What’s in the Black Box Known as EMERGENCY DOSE ASSESSMENT

Whats in the Black Box Known as EMERGENCY DOSE ASSESSMENTStephen F. LaVieSr. Emergency Preparedness SpecialistNuclear Security and Incident ResponseDivision of Preparedness and [email protected]

EP Technology Course H-203

Nuclear Power SchoolBainbridge MDNuclear Power PrototypeWindsor Locks CTUSS Sam Rayburn SSBN635B


In this workshop, we are going to pry open the black box which is dose assessment. Well:

Discuss the role of dose assessments in an emergency responseIdentify the three steps in performing a dose assessmentLook at each of these three steps in some detailLook at the major methods, principles and assumptions involved in each stepConsider how these assumptions affect the usefulness and uncertainty of the dose assessment outputs

Abstract


In this workshop, my goal is not that you can leave here this morning and be able to write your own dose assessment program; nor that you become a dose assessment wizard on a particular dose assessment program.

Objective

Instead, I hope that by discussing the dose assessment internals, you will gain a fuller understanding of the capabilities, limitations, and uncertainties of dose assessment, and be a better consumer of the data our dose assessors prepare.

Non-Objectives


Purpose of Dose AssessmentThe NRC places strong emphasis on the use of plant condition assessments as a basis for emergency classifications and protective action recommendations (PARs)This emphasis was developed as a result of insights from severe accident assessments performed in the 1980s. These insights showed that:Timely protective actions, preferably prior to the start of the release, were necessary for protecting the publicData necessary for meaningful dose assessments may not be readily available and that such assessments could be uncertainAs such, dose assessments could not fully support anticipatory decision-making associated with initial protective actions


Purpose of Dose Assessment (Cont)That said, offsite dose assessment is conducted during emergency response to assess the radiological impact of accidental releases of radionuclides in order to provide appropriate protective actions for the workers and the publicFor rapidly evolving events, verify adequacy of plant condition-based initial PARsFor slowly evolving, less severe events, provide dose input to PAR decisionProvide dose input for emergency classification decisions for monitored releasesProvide a basis for extending or upgrading a PAREstablish priorities for field monitoringProvide a basis for comparing consequences of different plant response options (e.g., early CNMT venting as opposed to late CNMT failure)


Purpose of Dose Assessment (Cont)Dose assessment results can be used to generate PARs in cases for which the offsite consequences cannot be readily assessed on the basis of plant conditions, for example, events involving spent fuel, radioactive waste storage, etc.Inform offsite emergency worker turnback criteria (e.g., pocket dosimeter vs TEDE correlationsInform PAR decisions for personnel onsite (e.g., outside repair teams, security posts) or at emergency response facilitiesNRC considers the ability to perform dose assessments to be one of the four risk-significant planning standard functions in the reactor oversight program.


Steps in Dose Assessment


1.Source Term


Source TermThe magnitude and mix of radionuclides available for release to the environment, as well as their chemical and physical form, and the timing of their releaseThe release to the environment is not the source term, but rather is the result of the source term being acted upon by various transport and mitigative features and phenomena:Hold-up in the containmentPlateout and deposition within plant systemsFiltration systemsRadioactive decayCrude thumb rule: 0.5 curies per watt of reactor thermal power at time of shutdown


Release Mitigation Features


Source Term InventoryHundreds of possible radionuclides; 30-60 of which are significant to accident dose; two major groups:Fission products Iodines contribute to thyroid dose.Noble gases (Kr, Xe) contribute to whole body submersion or immersion dose from cloud shine.Others (Alkali Metals, Tellurium, Barium, Noble Metals, Lanthanides, Cerium) are released as aerosols that contribute to ground deposition / ingestion doses.Activation products Due to irradiation of reactor structural components or RCS impurities.Co, Fe, Mg, Zr, etc.


