, for eitherwaste, the stochastic risk that could result from unrestricted releaseof a disposal site might exceed acceptable levels, due to the concentra-tions of heavy metals that induce stochastic effects. Both of thesefactors indicate that these wastes may be classifiable as low-hazardonly if perpetual control would be maintained over near-surfacedisposal sites to prevent long-term exposures of inadvertent intrud-ers. Such a conclusion also was obtained in the example of uraniummill tailings discussed in Section 7.1.5.

It is not NCRP’s intent to develop specific recommendations con-cerning classification of wastes based on the example analyses dis-cussed in Sections 7.1.3 through 7.1.8. This is especially the casewhen wastes contain high concentrations of heavy metals (NCRP


did not investigate the classification of wastes that contain highconcentrations of organic hazardous chemicals). Rather, the intentof the example analyses is to indicate the importance of assumptionsabout exposure scenarios and allowable doses or risks in classifyingwaste. These assumptions are largely matters of judgment.

7.2 Legal and Regulatory Ramifications

If implemented, the risk-based waste classification system pre-sented in this Report would have impacts on the current wasteclassification systems for radioactive and hazardous chemical wastes.While it is expected that most wastes would be classified in accordancewith current plans for their disposal, there would be some notableimpacts on waste classification and waste management.

7.2.1 Establishment of an Exempt Waste Class

The most profound change in waste classification that would resultfrom implementation of the system proposed in this Report wouldbe the establishment of an exempt class of waste. This class wouldbe defined based on the principle that waste could be regulated asif it were nonhazardous if the hazardous constituents were presentin amounts sufficiently low that the risk from disposal would benegligible (de minimis). At present, EPA has not established generalprovisions for exemption of listed hazardous chemical wastes regu-lated under RCRA, and efforts by NRC to establish general condi-tions for exemption of radioactive wastes were halted at the directionof Congress.

The establishment of general exemption principles would providea major incentive for generators to modify or create processes thatresult in smaller volumes of concentrated hazardous wastes andlarger volumes of exempt wastes, with disposition of the latter poten-tially ranging from disposal in municipal/industrial landfills for non-hazardous waste to recycle or reuse as laws and regulations permit.This approach to waste management should be more cost-effective byallowing exempt materials to be managed at considerably less cost,commensurate with the risks they pose. It would also have otherenvironmental benefits associated with reducing the need to acquirefresh resources, thus heading in the direction of sustainability.

7.2 LEGAL AND REGULATORY RAMIFICATIONS / 349

7.2.2 Elimination of Source-Based Waste Classifications

In a risk-based hazardous waste classification system, waste wouldbe classified based on its intrinsic characteristics. Radioactive wastes(most notably high-level waste) presently are classified based essen-tially on their source, and this would change in a risk-based classifi-cation system. In the case of high-level waste, this change shouldhave few significant impacts, because much of what is now in thiswaste class would be classified as high-hazard waste and still requiredisposal in a geologic repository. Some wastes from fuel reprocessingand reprocessing wastes that are old or have had a significant frac-tion of the radionuclides removed might be classified as low-hazardwaste. Similarly, some waste that is not presently classified as high-level waste would probably be classified as high-hazard wastebecause of its relatively high concentrations of radionuclides; anexample is waste now classified as greater-than-Class-C low-levelwaste. Most transuranic radioactive waste probably would be classi-fied as high-hazard waste, but this would have no effect on manage-ment practices because most transuranic waste is destined forgeologic disposal.

Elimination of source-based waste classifications would also havesome impact on classification and management of hazardous chemi-cal wastes. For example, the identification of some listed hazardouswastes under RCRA based on the source of the waste (the ‘‘F ’’ and‘‘K’’ lists) and the distinction between hazardous wastes regulatedunder RCRA and those regulated under TSCA would be eliminated.

The most significant impact of eliminating source-based wasteclassifications is likely to be in the area of classifying and managinglarge-volume NORM wastes from mining and processing of mineralores to extract nonradioactive materials. These wastes presently areclassified and managed separately from uranium and thorium milltailings having similar properties, and many NORM wastes essen-tially are unregulated. As in the case of mill tailings, many of thesewastes could require special consideration because deep disposal ordisposal in near-surface facilities currently used for radioactive orhazardous chemical wastes might be impractical. Thus, the distinc-tion between large-volume low-hazard wastes and other low-hazardradioactive and chemical wastes would need to continue under a risk-based classification system, and this distinction could be specified bysubclassification of low-hazard waste.

7.2.3 Recognition of Permanent Disposal of HazardousChemical Wastes

Present methods of disposal of hazardous chemical wastes involveemplacement in a near-surface facility with the stipulation that


(1) the facility be actively maintained and monitored for as long asthe waste remains hazardous, (2) leachate be collected and treated,and (3) the future of the facility be considered at 30 y after closure.The long-term performance of a facility is not analyzed when itis permitted and waste acceptance criteria are established. As aconsequence, there is no assurance that hazardous waste facilitieswill be acceptable for permanent disposal of the potentially long-lived chemical wastes they contain. The approach to near-surfacedisposal of radioactive wastes is essentially the opposite of this.These facilities are planned to be permanent disposal sites, withwaste acceptance criteria established at each site that would ensureadequate long-term protection of the public (including inadvertentintruders) in the absence of institutional control, which is assumedto cease at about 100 y.

A risk-based waste classification system would be established byfocusing on risks that arise from disposal of hazardous wastes. Thus,the amounts of hazardous chemical wastes that would be acceptablefor near-surface disposal over the longer term would need to beevaluated. While NCRP believes that many hazardous chemicalwastes would continue to be acceptable for near-surface disposal, itshould be anticipated that this will not be the case for some wastesthat contain high concentrations of heavy metals; e.g., see Okrentand Xing (1993). As a result, some hazardous chemical wastes couldbe classified as high-hazard (see next section), and such a classifica-tion also could also mean that perpetual institutional control willbe required at some existing burial sites.

7.2.4 Establishing the Potential for High-HazardChemical Wastes

At the present time, there is essentially only one class of hazardouschemical waste (i.e., a waste either is hazardous or it is not), withoutregard for the amounts of hazardous substances in the waste. Estab-lishment of a risk-based waste classification system would allow forthe possibility of two classes of hazardous chemical waste based onthe amounts of hazardous substances, consistent with the presentsituation for radioactive waste, with the attendant implication thathigh-hazard chemical waste that contains the highest amounts ofhazardous substances would require a disposal technology substan-tially more isolating than a near-surface system.

High-hazard chemical waste could result from relatively high con-centrations of hazardous organic chemicals (e.g., dioxins) or persis-tent toxic substances (e.g., heavy metals). Some wastes may be


rendered less hazardous by treatment, such as incineration of somewastes that contain organic chemicals, but other wastes probablycannot be treated effectively to reduce the long-term hazard. NCRPdid not investigate in any detail the amounts of hazardous chemicalwaste that might be classified as high-hazard. However, based onthe study by Okrent and Xing (1993) discussed in Section 7.1.8and previous efforts to develop a category of extremely hazardouschemical waste (see Section 4.2.1.3), NCRP expects that someamount of hazardous chemical waste would be classified as high-hazard. This waste may require a disposition significantly differentfrom the present practice of emplacement in near-surface facilities.Deeper disposal facilities for solid hazardous chemical wastes do notpresently exist in the United States, and there are no plans for theirdevelopment. In considering options for deeper disposal of high-hazard chemical wastes, the costs and benefits would need toweighed against the monetary costs and health risks associated withmaintaining perpetual institutional control at near-surface dis-posal facilities.

7.2.5 Elimination of the Mixed Waste Category

Differences in current approaches to management of hazardouschemical and radioactive wastes come into full play in the case ofmixed waste, with the result being major procedural and institu-tional impediments to effective management of these wastes. Eitherof two major features of a risk-based waste classification systemwould essentially eliminate these impediments. In some cases, estab-lishment of an exemption principle could allow either the chemicalor radioactive component of mixed waste to be exempt from regula-tion as hazardous waste. These wastes then would be consideredhazardous by virtue of their radioactive or hazardous chemical con-stituents but not both (the waste would no longer be ‘‘mixed’’). As aresult, the wastes could be managed in a relatively straightforwardmanner. Beyond this, the full impact of risk-based waste classifica-tion would be the elimination of regulatory differences in approachesto management of radioactive and hazardous chemical wastes. Thisshould lead to a single, consistent set of regulations for managementand disposal of hazardous waste, regardless of whether it is classifiedas such because of the presence of hazardous chemicals, radio-nuclides, or both. If this is achieved, the issue of mixed wastebecomes moot.

Achieving the minimum benefits of exemption would require fewchanges in the existing regulatory infrastructure other than those


noted in Section 7.2.1. Achieving the full benefits of risk-based wasteclassification would require significant changes in existing regula-tions to eliminate important differences in approaches to risk assess-ment and risk management in general, approaches to treatment andcharacterization of waste, and approaches to ensuring adequate long-term protection of the public, including inadvertent intruders, fromdisposal in near-surface facilities.

7.2.6 Elimination of the Category of Waste Containing NaturallyOccurring and Accelerator-Produced Radioactive Material

NARM waste largely falls outside the existing federal infrastruc-ture for management of radioactive wastes (see Section 4.1.2.4),except when this type of waste is the responsibility of a federalagency (e.g., DOE). Establishment of a risk-based waste classifica-tion system would necessarily include NARM waste within the sameregulatory structure as other radioactive wastes. NCRP expects thatmuch of this waste, especially diffuse NORM waste that containsrelatively low concentrations of naturally occurring radionuclides,would be classified as exempt or low-hazard waste, resulting in verylittle change in current waste management practices. However, somediscrete NARM wastes that contain relatively high concentrationsof long-lived alpha-emitting radionuclides (e.g., radium) might beclassified as high-hazard waste and could require a more isolatingdisposal technology than near-surface disposal. Perhaps the greatestimpact of establishing a comprehensive waste classification systemon management of NARM waste would be to encourage the develop-ment of a consistent set of regulatory requirements for all suchwaste, instead of the variety of federal and state regulations forthese wastes that exist at the present time.

7.2.7 Impact on Subclassification of Waste Classes

Existing hazardous waste classification systems frequentlyinclude subclassifications of basic waste classes to facilitate wastemanagement (see Sections 2.2.4, 4.1.2 and 6.6). Examples includeClass-A, -B, and -C commercial low-level waste and remotely-handled and contact-handled transuranic waste. These waste sub-classifications are not expected to be significantly affected by a risk-based classification system unless particular wastes would not begenerally acceptable for the disposal using the intended technology.For example, there is no inherent incompatibility with the system


for classifying Class-A, -B, and -C low-level waste unless the wastewould be classified as exempt or high-hazard based on its intrinsiccharacteristics. Transuranic waste could be a subclass of high-hazard waste based, for example, on the heat generation rate (seeSection 4.1.3.1), and transuranic waste could be further subclassifiedas remotely-handled or contact-handled based on the need to protectworkers. If a waste were reclassified at the highest level (e.g., fromlow-level to exempt), then existing subclassifications would beaffected or, more likely, would become moot.

8. Conclusions andRecommendations

8.1 Conclusions

Based on the background information presented in Sections 2through 5 and the discussions on development of a new hazardouswaste classification system in Sections 6 and 7, NCRP has reachedthe following conclusions:

● Despite the best efforts of pollution prevention and recyclingprograms, hazardous wastes are being, and will continue to be,generated. Classification of hazardous waste is necessary forcost-effective waste management.

● The most appropriate primary basis for classification of hazard-ous waste is the risk to human health posed by waste. Further-more, the health risks of primary concern in classifying hazardouswastes are risks to the public that arise from waste disposal, sincepermanent disposal is the intended disposition of most wastematerials having no further use to their present custodian.

● Although the existing classification systems for radioactive andhazardous chemical wastes have worked adequately in manyrespects, they have resulted in a number of undesirable out-comes, such as excessive costs in managing and disposing ofsome wastes, considerable difficulties in managing and disposingof mixed wastes, an increasing need to accommodate exceptionalwastes that were not considered when the classification systemswere developed, and unwarranted neglect of some potentiallyimportant wastes. Existing systems are deficient primarilybecause most are not based on risk, the collective system does notunambiguously classify wastes, and some potentially importantwastes are not given due consideration.

● Classification of waste based on risk requires assessments ofhealth risks posed by waste. While many aspects of risk assess-ment are the same for radionuclides and hazardous chemicals,there are important differences that complicate the establish-ment of a comprehensive and risk-based waste classification

354

8.1 CONCLUSIONS / 355

system. The most important of these differences are summarizedas follows:– Dose-response relationships for substances that induce sto-

chastic effects: best estimates (MLE) for radionuclides butupper bounds (UCLs) for chemicals that induce stochasticeffects;

– Safety and uncertainty factors used in establishing exposurelimits that are intended to prevent deterministic responses:normally much larger for hazardous chemicals that inducedeterministic effects than for radionuclides;

– Primary measure of stochastic response: cancer fatalities forradionuclides but cancer incidence for hazardous chemicalsthat induce stochastic effects; and

– Accounting of organs at risk: all organs and tissues at risk forradionuclides but estimates of risk for chemicals that inducestochastic effects based usually on observed responses in asingle organ in laboratory animals.

Of these, the difference between best estimates of dose-responserelationships for radionuclides but UCLs for chemicals thatinduce stochastic effects is the most significant for risk-basedwaste classification. The eventual use of best estimates of dose-response relationships and incidence of health effects as theprimary measure of response is preferred by NCRP, althoughan acceptable interim waste classification system might beestablished using different approaches for radionuclides andchemicals that induce stochastic effects. Differences in theaccounting of organs and tissues at risk are not expected to beimportant because, in contrast to radionuclides, it is unlikelythat many chemicals that induce stochastic effects would inducehealth effects in several organs with a significant probability.

● Classification of waste based on risk requires assumptions aboutallowable risks from various waste dispositions, i.e., decisionsabout approaches to risk management. The paradigms for man-aging risk from exposure to radionuclides and hazardous chemi-cals are fundamentally different. After a practice is justified interms of a positive net benefit to society, risks from exposure toradionuclides are managed by establishing a limit on acceptable(barely tolerable) dose (and, therefore, risk) and then requiringthat doses (risks) be reduced below the limit ALARA. In contrast,risks from exposure to hazardous chemicals are managed byestablishing goals for acceptable (negligible) risk and allowingan increase (relaxation) of risks above the goals based on thespecific circumstances of a particular exposure situation (e.g.,

356 / 8. CONCLUSIONS AND RECOMMENDATIONS

cost-benefit). This difference has resulted in considerable confu-sion and misunderstanding about risks that are ‘‘acceptable’’and those that are ‘‘unacceptable.’’

● An ideal system for classifying hazardous wastes should be risk-based, applicable to all wastes that contain radionuclides orhazardous chemicals, internally consistent, based on intrinsicwaste properties, comprehensible, quantitative, and compatiblewith existing or feasible data and methods. To the extent thatthese attributes are lacking in a waste classification system,undesirable consequences are likely to result.

● Given an assumption that waste materials have no further bene-ficial use, a risk-based hazardous waste classification systemshould focus on classification of waste for purposes of disposal.Therefore, waste should be classified in relation to one of thethree types of disposal systems (technologies) in current use orunder development that are expected to be generally acceptablein protecting public health: municipal/industrial landfills; dedi-cated near-surface facilities for hazardous wastes; and deeper,highly isolating facilities, such as a geologic repository. Materi-als containing very low concentrations of hazardous substancesalso could be considered for other dispositions.

● The framework for a risk-based waste classification systemshould include three classes of waste: exempt waste, whichwould be managed as if it were nonhazardous and would begenerally acceptable for disposal in a municipal/industrial land-fill; low-hazard waste, which is any nonexempt waste that wouldbe generally acceptable for disposal in a dedicated near-surfacefacility for hazardous wastes; and high-hazard waste, whichwould generally require disposal in a facility considerably moreisolating than a near-surface facility for hazardous wastes.Exempt wastes also could be considered for recycling and benefi-cial use in commerce, consistent with laws and regulations gov-erning allowable dispositions of nonhazardous materials.

● The boundaries between waste classes should be quantified byuse of a risk index, which essentially is the ratio of a calculatedrisk from disposal of a hazardous waste using a generic type ofdisposal system to an allowable risk appropriate to the assumeddisposal system. In establishing the boundary between exemptand low-hazard waste, the allowable risk should be based on anegligible stochastic risk, and the deterministic risk should bezero with an ample margin of safety. In establishing the bound-ary between low-hazard and high-hazard waste, the allowablerisk should be based on an acceptable (barely tolerable) stochas-tic risk substantially above a negligible risk and a deterministic

8.1 CONCLUSIONS / 357

risk that is expected to be zero but with less conservatism thanin the boundary between exempt and low-hazard waste. Theboundaries between waste classes normally would be expressedin terms of limits on concentrations of hazardous substances,and rules for applying the limits to waste that contains mixturesof hazardous substances would be specified.

● The concept of a hypothetical inadvertent intruder at a near-surface waste disposal site, including permanent occupants ofa site after an assumed loss of institutional control, provides asuitable basis for defining exposure scenarios that would beused to calculate risks that arise from waste disposal and theboundaries between waste classes. For other dispositions ofwaste, alternative scenarios would need to be developed andevaluated.

● As with existing waste classification systems, a risk-based wasteclassification system should be flexible and continue to includeprovisions for regulators to make exceptions on a case-by-casebasis with appropriate due process.

