Public Health Risks of Substituting Mixed-Oxide For Uranium Fuel in Pressurized-Water Reactors

8/7/2019 Public Health Risks of Substituting Mixed-Oxide For Uranium Fuel in Pressurized-Water Reactors

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Science & Global Security, Volume 9 pp 33-79

2001 Taylor and Francis

0892-9882/01 $12.00 + .00

Public Health Risks ofSubstituting Mixed-Oxide ForUranium Fuelin Pressurized-Wa terReactors

Ed w in S.Ly m a n

a

The U.S. DepartmentofEnergy(DOE)hasawardedacontracttothe consortium

Duke CogemaStone andWebster(DCS)todispose ofupto 33 tonnesofexcessweap-

ons-grade plutonium(WG-Pu) byirradiatingitinthe formofmixed-oxide (MOX)fuel

infourU.S. commercialpressurized-waterreactors(PWRs). Thispaper estimatesthe

increase inrisktothe publicfromusingWG-MOXatthese reactorsand findsthatit

exceeds recently established Nuclear Regulatory Commission (NRC) guidelines.

Therefore, the NRCwillhave atechnical basisforprohibitingthe use ofMOXatthese

reactorsunlessthe riskthattheywill experience asevere accidentcan be significantly

reduced.

MOX fuel will displace a fraction of the low-enricheduranium (LEU) fuel that

thesereactorscurrentlyuse. BecauseMOXcoreshavegreater quantitiesofplutoniumandotheractinidesthan LEUcoresthroughouttheoperatingcycle, thesourcetermfor

radiologicalreleasescaused byseverereactoraccidentswill begreaterforMOX-fueled

PWRs. In this paper, the radiological consequences to the public from containment

failure or bypassaccidentsatMOX-fueledPWRsare calculated, andcomparedtothose

resultingfromthe same accidentsat LEU-fueledPWRs.

Thispaper findsthatcomparedto LEUcores, the numberoflatentcancerfatali-

ties (LCFs)resulting fromanaccidentwithcore meltand earlycontainment failure

would be higher by 39%, 81%or 131%forfullWG-MOXcores, dependingonthe frac-

tionofactinidesreleased(0.3%, 1.5%or 6%). Underthe DCSplan, inwhichWG-Pu

will be purifiedusinganaqueousprocessandonly 40%ofthe core will be loadedwith

WG-MOX, the numberofLCFswould be 11%, 25%or 30%higher, respectively. The

average LCFriskto individualswithintenmilesofasevere accidentapproximately

doublesforafullWG-MOXcore, andincreases by 26%foraDCScore.

Theoriginalversionofthismanuscriptwasreceived byScience&GlobalSecurity

on9 October 1998.

a Edwin LymanisscientificdirectoroftheNuclearControlInstituteinWashington, D.C.


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These resultsare ofparticularconcernforthe nuclearplantsinthe DCSconsor-

tium, Catawba and McGuire. These plantshave ice-condenser containments, which

SandiaNationalLaboratories estimatesare atleasttwoordersofmagnitude more vul-

nerable to earlyfailure thanothertypesofPWRcontainments.

The findingsofthispaperalsoapplytothe proposeduse ofWG-MOX inVVER-

1000 reactorsinRussia, whichmeetlessstringentsafetystandardsthanU.S. reactors.

INTRODUCTION

Pluton ium Disposition

InJanuar

y 1997,th

e U.S. Depart

ment

ofEn

ergy(DO

E)decided

top

ursuea

dualtrackpolicyfordisposingofapproximately 50 tonnesofplutoniumpro-

ducedforweaponsprogramsthathave beendeclaredexcesstomilitaryneeds.

The two tracksrefer todifferentapproaches forconvertingseparatedpluto-

niumintoadilute andhighlyradioactive formthatismore difficulttoreturn

toweapons.

Under one approach, known as can-in-canister immobilization (CIC),

plutonium will be incorporated into chemically stable ceramic discs. These

discswillinturn beembeddedincanistersofvitrified(glassified)high-level

radioactive waste (VHLW)atthe Defense Waste ProcessingFacility(DWPF)

atthe SavannahRiverSite (SRS)inSouthCarolina. DOEisplanningtouse

CICforapproximately 17 tonnesof excessplutonium in impure forms. The

CICfacilitywill besitedatSRSadjacenttotheDWPF.Undertheotherapproach, plutoniumwill beusedtoproducemixedplu-

tonium-uraniumoxide(MOX)fuelassemblies, whichwill be irradiated ina

number of U.S. commercial light-waternuclear reactors (LWRs), displacing

some orall of the low-enricheduranium oxide (LEU) fuel the reactors cur-

rentlyuse. DOEisplanningtoutilizethisoptionfor 25.6 tonnesofweapons-

gradeplutonium(WG-Pu).

Both processes are regarded by most experts as roughly comparable in

theirabilitytorenderthe plutoniumasinaccessible asthe plutoniumincom-

mercialspentnuclearfuel, therebymeetingthe spentfuelstandarddefined

bytheNationalAcademyofSciences(NAS).

1

However, DOEdecidedtopursue

both tracks foranumber of reasons, one being the desirability ofhavinga

backupstrategyincase one approachdidnotsucceed.In 1998, DOEissuedaRequestforProposals, seekingvendorsinterested

inprovidingMOXfuelfabricationandirradiationservices. Ofthe three pro-

posals submitted, two were quickly eliminated for failing to meet basic

requirements. InMarch 1999, DOEsignedacontractwiththethirdparty, a

consortiumcalledDuke CogemaStone &Webster(DCS), whichincludedthe


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Pu blic H e a lth Risks ofSu bstitu tin g Mix e d - O x id e for Ura niu mFu el

35

U.S. utilitiesDukePowerandVirginiaPower, theFrenchnationalfuelcycle

companyCogema, andthe Stone &Websterarchitect-engineeringfirm.

2

Accordingtothe contract, DCSwilldesign, buildandoperate aMOXfuel

fabricationplantatSRS. MOXfuelwillthen be irradiatedinsixpressurized-

waterreactors (PWRs)at three sites ---VirginiaPower'sNorthAnnaplant,

locatedabout 70 milesfromWashington, DC, andDukePower'sMcGuireand

Catawba plants, both situated within twenty miles of downtown Charlotte,

NorthCarolina. InJanuary 2000, DOEconfirmedthisplan initsRecordof

Decision (ROD)on the SurplusPlutoniumDispositionFinalEnvironmental

ImpactStatement. However, inApril 2000, VirginiaPowerwithdrewfromthe

consortium, leavingonlyfourreactorsintheMOXprogram.

Use ofMOXfuelonalarge scale will be anovelpractice foraU.S. nuclearutility. Althoughsome countriesinEurope have begunusingMOXfuelona

limited basis in LWRs, U.S. utilitieshave not followedsuit. This isaresult

bothofthe U.S. non-proliferationpolicyadoptedinthe late 1970s(andreaf-

firmed bytheClintonAdministration)whichledtomoratoriaoncommercial

spentfuelreprocessingandMOXrecycling, and bythe poor economicsofMOX

fuel, whichisseveraltimesmore expensive thanLEU.

Environmenta lImpacts of MOX Use

The immobilizationandMOXapproaches, which bothrequire large-scale han-

dlingand processing of plutonium, will be expensive and will pose risks tohumanhealth and the environment. However, these risks are likely to be

small incomparison tothose thatwereencounteredwhen thematerialwas

produced.

Manyarms-controladvocates believe thatthe costsofplutoniumdisposi-

tionare justified bythe security benefits. Some observershave arguedfurther

thatdifferences incostandrisk between the twodisposition tracksare not

importantconsiderations,

3

aview thatdoesnottake intoaccount fiscaland

politicalconstraints. ColdWar-sized budgetsare not beingmade available for

disarmamentactivities, andthe plutoniumdispositionprogramisunderpres-

sure tominimize costs. Also, many environmentalgroupsandcitizens'groups

nearaffectedsitesoppose dispositionactivitiesunlesstheyhave low environ-

mentalandpublichealthimpacts.Costand publichealth impact were indeed major considerations in the

processDOEusedtoselectMOXandimmobilizationfromthe large numberof

dispositionoptionsthatwere initiallyproposed. Indecidingonthe dualtrack

policy, DOE argued that there were no significant differences between the


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MOXandimmobilizationoptionswithregardtothesecriteria. However, its

analysisofthisissue wasinadequate.