Source Term Inventory(Typical)VOLATILITY

Nuclide GroupInventory, CiAccident Release FractionNoble Gases (Xenon, Krypton)6.4E+81.0Halogens (Iodine, Bromine)8.2E+80.6-0.8Alkali Metals (Cesium, Rubidium)5.6E+70.6-0.8Tellurium Group (Te, Sb, Se, Ba, Sr)9.7E+80.3Noble metals (Ru, Rh, Pd, Mo, Tc, Co)8.2E+80.005Cerium (Ce, Pu, Np)2.7E+90.0006Lanthanides (La, Zr, Nd, Eu, Nb, Pm, Pr, Sm, Y, Cm, Am)1.6E+90.0005Inventory based on 3500 MWt with 60,000 MWD/MTU burnup; Release fractions from NUREG-1465 (all release phases)


Reactor Radionuclide Inventories(Typical)


Source Term StepsEstimate the inventory of radioactive material available for release

Estimate the fraction of the inventory released from the primary fission product barriers

Estimate the fraction removed on the way to the environment (e.g., filters, sprays)

Estimate the amount of radioactive material released to the environment


Radiation Monitors


Radiation Monitor CalibrationA radiation detector generates a electronic signal that is proportional to the energy deposited or the rate at which the radiation enters the detector.During manufacture, the vendor performs a primary calibration that determines the response of the detector to particular radiation sources, chosen to be representative of the radionuclides expected to be monitored, often in terms of uCi/cc/cpm. This information is provided to the purchaser.The response of the detector to radiation is dependent on the type of radiation and the energy of its emissions; the response is seldom linear and the uCi/cc/cpm value varies from radionuclide to radionuclide.As such, the response of the detector depends on the radionuclide mix in the release stream.Newer radiation monitor systems display results in terms of uCi/cc or uCi/sec; this indication is accurate only if the radionuclides in the release stream are comparable to the radionuclides used in calibration and calculation of the engineering unit conversion that converts cpm to the activity units.


Release Estimation Methods #1Release stream radiation monitor readings Continuously monitor the radioactivity release to the environment; indicate in the control roomNoble gas, iodine, particulate channelsIndividual isotope data generally not availableProvides immediate direct measurement of the release to the environment, provided:The release to the environment is monitoredThe release is ongoing.Sources used for calibration may not be representative of the radionuclides in a particular release stream during an accident


Release Estimation Methods #2Process radiation monitor readings Continuously monitor the radioactivity contained in plant systems; indicate in the control roomNoble gas, iodine, particulate channelsIndividual isotope data generally not availableProvides immediate direct measurement of radioactive contamination of plant systems--but not the release to the environmentIndications can be used to project the radioactive inventory available for release to the environmentRelease does not need be ongoingSources used for calibration and development of correlations may not be representative of the radionuclides in a particular release stream during an accident


Release Estimation Methods #3Area radiation monitor readings Continuously monitor the ambient dose rate at various plant locations; indicate in the control room and locallyPrimarily a radiation worker occupational dose controlContainment high range area radiation monitorsDoes not measure the release to the environmentIndications can be used to project the radioactive inventory available for release to the environmentRelease does not need be ongoingSources used for calibration and development of correlations may not be representative of the radionuclides in a particular release stream during an accident


Release Estimation Methods #4Release stream sample analysis Most effluent release streams have provision for manual or automatic sampling; plants required to have arrangements for analyzing high activity samplesSample analysis results provide direct measurement of the isotopic release to the environment, provided:The release to the environment can be sampledThe release is ongoingIsotopic results possible


Release Estimation Methods #5Backcalculation from Field Measurements All licensees have capability of dispatching field survey teams into the environment to obtain dose rate and airborne activity measurementsGiven available field measurements, one can backcalculate an estimate of the release rate from the plant; provided:The release is ongoingThe location of the field team with relation to the plume centerline is knownThe atmospheric dispersion to the field team location is knownAlthough the field measurements may be direct, the backcalculated release rate is highly uncertain


Release Estimation Methods #6Safety Analysis Report Applicants for licensees perform a series of design basis accident analyses as part of licensing.Highly stylized analyses; may not be representative of an actual eventShould not be used directly; may be overconservativeData from these analyses can inform source term decisionsSevere accident analysesAll licensees have performed probabilistic risk analyses (PRA) to assess core damage frequency and large early release fractions (level I PRA). Some licensees have also performed analyses of source term magnitude (level II PRA) and dose consequences (level III PRA)Accidents addressed in PRA may not be representative of accident at handPRA analyses are best estimateData from these analyses can inform source term decisions