● Development of a comprehensive and risk-based hazardouswaste classification system, in which waste classes are definedin relation to types of disposal systems that are expected tobe generally acceptable in protecting public health, would notobviate the need to establish waste acceptance criteria at eachdisposal site based on the characteristics of the site and engi-neered disposal facility and the properties of wastes intendedfor disposal therein. The primary purposes of a hazardous wasteclassification system are to facilitate cost-effective managementand disposal of waste and effective communication on wastematters.

● Establishing subclassifications of the basic waste classes to facil-itate waste management and disposal is likely to be desirable.Subclassifications of basic waste classes should be based on con-siderations of risk and could take into account, for example,differences in engineered systems required to manage wastesin the same class with different physical, chemical or radiologicalproperties. Consideration of cost-benefit in managing and dis-posing of wastes that pose similar risks but have greatly differ-ent volumes also would be important in subclassifying basicwaste classes.

● A risk-based waste classification system should include explic-itly justified degrees of conservatism in protecting public health.

● Differences in the meanings of commonly used terms betweenthe radiation and hazardous chemical communities presently

358 / 8. CONCLUSIONS AND RECOMMENDATIONS

constitute a significant impediment to establishing a comprehen-sive and risk-based hazardous waste classification system. Themost important example, as noted above, is the different mean-ings attached to the terms ‘‘acceptable’’ and ‘‘unacceptable’’ indescribing risks.

● Examples of how existing wastes might be classified under therisk-based system recommended in this Report, based on suit-able precedents for defining allowable risks associated with dif-ferent types of disposal systems, indicate that many hazardouswastes would continue to be managed essentially in the samemanner as presently foreseen. However, NCRP expects that sig-nificant volumes of waste currently managed as radioactive orchemically hazardous waste could be classified as exempt, basedon the low concentrations of hazardous substances they contain,thus resulting in substantially less expenditures of resources inmanaging these materials. NCRP also expects that some of themost hazardous chemical wastes could be classified as high-hazard based on a conclusion that they may not be generallyacceptable for near-surface disposal.

● In a risk-based hazardous waste classification system, classifi-cation and disposal of uranium mill tailings would continue torequire special considerations, due to the high concentrationsof radium and emanation rates of radon compared with averagesoil and rock and the very large waste volumes. NCRP believesthat most uranium mill tailings could be classified as low-hazardwaste, but only under conditions of perpetual institutional con-trol over disposal sites to preclude permanent occupancy by aninadvertent intruder. If perpetual institutional control is notassumed, most mill tailings would be classified as high-hazardwaste based on the high risk to a permanent resident on atailings pile. Regardless of how mill tailings are classified, how-ever, near-surface disposal probably will continue to be the pre-ferred option, because disposal of the very large volumes of thesematerials far below ground has been deemed impractical. Simi-lar considerations could apply to large volumes of other ore pro-cessing wastes that contain highly elevated levels of naturallyoccurring radionuclides or hazardous chemicals.

8.2 Recommendations

As a result of the foregoing conclusions, NCRP recommends thatthe present classification systems for radioactive and hazardous

8.2 RECOMMENDATIONS / 359

chemical wastes be replaced over time by the comprehensive andrisk-based hazardous waste classification system described in thisReport. Such a replacement would need to be undertaken carefullyand in recognition that existing systems for waste classification andwaste management, despite their shortcomings, have been adequatein protecting human health. In establishing a new hazardous wasteclassification system that would be an improvement on the existingsystems, there is a need to ensure that current approaches to man-agement and disposal of radioactive and hazardous chemical wasteswould not be unduly disrupted.

The principal elements of an approach to establishing the hazard-ous waste classification system recommended in this Report shouldinclude the following:

● Use of the foundations and framework for risk-based waste clas-sification as a blueprint for an improved classification system;

● A forthright approach to establishing such a system by planningthe entire process at the outset, included needed changes inexisting laws and regulations, as well as additional needs ofdata or scientific understanding;

● A striving to embody all the desired attributes of the new system,while recognizing that this may take many years and that anumber of important benefits can be obtained by interim imple-mentation of parts of the system. The most important areas inwhich interim implementations are likely to be beneficial includethe establishment of exemption levels for radionuclides and haz-ardous chemicals in waste, to allow hazardous wastes to bemanaged as nonhazardous material or to allow mixed waste tobe managed as radioactive or hazardous chemical waste only,and the elimination of source-based definitions of hazardouswastes, especially radioactive wastes.

● Ensuring that a new hazardous waste classification system,although it may be an improvement over existing classificationsystems in its principles and approaches to waste classification,does not become so rigid as to preclude the types of judgmentand flexibility that are needed to accommodate the special situa-tions and practicalities that will inevitably arise.

Glossary

absorbed dose, chemicals: The amount of a substance crossing a specificabsorption barrier (e.g., the exchange boundaries of the gastrointestinaltract, lungs, and skin) through uptake processes.

absorbed dose, ionizing radiation (D): The quotient of d� by dm, whered� is the mean energy imparted by ionizing radiation to the matter in avolume element and dm is the mass of the matter in that volume element:D � d�/dm. For purposes of radiation protection and assessing healthrisks in general terms, the quantity normally calculated is the averageabsorbed dose in an organ or tissue (T): DT � d�T/mT, where d�T is thetotal energy imparted in an organ or tissue of mass mT. The SI unit ofabsorbed dose is the joule per kilogram (J kg�1), and its special name is thegray (Gy). In conventional units often used by federal and state agencies,absorbed dose is given in rads; 1 rad � 0.01 Gy.

activity: The rate of transformation (or ‘‘disintegration’’ or ‘‘decay’’) of radioac-tive material. The SI unit of activity is the reciprocal second (s�1), and itsspecial name is the becquerel (Bq). In conventional units often used byfederal and state agencies, activity is given in curies (Ci); 1 Ci � 3.7 � 1010 Bq.

acutely toxic hazardous waste: A listed hazardous chemical waste regu-lated under the Resource Conservation and Recovery Act (RCRA) anddesignated by Hazard Code ‘‘H’’ in 40 CFR Part 261, Subpart D (EPA,1980b) including all ‘‘P’’ listed wastes (waste codes beginning with ‘‘P’’)and F020, F021, F022, F023, F026, and F027 listed wastes. Acutely toxichazardous waste is subject to more stringent requirements on accumula-tion and generation than other types of hazardous chemical waste regu-lated under RCRA.

administered dose, chemicals: The amount of a substance given to a testsubject (human or animal), especially by ingestion or inhalation, usuallyfor the purpose of determining dose-response relationships.

agent: An active force (e.g., ionizing radiation) or substance producing aneffect.

Agreement State: Any state with which the U.S. Nuclear RegulatoryCommission has entered into an effective licensing agreement underSection 274(b) of the Atomic Energy Act to enable the state to regulatesource, special nuclear, and byproduct materials.

alpha radiation: Energetic nuclei of helium atoms, consisting of two pro-tons and two neutrons, emitted spontaneously from nuclei in the decayof some radionuclides. Alpha radiation is weakly penetrating, and can bestopped by a sheet of paper or the outer dead layer of skin. Also calledalpha particle and sometimes shortened to alpha (e.g., alpha-emittingradionuclide).

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GLOSSARY / 361

annual dose, ionizing radiation: As normally used in radiation protec-tion, the sum of the dose received in a year from external radiation andthe committed dose due to intakes of radionuclides in a year. Definitionapplies most often to annual effective dose or annual effective dose equiva-lent, but also may be applied to annual dose equivalent and annual equiva-lent dose when prevention of deterministic responses in individual organsor tissues is of concern.

antagonistic: Situations in which the total response from simultaneousexposure of an organism to two or more hazardous agents is less thanthe sum of the responses from separate exposures to each agent.

applied dose, chemicals: The amount of a substance in contact with theprimary absorption boundaries of an organism (e.g., gastrointestinal tract,lung, skin) and available for absorption.

as low as reasonably achievable (ALARA): An approach to radiationprotection in which radiation exposures (both individual and collective,to the workforce and general public) are maintained as low as social,technical, economic, practical, and public policy considerations permit.ALARA is not a dose limit but is a process, which has the objective ofreducing doses as far below applicable limits as reasonably achievable.

Atomic Energy Act (AEA): Law passed originally in 1946 and extensivelyrevised in 1954 that governs the production and use of radioactive materi-als (i.e., source material, special nuclear material, and byproductmaterial) for defense and peaceful purposes and the regulation of suchradioactive materials to protect public health and safety. Act provides theauthority for licensing of commercial nuclear activities by the U.S. NuclearRegulatory Commission and Agreement States, and regulation by theU.S. Department of Energy of its atomic energy defense, research, anddevelopment activities.

background level: Levels (e.g., concentrations) of agents, especially haz-ardous agents in the environment, whose occurrence is not related tohuman activities at a site. Background sources may be naturally occurringor man-made (e.g., global fallout from atmospheric testing of nuclearweapons).

background radiation: Ionizing radiation that occurs naturally in theenvironment, including: cosmic radiation; radiation emitted by naturallyoccurring radionuclides in air, water, soil, and rock; radiation emitted bynaturally occurring radionuclides in the tissues of organisms (e.g., due toingestion or inhalation); and radiation emitted by man-made materialscontaining incidental amounts of naturally occurring radionuclides (e.g.,building materials). In the United States, the average annual effectivedose due to natural background radiation is about 1 mSv, excluding thedose due to indoor radon, and the average annual effective dose due toindoor radon is about 2 mSv.

becquerel (Bq): The special name for the SI unit of activity; 1 Bq � 1 s�1.below regulatory concern (BRC): Definable amounts of hazardous sub-

stances in a material such that the material can be exempted from regula-tions governing particular practices or sources (e.g., management anddisposal of hazardous wastes) based on considerations that the costs of

362 / GLOSSARY

regulating the materials generally are disproportionate to the low healthrisks to the public posed by the materials (application of ALARA principle).Amounts of hazardous substances that are BRC can depend on the particu-lar practice or source, and they can be substantially above levels generallyconsidered de minimis.

benchmark dose, chemicals: A dose of a hazardous substance correspond-ing to a specified level of response in a study population, obtained bystatistical fitting of a dose-response model to dose-response data. Thebenchmark dose often is taken to be the dose resulting in a response of 10percent. For purposes of health protection, the lower 95 percent confidencelimit of the benchmark dose often is taken as the point of departure inestablishing a safe dose of a hazardous substance.

best demonstrated available technology (BDAT): Technologies fortreatment of hazardous materials that have been shown to yield the greatestenvironmental benefit among competing, practically available technologies.

beta radiation: An energetic electron or positron emitted spontaneouslyfrom nuclei in the decay of some radionuclides. Beta radiation is not highlypenetrating, and the highest-energy radiations can be stopped by a fewcentimeters of plastic or aluminum. Also called beta particle and some-times shortened to beta (e.g., beta-emitting radionuclide).

biokinetic model: A model describing the time course of the absorption,distribution, metabolism, and excretion of a substance (e.g., a drug orhazardous substance) introduced into the body of an organism.

biologically effective dose, chemicals: The amount of a deposited orabsorbed substance that reaches the cells or target tissues where anadverse effect occurs, or where the substance interacts with a mem-brane surface.

biosphere: The life zone of Earth, including the lower part of the atmo-sphere, the hydrosphere, soil, and the lithosphere to a depth of about 2 km.

buried waste: Waste that has been emplaced in a near-surface facility.byproduct material: (1) Any radioactive material (except special nuclear

material) yielded in, or made radioactive by, exposure to the radiationincident to the process of producing or utilizing special nuclear material;and (2) the tailings or waste produced by the extraction or concentrationof uranium or thorium from any ore processed primarily for its sourcematerial content. Ore bodies depleted by uranium solution extractionoperations and which remain underground do not constitute byproductmaterial.

cap: The soil applied over waste at the end of each working day at a landfill;a permanent layer of impervious material (e.g., clay, polyethylene or poly-vinyl chloride liner) installed above the waste upon closure of a landfill.

carcinogen: An agent that can cause cancer; frequently used as a synonymfor stochastic agent.

characteristically hazardous waste: A hazardous chemical waste regu-lated under the Resource Conservation and Recovery Act (RCRA) anddesignated by Hazard Code ‘‘D’’ in 40 CFR Part 261, Subpart C (EPA,1980b). A waste is hazardous by characteristics if it is ignitable, corro-sive, reactive, or toxic.

GLOSSARY / 363

commercial waste: Waste generated in any activity by a nongovernmen-tal entity.

committed dose, ionizing radiation: The dose received over a specifiedtime period following an intake of a radionuclide by inhalation, ingestion,or dermal absorption. For adults, the committed dose usually is the dosereceived over 50 y; for children, the committed dose usually is the dosereceived from age at intake to age 70. Definition applies to committeddose equivalent, committed effective dose, committed equivalent dose, andcommitted effective dose equivalent.

Comprehensive Environmental Response, Compensation, and Lia-bility Act (CERCLA): Law, also known as ‘‘Superfund,’’ passed in 1980and amended by the Superfund Amendments and Reauthorization Act(SARA) of 1986 and later amendments, that governs federal responseand compensation for unpermitted and uncontrolled releases, includingthreats of release, of hazardous substances to the environment. An ‘‘unper-mitted’’ release is any release that is not properly regulated under otherlaws. An important focus of CERCLA/SARA is remediation of old, unper-mitted waste disposal sites that are closed or inactive. Basic objectives ofthe Superfund program are to protect human health and the environmentin a cost-effective manner, maintain this protection over time, and mini-mize the amounts of untreated waste in the environment.

confidence interval: A measure of the extent to which an estimate of risk,dose, or other parameter is expected to lie within a specified interval. Forexample, a 90 percent confidence interval of a risk estimate means that,based on the available information, the probability is 0.9 that the truebut unknown risk lies within the specified interval (see lower confidencelimit and upper confidence limit).

contact-handled transuranic waste: Containerized transuranic wastefor which the external dose-equivalent rate at the surface of the containerdoes not exceed 2 mSv h�1.

containment: The confinement of waste within a designated boundary(see isolation).

contaminant: Includes, but is not limited to, any chemical element, sub-stance, compound, or mixture, including disease-causing substances,which after release into the environment and upon external exposure,ingestion, inhalation, or assimilation into any organism, either directlyfrom the environment or indirectly by ingestion through food chains,will or may reasonably be anticipated to cause death, disease, behavioralabnormalities, cancer, genetic mutation, physiological malfunctions includ-ing malfunctions in reproduction, or physical deformations in such organismsor their offspring.

corrosive: A characteristic of solid hazardous waste regulated under theResource Conservation and Recovery Act (RCRA) and defined in 40 CFRPart 261.22 (EPA, 1980b). A solid waste is corrosive if it (1) is aqueous andhas a pH �2 or �12.5, or (2) is a liquid and corrodes SAE 1020 steel at arate �6.35 mm y�1 at a test temperature of 55 °C.

cosmic radiation: Ionizing radiation, including electromagnetic radiationand energetic particles, originating in space or produced by interactions ofsuch radiation with constituents of Earth’s atmosphere.

364 / GLOSSARY

cover: (see cap).critical organ: The most sensitive organ to the toxic effects of a hazardous

agent; the organ receiving the highest dose or experiencing the highest riskof an adverse effect from exposure to a hazardous agent.

curie: (see activity).deep-well injection: The subsurface emplacement of fluids through a bored,

drilled or driven well, or through a dug well whose depth is greater thanthe largest surface dimension.

defense waste: Radioactive waste generated in any activity performed inwhole or in part in support of the U.S. Department of Energy’s atomic energydefense activities.

degradation rate: The rate at which a chemical is broken down in the environ-ment by hydrolysis, photodegradation, or soil metabolism.

delisting: The process of exempting a listed hazardous chemical waste, amixture of a listed and solid waste, or a ‘‘derived-from’’ waste from require-ments for regulation as hazardous waste under the Resource Conservationand Recovery Act (RCRA), as specified in 40 CFR Part 260.20 and 260.22(EPA, 1986b). Delisting provisions do not apply to characteristically hazard-ous waste, which must be treated to remove any hazardous characteristics.

delivered dose, chemicals: The amount of a substance available for interac-tion with a particular organ or cell.

de manifestis: As applied to hazardous substances, a dose or risk that wouldgenerally be considered so high that action to reduce dose or risk normallyshould be undertaken without regard for cost or other circumstances.

de minimis: As applied to hazardous substances, a dose or risk that wouldgenerally be considered negligible for any exposure situation, without regardfor whether such a dose or risk is reasonably achievable for a particularsource or practice. If doses or risks are below de minimis levels, efforts tocontrol exposures generally would be unwarranted (see below regula-tory concern).

derived-from rule: Rule established under the Resource Conservation andRecovery Act (RCRA) in 40 CFR Part 261.3(c) (EPA, 1980b), which statesthat any solid waste derived from the treatment, storage, or disposal of alisted hazardous chemical waste is itself a hazardous waste, regardless ofthe concentrations of listed wastes, unless such waste is delisted.

deterministic agent: An agent that can produce a deterministic responsein organisms.

deterministic response: An adverse effect on organisms for which the sever-ity varies with the magnitude of the dose, and for which a threshold usuallyexists. Deterministic responses often occur relatively soon after an exposure.

detriment: A measure of stochastic response from exposure to ionizing radia-tion which takes into account the probability of fatal cancers, the probabilityof severe hereditary effects, the probability of nonfatal cancers weighted bythe lethality fraction, and relative years of life lost per fatal health effect(ICRP, 1991).

disposal: Placement of waste in a facility designed to isolate waste from theexposure environment of humans without an intention to retrieve the waste,irrespective of whether such isolation permits recovery of the waste.