Toaccuratelycompare the environmental impactsofthe twodisposition

approaches, one mustdetermine the increase inriskassociatedwithmodify-

ing existing processes to accommodate plutonium disposition. The MOX

approachconsistsofseveralstages, eachofwhichcanhaveasignificantenvi-

ronmentalandpublichealthimpact. AplantforfabricationofMOXfuelwill

be builtandoperated, andthe fuelwill be shippedtoreactorsitesandirradi-

ated. Afterward, the spentMOXfuelwillhave to be storedonsite untilageo-

logicrepository becomesavailable.

Bycomparison, theenvironmental impactsofCIC immobilizationresult

primarilyfromoperationofthe ceramicimmobilizationplant. Thisplantwillbe similartothe MOXfabricationplantinmanyways, anditwillhave similar

(ifnotlower)impacts. Moreover, the CICprocessis beingdesignedandtested

to ensure thatthere isnoimpactonthe safe operationofthe DWPF, andpre-

liminary results have been encouraging.

4

The risks associated with MOX

transportationtoreactorsitesandirradiationare not encounteredinthe CIC

process. Thus, acomparative analysisofthe twooptionswillnot be complete

withoutanassessmentofthe risksofMOXirradiation.

MOX Use and Sev ere AccidentRisk

Probabilistic Risk Assessment

Thetoolsofprobabilisticriskassessment(PRA)can beusedtoestimatethe

risktothe publicfromthe operationofnuclearpowerplantswithWG-MOX

fuel. PRAconsistsofthree steps:

5

1. Identificationanddelineationofthe combinationsofeventsthat, if

theyoccur, couldleadtoanaccident(orotherundesiredevent);

2. Estimationofthechanceofoccurrenceforeachcombination;and

3. Estimationoftheconsequencesassociatedwitheachcombination.

Tota

lrisk

isth

en

defin

edas

th

e productof

the p

robabili

tyan

dth

e cons

e-quencesofeachcombination, summedoverallcombinations.

PRAs are carried out in three stages. A Level 1 PRA identifies all

sequencesof eventsthatcouldresultincore damage and estimatestheirfre-

quencies of occurrence. Summing the frequenciesof these events yields the

core damage frequency(CDF), the average annualprobabilitythatcore dam-


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age will occur. A Level 2 PRA evaluates containment performance for each

accidentsequence that leadstocore damage, andcalculatesthe radiological

release for eachsequence inwhichcontainment iscompromised. ALevel 3

PRA estimatesthe consequences(i.e. promptfatalitiesandlatentcancerfatal-

itiesamong the public)of the sequences involvingradiologicalreleases, and

combinesallelementsintoameasureofthetotalrisktothepublic. Level 3

PRAanalysisusescomputercodesthatmodelradionuclide dispersalandradi-

ation exposure ofthe population. These calculationsrequire asinputradionu-

clide release fractionsobtainedfrom Level 2 PRA, whichare highlyuncertain

insome cases.

PRAs are huge, complex calculations and their results contain large

uncertainties. Therefore, they are more useful for ranking the relative risksignificance ofdifferent eventsthanforprovidingmeaningfulvaluesofabso-

lute risk. Consequently, the U.S. NuclearRegulatoryCommission(NRC)has

beenslowtoincorporate PRA-basedanalysisintoitsregulations, althoughit

hasrecentlyadoptedapolicytoincreaseuseofPRA(see below).

Atthe requestofthe NRC, allU.S. nuclearplantsconductedLevel 1 and 2

PRAs forinternal events for the IndividualPlantExamination (IPE) pro-

gram.

6

Subsequently, external events(seismic, tornadoand fire risks)were

analyzedforthe IndividualPlantExaminationofExternalEvents(IPEEE).

WhiletheNRCcollectedandanalyzedtheIPEresults, itdidnotpeer-review

or endorse them.

7

Infact, because the methodologyandassumptionsvaried

widelyfromplanttoplant, the overallresultswere inconsistentinimportant

respects. Inthe future,PRAsusedinlicensingwillhave toconformto quality-

controlandpeer-reviewstandardsnowunderdevelopment.

Mostplantshavenotcompletedcredible Level 3 PRAs. TheNRC itself

conducted peer-reviewedLevel 3 PRAs forfive U.S. nuclearplantsandpre-

sentedthe resultsinthe 1990 reportNUREG-1150.

8

It will not be a simple undertaking to conduct PRAs for MOX-fueled

LWRs. Ingeneral, the substitutionofWG-MOXfor LEUfuelwillaffect both

theprobabilityofoccurrenceand the consequencesof each reactoraccident

sequence, requiring extensive modificationofLEU-basedPRAs. The difficulty

will be compounded by the relative lack of experience with the use ofWG-

MOXfuelandthe absence ofdataonmanytechnicalaspectsofMOXuse.

The taskcan be simplified by focusingonthe reactoraccidents thatare

thelargestcontributorstotheoverallrisktothepublic: thebeyond-design-

basisorsevereaccidentsthatinvolve extensive core damage andfailure (or

bypass)ofthe reactorcontainment. Animportantsubsetofsevere accidents

are those thatresultinlarge radiologicalreleases before the localpopulation

has evacuated, increasing the chances for earlyhealth effects. If the use of


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MOXfuelweretoresultinanincreaseintheconsequencesand/orprobabili-

tiesof these accidents, then the totalriskto the publicwould increase bya

similarpercentage.

The mostdifficultpartofthisassessmentisthe calculationofthe proba-

bilitiesofsevere accidentsatMOX-fueledplants. Todo thisaccurately, the

computercodesused byU.S. utilitiesandtheNRCtoanalyzeaccidentswill

have to be modified to incorporate WG-MOX-specific parameters.

9

Some of

these parametershave not beenfully benchmarked evenforRG-MOXfuel, for

whichthere isconsiderablymore experimentaldata. Nonetheless, the NRC

hasstatedthatthe use ofMOXwillnothave a big effectonaccidentprogres-

sionandconsequently thatitappears likely that theprobabilityofsevere

accidents will not change.

10

Consistent with this assessment, this paperassumes thatsevere accidentprobabilitiesare the same for LEUandMOX

cores.

Incontrast, assessingthe consequencesofsevere accidentsatMOX-fueled

plants is straightforward, especially under the assumption that MOX and

LEU fuel rods behave similarlyunder severe accident conditions.

11

In that

case, the radionuclide release fractions(the fractionsofthe reactorcore radio-

nuclide inventories that are released during an accident) are the same for

bothtypesoffuel, andthe differencesinconsequencesare due entirelytotheir

differentradionuclideinventories.

Byassuming thatsevere accidentprobabilitiesandradionuclide release

fractionsare the same for bothLEUandMOXcores, riskcalculationsare sim-

plified. Level 2 PRAresultsfor LEUcorescan be adjustedforMOXcores by

using the appropriate radionuclide inventories. This is the approach used

here.

However, there are technicalaspectsofthe use ofMOXfuelthatcanaffect

the validityofthese twoassumptions. Thisissue isdiscussedinmore detail

below.

Impact of MOX Fuel on Accident Consequences

Throughoutthe operatingcycle, MOXcoreshave largerinventoriesthan LEU

coresofmosttransuranic(TRU)radionuclides, includingplutonium-239 (Pu-

239), americium-241 (Am-241) and curium-242 (Cm-242). Since many of

these radionuclides are long-lived alpha-emitters, with relatively highradiotoxicities if inhaledor ingested, small releases duringanaccident can

contribute significantlytopublicradiation exposure.