Release Estimation Methods #7Assessment using accident assessment tools These tools provide a flexible capability to model diverse accident conditionsConsiderable modeling time is requiredExtensive data needed to support calculation; much of which may not be readily available during the early phase of an accidentNot deemed feasiblePre-calculated assumptions from severe accident insightsRASCAL and the Response Technical Manual (RTM) contain a method based on insights of severe accident analysesThere is a small set of accident conditions that dominate any accident sequenceThere are parameters that can characterize these dominant sequencesThese parameters can be recognized or characterized during an accident


Release DurationProtective action guides are expressed as projected avoided doseA good estimate of the release duration is critical to proper protective action decisionsRelease durations can be difficult to estimateRequires the active participation of the technical support center (TSC) staff as well as the dose assessors.Default release durations (e.g., use four hours) should be used only as the last resortAlthough we want to issue an adequate PAR, we also need to avoid overconservative PARsOverconservative PARs can place the public at unnecessary riska non-conservative situationShort release durations (e.g., 1 hour with dose assessments repeated every hour) could result in no PAR when one would be indicated when a more appropriate release duration was used


Welcome to the workshop session, named:

Whats in the Black Box Known as Emergency Dose Assessment?

This is a topic I could discuss for long periods of time. We dont have the luxury of time. We have about

My name is Steve LaVie, I have been involved in nuclear power and, in particular, dose assessment for many years.I got my start in the nuclear industry during my service in the Naval Nuclear Power program

I served on a fleet ballistic missile submarine as the lead Engineering Laboratory Technicianthe civilian worlds radiation control and chemistry technicians.

I also qualified as a Engineering Watch Supervisorthe civilian worlds shift foreman.So now that you know who I am, why are we here, and what are we going to be doing.Discuss the role of dose assessments in an emergency responseIdentify the three steps in performing a dose assessment source term, dispersion, dose calculationLook at each of these three steps in some detailLook at the major methods, principles and assumptions involved in each stepConsider how these assumptions affect the usefulness and uncertainty of the dose assessment outputs

I hope you all saw the abstract for this session. My presentation will likely be different from other dose assessment presentations you may have attended.

It is NOT my objective to turn you into a dose assessment wizard or to teach you how to write your own dose projection software

It is my objective, that by discussing what goes on in the black box, that you can gain a fuller understanding of the capabilities, limitations, and uncertainties involvedto make you a better consumer of the data your dose assessors provide

A Caveat: My presentation today is a technical rather than a regulatory presentation. Please do not interpret what I say as inflexible requirements that must be met. The NRC has published little guidance on dose assessmentmy presentation is not intended to fill that gap.

Any specific reference to a particular dose assessment program or methodology should not be viewed as a recommendation on the part of the NRC.

Some of you may be aware of the NRCs position on basing protective action recommendations on plant condition assessments. Where did this position come from?

The NRC places strong emphasis on the use of plant condition assessments as a basis for emergency classifications and protective action recommendations (PARs)This emphasis was developed as a result of insights from severe accident assessments performed in the 1980s. These insights showed that:Timely protective actions, preferably prior to the start of the release, were necessary for protecting the publicData necessary for meaningful dose assessments may not be readily available and that such assessments could be uncertainAs such, dose assessments could not fully support anticipatory decision-making associated with initial protective actions

Given that, why are we even discussing dose assessment?Offsite dose assessment is conducted during emergency response to assess the radiological impact of accidental releases of radionuclides in order to provide appropriate protective actions for the workers and the public.

For rapidly evolving events, verify adequacy of plant condition-based initial PARs.For slowly evolving, less severe events, provide dose input to PAR decision.Provide dose input for emergency classification decisions for monitored releases.Provide a basis for extending or upgrading a PAR.Establish priorities for field monitoring.Provide a basis for comparing consequences of different plant response options (e.g., early CNMT venting as opposed to late CNMT failure)

Dose assessment results can be used to generate PARs in cases for which the offsite consequences cannot be readily assessed on the basis of plant conditions, for example, events involving spent fuel, radioactive waste storage, etc.Inform offsite emergency worker turnback criteria (e.g., pocket dosimeter vs TEDE correlationsInform PAR decisions for personnel onsite (e.g., outside repair teams, security posts) or at emergency response facilitiesNRC considers the ability to perform dose assessments to be one of the four risk-significant planning standard functions in the reactor oversight program.