GLOSSARY / 365

disposal facility: The land, structures, and equipment used for the disposalof waste.

disposal, geologic: Isolation of waste using a system of engineered and natu-ral barriers at a depth of up to several hundred meters below ground in ageologically stable formation (see geologic repository).

disposal, near-surface: Disposal of waste, with or without engineered barri-ers, on or below the ground surface, where the final protective covering ison the order of a few meters thick, or in mined openings within a few tensof meters of Earth’s surface (see land disposal facility).

disposition: Reuse, recycling, sale, transfer, storage, treatment, consumption,or disposal.

dosage, chemicals: Dose or dose rate normalized to body weight of an exposedorganism; e.g., mg kg�1 or mg (kg d)�1.

dose: General term used to quantify extent of exposure to hazardous agentsin assessing health risks to humans or other organisms; usually refers toadministered dose or dosage (but sometimes to absorbed dose, applieddose, delivered dose, or potential dose) for hazardous chemicals, or toaverage absorbed dose, equivalent dose (or average dose equivalent),or effective dose (or effective dose equivalent) for ionizing radiation.

dose assessment: (see exposure assessment).dose equivalent, ionizing radiation (H): The absorbed dose (D) at a point

in tissue weighted by the quality factor (Q) for the type and energy of theradiation causing the dose: H � D � Q. For purposes of radiation protectionand assessing health risks in general terms, and especially prior to introduc-tion of the equivalent dose and as used by federal and state agencies, doseequivalent often refers to the average absorbed dose in an organ or tissue(T) weighted by the average quality factor (Q) for the particular type ofradiation: HT � DT � Q. The SI unit of dose equivalent is the joule perkilogram (J kg�1), and its special name is the sievert (Sv). In conventionalunits often used by federal and state agencies, dose equivalent is given inrem; 1 rem � 0.01 Sv.

dose rate: Dose per unit time; often expressed as an average over some timeperiod (e.g., a year or a lifetime) (see dosage).

dose-response assessment: A determination of the relationship between thedose of a hazardous agent and the probability that a specific response willoccur in an organism during its lifetime. Dose-response assessment is thesecond step of a risk assessment.

effect: An observable change in an organ or tissue resulting from exposureto a hazardous agent (see response).

effective dose equivalent, ionizing radiation (HE): The sum over specifiedorgans and tissues (T) of the average dose equivalent in each tissue weightedby th e t i ssu e w eig hting fac tor (w T ) : H E � � w T H T , where� wT � 1 (ICRP, 1977) (now superseded by the effective dose, but oftenused by federal and state agencies).

effective dose, ionizing radiation (E): The sum over specified organs andtissues (T) of the equivalent dose in each tissue weighted by the tissueweighting factor (wT): E � � wTHT, where � wT � 1 (ICRP, 1991) (super-sedes the effective dose equivalent).

366 / GLOSSARY

effluents: Waste materials discharged into the environment.Emergency Response and Community Right-To-Know Act (ERCRA):

Title III of the Superfund Amendments and Reauthorization Act (SARA) of1986, but a free-standing title and not part of CERCLA (Superfund). Actaddresses emergency planning for releases of hazardous substances, commu-nity right-to-know reporting of hazardous chemicals, and reportable quanti-ties of emissions.

engineered barrier: A man-made structure or device intended to improvethe capability of a disposal facility to contain or isolate waste.

equivalent dose, ionizing radiation (H): A quantity developed for purposesof radiation protection and assessing health risks in general terms, definedas the average absorbed dose in an organ or tissue (T) weighted by theradiation weighting factor (wR) for the type and energy of the radiationcausing the dose: H � DT � wR (ICRP, 1991). The SI unit of equivalentdose is the joule per kilogram (J kg�1), and its special name is the sievert(Sv). In conventional units often used by federal and state agencies, equiva-lent dose is given in rem; 1 rem � 0.01 Sv (see dose equivalent).

exempt material: Material that is excluded from regulation as hazardousmaterial.

exposure: General term describing contact of an organism with a hazardousphysical, chemical, radiological, or biological agent through dermal absorp-tion, inhalation, ingestion, or external irradiation. Exposure is quantifiedas the amount of the hazardous agent at the exchange boundaries of anorganism (e.g., gut, lungs, skin) and available to give a dose.

exposure assessment: A specification of the population potentially exposedto hazardous agents and the pathways and routes by which exposure canoccur, and quantification of the magnitude, duration, and timing of theexposures and resulting doses that organisms might receive; also may bereferred to as dose assessment. Exposure assessment is the third step ofa risk assessment.

exposure pathway: The physical course a hazardous agent takes from itssource to an exposed organism.

exposure route: The means of intake of a substance by an organism (e.g.,ingestion, inhalation, or dermal absorption).

exposure scenario: A credible series of events that could result in exposureof organisms to hazardous agents (e.g., after emplacement of hazardouswaste in a disposal facility and closure of the facility).

external exposure, ionizing radiation: Exposure of organs or tissues of anorganism due to radiation sources outside the body.

extremely hazardous substance: A hazardous substance regulated underthe Emergency Response and Community Right-To-Know Act (Title III ofSARA) that when released at levels above its reportable quantity specifiedin 40 CFR Part 355 (EPA, 1987b) requires emergency notification of localand state emergency response authorities. Extremely hazardous substancescould cause serious irreversible health effects from accidental releases andare most likely to induce serious acute reactions following short-term expo-sure; these substances have a median lethal concentration (LC50) in bodytissues of �50 mg kg�1 or an oral median lethal dose (LD50) of �25 mg kg�1.

GLOSSARY / 367

More generally, any hazardous substance having unusually high toxicitycompared with other hazardous substances.

gamma radiation or gamma ray: The electromagnetic radiation emitted inde-excitation of an atomic nucleus, and frequently occurring in the decay ofradionuclides. Sometimes shortened to gamma (e.g., gamma-emitting radio-nuclide). High-energy gamma radiation is highly penetrating and requiresthick shielding, such as up to 1 m of concrete or a few tens of centimeter ofsteel (see photon and x ray).

geologic repository: A system intended for disposal of hazardous wastes inexcavated geologic media; includes a subterranean mined facility for thedisposal of waste and the portion of the geologic setting that provides abarrier to the movement of hazardous substances in the waste (see dis-posal, geologic).

gray (Gy): The special name for the SI unit of absorbed dose; 1 Gy � 1 J kg�1.greater confinement disposal: Land disposal in a facility located at interme-

diate depths between those of a near-surface facility and a geologic repository.groundwater: Water below the land surface in a zone of saturation.half-life: The time period in which the activity of a radioactive material

decreases by half. Measured half-lives of radionuclides vary from millionthsof a second to billions of years.

hazard: An act or phenomenon that has the potential to produce harm orother undesirable consequences to humans or what they value. Hazardsmay arise from physical phenomena (e.g., radioactivity, sound waves, mag-netic fields, fire, floods, explosions), chemicals (e.g., ozone, mercury, dioxins,carbon dioxide, drugs, food additives), organisms (e.g., viruses, bacteria),commercial products (e.g., toys, tools, automobiles), or human behavior (e.g.,drunk driving, skiing, firing guns).

hazard identification: A qualitative process of determining whether expo-sure to an agent has the potential to increase the occurrence of adverse effectsin organisms. Hazard identification is the first step of a risk assessment.

Hazardous and Solid Waste Amendments: Amendments to the ResourceConservation and Recovery Act (RCRA) passed in 1984, which added theland disposal restrictions, minimum technology requirements, and expandedcorrective action authorities to the law.

hazardous waste: General term describing waste that is deemed to be ahazard to the health of humans or other organisms, due to the presence ofradionuclides or hazardous chemicals, to the extent that it must be regulated.Hazardous waste does not include biological, medical, or infectious wastes.

hazardous waste, chemical: Solid hazardous waste regulated under SubtitleC of the Resource Conservation and Recovery Act (RCRA) and defined in40 CFR Part 261.3 (EPA, 1980b) (see characteristically hazardous wasteand listed hazardous waste).

high-level radioactive waste: (1) The highly radioactive material resultingfrom the reprocessing of spent nuclear fuel, including liquid waste produceddirectly in processing and any solid material derived from such liquid wastethat contains fission products in sufficient concentrations; and (2) otherhighly radioactive material that the U.S. Nuclear Regulatory Commission,

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consistent with existing law, determines by rule requires permanent isola-tion.

ignitable: A characteristic of solid hazardous waste regulated under theResource Conservation and Recovery Act (RCRA) and defined in 40 CFRPart 261.21 (EPA, 1980b). A solid waste is ignitable if it (1) is a liquid, otherthan an aqueous solution containing less than 24 percent alcohol by volume,and has a flash point less than 60 °C, or (2) is not a liquid and is capable,under standard temperature and pressure, of causing fire through friction,absorption of moisture or spontaneous chemical changes and, when ignited,burns so vigorously and persistently that it creates a hazard.

impact: (see effect).inadvertent intruder: A person who might occupy a waste disposal site after

facility closure and engage in normal activities, such as agriculture, dwellingconstruction, permanent residence, or other pursuits, which might result inthe person being unknowingly exposed to hazardous agents in the waste.

institutional control: Control of a waste disposal site or other facility byan authority or institution designated under the laws of a country, state, orlocal authority. Institutional control may be active (e.g., monitoring of efflu-ents, surveillance, guards, fences, or remedial activities) or passive (e.g.,warning signs).

internal exposure, ionizing radiation: Exposure of organs or tissues of anorganism due to intakes of radionuclides (e.g., by ingestion, inhalation, ordermal absorption).

ionizing radiation: Any radiation capable of displacing electrons from atomsor molecules, thereby producing ions. Examples include alpha radiation,beta radiation, gamma radiation or x rays, and cosmic rays. The minimumenergy of ionizing radiation is a few electron volts (eV); 1 eV � 1.6 � 10�19 J.

isolation: The emplacement of waste at locations apart from the humanexposure environment, especially in a disposal facility (see containment).

isotopes: Different forms of a chemical element distinguished by having differ-ent numbers of neutrons in the atomic nucleus. An element may have manystable or unstable (radioactive) isotopes.

land disposal facility: The land, buildings, and equipment intended to beused for the disposal of wastes in a subsurface facility within the upper30 m of Earth’s surface or in an above-grade facility. A geologic repositoryis not considered a land disposal facility.

landfill: (see municipal/industrial landfill).linear energy transfer (LET): The quotient of dE by d�, where dE is the

energy lost by a charged particle in traversing a distance d� in a material:LET � dE/d�. The SI unit of LET is the joule per meter (J m�1). For purposesof radiation protection, LET normally is specified in water and given in unitsof keV �m�1.

listed hazardous waste: A hazardous chemical waste regulated under theResource Conservation and Recovery Act (RCRA) and designated as hazard-ous in 40 CFR Part 261.31–33 (EPA, 1980b), including: (1) wastes fromnonspecific sources (‘‘F’’ wastes), (2) wastes from specific sources (‘‘K’’ wastes),and (3) discarded commercial chemicals from any source (‘‘P’’ and ‘‘U’’wastes).

GLOSSARY / 369

lower bound: An estimate of exposure, dose, risk or other parameter that isexpected to be lower than that experienced by any individual in a population.

lower confidence limit: A measure of the extent to which an estimate ofrisk, dose, or other parameter is not expected to be less than a specifiedvalue. For example, the lower 95 percent confidence limit of a risk estimatemeans that, based on the available information, the probability is 0.05 thatthe true but unknown risk is less than the specified value (see confidenceinterval and upper confidence limit).

low-level radioactive waste: Radioactive waste that (A) is not high-levelradioactive waste, spent nuclear fuel, transuranic waste, or byproduct mate-rial as defined in Section 11(e)(2) of the Atomic Energy Act, and (B) the U.S.Nuclear Regulatory Commission, consistent with existing law, classifies aslow-level radioactive waste. The byproduct material referred to in Clause (A)essentially is uranium or thorium mill tailings. Low-level radioactivewaste does not include waste that contains naturally occurring and accel-erator-produced radioactive material (NARM).

lowest-observed-adverse-effect level (LOAEL): In dose-response exper-iments, the lowest dose of a hazardous agent at which there are statisti-cally or biologically significant increases in the frequency or severity ofadverse effects between the exposed population and an appropriate con-trol group.

maximally exposed individual: An individual assumed to be at greatestrisk from a given hazard.

maximum likelihood estimate (MLE): An estimate of the most probablelevel of a response resulting from a dose of a hazardous agent, or anestimate of the most probable dose, exposure, or other parameter; oftensynonymous with ‘‘best estimate.’’

migration: The transport of substances in the environment, usually bymeans of movement of air, surface water, or groundwater.

mill tailings: The residues from chemical processing of uranium or thoriumores for their source material content. Mill tailings are a form of byproductmaterial, as defined in Section 11(e)(2) of the Atomic Energy Act.

mixed waste: Waste that contains radionuclides (i.e., source, specialnuclear, or byproduct material), as defined in the Atomic Energy Act, andhazardous chemical waste regulated under the Resource Conservationand Recovery Act (RCRA). Mixed waste also may include (1) waste thatcontains radionuclides defined in the Atomic Energy Act and hazardouschemical waste regulated under the Toxic Substances Control Act (TSCA)and (2) waste that contains naturally occurring and accelerator-producedradioactive material (NARM) and hazardous chemical waste regulatedunder RCRA or TSCA.

mixture rule: Rule established under the Resource Conservation andRecovery Act (RCRA) in 40 CFR Part 261.3(a) (EPA, 1980b), which statesthat (1) any mixture of a solid waste and a characteristically hazardouswaste is itself a hazardous waste if the mixture retains the hazardouscharacteristic and (2) any mixture of a solid waste and a listed hazardouswaste is itself a hazardous waste, unless such waste is delisted.

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municipal/industrial landfill: A facility for disposal of solid waste thatmeets regulatory criteria established under Subtitle D of the ResourceConservation and Recovery Act (RCRA) or is otherwise acceptable fordisposal of nonhazardous waste. Also may be referred to as a sanitarylandfill in the literature.

naturally occurring and accelerator-produced radioactive material(NARM): Any naturally occurring radioactive material (NORM) orany radioactive material produced in an accelerator.

naturally occurring radioactive material (NORM): Any naturallyoccurring radioactive material that is not source material, specialnuclear material, or byproduct material.

near-surface disposal facility: (see land disposal facility).negligible: Same as de minimis in regard to doses or health risks.noncarcinogen: A hazardous agent that does not cause cancer; frequently

used as a synonym for deterministic agent.no-observed-adverse-effect level (NOAEL): In dose-response experi-

ments, a dose of a hazardous agent at which there are no statistically orbiologically significant increases in the frequency or severity of adversehealth effects between the exposed population and an appropriate controlgroup. Some effects may be produced at this dose, but they are not consid-ered to be adverse or precursors of specific adverse effects. In an experi-ment with more than one NOAEL, NOAEL normally is the highest dosewithout adverse effect.

nuclear fuel cycle: Activities associated with the production, utilization,and disposition of fuel for nuclear reactors, including power reactors,research reactors, and isotope production reactors, and byproducts relatedto such activities.

PCBs (polychlorinated biphenyls): A family of chemicals composed ofbiphenyl molecules that have been chlorinated to varying degrees.

performance assessment: A type of risk assessment in which the potentiallong-term impacts of hazardous waste disposal on human health andthe environment are evaluated for the purpose of determining whetherdisposal of specific wastes at specific sites should be acceptable.

persistence: The length of time that a contaminant persists in theenvironment.

pharmacokinetic model: (see biokinetic model).photon: A quantum of electromagnetic radiation, having no charge or mass,

that exhibits both particle and wave behavior, especially a gamma rayor an x ray.

post-closure: Times subsequent to cessation of waste emplacement activi-ties at a disposal facility and actions (e.g., construction of impermeablecaps, seals, surface markers) to prepare the disposal site for long-termwaste isolation.

potential dose, chemicals: The amount of a substance ingested, inhaled,or applied to the skin.

probabilistic risk assessment: A type of risk assessment in which probabi-listic methods are used to describe processes, events, and their conse-quences and to derive a distribution of risk based on repeated randomsampling of distributions of input variables.