Inventoriesof fissionproductsarealsodifferentin LEUandMOXcores,

because U-235 and Pu-239 have slightly different fission product spectra.

Also, atleastone TRUisotope, Np-239 (a beta-emitter), hasalowerinventory


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39

inaMOXcore. However, theresultspresented belowindicatethatthesedif-

ferencesare lesssignificantforriskthanthe increase inalpha-emitters.

12

DOE'sFebruary 1996 Storage andDispositionofWeapons-Usable Fissile

Materials DraftEnvironmental Impact Statement (DPEIS) didnotanalyze

the environmentalimpactsofaccidentsatMOX-fueled LWRs, butanalyzedan

LEU-fueled LWRinstead. DOEjustifiedthis byclaimingthat

...studies ... indicate that the use of MOX fuel in a ... LWR does not

increase the riskandconsequencesofaccidents. Thisresults from the fact

thatthe otherradioisotopesthatare releasedinanaccidenthave more seri-

ousimpactsonhumanhealththanthe Puusedinthe MOXfuel.

13

This is based on the assumption that the consequences of reactoracci-

dentsare dominated byreleasesofthe more volatile fissionproducts, suchasiodine-131 (I-131) and cesium-137 (Cs-137), while plutonium and other

actinides, which typicallyhave very lowvaporpressures (are low-volatile),

wouldnot be releasedtothe environmentinsignificant quantities.

However, DOE's statement is not consistent with the current state of

understandingofsevere accidents. Althoughmostactinidesare low-volatile

andare not easilyreleasedfrommoltenfuel, significantactinide releasesinto

the environmentcanoccurduringcertainaccidents. Althoughthese accidents

are expected tooccurvery infrequently, there are bothhistoricalprecedents

andregulatoryrequirementsforconsideringtheminsafetyanalyses.

The Chernobylaccidenthasdemonstratedthatsignificantandwide-rang-

ingdispersalof low-volatile radionuclides ispossible. Arecentreviewof the

Chernobylsource termhasconcluded that the release fraction foractinides

wasapproximately 3.5%. Moreover, dispersalofthese relativelyheavyaero-

sols wasnot limited to the immediate vicinity of the plant; fuel fragments

were discoveredasfarawayasGreece andGermany, overone thousandkilo-

metersaway.

14

One oftenhearsthe claimthataChernobyl-type accidentcannothappen

in the West because Western reactorshave robust containment structures,

andtheparticularaccidentsequencethatoccurredwasspecifictoChernobyl-

type (RBMK)reactors. However, while the presence ofacontainmentdome at

Westernreactorsreduces the riskofsuchaccidents, itdoesnot eliminate it

entirely. Analysts have identified hypothetical accident sequences at U.S.

LWRs which can lead to energetic mechanical dispersal of the fuel, cata-

strophicfailureor bypassofthecontainmentandsignificantreleasesoflow-

volatile core fragmentsinthe formofaerosols.

Uncertaintiesinthe low-volatile release fractionspredictedtoresultfrom

severe core damage range overseveralordersofmagnitude. The NRC esti-

matesthatlow-volatile releasesashighasseveralpercentofthe core inven-


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torycanoccur, andusesthisinformationinitsriskstudies.

15

One mechanismforcore dispersaland earlycontainmentfailure atPWRs

isknownashigh-pressure melt ejection(HMPE), inwhichthe reactorvessel

failsathighpressure afteracore melt. The moltenfuelisthendispersedinto

the containment in a shower of fragments, resulting in direct containment

heating (DCH), a veryhigh rate ofheat transfer to the containmentatmo-

sphere. The resultingrapidpressurizationofthe containmentcancause itto

fail. Aphenomenonof evengreaterconcern is the buildupofhydrogen from

the reactionofzirconiumcladdingandwater, whichcanalsoleadtoan explo-

sioncapable offragmentingfueland breachingthe containment.

TheNuclearControlInstitute(NCI)citedtheseissuesinitscommentson

the DPEIS, andDOEsubsequentlyreviseditsanalysis. The December 1996Final Programmatic Environmental Impact Statement (FPEIS) estimated

thatusingafullcore ofMOXinan existing LWRwouldchange the numberof

latentcancerfatalities(LCFs)resultingfromasevere accident by+8%to-7%:

in other words, thenumber of LCFs could actually decrease as a result of

switching toMOX fuel. However, DOE'scalculation containedanumberof

flawswhichcastdoubtonitsaccuracy.

16

Forthisreason, NCIundertookthe

presentstudy.

17

Thispaper finds, contrarytoDOE'sassertions, thatthere are significant

publichealthrisksassociatedwiththeMOXapproach. Thisisdueprimarily

tothe findingthatthe consequencesofsevere accidentsatMOX-fueledLWRs

will be greaterthanthose atLEU-fueledLWRs, asaresultofthe largerinven-

toriesofplutoniumandotheractinidesinMOXcores.

The riskofcontainmentfailure and energeticfueldispersalisofparticu-

larconcernforthereactorsthathave beenselectedfortheU.S. MOXprogram.

All four reactors have ice-condenser containments, which are considerably

smaller and weaker than the large dry containments present at most U.S.

PWRs. Arecentstudy, conductedforthe NRC bySandiaNational Laborato-

ries (SNL), concluded that ice condenser plants are at least two orders of

magnitudemorevulnerabletoearlycontainmentfailurethanothertypesof

PWRs.

18

The studycitesthe McGuire plantinparticularashavinganunac-

ceptablyhighprobabilityofcontainmentfailure. Commentingonthisfinding,

DanaPowers, Chairman of the NRCAdvisory Committee on Reactor Safe-

guards (ACRS), said that the selectionofMcGuire forMOX irradiationwas

maybeasuboptimalchoice.

19


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Impact of MOX Fuel on Ac cidentProbabilities

The riskanalysis in this paperassumes that the use of MOX fuel willnot

affectthe probabilitythatasevere accidentwilloccur. While thisassumption

isreasonable asa firstapproximation, there are anumberofdifferencesinthe

neutronicandthermomechanicalpropertiesofthe twotypesoffuelthatcould

affect theoutcomeofaccidentprecursors. Notallof thenegative impactsof

MOXfueluse can be mitigated bymodificationstothe core design. The ques-

tionofwhetherthe remainingdifferenceswillhave asignificant effectonthe

likelihoodofasevere accidentremainsopenandwilllikelyrequire consider-

able experimentalandanalyticalworktoresolve.

Pu-239 hashigherthermalabsorptionand fissioncross-sectionsthanU-235, resultinginalessthermalneutronspectruminMOXcores. Comparedto

anLEUcore, aWG-MOXcore willhave

20

amore negative moderatortemperature coefficient

agenerallymore negative Dopplercoefficient

reduceddelayedneutronfractionandpromptneutronlifetime

reducedindividualcontrolrodworth

reduced boronworth

increasedlocalpowerpeakingfactors

highercenterline fueltemperatures

To permit the use of full-core MOX, while preserving margins of safety

suchas the shutdownmargin,

21

modifications must be made to the reactor

core, suchasanincrease inthe numberand/or qualityofrodclustercontrol

assemblies (RCCAs). For cores with a MOX loadingno greater than 33%,

additional control rods maynot beabsolutelynecessary if one is willing to

accepta large reduction inthe shutdownmargin. AlthoughDCShasfound

that the shutdownmargin isminimalfora 40%MOX loading, ithasstated

thatitdoesnotintendtoinstalladditionalcontrolrodsinMcGuire andCat-

awba, although it may change the type of control rod material used in

McGuire. In contrast, PWRs inFrance thatuse 30% MOXcoreshave four

additionalRCCAsinstalled.

MOXcoresalsorequire agreaterconcentrationofthe neutron-absorbing

isotope B-10 inthe coolant, whichcan be accomplished byincreasingthe boric

acidconcentrationor byusing boronthathas beenenrichedinB-10. Because


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thereisalimitontheamountof boricacidthatcan beaddedtothecoolant

withoutcausingproblemsduringoperation, DCShassaidthatitisconsider-

ingthe use ofenriched boron.

