PDF 7.1 Defense in Depth(1)

7/27/2019 PDF 7.1 Defense in Depth(1)

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Nuclear Engineering Program

A Look at Nuclear Science

and Technology

Reactor Safety

L. R. Foulke

Module 7.1

Defense in Depth


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Primary Objectives of Reactor Safety

The primary objectives of reactor safety are:

Shutdown the reactor

Maintain it in a shutdown condition

Cool the core

Contain the radioactive material

How are these objectives accomplished in todaysreactors?


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Radioactive materials in a 3300-MW(t) lightwater reactor core grouped by relative volatility

Volatility Elements Inventory (Ci)

Noble Gases Krypton (Kr)

Xenon (Xe)

1.7E+8

2.2E+8

Very Volatile Iodine (I)

Cesium (Cs)

7.5E+8

2.3E+7

Moderately Volatile Tellurium (Te)

Strontium (Sr)Barium (Ba)

1.8E+8

3.5E+83.4E+8

Less Volatile Ruthenium (Ru)

Lanthanum (La)

Cerium (Ce)

2.4E+8

4.7E+8

3.9E+8

Table Source: See Note 1


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Energy Sources

Energy Sources During an Accident

Stored Energy

Nuclear Transients

Decay Heat

Chemical Reactions

External Events


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Energy Sources

Stored Energy in Water-Cooled Reactors

Pressure Suddenly Below Saturation

Flashes to Steam Vapor

Nuclear Transients

Reactivity Insertion

Power Level Increase Power Pulse


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Energy Sources

Decay Heat Fission-Product Decay

~7.5% of Full Power at Shutdown

Dies Out Slowly (-1/5 Power)

Potentially Large Energy Source

Melting / Destruction of Fuel w/o Cooling

Severe Flow Blockage

Loss of Coolant

Absence of Heat SinkP(t)

t

tt0


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Energy Sources

Chemical Reactions

Water Reactors

Zirconium / Cladding

Stainless Steel / Structures Oxidation Before Melting

Integrity

Fragmentation

Reaction Rate Increases With Temperature Energy Release ~ Amount of Metal Involved

Reaction releases hydrogen gas



Energy Sources

External Energy Sources Natural Events

Flood Hurricane Tornado Earthquake Tsunami

Man-Made Events

Aircraft Impact

Industrial Explosion

Variable / Tied to Site Selection



Engineered Safety Features

Image Source: See Note 2


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Reactor Safety Fundamentals What is the biggest risk to the public that is unique to

nuclear power reactors?

Release of radioactive materials.

Operating reactors contain an enormous inventory ofradioactive products (fuel, fission products, activationproducts)

Release is prevented by Multiple-Barrier Design

Pellet Cladding

Reactor Primary Coolant System

Containment / Safety Systems


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Multiple Barriers

1st & 2nd BarriersPellet & Cladding

4th BarrierReactor

Containment

3rd BarrierPrimary-System Boundary

Image Source: See Note 3


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What is Defense in Depth?

Not defined in legislation or regulations

Multiple barriers to release of radionuclides

ceramic fuel metallic cladding

pressure boundary of reactor coolant system

containment exclusion area, low population zone, offsite

emergency response plan


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Defense-in-Depth

All commercial reactors are designed with a three-tiereddefense-in-depth strategy for protecting the reactor,workers, and the public

Prevention

Design maintenance and operation procedures to reduce thechances of an incident occurring.

Redundant systems to protect against some mechanicalfailures.

Protection

Design features and procedures to halt / deal with incidentsbefore they become worse (cause damage).

Mitigation

Limit the consequences of accidents that occur.


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Incident Prevention

Prevention - Avoid operational occurrences (or accidents)that can cause

System damage

Loss of fuel performance

Abnormal release of radioactive materials Prevention Elements

High reliability components Inherently stable operating characteristics Safety margins

Testing and inspections Instrumentation and automatic control Training Quality assurance


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Incident Protection

Protection Recognize / accept inevitable failures and errors

Halt unlikely incidents and/or transients

Provide automatic and manual systems to quickly shutdown(trip/scram) the reactor

Postulate and analyze every reasonably conceivablefailure; prepare protective actions for eachincident/accident.