GLOSSARY / 371

probability coefficient: The nominal probability of an adverse stochasticresponse per unit dose of a hazardous agent assuming a linear, nonthresh-old dose-response relationship; often called a ‘‘risk factor’’ or ‘‘risk coeffi-cient’’ in the literature (see slope factor).

pyrophoric: Term applied to materials that will ignite spontaneously inair at a temperature of 54 °C (130 °F) or lower.

quality factor (Q): A dimensionless factor developed for purposes of radia-tion protection and assessing health risks in general terms which accountsfor the relative biological effectiveness of different radiations in pro-ducing stochastic responses and is used to relate absorbed dose (D)at a point to dose equivalent (H): H � D � Q. The quality factor is aprescribed function of linear energy transfer (LET) in water (ICRP,1991), and is defined with respect to the particular type and energy ofradiation incident on tissue at the point of interest. Prior to introductionof the radiation weighting factor and as often used by federal andstate agencies, an average quality factor (Q) for any energy of a particularradiation type (e.g., one for all photons and electrons, 20 for all alpha parti-cles) is used to relate the average absorbed dose in an organ or tissue (DT)to the average dose equivalent in that organ or tissue (HT): HT � DT � Q.

rad: (see absorbed dose).radiation weighting factor (wR): A dimensionless factor developed for

purposes of radiation protection and assessing health risks in generalterms which accounts for the relative biological effectiveness of differ-ent types (and, in some cases, energies) of radiations in producing stochas-tic responses and is used to relate the average absorbed dose in an organor tissue (T) to equivalent dose: HT � DT � wR (ICRP, 1991). Theradiation weighting factor is intended to supersede the average qualityfactor (Q), and is defined with respect to the type and energy of theradiation incident on the body or, in the case of sources within the body,emitted by the source. Values of wR include one for all photons and elec-trons and 20 for all alpha particles. The radiation weighting factor (wR)is independent of the tissue weighting factor (wT).

radioactive waste: Solid, liquid, or gaseous materials of no value contain-ing radionuclides in sufficient amounts that the waste must be regulatedas a hazardous material.

radioactivity: The property or characteristic of an unstable atomic nucleus tospontaneously transform with the emission of energy in the form of radiation.

radionuclide: A naturally occurring or artificially produced radioactiveelement or isotope.

radon: A colorless, odorless, naturally occurring, and radioactive gaseouselement formed by radioactive decay of isotopes of radium.

reactive: A characteristic of solid hazardous waste regulated under theResource Conservation and Recovery Act (RCRA) and defined in 40 CFRPart 261.23 (EPA, 1980b). A solid waste is reactive if it has any of thefollowing properties: (1) it is normally unstable and readily undergoesviolent change without detonating; (2) it reacts violently with water; (3) itforms potentially explosive mixtures with water; (4) when mixed withwater, it generates toxic gases, vapors or fumes in a quantity sufficient

372 / GLOSSARY

to present a danger to human health or the environment; (5) it is a cyanideor sulfide bearing waste which, when exposed to pH conditions between2 and 2.5, can generate toxic gases, vapors or fumes in a quantity sufficientto present a danger to human health and the environment; (6) it is capableof detonation or explosive reaction if it is subjected to a strong initiatingsource or if heated under confinement; (7) it is readily capable of detona-tion or explosive decomposition or reaction at standard temperature andpressure; or (8) it is a forbidden, Class-A, or Class-B explosive as definedby the U.S. Department of Transportation.

recycling: The use or reuse of a waste material as an effective substitutefor a commercial product, as an ingredient, or as feedstock in an industrialor energy producing process; the reclamation of useful constituent frac-tions within a waste material; removal of contaminants from a waste toallow it to be reused.

reference dose (RfD): An estimate, with uncertainty spanning perhapsan order of magnitude or greater, of a daily exposure level for the humanpopulation, including sensitive subpopulations, that is likely to be withoutan appreciable risk of deleterious effects. RfDs normally are estimatedonly for noncarcinogenic (deterministic) hazardous chemicals, and maybe developed for chronic (7 y to lifetime), subchronic (two weeks to 7 y),or acute (single event) exposures. RfDs are obtained from LOAELs orNOAELs by applying various safety and uncertainty factors, and are notintended to represent thresholds for adverse health effects.

relative biological effectiveness: For a specific radiation (A), the ratioof the absorbed dose of a reference radiation required to produce a specificlevel of response in a biological system to the absorbed dose of radiation (A)required to produce an equal response. The reference radiation normally isgamma rays or x rays with an average linear energy transfer (LET) of3.5 keV �m�1 or less.

rem: (see dose equivalent or equivalent dose).remotely-handled transuranic waste: Containerized transuranic waste

for which the external dose-equivalent rate at the surface of the containerexceeds 2 mSv h�1.

repository: (see geologic repository).Resource Conservation and Recovery Act (RCRA): Law passed in

1976, and amended in 1980 and again in 1984 by the Hazardous and SolidWaste Amendments, that governs the generation, transport, treatment,storage, and disposal of solid hazardous waste and disposal of nonhazard-ous solid waste in municipal/industrial landfills. Solid hazardous wastesregulated under RCRA are defined in 40 CFR Part 261, Subpart A (EPA,1980b), and specifically exclude source, special nuclear, and byproductmaterial as defined in the Atomic Energy Act. Objectives of RCRA includeprotection of human health and the environment, expeditious reductionor elimination of the generation of hazardous waste, and conservation ofenergy and natural resources (i.e., material recycling and recovery).

response: A significant adverse effect on an organism resulting from expo-sure to a hazardous agent. The determination of whether an effect is

GLOSSARY / 373

significant or adverse involves subjective aspects. Often referred to as an‘‘impact,’’ ‘‘biological endpoint,’’ or ‘‘effect.’’

retrieval: The intentional removal of waste from the location of its emplace-ment for disposal.

risk: The probability of harm, combined with the potential severity of thatharm. In the context of impacts on human health resulting from disposalof hazardous waste, ‘‘risk’’ is the probability of a response in an individualor the frequency of a response in a population taking into account: (1) theprobability of occurrence of processes and events that could result inrelease of hazardous substances to the environment and the magnitudeof such releases, (2) the probability that individuals or populations wouldbe exposed to the hazardous substances released to the environment andthe magnitude of such exposures, and (3) the probability that an exposurewould produce a response.

risk assessment: An analysis of the potential adverse impacts of an event(e.g., the release or threat of release of a hazardous substance) upon thewell-being of an individual or population. Risk assessment is a processby which information or experience concerning causes and effects undera set of circumstances is integrated with the extent of those circumstancesto quantify or otherwise describe risk. The process is multi-step and con-sists of hazard identification, dose-response assessment, exposureassessment, and risk characterization.

risk characterization: An integration and interpretation of the informa-tion developed during hazard identification, dose-response assessment,and exposure assessment to yield an estimate of risk to human health orother organisms, including an identification of limitations and uncertain-ties in the models and data. Risk characterization is the last step of arisk assessment.

risk management: The process by which results of risk assessments areintegrated with other information (e.g., results of cost-benefit analysis,societal concerns) to make decisions about the need for, method of, andextent of risk reduction or control.

sievert (Sv): The special name for the SI unit of dose equivalent and equiva-lent dose; 1 Sv � 1 J kg�1.

slope factor: For hazardous chemicals that induce stochastic effects, aplausible upper-bound estimate (e.g., upper 95 percent confidence limit)of the probability of cancer incidence per unit intake (e.g., milligram intakeper kilogram body weight per day) of a hazardous substance over a lifetime.For radionuclides, the age-averaged best estimate of the probability ofcancer incidence per unit activity intake of a radionuclide or, in the caseof external exposure, per unit activity concentration of a radionuclide inthe environment; values apply to any intake (chronic or acute) of anyduration (see probability coefficient).

solid waste: Material regulated under the Resource Conservation andRecovery Act (RCRA) and defined in 40 CFR Part 261.2 and 261.4 (EPA,1980b); solid waste includes, but is not restricted to, material that hasbeen discarded, abandoned, or is inherently waste-like, and such wastecan be a solid, liquid, or gas.

374 / GLOSSARY

source material: (1) Uranium or thorium or any combination of uraniumor thorium in any physical or chemical form, or (2) ores that contain, byweight, 0.05 percent or more of uranium, thorium, or any combination ofuranium or thorium. Source material does not include special nuclearmaterial.

special nuclear material: (1) Plutonium, uranium enriched in the isotope233 or 235, and any other material that the U.S. Nuclear RegulatoryCommission determines to be special nuclear material, or (2) any materialartificially enriched in any of the foregoing. Special nuclear material doesnot include source material.

statistical significance: An inference that the probability is low that anobserved difference in quantities being measured is due to variability inthe data rather than an actual difference in the quantities themselves. Theinference that an observed difference is statistically significant normally isbased on a test to reject one hypothesis and accept another.

stochastic agent: An agent that can produce a stochastic response inorganisms.

stochastic response: An adverse effect on organisms for which the proba-bility of occurrence, but not the severity, is a function of dose withoutthreshold (e.g., cancer). In humans, stochastic responses may not occurfor many years after an exposure.

storage: Retention of waste with the intent to retrieve it for subsequentuse, processing, or disposal.

Superfund: The common name for the Comprehensive EnvironmentalResponse, Compensation, and Liability Act (CERCLA).

Superfund Amendments and Reauthorization Act (SARA): Amend-ments to CERCLA (Superfund) passed in 1986, which also include free-standing provisions in Title III (The Emergency Response and CommunityRight-To-Know Act), Title IV (The Radon Gas and Indoor Air QualityResearch Act), and Title V amending the Internal Revenue Code (TheSuperfund Revenue Act).

synergistic: Situations in which the total response from simultaneous expo-sure of an organism to two or more hazardous agents is greater than thesum of the responses from separate exposures to each agent.

tissue weighting factor (wT): A dimensionless factor which representsthe ratio of the stochastic responses attributable to a specific organ ortissue (T) to the total stochastic responses attributable to all organs andtissues when the whole body receives a uniform exposure to ionizingradiation. When calculating effective dose equivalent, the tissueweighting factor represents the probability of fatal cancers or severe hered-itary effects (ICRP, 1977). When calculating effective dose, the tissueweighting factor represents the total detriment (ICRP, 1991).

toxic: (1) Capable of producing injury, illness, or damage to living organismsthrough ingestion, inhalation, or absorption through any body surface.(2) A characteristic of solid hazardous waste regulated under the ResourceConservation and Recovery Act (RCRA) and defined in 40 CFR Part 261.24(EPA, 1980b). A solid waste is toxic if, when using the toxicity character-istic leaching procedure, the extract from a representative sample of

GLOSSARY / 375

the waste contains any of 40 contaminants (7 metals and 33 organiccompounds) at a concentration equal to or greater than the specifiedvalues. When the waste contains less than 0.5 percent filterable solids,the waste itself, after filtering, is considered to be the extract for thepurpose of determining whether it is toxic.

toxicity characteristic leaching procedure: A testing procedure speci-fied in 40 CFR Part 261, Appendix II (EPA, 1980b), used to determinewhether a solid waste is toxic under the Resource Conservation andRecovery Act (RCRA).

Toxic Substances Control Act (TSCA): Law passed in 1976 that governsthe regulation of toxic substances in commerce, with the objective of pre-venting human health and environmental problems before they occur.The manufacturing, processing, or distribution in commerce of toxic sub-stances may be limited or banned if EPA finds, based on results of toxicitytesting and exposure assessments, that there is an unreasonable risk ofinjury to human health or the environment. Important hazardous chemi-cals regulated under TSCA include, for example, dioxins, PCBs, andasbestos.

transportation: The movement of material by air, rail, highway, or water.transuranic waste: Radioactive waste containing more than 4 kBq g�1 of

alpha-emitting transuranium isotopes, with half-lives greater than 20 y,except for (1) high-level radioactive waste, (2) radioactive waste that theSecretary of the U.S. Department of Energy has determined, with concur-rence of the Administrator of the U.S. Environmental Protection Agency,does not need the degree of isolation required by the disposal regulationsin 40 CFR Part 191 (EPA, 1993a), or (3) radioactive waste that the U.S.Nuclear Regulatory Commission has approved for near-surface disposalon a case-by-case basis in accordance with 10 CFR Part 61 (NRC,1982a; 1989).

transuranium element: Chemical element with an atomic number greaterthan that of uranium (92) including, among others, neptunium, plutonium,americium, and curium (often referred to as transuranic element in theliterature).

treatment: Any method, technique, or process designed to change the physi-cal or chemical character of a hazardous material to render it less hazard-ous, safer to transport, store or dispose of, or to reduce its volume.

uncertainty: A lack of sureness or confidence about parameters, results ofmeasurements, predictions of models, or other factors. Uncertainty oftencan be reduced through further study (see variability).

upper bound: An estimate of exposure, dose, risk or other parameter thatis expected to be higher than that experienced by any individual in a popu-lation.

upper confidence limit (UCL): A measure of the extent to which anestimate of risk, dose, or other parameter is not expected to be greaterthan a specified value. For example, the upper 95 percent confidence limitof a risk estimate means that, based on the available information, theprobability is 0.05 that the true but unknown risk is greater than thespecified value (see confidence interval and lower confidence limit).

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variability: A heterogeneity, diversity, or range that characterizes a param-eter (e.g., differences in body weight in a population) or response (e.g.,differences in sensitivity to a hazardous agent in a population). Furtherstudy cannot reduce variability but may provide greater confidence inquantitative characterizations of variability (see uncertainty).

waste classification: Any grouping wastes having similar attributes.waste management: All activities associated with the disposition of waste

products after they have been generated, as well as actions to minimizethe production of waste.

waste minimization: Reduction, to the extent practicable, of the volumeor toxicity of hazardous waste prior to its treatment, storage, or disposal.

weight-of-evidence classification: A classification system for character-izing the extent to which available data indicate that an agent is ahuman carcinogen.

x ray: The electromagnetic radiation emitted in de-excitation of boundatomic electrons, and frequently occurring in the decay of radionuclides,referred to as characteristic x rays, or the electromagnetic radiation pro-duced in the deceleration of energetic charged particles (e.g., beta radia-tion) in passing through matter, referred to as continuous x rays orbremsstrahlung. Higher-energy x rays are penetrating and may requireshielding of up to a few tens of centimeter of concrete or a few centimetersof steel (see gamma radiation and photon).

Acronyms

AEA Atomic Energy ActALARA as low as reasonably achievableBDAT best demonstrated available technologyBRC below regulatory concernCERCLA Comprehensive Environmental Response,

Compensation, and Liability ActCFR Code of Federal RegulationsDDREF dose and dose-rate effectiveness factorDNA deoxyribonucleic acidED10 central estimate of dose causing increase in

effects (responses) of 10 percentFR Federal RegisterHEAST Health Effects Assessment Summary TablesIRIS Integrated Risk Information SystemLDR land disposal restrictionLEAF Legal Environmental Assistance FoundationLED10 lower confidence limit of dose causing increase in

effects (responses) of 10 percentLET linear energy transferLLRWPAA Low-Level Radioactive Waste Policy Amendments

ActLOAEL lowest-observed-adverse-effect levelMCL maximum contaminant levelMCLG maximum contaminant level goalMF modifying factorMLE maximum likelihood estimateMOE margin of exposureMTD maximum tolerated doseNARM naturally occurring and accelerator-produced

radioactive materialNOAEL no-observed-adverse-effect levelNORM naturally occurring radioactive materialNWPA Nuclear Waste Policy ActPB-PK physiologically-based pharmacokineticPCBs polychlorinated biphenylsRCRA Resource Conservation and Recovery ActRfD reference dose

377

378 / ACRONYMS

RI risk indexTSCA Toxic Substances Control ActUCL upper confidence limitUF uncertainty factorWIPP Waste Isolation Pilot PlantWIPPLWA Waste Isolation Pilot Plant Land Withdrawal Act

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WARRINER, J.B. and BENNETT, R.D. (1985). Alternative Methods for Dis-posal of Low-Level Radioactive Wastes—Task 2a: Technical Requirementsfor Below-Ground Vault Disposal of Low-Level Radioactive Waste, U.S.Nuclear Regulatory Commission Report NUREG/CR-3774, Volume 2(National Technical Information Service, Springfield, Virginia).

WATERS, R.D., CRUTCHER, M.D. and PARKER, F.L. (1993). ‘‘Hazardranking systems for chemical wastes and chemical waste sites,’’ pages 115 to170 in Hazard Assessment of Chemicals, Volume 8, Saxena, J., Ed. (AcademicPress, New York).

WEIL, C.S. (1972). ‘‘Statistics versus safety factors and scientific judgment inthe evaluation of safety for man,’’ Toxicol. Appl. Pharmacol. 21, 454–463.

WILSON, M.L., BARNARD, R.W., GAUTHIER, J.H., BARR, G.E., DOCK-ERY, H.A., DUNN, E., EATON, R.R., GUERIN, D.C., LU, N., MARTINEZ,M.J., NILSON, R., RAUTMAN, C.A., ROBEY, T.H., ROSS, B., RYDER,E.E., SCHENKER, A.R., SHANNON, S.A., SKINNER, L.H., HALSEY,W.G., GANSEMER, J., LEWIS, L.C., LAMONT, A.D., TRIAY, I.R.,MEIJER, A. and MORRIS, D.E. (1994). Total-System Performance Assess-ment for Yucca Mountain: SNL Second Iteration (TSPA-1993). ExecutiveSummary, Sandia National Laboratories Report SAND 93-2675 (NationalTechnical Information Service, Springfield, Virginia).

WILTSHIRE, S. and DOW, K. (1995). ‘‘Social and political considerations,’’pages 97 to 105 in Radioactive and Mixed Waste—Risk as a Basis for WasteClassification, NCRP Symposium Proceedings No. 2 (National Council onRadiation Protection and Measurements, Bethesda, Maryland).

WIPPLWA (1992). Waste Isolation Pilot Plant Land Withdrawal Act. PublicLaw 102-579 (October 30), 106 Stat. 4777, as amended (U.S. GovernmentPrinting Office, Washington).

400 / REFERENCES

WVDPA (1980). West Valley Demonstration Project Act. Public Law 96-368(October 1), 94 Stat. 1347 (U.S. Government Printing Office, Washington).

YU, C., ZIELEN, A.J., CHENG, J.J., YUAN, Y.C., JONES, L.B., LEPOIRE,D.J., WANG, Y.Y., LOUREIRO, C.O., GNANAPRAGASAM, E., FAIL-LACE, E., WALLO, A., III, WILLIAMS, W.A. and PETERSON, H. (1993).Manual for Implementing Residual Radioactive Material Guidelines UsingRESRAD, Version 5.0, Argonne National Laboratory Report ANL/EAD/LD-2 (National Technical Information Service, Springfield, Virginia).

The NCRP

The National Council on Radiation Protection and Measurements is anonprofit corporation chartered by Congress in 1964 to:1. Collect, analyze, develop and disseminate in the public interest informa-

tion and recommendations about (a) protection against radiation and(b) radiation measurements, quantities and units, particularly those con-cerned with radiation protection.

2. Provide a means by which organizations concerned with the scientificand related aspects of radiation protection and of radiation quantities,units and measurements may cooperate for effective utilization of theircombined resources, and to stimulate the work of such organizations.