22

Powerpeaking, caused by the large differences in flux betweenadjacent

MOXand LEUassemblies, alsomust be taken intoaccount inpartialMOX

coredesign. Thisrequiresuseofzonedfuelassembliescontainingfuelpins

withseveraldifferentplutonium enrichmentlevels, aswellascarefulplace-

mentofMOXassemblies inthe core. Evenwith these modifications, power

peaking still can occur due to the heterogeneous distribution of plutonium

withinthe MOXfuelpellets. (Variationsinisotopiccontentcanalsoresultin

significantpowerpeaking, althoughthisismoreofaconcernwithRG-Pu.)

AnotherpropertyofMOXfuelthatcouldaffectradionuclide release dur-ingaccidentsisthe inferiorphysical behaviorofMOXpelletstothatofUO

2

pelletsdue tohighertemperature andgreater fissiongasrelease, especially

at burnups greater than 35 GWD/t.

23

The heterogeneous microstructure of

MOX fuel (resulting from thepresenceofplutoniumclusters)results in the

appearance ofhotspotsofveryhighlocal burnup, inwhichhighconcentra-

tionsoffissiongasaccumulate.

24

Duringreactivityinsertionaccidents(RIAs),

thisgascan be releasedsuddenly, causingfuelrodcladdingfailure andfuel

pelletfragmentation. Thisproblemis exacerbated bythe factthatMOXfuel

hasa lower thermalconductivityandahighercenterline temperature than

LEUfuel.

Basedonthe resultsofaseriesofRIAtestsatthe CabrireactorinFrance,

French regulators have concluded that MOX fuel shows a higher failure

potential thanUO

2

atcomparable burnup

25

and that there isaveryhigh

potentialforruptureofMOXfuelduringRIAs.

26

Inoneexperiment, aMOX

fuel rod segment with a burnup of 55 GWD/t (which is typical of burnups

attained inU.S. PWRstoday) experiencedaviolentrupture anddispersalof

fuelparticlesatapeakenthalpyof120 cal/g, while an LEUrodofcomparable

burnup was able to withstand a similar peak enthalpy without rupture.

Althoughsomehigh-burnup, highlycorroded LEUrodsalsofailedduringthe

testseries, researchersconcludedthatthe failure ofthe MOXrodwasunique

because itdidnothave aheavilycorrodedcladding.

Unresolved safety questions of MOX fuel performance have ledFrench

regulatoryauthoritiestorestrictthe irradiationtime ofMOXfuelassemblies

tothreeannualcycles, oranassembly-averaged burnupof41 GWD/t, whereas

the LEUlimitis 52 GWD/t. Asaresult, FrenchplantswithpartialMOXcores

continue to operate on annual cycles while other plantshave been able to

switchto 18-monthcycles, whichlowerstheircosts byreducingthe frequency

ofrefuelingoutages. Evenat lower burnups, however, the Cabri testseries


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found that fissiongasrelease in MOX fuel wassignificantlyhigher than in

LEUfuel.

More evidence of problems with MOX fuelhassurfacedatanother fuel

testfacilityinFrance knownasVERCORS, where itwasobservedthatduring

the earlystagesofcore degradation, releasesofvolatile radionuclides from

MOXaremoreextensivethanfromconventionalfuelsatsimilarlevelsofbur-

nupwhichisconsistentwiththe peculiarnature ofporositythatdevelopsin

MOXduring burnup.

27

Inparticular, inatestinwhichspentfuelwasheldat

atemperature of1780 Kforone hour, the cesiumrelease fractionforaMOX

fuelrodwitha burnupof 41 GWD/twas 58%, comparedtoonly 18% foran

LEUrodwitha burnupof47 GWD/t.

28

The Cabritestssuggestthatthe probabilitythatanRIAwillcause signif-icant fuel damage and progress to a severe accident may be greater when

MOXfuelisinthe core. Therefore, the probabilitiesofsome severe accidents

may increase whenMOX issubstitutedfor LEU inPWRs, unlessstrict bur-

nuplimitsareimposedonMOXassemblies.

The VERCORS experimentssuggestthatMOXradionuclide release frac-

tionsmayneed be changedtotake intoaccountthe greaterobservedreleases

of volatile radionuclides. Thiswould result inmore severe consequencesof

MOX-fueledaccidentsthanare estimatedinthispaper.

DCShasstatedthatinitiallyitplanstoirradiateMOXfuelforonlytwo

18-monthcycles, with limitsonassembly-averagedandpeakrod burnupsof

45 GWD/tand 50 GWD/t, respectively.

29

However, peakpellet

burnupsinthis

case will exceed 55 GWD/t, the regionofconcernidentifiedinthe Cabritest.

Infact, the MOXrodthatfailedduringthe Cabritestwastakenfromafuel

assemblywithanaverage burnupof46 GWD/t. Moreover, DCShassaidthat

itintendsto eventuallyirradiate MOXforthree 18-monthcycles, whichwould

require raisingthe MOXpeakrod burnuptoover 60 GWD/t, welloutside of

the current experimentaldatabase.

NuclearRegulatory Issues

InordertoobtainregulatoryapprovalforusingMOXfuel, DCSwillhave to

applytotheNRCforamendmentstoitsreactoroperatinglicenses. Adetailed

understandingofthe increasedrisksassociatedwithMOXfuelwill be impor-

tantforthe license amendmentprocess.This paper proposes anapproach for judging the safety of MOXuse in

LWRs according to the risk-informed regulatory procedures now being

adopted bytheNRC.

30

InJuly 1998, theNRCissuedRegulatoryGuide(RG)

1.174, which isone ofthe firstsystematicapplicationsofPRA in itsregula-

tions.

31

RG 1.174 providesamethodologyforrankingproposedchangestoa


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nuclearplantaccordingtotheirrisksignificance. Changeswhicharedeter-

mined (byPRAmethods) tosubstantially increase risk to the publicwill be

subjecttogreaterregulatoryattention bythe NRC. Notably, RG 1.174 defines

anupperlimitforacceptable levelsofriskincrease.

InJanuary 2000 the CommissiongrantedNRCstaffthe authority(inspe-

cialcases)touserisk-informedanalysis, suchasRG 1.174 guidelines, in its

reviewsoflicense amendmentrequests.

32

Ifthe NRCdeterminesthatapro-

posedlicense amendmentwouldresultinanunacceptable increase inthe risk

ofasevere accident, itwill be able torejectthe requestorattachsignificant

conditionstoitsapproval, evenifthe requestsatisfiesallregulatoryrequire-

ments.

RG 1.174 can be used to evaluate the regulatory implications of theincreasedriskassociatedwithuse ofMOXfuel. The calculationspresentedin

thispaperindicate thatthe DCSMOXplanislikelytocause anincrease in

risktothe publicthatwill exceedthe upperlimitspecifiedinRG 1.174. This

shouldprovidea basisfortheNRC, inthecourseofMOXlicenseamendment

reviews, torequire acomprehensive analysisofthe severe accidentrisktothe

publicfromthe use ofMOXfuel. Tofulfillthisrequirement, DCSwillhave to

conductMOX-specific Level 3 PRAsforCatawbaandMcGuire, subjecttorig-

orousstandardsof qualitycontrolandpeerreview. If the magnitude of the

riskincreaseestimated belowisconfirmed bythisanalysis, theNRCwillhave

justification to require significant modifications to, or even reject, the DCS

MOXlicense amendmentrequests.

C alculation of MOX Seve re Accident Consequences

Core Inventories

This paper adopts a standard approach, using the SAS2H/ORIGEN-S com-

putercode, tocalculate LEUandWG-MOXcoreradionuclide inventoriesat

the end (EOC) and beginning (BOC) of an equilibrium cycle. SAS2H/ORI-

GEN-Sisamodule ofthe OakRidge NationalLaboratory(ORNL)SCALE 4.3

code which generates burnup-dependent cross sections and simulates fuel

irra

diat

ion

.