Accordingly, the NRC places oversight emphasis on dose assessment capabilities and performance during inspections and exercises. Licensees track their performance in performing dose assessments during drills and exercises as one of the emergency preparedness performance indicator metrics.

Three major steps in Dose Assessment:

First, you need to know whats being releasethe source term

Second, you need to know the effectiveness of atmospheric dispersion and the atmospheric transportcall it dispersion

Third, you need to convert the concentration at some downwind location into dosethe dose calculation.First, lets look at source termWhat is a Source term?The magnitude and mix of radionuclides available for release to the environment, as well as their chemical and physical form, and the timing of their releaseThe release to the environment is not the source term, but rather is the result of the source term being acted upon by various transport and mitigative features and phenomena:

Hold-up in the containmentPlateout and deposition within plant systemsFiltration systemsRadioactive decay

Crude thumb rule: about 0.5 curies per watt of reactor thermal power at time of shutdown

Lets take a quick look at some of the release mitigation features. This schematic identifies some of the major mechanisms and phenomena that act to reduce the release to the environment.

Containment sprays spray down the CNMT atmosphere and scrub out aerosol radioactive materials such as halogens. To some degree, fire suppression systems can perform the same function.

Filtration is the most common fission product release mitigation. Filters are located either on duct work to filter a process stream, or in recirculation systems to clean up a volume (such as the CNMT).

Larger aerosol particles will settle or deposit out on horizontal surfaces

Material in process streams (RCS, main steam lines in BWRs) can plateout on pipe walls.

Isolating a volume can minimize releases by holding up the material to allow for decay of other mechanisms to work. The majority of CNMT penetrations automatically isolate on a loss of coolant accident.The radionuclides that are of interest to emergency dose assessment can be categorized into two groups:Fission productsActivation productsFission products Iodines contribute to thyroid dose.Noble gases (Kr, Xe) contribute to whole body submersion or immersion dose from cloud shine.Others (Alkali Metals, Tellurium, Barium, Noble Metals, Lanthanides, Cerium) are released as aerosols that contribute to ground deposition / ingestion doses.Activation products Due to irradiation of reactor structural components or RCS impurities.Co, Fe, Mg, Zr, etc.

This is a table of the expected core inventory from a large reactor at the end of core life.

This radionuclides are broken down by groups, based somewhat on the volatility of the nuclides in that group.

The right hand column tabulates the fraction of the core inventory that is expected to be released from the fuel to the containment following a severe accident

.The key concepts to be taken away from this table are:

The reactor fuel in the core contains the greatest fission product inventory.

Other plant components have a much lower inventory of fission products.

Previous guidance has suggested that 20% of the fuel must be breeched to exceed PAGs.

The typical steps:

Estimate the inventory of radioactive material present

Estimate the fraction of the inventory released from the primary fission product barrier

Estimate the fraction removed on the way to the environment (e.g., filters, sprays)

Estimate the amount of radioactive material released to the environment

We are going to discuss radiation monitors in the next several slides.

Nuclear power plants have numerous radiation monitors installed within the planta typical reactor may have 100-150 monitor channels.

These monitors can be broken down into area radiation monitors, process monitors, and high range monitors.

Process monitors typically monitor the radioactive contamination being carried in process streams --- some perform control functions

An effluent monitor is a process monitor that monitors a liquid or gaseous release just before it enters the environment.

Following TMI, licensees installed high range monitors in several plant locations.A radiation detector generates a electronic signal that is proportional to the energy deposited or the rate at which the radiation enters the detector.

During manufacture, the vendor performs a primary calibration that determines the response of the detector to particular radiation sources, chosen to be representative of the radionuclides expected to be monitored, often in terms of uCi/cc/cpm. This information is provided to the purchaser.

The response of the detector to radiation is dependent on the type of radiation and the energy of its emissions; the response is seldom linear and the uCi/cc/cpm value varies from radionuclide to radionuclide.

As such, the response of the detector depends on the radionuclide mix in the release stream.