3. Develop basic concepts about radiation quantities, units and measure-ments, about the application of these concepts, and about radiationprotection.

4. Cooperate with the International Commission on Radiological Protection,the International Commission on Radiation Units and Measurements,and other national and international organizations, governmental andprivate, concerned with radiation quantities, units and measurementsand with radiation protection.The Council is the successor to the unincorporated association of scien-

tists known as the National Committee on Radiation Protection and Mea-surements and was formed to carry on the work begun by the Committeein 1929.

The participants in the Council’s work are the Council members andmembers of scientific and administrative committees. Council members areselected solely on the basis of their scientific expertise and serve as individu-als, not as representatives of any particular organization. The scientificcommittees, composed of experts having detailed knowledge and competencein the particular area of the committee’s interest, draft proposed recommen-dations. These are then submitted to the full membership of the Councilfor careful review and approval before being published.

The following comprise the current officers and membership of theCouncil:

Officers

President Thomas S. TenfordeVice President Kenneth R. KaseSecretary and Treasurer William M. BecknerAssistant Secretary Michael F. McBride

401

402 / THE NCRP

Members

John F. Ahearne Ethel S. Gilbert Bruce A. NapierLarry E. Anderson Joel E. Gray Carl J. PaperielloBenjamin R. Archer Andrew J. Grosovsky Ronald C. PetersenMary M. Austin-Seymour Raymond A. Guilmette R. Julian PrestonHarold L. Beck William R. Hendee Jerome S. PuskinEleanor A. Blakely John W. Hirshfeld Marvin RosensteinJohn D. Boice, Jr. David G. Hoel Lawrence N. RothenbergThomas B. Borak F. Owen Hoffman Henry D. RoyalAndre Bouville Geoffrey R. Howe Michael T. RyanLeslie A. Braby Kenneth R. Kase Jonathan M. SametDavi J. Brenner Ann R. Kennedy Stephen M. SeltzerAntone L. Brooks David C. Kocher Roy E. ShoreJerrold T. Bushberg Ritsuko Komaki Edward A. SicklesJohn F. Cardella Amy Kronenberg David H. SlineyShih-Yew Chen Charles E. Land Paul SlovicChung-Kwang Chou Susan M. Langhorst Daniel J. StromMary E. Clark Richard W. Leggett Louise C. StrongJames E. Cleaver Howard L. Liber Thomas S. TenfordeJ. Donald Cossairt James C. Lin Lawrence W. TownsendAllen G. Croff Jill Lipoti Lois B. TravisFrancis A. Cucinotta John B. Little Robert L. UllrichPaul M. DeLuca Jay H. Lubin Richard J. VetterCarter Denniston C. Douglas Maynard Daniel WartenbergGail de Planque Claire M. Mays David A. WeberJohn F. Dicello Barbara J. McNeil F. Ward WhickerSarah S. Donaldson Fred A. Mettler, Jr. Chris G. WhippleWilliam P. Dornsife Charles W. Miller J. Frank WilsonStephen A. Feig Jack Miller Susan D. WiltshireH. Keith Florig Kenneth L. Miller Marco ZaiderKenneth R. Foster William F. Morgan Pasquale ZanzonicoJohn F. Frazier John E. Moulder Marvin C. ZiskinThomas F. Gesell David S. Myers

Honorary Members

Lauriston S. Taylor, Honorary PresidentWarren K. Sinclair, President Emeritus; Charles B. Meinhold, President Emeritus

S. James Adelstein, Honorary Vice PresidentW. Roger Ney, Executive Director Emeritus

Seymour Abrahamson Patricia W. Durbin Robert J. NelsenEdward L. Alpen Keith F. Eckerman Wesley L. NyborgLynn R. Anspaugh Thomas S. Ely John W. Poston, Sr.John A. Auxier Richard F. Foster Andrew K. PoznanskiWilliam J. Bair R.J. Michael Fry Chester R. RichmondBruce B. Boecker Robert O. Gorson William L. RussellVictor P. Bond Arthur W. Guy Eugene L. SaengerRobert L. Brent Eric J. Hall William J. SchullReynold F. Brown Naomi H. Harley J. Newell StannardMelvin C. Carter Donald G. Jacobs John B. StorerRandall S. Caswell Bernd Kahn John E. TillFrederick P. Cowan Roger O. McClellan Arthur C. UptonJames F. Crow Dade W. Moeller George L. VoelzGerald D. Dodd A. Alan Moghissi Edward W. Webster

THE NCRP / 403

Lauriston S. Taylor Lecturers

Herbert M. Parker (1977) The Squares of the Natural Numbers in RadiationProtection

Sir Edward Pochin (1978) Why be Quantitative about Radiation Risk Estimates?Hymer L. Friedell (1979) Radiation Protection—Concepts and Trade OffsHarold O. Wyckoff (1980) From ‘‘Quantity of Radiation’’ and ‘‘Dose’’ to

‘‘Exposure’’ and ‘‘Absorbed Dose’’—An Historical ReviewJames F. Crow (1981) How Well Can We Assess Genetic Risk? Not VeryEugene L. Saenger (1982) Ethics, Trade-offs and Medical RadiationMerril Eisenbud (1983) The Human Environment—Past, Present and FutureHarald H. Rossi (1984) Limitation and Assessment in Radiation ProtectionJohn H. Harley (1985) Truth (and Beauty) in Radiation MeasurementHerman P. Schwan (1986) Biological Effects of Non-ionizing Radiations:

Cellular Properties and InteractionsSeymour Jablon (1987) How to be Quantitative about Radiation Risk EstimatesBo Lindell (1988) How Safe is Safe Enough?Arthur C. Upton (1989) Radiobiology and Radiation Protection: The Past

Century and Prospects for the FutureJ. Newell Stannard (1990) Radiation Protection and the Internal Emitter SagaVictor P. Bond (1991) When is a Dose Not a Dose?Edward W. Webster (1992) Dose and Risk in Diagnostic Radiology: How Big?

How Little?Warren K. Sinclair (1993) Science, Radiation Protection and the NCRPR.J. Michael Fry (1994) Mice, Myths and MenAlbrecht Kellerer (1995) Certainty and Uncertainty in Radiation ProtectionSeymour Abrahamson (1996) 70 Years of Radiation Genetics: Fruit Flies, Mice

and HumansWilliam J. Bair (1997) Radionuclides in the Body: Meeting the Challenge!Eric J. Hall (1998) From Chimney Sweeps to Astronauts: Cancer Risks in the

WorkplaceNaomi H. Harley (1999) Back to BackgroundS. James Adelstein (2000) Administered Radioactivity: Unde Venimus Quoque

ImusWesley L. Nyborg (2001) Assuring the Safety of Medical Diagnostic UltrasoundR. Julian Preston (2002) Developing Mechanistic Data for Incorporation into

Cancer Risk Assessment: Old Problems and New Approaches

Currently, the following committees are actively engaged in formulatingrecommendations:

SC 1 Basic Criteria, Epidemiology, Radiobiology and RiskSC 1-4 Extrapolation of Risks from Non-Human ExperimentalSystems to ManSC 1-7 Information Needed to Make Radiation ProtectionRecommendations for Travel Beyond Low-Earth OrbitSC 1-8 Risk to Thyroid from Ionizing RadiationSC 1-10 Review of Cohen’s Radon Research MethodsSC 1-11 Radiation Protection and Measurement for NeutronSurveillance ScannersSC 1-12 Exposure Limits for Security Surveillance Devices

SC 9 Structural Shielding Design and Evaluation for Medical Use ofX Rays and Gamma Rays of Energies Up to 10 MeV

404 / THE NCRP

SC 46 Operational Radiation SafetySC 46-8 Radiation Protection Design Guidelines for ParticleAccelerator FacilitiesSC 46-13 Design of Facilities for Medical Radiation TherapySC 46-16 Radiation Protection in Veterinary MedicineSC 46-17 Radiation Protection in Educational InstitutionsSC 57-15 Uranium RiskSC 57-17 Radionuclide Dosimetry Models for Wounds

SC 64 Environmental IssuesSC 64-22 Design of Effective Effluent and EnvironmentalMonitoring ProgramsSC 64-23 Cesium in the Environment

SC 72 Radiation Protection in MammographySC 85 Risk of Lung Cancer from RadonSC 87 Radioactive and Mixed Waste

SC 87-1 Waste Avoidance and Volume ReductionSC 87-3 Performance AssessmentSC 87-5 Risk Management Analysis for Decommissioned Sites

SC 89 Nonionizing Electromagnetic FieldsSC 89-3 Biological Effects of Extremely Low-Frequency Electricand Magnetic FieldsSC 89-4 Biological Effects and Exposure Recommendations forModulated Radiofrequency FieldsSC 89-5 Biological Effects and Exposure Criteria forRadiofrequency Electromagnetic FieldsSC 89-6 Wireless Telecommunications Safety Issues for BuildingOwners and Managers

SC 91 Radiation Protection in MedicineSC 91-1 Precautions in the Management of Patients Who HaveReceived Therapeutic Amounts of RadionuclidesSC 91-2 Radiation Protection in Dentistry

SC 92 Public Policy and Risk CommunicationSC 93 Radiation Measurement and Dosimetry

In recognition of its responsibility to facilitate and stimulate cooperationamong organizations concerned with the scientific and related aspects ofradiation protection and measurement, the Council has created a categoryof NCRP Collaborating Organizations. Organizations or groups of organiza-tions that are national or international in scope and are concerned withscientific problems involving radiation quantities, units, measurements andeffects, or radiation protection may be admitted to collaborating status bythe Council. Collaborating Organizations provide a means by which the

THE NCRP / 405

NCRP can gain input into its activities from a wider segment of society.At the same time, the relationships with the Collaborating Organizationsfacilitate wider dissemination of information about the Council’s activities,interests and concerns. Collaborating Organizations have the opportunityto comment on draft reports (at the time that these are submitted to themembers of the Council). This is intended to capitalize on the fact thatCollaborating Organizations are in an excellent position to both contributeto the identification of what needs to be treated in NCRP reports and toidentify problems that might result from proposed recommendations. Thepresent Collaborating Organizations with which the NCRP maintains liai-son are as follows:

Agency for Toxic Substances and Disease RegistryAmerican Academy of DermatologyAmerican Academy of Environmental EngineersAmerican Academy of Health PhysicsAmerican Association of Physicists in MedicineAmerican College of Medical PhysicsAmerican College of Nuclear PhysiciansAmerican College of Occupational and Environmental MedicineAmerican College of RadiologyAmerican Dental AssociationAmerican Industrial Hygiene AssociationAmerican Institute of Ultrasound in MedicineAmerican Insurance Services GroupAmerican Medical AssociationAmerican Nuclear SocietyAmerican Pharmaceutical AssociationAmerican Podiatric Medical AssociationAmerican Public Health AssociationAmerican Radium SocietyAmerican Roentgen Ray SocietyAmerican Society for Therapeutic Radiology and OncologyAmerican Society of Health-System PharmacistsAmerican Society of Radiologic TechnologistsAssociation of University RadiologistsBioelectromagnetics SocietyCampus Radiation Safety OfficersCollege of American PathologistsConference of Radiation Control Program Directors, Inc.Council on Radionuclides and RadiopharmaceuticalsDefense Threat Reduction AgencyElectric Power Research InstituteFederal Communications CommissionFederal Emergency Management AgencyGenetics Society of AmericaHealth Physics SocietyInstitute of Electrical and Electronics Engineers, Inc.

406 / THE NCRP

Institute of Nuclear Power OperationsInternational Brotherhood of Electrical WorkersNational Aeronautics and Space AdministrationNational Association of Environmental ProfessionalsNational Electrical Manufacturers AssociationNational Institute for Occupational Safety and HealthNational Institute of Standards and TechnologyNuclear Energy InstituteOffice of Science and Technology PolicyOil, Chemical and Atomic WorkersRadiation Research SocietyRadiological Society of North AmericaSociety for Risk AnalysisSociety of Chairmen of Academic Radiology DepartmentsSociety of Nuclear MedicineSociety of Skeletal RadiologyU.S. Air ForceU.S. ArmyU.S. Coast GuardU.S. Department of EnergyU.S. Department of Housing and Urban DevelopmentU.S. Department of LaborU.S. Department of TransportationU.S. Environmental Protection AgencyU.S. NavyU.S. Nuclear Regulatory CommissionU.S. Public Health ServiceUtility Workers Union of America

The NCRP has found its relationships with these organizations to beextremely valuable to continued progress in its program.

Another aspect of the cooperative efforts of the NCRP relates to theSpecial Liaison relationships established with various governmental organi-zations that have an interest in radiation protection and measurements.This liaison relationship provides: (1) an opportunity for participating orga-nizations to designate an individual to provide liaison between the organiza-tion and the NCRP; (2) that the individual designated will receive copies ofdraft NCRP reports (at the time that these are submitted to the membersof the Council) with an invitation to comment, but not vote; and (3) that newNCRP efforts might be discussed with liaison individuals as appropriate, sothat they might have an opportunity to make suggestions on new studiesand related matters. The following organizations participate in the SpecialLiaison Program:

Australian Radiation LaboratoryBundesamt fur Strahlenschutz (Germany)Canadian Nuclear Safety CommissionCentral Laboratory for Radiological Protection (Poland)

THE NCRP / 407

China Institute for Radiation ProtectionCommisariat a l’Energie AtomiqueCommonwealth Scientific Instrumentation Research Organization

(Australia)European CommissionHealth Council of the NetherlandsInternational Commission on Non-Ionizing Radiation ProtectionJapan Radiation CouncilKorea Institute of Nuclear SafetyNational Radiological Protection Board (United Kingdom)Russian Scientific Commission on Radiation ProtectionSouth African Forum for Radiation ProtectionWorld Association of Nuclear Operations

The NCRP values highly the participation of these organizations in theSpecial Liaison Program.

The Council also benefits significantly from the relationships establishedpursuant to the Corporate Sponsor’s Program. The program facilitates theinterchange of information and ideas and corporate sponsors provide valu-able fiscal support for the Council’s program. This developing program cur-rently includes the following Corporate Sponsors:

3M Corporate Health PhysicsAmersham HealthDuke Energy CorporationICN Biomedicals, Inc.Landauer, Inc.Nuclear Energy InstitutePhilips Medical SystemsSouthern California Edison

The Council’s activities are made possible by the voluntary contributionof time and effort by its members and participants and the generous supportof the following organizations:

3M Health Physics ServicesAgfa CorporationAlfred P. Sloan FoundationAlliance of American InsurersAmerican Academy of DermatologyAmerican Academy of Health PhysicsAmerican Academy of Oral and Maxillofacial RadiologyAmerican Association of Physicists in MedicineAmerican Cancer SocietyAmerican College of Medical PhysicsAmerican College of Nuclear PhysiciansAmerican College of Occupational and Environmental MedicineAmerican College of RadiologyAmerican College of Radiology Foundation

408 / THE NCRP

American Dental AssociationAmerican Healthcare Radiology AdministratorsAmerican Industrial Hygiene AssociationAmerican Insurance Services GroupAmerican Medical AssociationAmerican Nuclear SocietyAmerican Osteopathic College of RadiologyAmerican Podiatric Medical AssociationAmerican Public Health AssociationAmerican Radium SocietyAmerican Roentgen Ray SocietyAmerican Society of Radiologic TechnologistsAmerican Society for Therapeutic Radiology and OncologyAmerican Veterinary Medical AssociationAmerican Veterinary Radiology SocietyAssociation of University RadiologistsBattelle Memorial InstituteCanberra Industries, Inc.Chem Nuclear SystemsCenter for Devices and Radiological HealthCollege of American PathologistsCommittee on Interagency Radiation Research and Policy

CoordinationCommonwealth EdisonCommonwealth of PennsylvaniaConsolidated EdisonConsumers Power CompanyCouncil on Radionuclides and RadiopharmaceuticalsDefense Nuclear AgencyEastman Kodak CompanyEdison Electric InstituteEdward Mallinckrodt, Jr. FoundationEG&G Idaho, Inc.Electric Power Research InstituteElectromagnetic Energy AssociationFederal Emergency Management AgencyFlorida Institute of Phosphate ResearchFlorida Power CorporationFuji Medical Systems, U.S.A., Inc.Genetics Society of AmericaHealth Effects Research Foundation (Japan)Health Physics SocietyInstitute of Nuclear Power OperationsJames Picker FoundationMartin Marietta CorporationMotorola FoundationNational Aeronautics and Space AdministrationNational Association of Photographic Manufacturers

THE NCRP / 409

National Cancer InstituteNational Electrical Manufacturers AssociationNational Institute of Standards and TechnologyNew York Power AuthorityPicker InternationalPublic Service Electric and Gas CompanyRadiation Research SocietyRadiological Society of North AmericaRichard Lounsbery FoundationSandia National LaboratorySiemens Medical Systems, Inc.Society of Nuclear MedicineSociety of Pediatric RadiologyU.S. Department of EnergyU.S. Department of LaborU.S. Environmental Protection AgencyU.S. NavyU.S. Nuclear Regulatory CommissionVictoreen, Inc.Westinghouse Electric Corporation

Initial funds for publication of NCRP reports were provided by a grantfrom the James Picker Foundation.

The NCRP seeks to promulgate information and recommen-dationsbased on leading scientific judgment on matters of radiation protection andmeasurement and to foster cooperation among organizations concerned withthese matters. These efforts are intended to serve the public interest andthe Council welcomes comments and suggestions on its reports or activitiesfrom those interested in its work.