33

Threecaseswereanalyzed. The first is basedonDOE'sSurplusPluto-

niumDispositionDraftEIS (DEIS), whichassumeduse ofadryprocessfor

convertingplutoniumpitstooxide andfullMOXreactorcore loadings.

34

The

fullMOXcore designwasadaptedfroma 1996 Westinghouse reportwritten

undercontractforDOE, whichNCIacquiredundertheFreedomofInforma-


13/47


14/47

Ly m a n

46

Table 2 specifies the isotopic compositions of the fresh fuel assemblies.

TheWG-Puisotopiccompositionswere derivedfromthe reference composition

inthe DOEDEIS.

43

Inthe DCScase, the calculationassumesthatthree years

willelapse betweenthePupolishingstepandtheloadingoftheMOXfuelinto

the reactor. Immediatelyafterpolishing, the concentrationofAm-241 will be

reducedfromabout 0.8 weight-percenttoafewpartspermillion. However, it

thenwillsteadilyincrease asaresultofPu-241 decay.

Table 1:LEU and W G-M OX C ore C h a ra ct e ristics

De sign Parameter LEU DEIS /DCS WG-MOX

N u m b e r o ffe e d asse m b li es 88 92

To t a l n u m b e r o f a sse m b li es 193 193

Fe e d Lo a d in g (tH M) 37.2 38 .9

C ore to ta llo a d in g (tH M) 81.6 81 .7

Av era g e

fe e d e n

ric h m e nt (w / o) 4

.

25 (U-235) 4.

54/4.

43 (W G-Pu+A m-241)To t a l Pu use d p e r y e ar (t) 0 1 .16/0 .46

Av e ra g e d isc h a rg e b urnu p(M WD / THM)

44 ,080 44,080

C y c le leng th (M WD / THM) 20,100 21,010

C y c le leng th ( e ffe ctiv e fu ll- p o w er d ays) 460 481

O u t a g e l en gth (d ays) 40 40

C ore p o w e r (M W

th

) 3565 3565

So lu b le b oro n B-10 c o nt e nt ( a /o) 19.8 40

M id-cyc l e b oron c onc en tra tio n (p p m) 1100 550


15/47


47

Table 2 alsodisplaysthe compositionofthe RG-Puusedinthe RG-MOX

calculation, whichassumesthatthe RG-PuisobtainedfromLEUspentfuel

irradiatedtoa burnupof44 GWD/tandcooledfornine years before reprocess-

ing, andtheRG-MOXfuelisstoredforthreeyears before beingloadedinthe

reactor, the assumptions used by the French utility Electricit de France(EDF).

Otherdetailsneededforthe calculationare providedinTablesA.1 andA.2

ofAppendixA.

Results

TheinventoriesofPuandotheractinidesinMOXcoresarefunctionsofthe

isotopiccontentofthe Puinthe freshfuel, the initialPucore loadingandthe

irradiationtime. AtEOC, the DEIScore contains, comparedtothe LEUcore,

quantitiesofPu-239, Am-241 andCm-242 thatare greater byfactorsof 3, 7

and 7, respectively. AtBOC, these factorsare 11, 22 and 13.

Inth

eDCScoreat

E

OC, with

asmalle

rMO

X

coref

raction

an

dlowerin

i-tial Am-241 content, the inventories of most transuranics will be about 2

timesgreaterthanthose inthe LEUcore.

44

Results of the calculation for fifty-eight radionuclides are presented in

TablesA.3-A.5 ofAppendixA.

Table 2:In iti a lIso t o p ic C om p ositio n o fFu e l

LEU W G -M O X(D EIS/ D CS)

RG - M O X

Ura n iu m c o m p osition (w/o )

U-234U-235U-236U-238

0.044.250.0195 .7

0.0020.20.00199 .797

0.0020.20.00199 .797

Pluton iu m c o m p osition (w/o )

Pu-238Pu-239Pu-240Pu-241Pu-242

----------

0.04/0 .0492 .37/93.086.49/6 .540.24/0 .210.1 /0.1

2.356 .224 .29.06.9

A m-241 -- 0.76/0.03 1.4


16/47

Ly m a n

48

Consequence Assessment

Usingthe core radionuclide inventoriestabulatedinAppendixA, one cancal-

culate the radiologicalconsequencesofsevere accidents. The NRC-approved

MACCS2 code

45

wasusedtoestimatetheconsequencesofthreesevereacci-

dentsforLEU, DEISandDCScores. Arepresentative four-loopPWRwithan

ice-condensercontainmentwaschosenforanalysis.

The indicatorsused to measure accident consequences are (1) the total

number of expected latent cancer fatalities (LCFs) among the population

withinathousandmilesoftheplant, (2)thetotalnumberofpromptfatalities

(PFs)caused byacute radiation exposure, and(3)the average LCFrisktoan

individual within ten miles of the plant.

46

The calculation only considered

radiation exposures received duringa one-week emergency phase following

the accident.

TheMACCS2 databasewasupdated byreplacingtheoutdateddosecon-

version factors in the model by the mostrecent compilationof the Interna-

tionalCommissiononRadiologicalProtection(ICRP), ICRPPublication 72.

47

These factorsare based ona revised model of the human respiratory tract

whichreducesthe dose frominhalationofactinides.

48

However, otherinternal

featuresoftheMACCS2 codemayresultinunderestimatesoftheradiological

riskassociatedwith exposure toalpha-emitters.

49

In the calculations, generic parameters were used for population and

atmospheric data.

50

Since the quantities of most interest are the ratios of

r

esu

lts

for

W

G-MOX

an

d LE

Ucor

es

, mor

e deta

ileds

it

e chara

ct

er

izat

ion

was

notnecessary. Whiletheabsolutevaluesoftheaccidentconsequencesdepend

strongly on these parameters, the consequence ratios donot. Varying these

parametershadonlyasmall effect(lessthan 10%)onconsequence ratios.

Severeaccidentsourceterms

The MACCS2 code requiresas inputasourceterm

, the specificradionuclide

releasethatoccursduringanaccident. Thesourcetermisusuallyexpressed

asasetofreleasefractions

---thefractionofthecoreinventoryofeachradio-

nuclide that is released to the environment. The timingand magnitude of

the release of a particular radionuclide from failed fuel rods is determined

bythe plantconditionsassociatedwith eachaccidentsequence andthe ther-

moch

emicalp

rope

rt

iesof

thera

dionu

clide.Volatile and semi-volatile radionuclides, such as the noble gases,

iodine, tellurium and cesium, are usually released from melted fuel in the

form of gases or very fine aerosols that can easily escape into the environ-

ment througha breach inthe containment. Incontrast, mostactinide met-

alsand oxideshave extremelyhigh meltingand boiling points, and will be


17/47


49

releasedinlarge quantitiesonlyifaviolentexplosioncausesmechanicaldis-

persalofthe moltenuraniumoxide core. Nevertheless, mechanicaldispersal

offuelispossible duringsevere accidents.

Tosimplifythe analysisofaccidentconsequences, radionuclideswithsim-

ilar thermochemical propertiesare grouped together ina release class. All

members of a release class are assumed to behave identically during the

course ofanaccident. Puisassignedtothe cerium(Ce)groupandNp, Amand

Cmare assignedtothe lanthanum(La)group.51

Table 3 displaysa set of simplified severe accident source terms which

were derivedfromthe NRC'sPRAforthe Sequoyahplant.52 Sequoyah, afour-

loopWestinghousePWRwithanicecondensercontainment, isverysimilarto

the DCS Catawba and McGuire plants. The Sequoyah analysis was usedbecause peer-reviewedPRAsforCatawbaandMcGuire are notpubliclyavail-

able.53

These source termsare associatedwiththe three differentplant(core and

containment)damage statesthatcontribute tothe riskoflarge earlyreleases.