Newer radiation monitor systems display results in terms of uCi/cc or uCi/sec; this indication is accurate only if the radionuclides in the release stream are comparable to the radionuclides used in calibration and calculation of the engineering unit conversion that converts cpm to the activity units.

Theres no source term meter in the control room, so the source terms used in emergency dose assessment need to be estimated from available data. The next several slides will discuss some of the ways environmental releases can be estimated.

Each approach has its pros and cons. All of these approaches should be considered. First lets look at release stream radiation monitor readings

Continuously monitor the radioactivity release to the environment; indicate in the control roomNoble gas, iodine, particulate channelsIndividual isotope data generally not availableProvides immediate direct measurement of the release to the environment, provided:The release to the environment is monitoredThe release is ongoing.Sources used for calibration may not be representative of the radionuclides in a particular release stream during an accident

Method 2 is process radiation monitor readings:

Continuously monitor the radioactivity contained in plant systems; indicate in the control roomNoble gas, iodine, particulate channelsIndividual isotope data generally not availableProvides immediate direct measurement of radioactive contamination of plant systems--but not the release to the environmentIndications can be used to project the radioactive inventory available for release to the environmentRelease does not need be ongoingSources used for calibration and development of correlations may not be representative of the radionuclides in a particular release stream during an accident

Method 3 is area radiation monitors:Continuously monitor the ambient dose rate at various plant locations; indicate in the control room and locallyPrimarily a radiation worker occupational dose controlContainment high range area radiation monitorsDoes not measure the release to the environmentIndications can be used to project the radioactive inventory available for release to the environmentRelease does not need be ongoingSources used for calibration and development of correlations may not be representative of the radionuclides in a particular release stream during an accident

Most effluent release streams have provision for manual or automatic sampling; plants required to have arrangements for analyzing high activity samplesSample analysis results provide direct measurement of the isotopic release to the environment, provided:The release to the environment can be sampledThe release is ongoingIsotopic results possible

Inherent delay in getting results

All licensees have capability of dispatching field survey teams into the environment to obtain dose rate and airborne activity measurementsGiven available field measurements, one can backcalculate an estimate of the release rate from the plant; provided:The release is ongoingThe location of the field team with relation to the plume centerline is knownThe atmospheric dispersion to the field team location is knownAlthough the field measurements may be direct, the backcalculated release rate is highly uncertainSome may use installed environs monitors, but the same uncertainties applyis the monitor on the plume centerline?

Safety Analysis Report Applicants for licensees perform a series of design basis accident analyses as part of licensing.Highly stylized analyses; may not be representative of an actual eventShould not be used directly; may be overconservativeData from these analyses can inform source term decisionsSevere accident analysesAll licensees have performed probabilistic risk analyses (PRA) to assess core damage frequency and large early release fractions (level I PRA). Some licensees have also performed analyses of source term magnitude (level II PRA) and dose consequences (level III PRA)Accidents addressed in PRA may not be representative of accident at handPRA analyses are best estimateData from these analyses can inform source term decisionsAssessment using accident assessment tools These tools provide a flexible capability to model diverse accident conditionsConsiderable modeling time is requiredExtensive data needed to support calculation; much of which may not be readily available during the early phase of an accidentNot deemed feasiblePre-calculated assumptions from severe accident insightsRASCAL and the Response Technical Manual (RTM) contain a method based on insights of severe accident analysesThere is a small set of accident conditions that dominate any accident sequenceThere are parameters that can characterize these dominant sequencesThese parameters can be recognized or characterized during an accident

I want to talk about release duration. Some of our sites have had problems in this area.

Protective action guides are expressed as projected avoided doseA good estimate of the release duration is critical to proper protective action decisionsRelease durations can be difficult to estimateRequires the active participation of the technical support center (TSC) staff as well as the dose assessors.Default release durations (e.g., use four hours) should be used only as the last resortAlthough we want to issue an adequate PAR, we also need to avoid overconservative PARsOverconservative PARs can place the public at unnecessary riska non-conservative situationShort release durations (e.g., 1 hour with dose assessments repeated every hour) could result in no PAR when one would be indicated when a more appropriate release duration was used

What’s in the Black Box Known as EMERGENCY DOSE ASSESSMENT

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