NCRP Publications

Information on NCRP publications may be obtained from the NCRPwebsite (http://www.ncrp.com), e-mail ([email protected]), by telephone(800-229-2652, Ext. 25), or fax (301-907-8768). The address is:

NCRP Publications7910 Woodmont AvenueSuite 400Bethesda, MD 20814-3095

Abstracts of NCRP reports published since 1980, abstracts of all NCRPcommentaries, and the text of all NCRP statements are available at theNCRP website. Currently available publications are listed below.

NCRP Reports

No. Title

8 Control and Removal of Radioactive Contamination inLaboratories (1951)

22 Maximum Permissible Body Burdens and Maximum PermissibleConcentrations of Radionuclides in Air and in Water forOccupational Exposure (1959) [Includes Addendum 1 issued inAugust 1963]

25 Measurement of Absorbed Dose of Neutrons, and of Mixtures ofNeutrons and Gamma Rays (1961)

27 Stopping Powers for Use with Cavity Chambers (1961)30 Safe Handling of Radioactive Materials (1964)32 Radiation Protection in Educational Institutions (1966)35 Dental X-Ray Protection (1970)36 Radiation Protection in Veterinary Medicine (1970)37 Precautions in the Management of Patients Who Have Received

Therapeutic Amounts of Radionuclides (1970)38 Protection Against Neutron Radiation (1971)40 Protection Against Radiation from Brachytherapy Sources (1972)41 Specification of Gamma-Ray Brachytherapy Sources (1974)42 Radiological Factors Affecting Decision-Making in a Nuclear

Attack (1974)

410

NCRP PUBLICATIONS / 411

44 Krypton-85 in the Atmosphere—Accumulation, BiologicalSignificance, and Control Technology (1975)

46 Alpha-Emitting Particles in Lungs (1975)47 Tritium Measurement Techniques (1976)49 Structural Shielding Design and Evaluation for Medical Use of

X Rays and Gamma Rays of Energies Up to 10 MeV (1976)50 Environmental Radiation Measurements (1976)52 Cesium-137 from the Environment to Man: Metabolism and Dose

(1977)54 Medical Radiation Exposure of Pregnant and Potentially

Pregnant Women (1977)55 Protection of the Thyroid Gland in the Event of Releases of

Radioiodine (1977)57 Instrumentation and Monitoring Methods for Radiation

Protection (1978)58 A Handbook of Radioactivity Measurements Procedures, 2nd ed.

(1985)60 Physical, Chemical, and Biological Properties of Radiocerium

Relevant to Radiation Protection Guidelines (1978)61 Radiation Safety Training Criteria for Industrial Radiography

(1978)62 Tritium in the Environment (1979)63 Tritium and Other Radionuclide Labeled Organic Compounds

Incorporated in Genetic Material (1979)64 Influence of Dose and Its Distribution in Time on Dose-Response

Relationships for Low-LET Radiations (1980)65 Management of Persons Accidentally Contaminated with

Radionuclides (1980)67 Radiofrequency Electromagnetic Fields—Properties, Quantities

and Units, Biophysical Interaction, and Measurements (1981)68 Radiation Protection in Pediatric Radiology (1981)69 Dosimetry of X-Ray and Gamma-Ray Beams for Radiation

Therapy in the Energy Range 10 keV to 50 MeV (1981)70 Nuclear Medicine—Factors Influencing the Choice and Use of

Radionuclides in Diagnosis and Therapy (1982)72 Radiation Protection and Measurement for Low-Voltage Neutron

Generators (1983)73 Protection in Nuclear Medicine and Ultrasound Diagnostic

Procedures in Children (1983)74 Biological Effects of Ultrasound: Mechanisms and Clinical

Implications (1983)75 Iodine-129: Evaluation of Releases from Nuclear Power

Generation (1983)77 Exposures from the Uranium Series with Emphasis on Radon

and Its Daughters (1984)78 Evaluation of Occupational and Environmental Exposures to

Radon and Radon Daughters in the United States (1984)

412 / NCRP PUBLICATIONS

79 Neutron Contamination from Medical Electron Accelerators(1984)

80 Induction of Thyroid Cancer by Ionizing Radiation (1985)81 Carbon-14 in the Environment (1985)82 SI Units in Radiation Protection and Measurements (1985)83 The Experimental Basis for Absorbed-Dose Calculations in

Medical Uses of Radionuclides (1985)84 General Concepts for the Dosimetry of Internally Deposited

Radionuclides (1985)85 Mammography—A User’s Guide (1986)86 Biological Effects and Exposure Criteria for Radiofrequency

Electromagnetic Fields (1986)87 Use of Bioassay Procedures for Assessment of Internal

Radionuclide Deposition (1987)88 Radiation Alarms and Access Control Systems (1986)89 Genetic Effects from Internally Deposited Radionuclides (1987)90 Neptunium: Radiation Protection Guidelines (1988)92 Public Radiation Exposure from Nuclear Power Generation in

the United States (1987)93 Ionizing Radiation Exposure of the Population of the United

States (1987)94 Exposure of the Population in the United States and Canada

from Natural Background Radiation (1987)95 Radiation Exposure of the U.S. Population from Consumer

Products and Miscellaneous Sources (1987)96 Comparative Carcinogenicity of Ionizing Radiation and

Chemicals (1989)97 Measurement of Radon and Radon Daughters in Air (1988)99 Quality Assurance for Diagnostic Imaging (1988)100 Exposure of the U.S. Population from Diagnostic Medical

Radiation (1989)101 Exposure of the U.S. Population from Occupational Radiation

(1989)102 Medical X-Ray, Electron Beam and Gamma-Ray Protection for

Energies Up to 50 MeV (Equipment Design, Performance andUse) (1989)

103 Control of Radon in Houses (1989)104 The Relative Biological Effectiveness of Radiations of Different

Quality (1990)105 Radiation Protection for Medical and Allied Health Personnel

(1989)106 Limit for Exposure to ‘‘Hot Particles’’ on the Skin (1989)107 Implementation of the Principle of As Low As Reasonably

Achievable (ALARA) for Medical and Dental Personnel (1990)108 Conceptual Basis for Calculations of Absorbed-Dose

Distributions (1991)109 Effects of Ionizing Radiation on Aquatic Organisms (1991)110 Some Aspects of Strontium Radiobiology (1991)


111 Developing Radiation Emergency Plans for Academic, Medical orIndustrial Facilities (1991)

112 Calibration of Survey Instruments Used in Radiation Protectionfor the Assessment of Ionizing Radiation Fields andRadioactive Surface Contamination (1991)

113 Exposure Criteria for Medical Diagnostic Ultrasound: I. CriteriaBased on Thermal Mechanisms (1992)

114 Maintaining Radiation Protection Records (1992)115 Risk Estimates for Radiation Protection (1993)116 Limitation of Exposure to Ionizing Radiation (1993)117 Research Needs for Radiation Protection (1993)118 Radiation Protection in the Mineral Extraction Industry (1993)119 A Practical Guide to the Determination of Human Exposure to

Radiofrequency Fields (1993)120 Dose Control at Nuclear Power Plants (1994)121 Principles and Application of Collective Dose in Radiation

Protection (1995)122 Use of Personal Monitors to Estimate Effective Dose Equivalent

and Effective Dose to Workers for External Exposure to Low-LET Radiation (1995)

123 Screening Models for Releases of Radionuclides to Atmosphere,Surface Water, and Ground (1996)

124 Sources and Magnitude of Occupational and Public Exposuresfrom Nuclear Medicine Procedures (1996)

125 Deposition, Retention and Dosimetry of Inhaled RadioactiveSubstances (1997)

126 Uncertainties in Fatal Cancer Risk Estimates Used in RadiationProtection (1997)

127 Operational Radiation Safety Program (1998)128 Radionuclide Exposure of the Embryo/Fetus (1998)129 Recommended Screening Limits for Contaminated Surface Soil

and Review of Factors Relevant to Site-Specific Studies (1999)130 Biological Effects and Exposure Limits for ‘‘Hot Particles’’ (1999)131 Scientific Basis for Evaluating the Risks to Populations from

Space Applications of Plutonium (2001)132 Radiation Protection Guidance for Activities in Low-Earth Orbit

(2000)133 Radiation Protection for Procedures Performed Outside the

Radiology Department (2000)134 Operational Radiation Safety Training (2000)135 Liver Cancer Risk from Internally-Deposited Radionuclides

(2001)136 Evaluation of the Linear-Nonthreshold Dose-Response Model for

Ionizing Radiation (2001)137 Fluence-Based and Microdosimetric Event-Based Methods for

Radiation Protection in Space (2001)138 Management of Terrorist Events Involving Radioactive Material

(2001)139 Risk-Based Classification of Radioactive and Hazardous

Chemical Wastes (2002)140 Exposure Criteria for Medical Diagnostic Ultrasound: II. Criteria

Based on All Known Mechanisms (2002)


141 Managing Potentially Radioactive Scrap Metal (2002)142 Operational Radiation Safety Program for Astronauts in Low-

Earth Orbit: A Basic Framework (2002)

Binders for NCRP reports are available. Two sizes make it possibleto collect into small binders the ‘‘old series’’ of reports (NCRP ReportsNos. 8-30) and into large binders the more recent publications (NCRP ReportsNos. 32-142). Each binder will accommodate from five to seven reports. Thebinders carry the identification ‘‘NCRP Reports’’ and come with label holderswhich permit the user to attach labels showing the reports contained ineach binder.

The following bound sets of NCRP reports are also available:

Volume I. NCRP Reports Nos. 8, 22Volume II. NCRP Reports Nos. 23, 25, 27, 30Volume III. NCRP Reports Nos. 32, 35, 36, 37Volume IV. NCRP Reports Nos. 38, 40, 41Volume V. NCRP Reports Nos. 42, 44, 46Volume VI. NCRP Reports Nos. 47, 49, 50, 51Volume VII. NCRP Reports Nos. 52, 53, 54, 55, 57Volume VIII. NCRP Report No. 58Volume IX. NCRP Reports Nos. 59, 60, 61, 62, 63Volume X. NCRP Reports Nos. 64, 65, 66, 67Volume XI. NCRP Reports Nos. 68, 69, 70, 71, 72Volume XII. NCRP Reports Nos. 73, 74, 75, 76Volume XIII. NCRP Reports Nos. 77, 78, 79, 80Volume XIV. NCRP Reports Nos. 81, 82, 83, 84, 85Volume XV. NCRP Reports Nos. 86, 87, 88, 89Volume XVI. NCRP Reports Nos. 90, 91, 92, 93Volume XVII. NCRP Reports Nos. 94, 95, 96, 97Volume XVIII. NCRP Reports Nos. 98, 99, 100Volume XIX. NCRP Reports Nos. 101, 102, 103, 104Volume XX. NCRP Reports Nos. 105, 106, 107, 108Volume XXI. NCRP Reports Nos. 109, 110, 111Volume XXII. NCRP Reports Nos. 112, 113, 114Volume XXIII. NCRP Reports Nos. 115, 116, 117, 118Volume XXIV. NCRP Reports Nos. 119, 120, 121, 122Volume XXV. NCRP Report No. 123I and 123IIVolume XXVI. NCRP Reports Nos. 124, 125, 126, 127Volume XXVII. NCRP Reports Nos. 128, 129, 130Volume XXVIII. NCRP Reports Nos. 131, 132, 133Volume XXIX. NCRP Reports Nos. 134, 135, 136, 137

(Titles of the individual reports contained in each volume aregiven above.)


NCRP Commentaries

No. Title

1 Krypton-85 in the Atmosphere—With Specific Reference to thePublic Health Significance of the Proposed Controlled Releaseat Three Mile Island (1980)

4 Guidelines for the Release of Waste Water from NuclearFacilities with Special Reference to the Public HealthSignificance of the Proposed Release of Treated Waste Watersat Three Mile Island (1987)

5 Review of the Publication, Living Without Landfills (1989)6 Radon Exposure of the U.S. Population—Status of the Problem

(1991)7 Misadministration of Radioactive Material in Medicine—

Scientific Background (1991)8 Uncertainty in NCRP Screening Models Relating to Atmospheric

Transport, Deposition and Uptake by Humans (1993)9 Considerations Regarding the Unintended Radiation Exposure of

the Embryo, Fetus or Nursing Child (1994)10 Advising the Public about Radiation Emergencies: A Document

for Public Comment (1994)11 Dose Limits for Individuals Who Receive Exposure from

Radionuclide Therapy Patients (1995)12 Radiation Exposure and High-Altitude Flight (1995)13 An Introduction to Efficacy in Diagnostic Radiology and Nuclear

Medicine (Justification of Medical Radiation Exposure) (1995)14 A Guide for Uncertainty Analysis in Dose and Risk Assessments

Related to Environmental Contamination (1996)15 Evaluating the Reliability of Biokinetic and Dosimetric Models

and Parameters Used to Assess Individual Doses for RiskAssessment Purposes (1998)

Proceedings of the Annual Meeting

No. Title

1 Perceptions of Risk, Proceedings of the Fifteenth AnnualMeeting held on March 14-15, 1979 (including Taylor LectureNo. 3) (1980)

3 Critical Issues in Setting Radiation Dose Limits, Proceedings ofthe Seventeenth Annual Meeting held on April 8-9, 1981(including Taylor Lecture No. 5) (1982)

4 Radiation Protection and New Medical Diagnostic Approaches,Proceedings of the Eighteenth Annual Meeting held on April6-7, 1982 (including Taylor Lecture No. 6) (1983)


5 Environmental Radioactivity, Proceedings of the NineteenthAnnual Meeting held on April 6-7, 1983 (including TaylorLecture No. 7) (1983)

6 Some Issues Important in Developing Basic Radiation ProtectionRecommendations, Proceedings of the Twentieth AnnualMeeting held on April 4-5, 1984 (including Taylor Lecture No. 8)(1985)

7 Radioactive Waste, Proceedings of the Twenty-first AnnualMeeting held on April 3-4, 1985 (including Taylor Lecture No. 9)(1986)

8 Nonionizing Electromagnetic Radiations and Ultrasound,Proceedings of the Twenty-second Annual Meeting held onApril 2-3, 1986 (including Taylor Lecture No. 10) (1988)

9 New Dosimetry at Hiroshima and Nagasaki and Its Implicationsfor Risk Estimates, Proceedings of the Twenty-third AnnualMeeting held on April 8-9, 1987 (including Taylor Lecture No. 11)(1988)

10 Radon, Proceedings of the Twenty-fourth Annual Meeting heldon March 30-31, 1988 (including Taylor Lecture No. 12) (1989)

11 Radiation Protection Today—The NCRP at Sixty Years,Proceedings of the Twenty-fifth Annual Meeting held onApril 5-6, 1989 (including Taylor Lecture No. 13) (1990)

12 Health and Ecological Implications of RadioactivelyContaminated Environments, Proceedings of the Twenty-sixthAnnual Meeting held on April 4-5, 1990 (including TaylorLecture No. 14) (1991)

13 Genes, Cancer and Radiation Protection, Proceedings of theTwenty-seventh Annual Meeting held on April 3-4, 1991(including Taylor Lecture No. 15) (1992)

14 Radiation Protection in Medicine, Proceedings of the Twenty-eighth Annual Meeting held on April 1-2, 1992 (includingTaylor Lecture No. 16) (1993)

15 Radiation Science and Societal Decision Making, Proceedings ofthe Twenty-ninth Annual Meeting held on April 7-8, 1993(including Taylor Lecture No. 17) (1994)

16 Extremely-Low-Frequency Electromagnetic Fields: Issues inBiological Effects and Public Health, Proceedings of theThirtieth Annual Meeting held on April 6-7, 1994 (notpublished).

17 Environmental Dose Reconstruction and Risk Implications,Proceedings of the Thirty-first Annual Meeting held on April12-13, 1995 (including Taylor Lecture No. 19) (1996)

18 Implications of New Data on Radiation Cancer Risk,Proceedings of the Thirty-second Annual Meeting held onApril 3-4, 1996 (including Taylor Lecture No. 20) (1997)

19 The Effects of Pre- and Postconception Exposure to Radiation,Proceedings of the Thirty-third Annual Meeting held on April2-3, 1997, Teratology 59, 181–317 (1999)


20 Cosmic Radiation Exposure of Airline Crews, Passengers andAstronauts, Proceedings of the Thirty-fourth Annual Meetingheld on April 1-2, 1998, Health Phys. 79, 466–613 (2000)

21 Radiation Protection in Medicine: Contemporary Issues,Proceedings of the Thirty-fifth Annual Meeting held on April7-8, 1999 (including Taylor Lecture No. 23) (1999)

22 Ionizing Radiation Science and Protection in the 21st Century,Proceedings of the Thirty-sixth Annual Meeting held on April5-6, 2000, Health Phys. 80, 317–402 (2001)

23 Fallout from Atmospheric Nuclear Tests—Impact on Science andSociety, Proceedings of the Thirty-seventh Annual Meetingheld on April 4-5, 2001, Health Phys. 82, 573–748 (2002)

Lauriston S. Taylor Lectures

No. Title

1 The Squares of the Natural Numbers in Radiation Protection byHerbert M. Parker (1977)

2 Why be Quantitative about Radiation Risk Estimates? by SirEdward Pochin (1978)

3 Radiation Protection—Concepts and Trade Offs by Hymer L.Friedell (1979) [Available also in Perceptions of Risk, seeabove]

4 From ‘‘Quantity of Radiation’’ and ‘‘Dose’’ to ‘‘Exposure’’ and‘‘Absorbed Dose’’—An Historical Review by Harold O. Wyckoff(1980)

5 How Well Can We Assess Genetic Risk? Not Very by James F.Crow (1981) [Available also in Critical Issues in SettingRadiation Dose Limits, see above]

6 Ethics, Trade-offs and Medical Radiation by Eugene L. Saenger(1982) [Available also in Radiation Protection and NewMedical Diagnostic Approaches, see above]

7 The Human Environment—Past, Present and Future by MerrilEisenbud (1983) [Available also in EnvironmentalRadioactivity, see above]

8 Limitation and Assessment in Radiation Protection by Harald H.Rossi (1984) [Available also in Some Issues Important inDeveloping Basic Radiation Protection Recommendations, seeabove]

9 Truth (and Beauty) in Radiation Measurement by John H.Harley (1985) [Available also in Radioactive Waste, see above]

10 Biological Effects of Non-ionizing Radiations: Cellular Propertiesand Interactions by Herman P. Schwan (1987) [Available alsoin Nonionizing Electromagnetic Radiations and Ultrasound,see above]


11 How to be Quantitative about Radiation Risk Estimates bySeymour Jablon (1988) [Available also in New Dosimetry atHiroshima and Nagasaki and its Implications for RiskEstimates, see above]

12 How Safe is Safe Enough? by Bo Lindell (1988) [Available alsoin Radon, see above]

13 Radiobiology and Radiation Protection: The Past Century andProspects for the Future by Arthur C. Upton (1989) [Availablealso in Radiation Protection Today, see above]

14 Radiation Protection and the Internal Emitter Saga by J. NewellStannard (1990) [Available also in Health and EcologicalImplications of Radioactively Contaminated Environments, seeabove]

15 When is a Dose Not a Dose? by Victor P. Bond (1992) [Availablealso in Genes, Cancer and Radiation Protection, see above]

16 Dose and Risk in Diagnostic Radiology: How Big? How Little? byEdward W. Webster (1992)[Available also in RadiationProtection in Medicine, see above]

17 Science, Radiation Protection and the NCRP by Warren K.Sinclair (1993)[Available also in Radiation Science andSocietal Decision Making, see above]

18 Mice, Myths and Men by R.J. Michael Fry (1995)19 Certainty and Uncertainty in Radiation Research by Albrecht M.

Kellerer. Health Phys. 69, 446–453 (1995).20 70 Years of Radiation Genetics: Fruit Flies, Mice and Humans

by Seymour Abrahamson. Health Phys. 71, 624–633 (1996).21 Radionuclides in the Body: Meeting the Challenge by William J.