Eachone correspondstoasevere accidentthatresultsinlossofcore cooling,

core melt, breachofthe pressure vesselandradiologicalrelease tothe envi-

ronment. Eachsource term comprises two plumes, the first consistingofa

shortdurationpulse correspondingto breach(or bypass)ofthe containment

an

dth

es

econ

d, lon

ger

pu

ls

e corr

es

pon

din

gtosu

bs

equ

ent

r

eleas

es

fromco

r

e-concrete interactions.

The three eventsare distinguished bythe timingandmode ofradionuclide

releasetotheenvironment. ST-1 andST-2 arerepresentativeofveryearly

and early containment failures, occurring prior to and concurrent with

breach of the pressure vessel, respectively. ST-3 is a containment bypass

Table 3:So urc e Te rmsFor C onseq ue nc e C a lc u l a tio ns

So urc et e rm

Tim e o fRe le ase(h )

Re l e ased ura tio n(s)

R a d io n u c li d e re l e ase fra ctio ns

Kr I C s Te Sr Ru La C e

ST-1(f1=2.8E-7)

5.56.0

2007200

10

0.370.22

0.270.35

0.130.30

0.0250.13

8E-33E-3

1.6E-30.013

8E-30.018

ST-2(f2=3.6-E6)

ST-3(f3=3.1E-6)

6.06.06

1.01.5

2007200

18007200

10

10

0.050.13

0.0750.04

0.040.15

0.060.06

0.020.11

0.020.05

4E-30.045

5E-30.02

1E-31E-3

1E-36E-4

2E-45E-3

3E-43E-3

1E-36E-3

1E-33E-3


18/47

Ly m a n 50

event, inwhichreleasetotheenvironmentoccursthrough breachofasteam

generatororanother barrier betweenthe primarycoolantsystemandsystems

outside ofcontainment. The probabilityofvery earlycontainmentfailure (ST-

1)isassociatedwithhydrogencombustion, andforPWRsitisthoughtto be

significantonlyforice-condensercontainments.

The sum of the frequencies of all the accidents corresponding to each

source term in Table 3 equals the frequency of occurrence f, which is the

expectedprobabilityperyearofanaccidentresultinginthe specifiedradionu-

clide release. (These valuestake intoaccountinternalinitiating eventsonly

--- that is, they donot consider the possibility of earthquakes, floods, high

windsor fires.)

The three source termsinTable 3 accountforallsevere accidentsthatareassociated with a significant risk ofPFs and LCFs (from early exposures).

The sum of their frequencies equals the large early release frequency

(LERF), which is defined in NRC RG 1.174 asthe frequencyof those acci-

dentsleadingtosignificant, unmitigatedreleasesfromcontainmentinatime

frame priorto effective evacuationofthe close-inpopulationsuchthatthere is

apotentialfor earlyhealth effects.

Allotheraccidentsfallintotwootherplantdamage states: eitherlate

containmentfailure (twenty-fourhoursafterthe startofthe accident), orno

containmentfailure. Neitherofthesehasthepotentialforcausingearlyfatal-

ities;theytherefore donotcontribute tothe LERF. Moreover, theircontribu-

tion to the overall risk to the public of latent cancer fatalities isnegligible

comparedto earlycontainmentfailure or bypassaccidents. The only excep-

tion is for late containment failure, in the unlikely event thata significant

fractionofthenearbypopulationhadnot beenevacuatedafulldayafterthe

startofthe accident.

Results

ResultsoftheMACCS 2 calculationsshowthathighertransuranicinven-

toriesinMOXcoresleadtoincreasedsevere accidentconsequences.54Table 4

presents the resultsobtained byaveraging the LCFscaused bysevere acci-

dentsatEOCandBOC. The cycle-averagednumberof LCFsis 81%to 96%

greaterforaDEIScore (correspondingto 1,440 to 6,165 additional LCFs), and

20%to 25%g

reat

erfo

raDCSco

re(co

rrespon

din

gto 350

to 1,855

addi

tiona

lLCFs). The EOC and BOC results can be found in Tables B.1 and B.2 of

AppendixB.


19/47

Pu blic H e a lth Risks ofSu bstitu tin g Mix e d - O x id e for Ura niu mFu el 51

The fractional increase in LCFs isapproximately four timessmaller for

the DCScore thanforthe DEIScore, reflecting both the reducedMOXcore

loadingandthe effectofAm-241 removalfromPupolishing.55Fora fixedcore

loading, removalofAm-241 reducesthe numberofLCFs byabout 10%.

Smaller increases (around 40% for the DEIS core and 5% for the DCS

core)occur in thenumberofPFsresulting fromearlycontainment failures.

For containment bypass, the number ofPFs was found to decrease by 12%

(from 33

to 29). Howeve

r,th

etota

lnu

mberof

PFsisapp

roxim

ately

ahun

dred

timessmallerthanthe numberofLCFs.

Sensitivityanalysis

A sensitivity analysis was carried out to assess the impact of varying the

actinide release fractions.

The three PWRsource termspresentedinTable 3 are basedonthe mean

values of distributions of experimental and analytical data for the release

fractions in each class. Use of mean values isappropriate for volatile and

semi-volatileradionuclides, becausetherangeofuncertaintyis believedto be

within an order of magnitude. For low-volatile radionuclides like the

actinides, however, the release fractionscanvary byseveralordersofmagni-

tude, dependingondetailsof the accidentprogression. For thisreason, the

MACCS2 calculationwasrepeatedforarange ofactinide release fractions.

Threesourcetermswerecompared(Table 5). ST-MisthesameasST-1 of

Table 3, withactinide release fractionsonthe orderof 1%. ST-HandST-L,

whichincorporate highandlowvaluesofthe actinide release fractions, were

Table 4:C on se q u e n c es o fse v e re re a ct or a c cid e n ts ( c y c le a v e ra g e).

LEU W G -M O X W G -M O X/ LEU R a tio

So urc e te rm: DEIS c ore D CS c ore DEIS c ore D CS c ore

ST-1Late nt c an c e rfa t a litiesPro m p t fa t a lities

7,56562

13 ,73088

9,42064

1.811.42

1.251.03


2,51520

4,92028

3,08021

1.961.40

1.221.05


1,73533

3,17528

2,08529

1.830.85

1.200.88


20/47

Ly m a n 52

basedonthelimitsofthedatadistributions.56 Theresultsindicatethatfora

DEIS core, the consequences depend strongly on the magnitudes of the

actinide release fractions. Forthe DCScore withAm-241 removal, the depen-

dence onactinide release fractionsislesspronounced.

Forthe DEIScore, the cycle-averagedincrease inLCFsrangesfrom 39%

to 131%forthe three release fractions, correspondingtoanincrease of1,730

to 18,185 LCFsforthe genericsite. Inthe worstcase (ST-H), the numberof

additional LCFsisabout 60%ofthe totalnumberofcancersworldwide pre-dictedtoresultfromtheChernobylaccident. FortheDCScore, thecycle-aver-

agedLCFsare 11%to 30%greaterthanforLEUcores, correspondingto 510

to 4,185 additionalLCFs.57

ResultsatEOCandBOCare givenin TablesB.3 andB.4 ofAppendixB.

ResultsforRG-MOXwere alsocalculatedatEOCasafunctionofactinide

release fraction (AppendixC). Compared toan LEUcore, Am-241 andCm-

242 inventoriesatEOCare 20 and 27 timesgreater, respectively. The corre-

spondingnumberofLCFsisgreater by 123%to 486%, approximatelyafactor

offourgreaterthanforWG-MOX. Thissignificantincrease inriskshould be

animportantconsiderationfornationssuchasFrance andJapanthatare cur-

rentlyusingorplanningtointroduceRG-MOXintotheirnuclearplants.

Insummary, we findthatthe numberofLCFsresultingfromshort-termexposuresfollowingasevere reactoraccidentwill be significantlygreaterfor

PWRs with either full or partialWG-MOX cores than forPWRs with LEU

cores. Inno case was the number of LCFs smaller for the WG-MOX core,

unlikesomeoftheresultsreportedintheDOEDEIS.