Fast-acting shutdown (trip/scram)

Pressure relief (prevent ruptures)

Interlocks

Automatic monitoring / safety-system initiation


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Incident Mitigation

Mitigation

Limit Consequences of Accidents

Evaluate Low Probability Severe Core-Damaging Accident

Establish Engineering Safety SystemsPerformance Criteria

Measures Emergency Feed / Core-Cooling / Electric Power

Containment

Emergency Planning


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Other Safety Factors

Other nuclear safety factors

Strong Technical / Scientific Emphasis

Free / Open International Exchange

Knowledge / Experience

Feedback to the Design Process

Voluntary Peer Oversight / Regulatory

Controls Independent Verification

Independent verification does NOT Replace ResponsibleDesign/Operation


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Why Defense in Depth? A way to compensate for uncertainty

In 1950-1960 time frame there was little experiencewith nuclear power plant operation

Idea was to postulate a variety of design-basisaccidents and show deterministically that they wouldnot result in core damage

No defensible estimates of the relative likelihoods of

potential accidents existed Focus was on design-basis large-break loss of coolant

accidents (LOCAs)


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What is wrong with Defense in

Depth? Its expensive

It creates complexity

It is unbounded

It may or may not work


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Reactor Safety Analyses

Part of the licensing procedure for every commercialreactor design is to prove that the reactor can operatesafely in normal operating conditions as well as selectedaccident conditions

The safety analysis shows the NRC

The plant is designed to remove heat from the core under allconditions

The plant can handle transients

The plant/core can survive design-basis severe accidents

The engineered safety systems designed to cope with off-normalconditions.


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NRC Safety Goals(http://www.nrc.gov/reading-rm/doc-collections/commission/policy/51fr30028.pdf)

Issued in 1986, the goals expressed "theCommission's views on the level of risks to publichealth and safety that the industry should strive

for in its nuclear power plants.

Two qualitative goals were established:

Individual members of the public should beprovided a level of protection from theconsequences of nuclear power plantoperation such that individuals bear nosignificant additional risk to life and health.
http://www.nrc.gov/reading-rm/doc-collections/commission/policy/51fr30028.pdfhttp://www.nrc.gov/reading-rm/doc-collections/commission/policy/51fr30028.pdfhttp://www.nrc.gov/reading-rm/doc-collections/commission/policy/51fr30028.pdfhttp://www.nrc.gov/reading-rm/doc-collections/commission/policy/51fr30028.pdfhttp://www.nrc.gov/reading-rm/doc-collections/commission/policy/51fr30028.pdfhttp://www.nrc.gov/reading-rm/doc-collections/commission/policy/51fr30028.pdfhttp://www.nrc.gov/reading-rm/doc-collections/commission/policy/51fr30028.pdf


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Societal risks to life and health from nuclear power

plant operation should be comparable to or lessthan the risks of generating electricity by viablecompeting technologies and should not be asignificant addition to other societal risks.

To quantify these goals, two quantitative healthobjectives (QHOs) were also established:

The risk to an average individual in the vicinity ofa nuclear power plant of prompt fatalities thatmight result from reactor accidents should not

exceed one-tenth of one percent (0.1 percent) ofthe sum of prompt fatality risks resulting fromother accidents to which members of the U.S.population are generally exposed.


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The risk to the population in the area near anuclear power plant of cancer fatalities that

might result from nuclear power plantoperation should not exceed one-tenth ofone percent (0.1 percent) of the sum ofcancer fatality risks resulting from all other

causes.


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1. Haskin, F. E. & Camp, A. L. (1994).Perspectives onReactor Safety. Table 5.1-14. NUREG/CR-6042.Office of Nuclear Regulatory Research, U.S. NuclearRegulatory Commission.http://pbadupws.nrc.gov/docs/ML0727/ML072740014.pdf

2. Reprinted with permission from Nuclear EnergyInstitute.http://www.nei.org/corporatesite/media/filefolder/c

ontainment_wall_construction.jpg3. Reprinted with permission from the USNRC.

http://www.nrc.gov/about-nrc/regulatory/research/soar/soarca-accident-progression.html

Image Source Notes
http://pbadupws.nrc.gov/docs/ML0727/ML072740014.pdfhttp://pbadupws.nrc.gov/docs/ML0727/ML072740014.pdfhttp://www.nei.org/corporatesite/media/filefolder/containment_wall_construction.jpghttp://www.nei.org/corporatesite/media/filefolder/containment_wall_construction.jpghttp://www.nrc.gov/about-nrc/regulatory/research/soar/soarca-accident-progression.htmlhttp://www.nrc.gov/about-nrc/regulatory/research/soar/soarca-accident-progression.htmlhttp://www.nrc.gov/about-nrc/regulatory/research/soar/soarca-accident-progression.htmlhttp://www.nrc.gov/about-nrc/regulatory/research/soar/soarca-accident-progression.htmlhttp://www.nrc.gov/about-nrc/regulatory/research/soar/soarca-accident-progression.htmlhttp://www.nrc.gov/about-nrc/regulatory/research/soar/soarca-accident-progression.htmlhttp://www.nei.org/corporatesite/media/filefolder/containment_wall_construction.jpghttp://www.nei.org/corporatesite/media/filefolder/containment_wall_construction.jpghttp://pbadupws.nrc.gov/docs/ML0727/ML072740014.pdfhttp://pbadupws.nrc.gov/docs/ML0727/ML072740014.pdf

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