Bair. Health Phys. 73, 423–432 (1997).22 From Chimney Sweeps to Astronauts: Cancer Risks in the Work

Place by Eric J. Hall. Health Phys. 75, 357–366 (1998).23 Back to Background: Natural Radiation and Radioactivity

Exposed by Naomi H. Harley. Health Phys. 79, 121–128(2000).

24 Administered Radioactivity: Unde Venimus Quoque Imus byS. James Adelstein. Health Phys. 80, 317–324 (2001).

25 Assuring the Safety of Medical Diagnostic Ultrasound by WesleyL. Nyborg. Health Phys. 82, 578–587 (2002)

Symposium Proceedings

No. Title

1 The Control of Exposure of the Public to Ionizing Radiation inthe Event of Accident or Attack, Proceedings of a Symposiumheld April 27-29, 1981 (1982)

2 Radioactive and Mixed Waste—Risk as a Basis for WasteClassification, Proceedings of a Symposium held November 9,1994 (1995)


3 Acceptability of Risk from Radiation—Application to HumanSpace Flight, Proceedings of a Symposium held May 29, 1996(1997)

4 21st Century Biodosimetry: Quantifying the Past and Predictingthe Future, Proceedings of a Symposium held on February 22,2001, Radiat. Prot. Dosim. 97, No. 1, 7–80 (2001)

NCRP Statements

No. Title

1 ‘‘Blood Counts, Statement of the National Committee onRadiation Protection,’’ Radiology 63, 428 (1954)

2 ‘‘Statements on Maximum Permissible Dose from TelevisionReceivers and Maximum Permissible Dose to the Skin of theWhole Body,’’ Am. J. Roentgenol., Radium Ther. and Nucl.Med. 84, 152 (1960) and Radiology 75, 122 (1960)

3 X-Ray Protection Standards for Home Television Receivers,Interim Statement of the National Council on RadiationProtection and Measurements (1968)

4 Specification of Units of Natural Uranium and NaturalThorium, Statement of the National Council on RadiationProtection and Measurements (1973)

5 NCRP Statement on Dose Limit for Neutrons (1980)6 Control of Air Emissions of Radionuclides (1984)7 The Probability That a Particular Malignancy May Have Been

Caused by a Specified Irradiation (1992)8 The Application of ALARA for Occupational Exposures (1999)9 Extension of the Skin Dose Limit for Hot Particles to Other

External Sources of Skin Irradiation (2001)

Other DocumentsThe following documents of the NCRP were published outside of the

NCRP report, commentary and statement series:

Somatic Radiation Dose for the General Population, Report ofthe Ad Hoc Committee of the National Council on RadiationProtection and Measurements, 6 May 1959, Science, February19, 1960, Vol. 131, No. 3399, pages 482-486

Dose Effect Modifying Factors In Radiation Protection, Report ofSubcommittee M-4 (Relative Biological Effectiveness) of theNational Council on Radiation Protection and Measurements,Report BNL 50073 (T-471) (1967) Brookhaven NationalLaboratory (National Technical Information ServiceSpringfield, Virginia)

Index

Absorbed dose, radiation 360use in risk assessment

129–130, 138, 140use in waste classification

166–167Accelerator-produced waste

see Radioactive materials,definitions, NARM; Wasteclassification system, existingradioactive, NARM waste

ALARA (as low as reasonablyachievable) 33–36, 133,147–150, 152, 155–159,163–164, 196–197, 231,236–237, 268–269, 315, 355,361

Allowable dose or risk 30–33,160, 246, 266, 271, 275–276,278, 318, 321, 324–325acceptable 3, 28–29, 33–39,

41–42, 48, 55, 147–150, 153,155–159, 164, 268–269,272–273, 277–280, 295–296,309, 312–313, 315, 318–320,355–356, 358

background risks, use inestablishing 40, 42, 145–146,278–280, 283

barely tolerablesee acceptable

carcinogenic responsessee stochastic responses

de minimissee negligible

deterministic responses 34, 36,39–42, 276–278, 312–313

goals 34, 150–153, 163,268–269

limits 33, 147–149, 155–157,163, 268–269

420

negligible 3, 28, 33–41, 48, 55,147, 149–150, 153, 155–159,163–164, 268–269, 272–273,276–279, 295–296, 309,312–313, 315, 318–320,355–356, 364, 370

nomenclature issues 29, 33–36,155–159, 163–164, 252–253,268, 309, 355–358

noncarcinogenic responsessee deterministic responses

stochastic responses 33–34, 36,39–42, 278–280, 312–313

trivialsee negligible

see also ALARA; Referencedose; Risk managementparadigms; Unacceptabledose or risk

Animal studiesprotocols for 81–85use in hazard identification and

dose-response assessment 47,78–87, 99–102, 104–107, 122,124, 126–129, 261, 263–265,312

see also Dose-responseassessment, basis

Asbestos 22, 211, 233, 241Atomic Energy Act (AEA) 8,

23–25, 28, 33–34, 36, 53–54,146, 150, 156, 171–172, 182,187–189, 191–192, 194–195,212, 220–223, 229–235,241–242, 250, 252, 273, 281,316, 361

Below regulatory concern (BRC)196–199, 202–204, 209, 327,361

INDEX / 421

Benchmark dosedefinition and determination

47, 109–111, 362use in cancer risk assessment

115–117use in estimating threshold

doses 47–48, 142, 264–265,312, 320

use in exempt wasteclassification 37, 276–277

use in low-hazard wasteclassification 41, 277

see also Waste classificationsystem, NCRP

Beneficial uses of waste 2, 14–15,21, 27, 38, 52, 66, 166,198–200, 208, 215–216, 247,356

Biohazardous waste 57, 213Biokinetic models

see Pharmacokinetic modelsBiologically-based models of

cancer 103, 112, 119–121Byproduct material

see Radioactive materials,definitions

Cancerlethality fractions 135–137,

259–262models

see Biologically-based modelsof cancer

Carbon tetrachloride 118–119,338

CERCLA (ComprehensiveEnvironmental Response,Compensation, and LiabilityAct) 33–35, 152–153, 172, 214,281, 313, 363

Characteristically hazardouswastesee Waste classification system,

existing chemicalChromium, toxicity 81Classification of existing wastes

see Waste classification system,NCRP, implications

Clean Air Act (CAA) 152, 195,215–216

Clean Water Act (CWA) 211–212,215–216, 221, 241

Coal-burning power plant waste21, 215–216

Committed dose, radionuclides138, 144, 363

Consumer products 14, 197–198,209

Corrosive waste 363see Waste classification system,

existing chemical,characteristically hazardouswaste

Critical effect 78–80, 103–106,112see also Hazard identification,

chemicalCritical organ, chemicals 50, 246,

289, 291, 338see also Critical effect

Critical responsesee Critical effect

Decommissioning waste 19, 206,208

Deep-well injection 217Delisting

see Waste classification system,existing chemical, listedwaste

de manifestis dose or risksee Unacceptable dose or risk

Derived-from rulesee Waste classification system,

existing chemical, listedwaste

Deterministic responses,definitionsee Dose-response assessment

Dioxins 22, 118–119, 127, 129,211, 218, 225, 233, 241, 350

Disposal technologiessee Waste disposal technologies

Department of Energy (U.S.)Organization Act 172

422 / INDEX

Dosemeanings for chemicals and

radiation 29, 88, 365utility for radiation risk

assessment 129–131,138–140

Dose and dose-rate effectivenessfactor 133–134

Dose-response assessmentbasis for 99, 102carcinogens

see stochastic responsescharacterization 122–123comparison for chemicals and

radionuclides, deterministicresponses 29, 141–142,161–162, 311–312see also use in risk

assessment andmanagement

comparison for chemicals andradionuclides, stochasticresponses 29, 44–46, 114,142–145, 162–163, 237–239,310–311, 320see also use in risk

assessment andmanagement

confidence level in 109database 105, 132deficiencies, chemicals 123–129definition 88, 365deterministic responses,

chemicals 102–111deterministic responses,

definition 74, 318, 364deterministic responses,

estimation of thresholds110–111, 131, 264–265

deterministic responses,radionuclides 131

epidemiological studies 75–76,78–79, 81, 85–86, 99–100,102, 114, 127–128, 131–134,239

extrapolation models 99–102,122, 124–125

linearized-multistage model 45,120–122, 125, 265, 310

lower confidence limit or boundsee upper or lower confidence

limits or boundsmaximum likelihood estimates

(MLE) 114–115, 122,126–127, 132–133, 141–142,145, 265–266, 310–311, 320,369

modeling, use of 109–111,120–122, 126–127

statistical models 113–115stochastic responses, chemicals

111–122, 265–266stochastic responses, definition

74, 318, 374stochastic responses,

radionuclides 131–134,265–266

thresholdsee deterministic responses,

estimation of thresholdsuncertainties 123–125,

133–134, 141uncertainties, comparison for

radionuclides and chemicals134, 239

upper or lower confidence limitsor bounds 110, 113–115, 122,126–127, 141–142, 145,161–162, 265–266, 310–311,320, 369, 375

use in risk assessment andmanagement 75–77,145–146, 161–163

Dose-response relationshipsee Dose-response assessment

Effective dose 49, 138–140,143–144, 235, 287, 297, 365

Emergency Response andCommunity Right-to-Know Act21, 214, 366

Energy Reorganization Act 172Environmental standards

air 152, 195

INDEX / 423

drinking water 126, 151–152,154–155, 157, 213–214, 232,234, 338

remediation 151–153, 198Epidemiological studies

see Dose-response assessmentEquivalent dose 130–131,

137–140, 235, 366Exemptions, existing

chemical waste 21–22,215–216, 246–247

radioactive waste 11, 14,168–169, 171, 195, 197–200,247

source and byproduct materials14, 197–200, 247, 302,327–328, 362, 374

see also Waste classificationsystem, existing chemical;Waste classification system,existing radioactive

Exempt waste 366see Below regulatory concern;

Exemptions, existing; IAEA;Waste classification system,NCRP

Exposure assessment 88–92applied to waste classification

96–98see also Inadvertent intrusion,

general; Waste classificationsystem, NCRP, exposurescenarios

Extremely hazardous waste 21,214, 217, 245, 305, 351, 366

Federal Facility Compliance Act220, 226–228, 249

Geologic repository 367see Waste disposal technologies

Greater confinement disposal 367see Waste disposal technologies

Hazard identification, chemical76–87, 367deterministic responses 78–82stochastic responses 82–87

weight-of-evidence judgments78, 82, 84, 86–87

Hazard identification, radiation76

Hazardous and Solid WasteAmendments 367see RCRA

Hazardous, definition 6, 57, 367Hazardous material life cycle

58–59Hazardous waste

chemical waste identification87–88

definition 20–21, 212–214, 367see also Waste classification

system, existing chemical;Waste classification system,existing radioactive

Hazardous waste sites, screeningor ranking 7, 66–67, 98

Health effects, typessee Measure of response

Heat generating wastes 17, 19,69, 172, 176, 179–180, 201,204, 206–208, 210, 306–307,353

Heavy metals, waste containing20–21, 24–25, 51–52, 307–308,334, 336–342, 344–348, 350

High-hazard wastesee Waste classification system,

NCRPHigh-level waste 367

see IAEA; Waste classificationsystem, existing radioactive

IAEA (International AtomicEnergy Agency),recommendations onradioactive waste classificationcomparison with U.S. system

17, 20, 209–211early recommendations

203–205exempt waste 17–18, 20, 39,

205–206, 278exemption principles 34, 149,

208–211

424 / INDEX

long-lived wastesee low- and intermediate-

level wastelow- and intermediate-level

waste 17–19, 205–207,306–307

high-level waste 17, 19,205–208, 307

relationship to disposaltechnologies 17, 205, 207–208

short-lived wastesee low- and intermediate-

level wastewaste containing long-lived,

naturally occurringradionuclides 19–20, 206, 208

ICRP (International Commissionon Radiological Protection),recommendations on radiationprotection 135–140

Ignitable waste 368see Waste classification system,


Immobilization of hazardouswaste 20–21, 59, 191–192, 194,215, 218

Inadvertent intrusion, general32–33, 96–98see also Waste classification

system, NCRP, exposurescenarios

Incidental waste 9–11, 168–169,177, 180

Incineration of hazardous waste20, 59, 215, 217–218, 225,245–246, 350–351

Industrial waste managementsee Waste disposal technologies,

municipal landfillInstitutional control 368

chemical waste facilities 21–23,25–26, 41–43, 218–219, 246,249, 273, 281–282, 298,303–304, 314, 316, 349–351

radioactive waste facilities 12,23, 41–43, 190, 207, 231, 273,

279–280, 282, 284, 298,303–304, 314, 316, 350

role in waste classification12–13, 32, 41–43, 51–52,96–97, 190, 207, 267, 273,279–282, 298–299, 302–303,313–314, 320, 329, 331–336,345–347, 350, 357–358

Intolerable dose or risksee Unacceptable dose or risk

Land disposalsee Waste disposal technologies,

near-surface disposalLand disposal restrictions,

chemical waste 218, 223,225–226, 229–232

Lethality fractionssee Cancer

Life cyclesee Hazardous material life

cycleLinearized-multistage dose-

response modelsee Dose-response assessment

Liquid scintillation materials 14,197, 227

Liquid wastes 69, 172–178,180–181, 193, 204, 212–213,217, 220, 227, 230, 252

Listed hazardous wastesee Waste classification system,

existing chemicalLOAEL (lowest-observed-adverse-

effects level)definition and determination

34, 103–105, 109–112, 369use in estimating threshold

doses 312use in health protection 34,

106–108, 112, 269, 345use in waste classification

system, NCRP 39, 277Low-hazard waste

see Waste classification system,NCRP

Low-Level Radioactive WastePolicy Act 182, 187, 189

INDEX / 425

Low-Level Radioactive WastePolicy Amendments Act187–190, 222

Low-level wastesee Waste classification system,

existing radioactive

Margin of exposure analysis116–117

Marine Protection, Research andSanctuaries Act 177

Maximum tolerated dose 84, 116Measure of response

fatalities 29, 44, 55, 73, 94,134–135, 141–143, 145,162–163, 258–263, 310, 355

incidence 29, 44, 55, 73–74,82–84, 94, 101–102, 115, 129,133–135, 137, 141–143, 145,161–163, 238–239, 258–263,310–311, 335–338, 355, 364

total detriment 135–140, 143,258, 260–262

Mill tailingssee Waste classification system,

existing radioactiveMining wastes 1, 14, 19, 43, 52,

191, 194–196, 205–206, 208,212, 282, 303, 307, 335, 349

Mixed wastecompliance difficulties 25, 53,

65, 220–221, 225–229,233–235, 242, 249–250, 354

definition 24, 165–166, 212,219–224, 233, 241, 369

dual regulation, consequences24, 220–235, 249–250

exemption from dual regulation24, 229–232, 250

National Capacity Variance225–226

regulatory guidance 224–225sources and amounts 220,

227–228state regulation 222–223, 233treatment standards 24, 225,

229Mixture rule

see Waste classification system,existing chemical, listedwaste

Modifying factorsee Reference dose; Waste

classification system, NCRP,Risk index

Municipal waste managementsee Waste disposal technologies,

municipal landfill

NARM (naturally occurring andaccelerator-producedradioactive material)see Radioactive materials,

definitions; Wasteclassification system, existingradioactive

National Energy Policy Act 28,199, 270, 315

National Security and MilitaryApplications of Nuclear EnergyAuthorization Act 38, 182,185–186