Table 5:Se nsitiv ity o f c o nse q u e n c es to a ct in id e re le ase fra c tion ( cyc le a v e ra g e).


So urc e t e rm: DEIS c ore D CS c ore DEIS c ore D CS c ore

ST-M (La R.F.=0.015)Lat ent c an c e rfa t a lit ies

Pro m p t fa t a liti es7,565

62

13 ,730

88

9,420

64

1.81

1.50

1.25

1.03

ST-H (La R.F.=0 .06)Lat ent c an c e rfa t a lit iesPro m p t fa t a liti es

13 ,915234

32 ,100407

18 ,100248

2.311.74

1.301.06

ST-L (La R.F.=0 .003)Lat ent c an c e rfa t a lit iesPro m p t fa t a liti es

4,48026

6,21032

4,99027

1.391.23

1.111.04


21/47


Thecalculationofconsequencesdue to long-term, chronicexposures fol-

lowing the accident is beyond the scope of this paper. Such calculations

require the specification of many more parameters than calculations

restrictedtothe periodimmediatelyafterthe accidentandare associatedwith

considerablygreateruncertainties. However, one may expectthatthe costof

landdecontaminationwill increaseasaresultofthegreaterfalloutoflong-

livedactinides.

A Risk-Informed Ana lysis of MOX Loading

ComparedtoanLEUcore, the numberoflatentcancerfatalitiescaused bya

severe accidenthas beencalculatedabove to be greateronaverage byabout

85% for a DEIS core or by 22% for a DCS core. Therefore, the total risk,

definedasthe sumofthe productsofthe probabilityandconsequencesforall

accidentsequences, willalso begreater forMOXcores. In thissection, the

increase inriskassociatedwithsubstitutingMOX forLEU is estimated for

these twocases.

Because the baseline riskofnuclearpowerplantoperationis believedto

be small, these risk increasesare also likely to be small. To judge whether

suchincreaseswill beacceptabletotheNRC, however, theymust beevaluated

inaccordance withthe risk-informedregulatoryprinciplesthatthe NRCis

nowincorporatingintoitsregulations. Incontrasttothe deterministicregula-

tionsthatare nowthe norm, arisk-informedapproachrepresentsaphiloso-

phy whereby risk insights are considered together with other factors to ...

better focus ... regulatory attention on ... issues commensurate with their

importance topublichealthandsafety.58

One canassessthe regulatorysignificance of the increasedriskofMOX

use by using the methodology of NRC Regulatory Guide (RG) 1.174 (see

above). One of the main principlesupon which RG 1.174 is based is that

whenproposedchangesresultinanincreasein ... risk, theincreasesshould

be small....

Because the calculationofrisktothe publiccan be verycomplicated, RG

1.174 definessimplersurrogateobjectivesthatcan be usedtomeetitsguide-

lin

es. O

ne of

these objec

tive

sis

express

edin

termsof

the pl

antLE

RF.

The RG 1.174 guidelines for NRC consideration of license amendment

applicationsresultinginincreasesinLERF(positive LERF)are asfollows

(RY=reactor-year):


22/47

Ly m a n 54

LERF< 10-7/RY:applicationwill beconsideredregardlessofwhetherthere

isacalculationoftotalLERF.

10-7/RY< LERF< 10-6/RY:application will be consideredonly if it can be

reasonablyshownthatthe total LERFislessthan 10-5/RY.

10-6/RY


23/47


contributetothe LERFdonotchangewhenWG-MOXissubstitutedfor LEU,

then LERFeff can be relatedtothe change inrisk Raccordingtothe fol-

lowingrelation(Eq. 1):

(1)

where arethefrequenciesofoccurrenceoftheaccidentsthatcontributeto

the LERF(i.e. ), andCiistheithconsequencecoefficient,which

isdefinedinthispaperasthe average risktoanindividualwithintenmilesof

anuclearplantofcontractinga fatalcancer, given the occurrence of the ith

plantdamage state.62 The annual LCFrisk(asdefinedinthe NRCQuantita-tiveHealthObjectives)isthengiven by . (Ralsoincludesaddi-

tional terms representing late containment failure and no containment

failure, butthe valuesoffiCiforthese plantdamage statesare negligible in

comparisontothe otherterms.)

Table 6 listsvaluesforthe LCFconsequence coefficientsCiforthe three

sourcetermsin Table 3, averagedoveracycle. FortheDEIScore, theconse-

quence coefficients are 74% to 116% greater than thoseassociated with an

LEUcore forthe three source terms. For the DCScore, the coefficientsare

greater by 21%to 30%.

Forthe Sequoyahplant, the LERFisthe sumofthe three accidentfre-

quencieslistedin Table 6:very earlycontainmentfailure (duringcore degra-

dation), containment failure at vessel breach, and containment bypass.

Utilizingthefrequencydataandconsequencecoefficientsgivenin Table 6, one

findsforthe DCScore that

Table 6: Lat ent c an c e r c o nse q u e n c e c o e ffic ie nts



ST-1 (f1=2 .8x10-7) 0.0214 0.0372 0.0265 1.74 1.24

ST-2 (f2=3 .6x10-6) 0.0128 0.0276 0.0166 2.16 1.30

ST-3 (f3=3 .1x10-6) 0.0114 0.0208 0.0138 1.83 1.21

LERFef f LERF R R LERF 1( R )i f( iCi)=

fi

LERF ifi

=

R if(iC

i)=


24/47

Ly m a n 56

LERF=7.0x10-6,

RLEU=8.74x10-8,

RWG-MOX=1.10x10-7and

R/R=0.26.

Therefore, usingEq. 1, LERFeff=1.8x10-6, which exceeds by 80%the RG

1.174 thresholdof1x10-6for LERF. RepeatingthecalculationfortheDEIS

core, one findsthat R/R=0.99 and LERFeff=6.9x10-6, whichisnearlyseventimesgreaterthanthe limit.

This calculation is based on NUREG-1150, which does not take into

account externalinitiating events. Since external eventswillcontribute sub-

stantiallyto both LERFand LERFeff, itisnecessarytoincludetheminthe

analysis.

External eventsare analyzed inthe McGuire andCatawbaIPEsubmit-

tals. Asdiscussedpreviously, the IPEshave not beenfullypeer-reviewed, so

these resultsshould be consideredpreliminary. The total LERFs (including

bothinternalandexternalevents)calculated byDukePowerare 4.7x10-6and

6.3x10-6 for McGuire and Catawba, respectively.63 Using these values, one

findsthatthe correspondingvaluesof LERFeffforthe DCSMOXplanare

1.2x10-6 and 1.6x10-6. In bothcases, the 1x10-6thresholdwould be exceeded,

and the NRC willhave justification on risk grounds to prohibit the use of

MOXinMcGuireandCatawba, ortorequiresignificantrestrictionsontheir

licensestolowerthe operatingriskofthe plants before MOXcan be loaded. A

similarconclusionislikelytoapplyformostothernuclearplantsinthe U.S.64

The safety case for MOX use in McGuire and Catawba appears even

weakerinlightofarecentSNL study findingthatthe Duke PowerIPEssig-

nificantlyunderestimatedthevulnerabilityofice-condensercontainmentsto

earlyfailure fromhydrogencombustionduringsevere accidents. SNLcalcu-

latesthatthe probabilityof earlycontainmentfailure (givencore damage)at

McGuire is 13.9%forinternal events, afactorofsevengreaterthanthe value

estimated in the IPE.65 Using this result, the McGuire LERF for internal

eventsincreasesto 6.7x10-6. Ifexternaleventsareconsidered, thetotal LERF

will exceed 1x10-5.66According to RG 1.174, plants with LERFs exceeding

1x10-5 cannot make changes that would increase the LERF by more than

1x10-7. Since LERFeffforthe DCScore ismore thanafactoroftengreater

thanthisthreshold, thisanalysisstrengthensthe conclusionthatthe use of


25/47


MOXatMcGuirewouldposeanunacceptableriskandshouldnot beallowed.