Natural background riskschemicals 40, 42, 145–146,

278–280radiation 40, 42, 131, 133,

145–146, 199, 237, 278–280use in classifying waste 40, 42,

278–280Naturally occurring hazardous

substanceschemicals 1, 25, 31, 40, 43, 52,

145–146, 216, 278–279, 282,334

radionuclides 1, 19–20, 25, 31,40, 43, 51, 145, 149, 171, 174,196, 198–199, 205–206,208–209, 216, 245, 279, 282,334

Naturally-occurringradionuclides, waste containingsee NARM, NORM

NCRP recommendationsapplication to waste

classification 37, 39, 41–42,237, 278–279

426 / INDEX

comparative carcinogenicity ofradiation and chemicals237–239

negligible individual dose 37,39, 237, 278

radiation protection 33, 41–42,235–237, 279

see also Waste classificationsystem, NCRP

Near-surface disposal 368, 370see Waste disposal technologies

NOAEL (no-observed-adverse-effects level)definition and determination

34, 103–105, 109–112, 370use in estimating threshold

doses 47–48, 104–106,264–265, 312

use in health protection 34,106–108, 112, 142, 269, 345

use in waste classificationsystem, NCRP 39, 277

Noncritical effect 104–105NORM (naturally occurring

radioactive material wastes)see Radioactive materials,

definitions; Wasteclassification system, existingradioactive

Nuclear fuel cycle 8, 167, 370Nuclear Waste Policy Act

(NWPA) 38, 177–182, 187, 189,200–202

PCBs (polychlorinated biphenyls)22, 211, 220, 233, 241, 370

Pharmacokinetic modelscompartment models 117, 131general 82–83, 126, 128, 141,

370physiologically-based models

110, 117–119, 131, 144Phosphogypsum 191, 195,

215–216Probability coefficient, stochastic

responses 44–46, 49, 99, 133,135–137, 139–140, 142–144,148, 236, 260–266, 276, 278,

287, 295, 310–311, 320, 337,344, 371see also Dose-response

assessment, stochasticresponses

Radiation protection standardssee ICRP; NCRP

recommendations, radiationprotection; Risk managementparadigms, stochasticresponses, radionuclides

Radiation weighting factor 130,132, 138–140, 371

Radioactive materials, definitionsbyproduct material 171, 362NARM 8, 170, 194, 370NORM 8, 170, 194–195, 370source material 171, 374special nuclear material 171,

374RCRA (Resource Conservation

and Recovery Act) 20–25, 28,33–34, 38–39, 41, 53, 152, 172,196–197, 211–224, 226–235,240–242, 245, 249–250, 252,270, 272–273, 278, 281, 316,339, 343, 346, 348–349

Reactive waste 371–372see Waste classification system,


Recycling and reusesee Beneficial uses of waste

Reference dose (RfD)definition and determination

34–36, 103–110, 142, 372modifying factor, U.S.

Environmental ProtectionAgency 106–109

uncertainty factor, U.S.Environmental ProtectionAgency 106–109

use in exempt wasteclassification 39–40, 277, 313,324–325, 337–338

INDEX / 427

use in health protection 34,105–108, 112, 142, 154–155,269, 337, 345

use in low-hazard wasteclassification 42, 277–278,313, 324, 337–338, 340, 342,344–345

Risk assessment, definition 75,373

Risk assessment processdescription 75–77site-specific 3, 5–6, 32, 38, 63,

69, 95–98, 160–161, 208, 244,267, 274, 301, 357

use in waste classification 63,95–99, 296–297

see also Hazard identification;Dose-response assessment;Exposure assessment; Riskcharacterization

Risk characterization 76, 92, 94,373

Risk, general considerationsbasis for waste classification 1,

6, 26, 28, 63–65, 72, 160,243–246, 256–258

definition 64, 73, 160, 373measures of 29, 44, 55, 65,

73–74, 246nomenclature issues

see Allowable dose or risk,nomenclature issues

see also Measure of response;Waste classification system,NCRP, development needs

Risk indexsee Waste classification system,

NCRPRisk management

ALARA, importance of 157,159, 164, 269see also Risk management

paradigms, stochasticresponses, reconciliation ofdifferences

relationship to risk assessment94–95, 145, 326

use in waste classification 31,46, 63, 67–68, 72, 77, 94–95,145, 160, 163, 239, 266,268–269, 295–296, 311, 321

see also Risk managementparadigms

Risk management paradigms,deterministic responseschemicals 34–35, 46, 141,

154–155, 276, 286, 288comparison of chemicals and

radionuclides 46–47,141–142, 146, 276

radionuclides 46, 131, 141, 276,286, 288

Risk management paradigms,stochastic responseschemicals 34–35, 150–154,

163–164, 268, 355–356comparison of chemicals and

radionuclides 29, 33, 35–36,150–152, 155–157, 163–164,253, 268–269, 355–356

organs accounted for 29, 44, 46,48–49, 101–102, 136–140,144, 162–163, 287, 297, 310,355see also Critical organ;

Effective doseradionuclides 33–34, 146–150,

163, 268, 272–273, 355reconciliation of differences 35,

157–159, 164, 309, 315

Safe Drinking Water Act 34, 146,151–152, 213

Safety factor approachsee Reference dose

Scrap metals, contaminated 2,343

Selenium, toxicity 70Sewage sludge 21, 211–212,

215–216, 241Shallow-land burial

see Waste disposal technologies,near-surface disposal

Single organ, chemicalssee Critical organ, chemicals

428 / INDEX

Slope factor 99, 335, 337–338,342, 344, 346–347, 373see also Probability coefficient,

stochastic responsesSolid waste, definition 24, 212,

221, 252Source constraints 148–149,

236–237see also Environmental

standardsSource material

see Radioactive materials,definitions

Special nuclear materialsee Radioactive materials,

definitionsStabilization of hazardous waste

see Immobilization ofhazardous waste

Stochastic responses, definitionsee Dose-response assessment

Structure-activity relationships78, 80, 82, 86, 141

Subclassification of waste classessee Waste classification system,

existing chemical; Wasteclassification system, existingradioactive; Wasteclassification system, general;Waste classification system,NCRP

Subseabed disposal 181Superfund

see CERCLA

Tissue weighting factor, radiation138–140, 143, 374

Toxic Substances Control Act(TSCA) 22, 172, 196, 211, 220,227–228, 232–233, 241, 375

Transuranic wastesee Waste classification system,

existing radioactive

Unacceptable dose or risk 33–36,148–150, 153, 155–159,163–164, 268–269, 309, 318,356, 358, 364

Uncertainty 375treatment in risk assessment

63, 94, 100–101, 103–111,113–115, 122–126, 133–134,141–142, 161–162, 263–264,266, 269, 271, 311–313,320–321, 345

types 94Uncertainty factor

see Reference dose (RfD),uncertainty factor, U.S.Environmental ProtectionAgency

Uranium Mill Tailings RadiationControl Act 191–192

Waste, definition 5, 57Waste classification, general

bases for waste classification62–63, 65

definition 5, 59, 376purpose 5, 60–62, 357see also Waste classification

system, generalWaste classification system,

alternative radioactivecharacteristics 240Kocher and Croff 200–202LeMone and Jacobi 203–204Smith and Cohen 202–203U.S. Nuclear Regulatory

Commission 200see also IAEA

Waste classification system,existing chemicalbases 1, 6, 54, 64–65, 216, 241,

245characteristically hazardous

waste 20–22, 213–218, 241,362–363, 368, 371–372

deficiencies 1–2, 4, 7, 25–26,64–66, 216, 241, 245–248,251–253

disposal requirements,relationship to wasteclassification 4, 21, 23, 218,241, 303, 305

exclusions 215–216, 241, 248

INDEX / 429

exemption 21, 53, 215, 219,247, 302, 328

hazardous waste, definition212–214, 241, 367

implications 23, 65–66, 214,216, 241, 245, 247, 249–253

industrial wastesee municipal waste

listed waste 21, 214–219,234–235, 241, 245, 247, 270,316, 364, 368–369

municipal waste 22, 38, 41,219, 272, 281

state programs 21, 216–217,241

subclassification 23, 214, 241,306

waste treatment, effect onclassification 20–21, 214–215

see also Extremely hazardouswaste; Land disposalrestrictions; Mixed waste;Waste classification system,general

Waste classification system,existing mixed wastesee Mixed waste

Waste classification system,existing radioactivebases 1, 6, 9, 13, 16, 54, 64–65,

173, 175–177, 240, 245, 251,253–254

comparison with IAEArecommendations 17, 20,209–211

deficiencies 1–2, 4, 7, 15–17,25–26, 64–66, 240, 245,247–248, 251–254, 314–315

disposal requirements,relationship to wasteclassification 4, 12–13,16–17, 175, 178–179,193–194, 210, 240

distinction between nuclearfuel-cycle and other wastes 8,28, 170–172, 240, 248, 270,314

exclusions 195

exemptions 11, 14, 168–169,196–200, 247, 270, 302,327–328, 366see also Below regulatory

concern; IAEAgreater-than-Class-C waste

see low-level waste, greater-than-Class-C

high-level waste, currentdefinition 10–11, 168–169,177–180, 367see also IAEA

high-level waste, disposalrequirements 12, 181–182,193–194

high-level waste, historicaldefinitions 172–174, 176

high-level waste, sources andproperties 9, 176–177,209–210

implications 9, 12–13, 180, 189,192–193, 209, 240, 247,251–254

incidental waste 9, 11, 169,177, 180, 343, 349

intermediate-level wastesee IAEA

internationalsee IAEA

low-level waste, currentdefinition 9–10, 65, 168,187–189, 369see also IAEA

low-level waste, disposalrequirements 12, 189–191,193–194

low-level waste, greater-than-Class-C 8, 12–13, 32, 52,190–191, 193, 201, 252, 304,306–307, 349

low-level waste, historicaldefinitions 172–175, 182–183

low-level waste, sources andproperties 187, 189, 209–210,307

low-level waste,subclassification 8, 13, 30–32,

430 / INDEX

53, 69, 170–171, 190, 245,251, 284, 306, 308, 352–353

mill tailings, definition 10, 168,191, 369see also IAEA

mill tailings, disposalrequirements 12, 43,191–192, 194, 282, 284,303–304, 335

mill tailings, properties 191,194, 284, 303–304, 306–307,334, 358

NARM waste 8, 11, 13–14, 168,170–172, 194–196, 240, 352see also NORM waste

NORM waste 8, 11, 13–14,170–171, 194–195see also NARM waste

reprocessing wastessee high-level waste; Liquid

wastessource-based definitions

see basesspent nuclear fuel, definition

10–11, 177–178see also high-level waste,

current definitionsummary 7–8, 10–11, 168–171transuranic waste, current

definition 10, 168, 183–185,375

transuranic waste, disposalrequirements 12, 174–175,185–187, 193–194

transuranic waste, historicaldefinition 174–175

transuranic waste, sources andproperties 184–185, 209

transuranic waste,subclassification 8, 13,170–171, 174, 184–185,306–307, 352–353

see also Mixed waste; Wasteclassification system, general

Waste classification system,generalcomparison of systems for

chemical and radioactivewastes 22–23

deficiencies 25–26, 64–66,243–255, 354

description 165–166desirable attributes 63,

243–255, 356disposal technologies assumed

61, 68generic disposal sites, focus on

63, 69, 77, 92, 160–161, 244subclassifications 68–69see also Waste classification,

general; Waste classificationsystem, existing chemical;Waste classification system,existing radioactive

Waste classification system,NCRPadvantages 1–2, 4, 55–56, 301,

359allowable dose or risk,

assumptions on 33, 272, 276,278, 312, 320see also Allowable dose or

riskallowable dose or risk,

acceptable for classifying low-hazard waste 3, 28, 41–44,48, 277–280, 312–313

allowable dose or risk,negligible for classifyingexempt waste 3, 28, 37,39–40, 44, 48, 276–279,312–313

applicability 7, 27, 66–67,69–70, 256, 258, 357

association with disposaltechnologies 1, 4, 26, 66, 68,256, 270, 272, 317, 356–357

assumptions 27–28, 258,263–264, 266, 268, 317

basic principles and framework1–3, 6, 26–27, 37–39, 54,63–64, 256–258, 270–274,317, 354, 356

benchmark dose, use inclassifying waste 37, 41,47–48, 276–277see also Benchmark dose

INDEX / 431

boundaries between wasteclasses, establishing 7, 28,270, 280, 295–297, 318–320,356–357

challenges in developing 28–29,354–356

classification of existing wastessee implications

conservative assumptions 40,45–47, 85, 98, 244, 266–267,277, 301, 311, 319–321, 357

development needs, dose-response assessment 55,309–312

development needs,establishing negligible andacceptable risks 55, 312–313

development needs, estimatinghealth impacts 55, 312–313

development needs, exposurescenarios 55, 313–314

development needs, legal andregulatory 314–316

development needs,nomenclature 309, 357–358see also Allowable dose or

risk, nomenclature issuesdevelopment needs,

reconciliation of riskmanagement paradigms 315

dose-response relationships,deterministic responses 28,47–48, 264–265, 286, 288,312, 320

dose-response relationships,maximum likelihoodestimates versus upperconfidence limits 45–46,265–266, 311, 320

dose-response relationships,stochastic responses 28,45–46, 265–266, 286–287,310–311

ecological impacts, neglect of69–70

example applications,assumptions 323–337,339–340, 342–347

example applications, chemicalwaste 346–347

example applications,commercial low-levelradioactive waste 332–333

example applications, U.S.Department of Energy low-level waste 328–332

example applications, electricarc furnace dust (mixedwaste) 336–347

example applications, exemptwastes 326–328

example applications, high-grade uranium ore residues335–336

example applications, uraniummill tailings 333–335

exempt waste, definition 2, 26,257, 272, 281, 317, 356

exempt waste, determination 3,37, 257, 272, 318–319,356–357

exempt waste, disposaltechnology 2, 26, 37, 40, 257,272, 281, 317, 356

exposure scenarios, general 3,32–33, 266–267, 274, 280,320, 357

exposure scenarios, inadvertentintrusion 32–33, 40–44,281–283see also Inadvertent

intrusion, generalexposure scenarios, site-specific

considerations 3, 267–268exposure scenarios, time period

for applying 43, 298–300high-hazard waste, definition 2,

26, 257, 274, 317, 356high-hazard waste,

determination 3, 43, 257,274, 318, 356–357

high-hazard waste, disposaltechnology 2, 26, 43–44, 257,274, 317, 356

implementation 295–300

432 / INDEX

implications, chemical wastes3–4, 51–52, 54, 301–305,308, 358

implications, exemption ofwaste 51, 53, 315, 348, 358

implications, laws andregulations 53–54

implications, mill tailings 43,51–52, 282, 302–304, 349,358

implications, mixed wastes 53,316, 351–352, 359

implications, NARM andNORM wastes 54, 302–303,314, 349, 352

implications, source-basedclassifications 54, 349

implications, subclassification69, 352–353, 357

legal and regulatoryimpediments 28, 269–270

low-hazard waste, definition 2,26, 257, 273, 317, 356

low-hazard waste,determination 3, 41, 273,318, 356–357

low-hazard waste, disposaltechnology 2, 26, 41–42, 257,273, 317, 356

measures of response inclassifying waste 44,262–263, 310–311see also Measure of response

nonexempt waste 2, 26–27, 38,50–51, 271–274, 284, 292,299–300, 314, 317, 356see also high-hazard waste;

low-hazard wasteRisk index, composite 48–51,

285–292Risk index, definition 30, 271,

275, 318, 356Risk index, deterministic

responses 31, 49–50, 275,288–291, 318

Risk index, modifying factor30–31, 40, 49–50, 271, 275,

283–284, 287–288, 290–291,293, 311

Risk index, stochastic responses31, 48–49, 275, 286–288, 318

Risk index, sum-of-fractionsrule 287, 290–291, 296, 318

Risk index, use in classifyingwaste 31, 44, 50–51,256–258, 291–295, 318–319,356–357

risk management paradigm35–37, 268–269

risk, use in classifying waste 1,3, 6, 26, 28–31, 63–64,256–258, 274–275

shortcomings 300–301subclassification 52–53, 303,

305–308summary 2–3, 38, 257,

317–321, 356–357Waste disposal, approaches and

regulationschemical waste 20–22,

214–215, 217–219, 241, 303,349–350see also Waste classification

system, existing chemical,characteristicallyhazardous waste; Wasteclassification system,existing chemical, listedwaste

comparisons of chemical andradioactive wastes 23–24,32–33, 248–250, 252,349–350

radioactive waste 12, 43,181–182, 185–187, 189–194,249–250, 282, 284, 303–304see also Waste classification

system, existing radioactiveWaste disposal technologies

geologic repository 10, 12–13,38–39, 68, 168–169,178–182, 190–191, 206–208,230–232, 256–257, 274, 283,324, 367

INDEX / 433

greater confinement disposal11, 38, 194, 304, 367

industrial landfillsee municipal landfill

municipal landfill 37–41, 68,196, 219, 257, 272–273, 281,324–325, 370

near-surface disposal 10–13,38–39, 68–69, 96–98, 161,168–169, 189–191, 217–219,223–224, 256–257, 266–267,273–274, 279–283, 313–314,324–325, 349–352, 368, 370

shallow-land burialsee near-surface disposal

Waste Isolation Pilot Plant 24,169, 185–186, 193, 223, 231,249–250

Waste Isolation Pilot Plant LandWithdrawal Act 183–186,231–232

Waste managementdefinition 59, 376description 58–59steps in 60–62

West Valley DemonstrationProject Act 177

Yucca Mountain repository 169,181–182, 187, 193

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