Therefore, unlessDCScanshowconvincinglythatthe McGuire LERFis

actually much smaller than the value based on recent SNL analysis, it

appearsunlikelythatitwill be permittedtoloadMOXinMcGuire.

Accordingtothese results, the riskincrease resultingfromthe substitu-

tionofpartialorfullcoresofWG-MOXfor LEUinmostU.S. PWRswillexceed

the RG 1.174 threshold. Thisdoesnotimplythatthe NRCwillprohibitMOX

use, especiallyifitconcludesthatthe non-quantifiable benefitsofplutonium

dispositionoutweighthe riskincrease. Nonetheless, ataminimum, anyutility

wishingtouse MOXwillhave tocarryoutadetailedlevel-3 PRAofitsplants

for both LEU and partial MOX cores, and mayhave significant and costly

restrictionsplacedonthe plantsoperatinglicenses.

MOX Core AccidentProbabilities

TheprecedingriskanalysisassumedthattheuseofMOXfuelwouldhaveno

effecton the probabilitiesofsevere accidents. However, asdiscussedabove,

MOXfuelandLEUfuelhave differentneutronicandthermomechanicalprop-

erties. As a result, the probabilities of severe accident sequences may

increase, decreaseorremainthesamewhenWG-MOXfuelisused. The qual-

itativeanalysis belowsuggeststhattheprobabilitiesofthemostrisk-signifi-

cant accidents at LEU-fueled PWRs will be comparable or even higher at

MOX-fueledPWRs. Therefore, the assumptionthatthe probabilitiesare thesame isnot likelytooverestimate the MOXriskandmay in factunderesti-

mateit.

AccidentprecursorsinPWRscan begroupedintothefollowingcategories:

those causing(1)anincrease inpower, (2)anincrease incoolanttemperature

orpressure and(3)adecrease incoolanttemperature orpressure.67Aprecur-

soroflimitingseverityinthe firstcategoryisarodclustercontrolassembly

(RCCA)ejection;inthesecond, aloss-of-coolantaccident(LOCA), andinthe

third, amainsteamline break(MSLB).

Basedonreactivityconsiderationsalone, precursorsinthe firstcategory

may be eithermore orlesssevere inMOXcores, those inthe secondmay be

lesssevere and those in the thirdwill be more severe. (However, when the

thermomechanical differences of the two fuel typesare taken intoaccount,

anyadvantagesofMOX become lessapparent.) Theoverall impactofMOX

fueluse onthe totalcore damage frequency(CDF)andLERFdependsonthe

relative frequenciesof these typesofaccidents, whichare different for each

nuclearplant. Aresolutionofthis questionwillrequire plant-specificPRAs


26/47

Ly m a n 58

forMOXcores. However, itispossibletomakesomegeneralobservations.

Rodclustercontrolassembly(RCCA)ejection

The primarycoolantsysteminPWRsismaintainedathighpressure. Ifarod

clustercontrolassembly(RCCA)were tocome loose fromitshousing, itcould

be ejected from the core. This would lead toa reactivity insertionaccident

(RIA)---arapidincreaseinreactivitywithlocalpowerspikesthatcouldcause

fueldamage.

Westinghouse hasanalyzedthe RCCA ejection eventforpartialWG-MOX

cores.68Itisdifficulttopredictwhetherthisaccidentwould be more severe in

aMOXcore, because itdependsonanumberofcompetingfactors, including

thewo

rthof

theejec

t

edRCCAan

dth

edelayed

neutr

on

fra

ct

ion

.For

a

part

ia

lMOX core, the worth ofan individual RCCA is reduced, and if ejected, the

increase inreactivitywould be smaller. However, the resultingrise inpower

could be more rapid because the delayed neutron fraction is also smaller.

AccordingtoWestinghouse, the smallerRCCAworth, alongwithmore nega-

tivereactivitycoefficients, morethancompensatefortheunfavorableimpact

ofthe delayedneutronfraction.

Inparticular, foranRCCA ejectionoccurringatthe end-of-cycle (atwhich

pointthe burnupswillalso be the greatest), the maximum enthalpywasfound

to be 150 cal/g. Because theWestinghouse analysisusesthe LEU-basedcrite-

rion that fuel cladding will remain intact provided the maximum fuel rod

enthalpy (absorbedheat) is below 200 cal/g, no fuel damage is predicted to

occur.However, theWestinghouse analysisdoesnottake intoaccountthe exper-

imental evidence from the Cabri testreactor thathigh-burnupMOX fuel is

lessresistant toreactivity insertions than LEU fuel. Infact, thecalculated

maximumenthalpyexceedsthe 120 cal/gvalueatwhichtheMOXrodrupture

wasobservedinthe Cabritest. Therefore, the increasedvulnerabilityofhigh

burnup MOX fuel could cancel out the beneficial effect of the reduced rod

worth, unlessstrict burnuplimitswere imposedonMOXfuel.

Moreover, DCS may change the control rod material in its plants to

increasetherodworths, inwhichcasetheassumptionthattheworthofthe

ejectedrodwould be lowerinaMOXcore wouldnot be valid. Therefore, itis

unlikelythatthistransientwould be more benignforDCScores.

Loss-of-coolantaccidents

Loss-of-coolantaccidents(LOCAs), initiated eitherdirectly(bypipe breaks)or

indirectly(bylossofcomponentcooling, resultingintemperature-inducedfail-

ure ofpumpseals), rankamongthe mostrisk-significantaccidents. For exam-


27/47


ple, theCatawbaIPEestimatesthat LOCAscontributemorethan 80%tothe

CDFforinternal events. The probabilitythataLOCAwillleadtocore dam-

age dependsonthe extenttowhichfuelcladdingisdamagedpriortostartup

of emergency core cooling, which in turn depends on the fuel rod tempera-

tures. Although the rate of decayheat generation will be slightly lower in

MOXfuelthanin LEUfuelimmediatelyafteratrip(becausethethermal flux

issmaller), the thermalconductivityofMOXfuelisabout 10%lower, andthe

centerline temperature about 50Chigher.

Because MOX fuelhasahigher centerline temperature than LEU fuel,

the rise incladdingtemperature andrate ofcladdingoxidationduringthe ini-

tialstagesofa LOCAcan begreaterthanfor LEUfuel, anditmay beharder

forMOXcorestomeetthe NRC'srequirementsforLOCAmitigation.69Insome transientsthatcause anincrease incoolanttemperature, suchas

alossofheatsink, the strongernegative temperature feedbackinMOXcores

couldinprinciple be beneficial bymore rapidlyforcingthe reactortoasubcrit-

icalstate. However, insuchaccidentsthereactorisexpectedtotripautomati-

callyon excessive temperature orpressure signals, afterwhichheatremoval

becomesthe mainconcern. Therefore, the only eventsthatmay benefitfrom

thispropertyofMOXfuelare those inwhichthe reactorfailstotrip(the so-

called anticipated transients without scram, orATWS), which have rela-

tively low frequencies inPWRs. In the Catawba IPE, for example, ATWS

eventscontributedlessthan 3%tothe internalCDF. Therefore, areductionin

the severityofATWS eventswouldonlyhave asmall effectonthe LERF.

Overcoolingtransientsandpressurizedthermalshock

The more negative moderatortemperature coefficientinMOXcorescanmake

overcooling eventsmore severe, since agivenrate oftemperature decrease is

associatedwithagreaterrateofreactivity increase. Overcoolingsequences

can be initiated bymanydifferent events, mostofwhichare associatedwitha

leakinthe secondarycoolantsystem. Thiscanoccur eitherfromapipe fail-

ure, suchasamainsteamline break(MSLB), orasaresultofareactortrip

followed by the failure of a secondary-side valve to close. Other types of

events, such as steam generator tube ruptures or small-break LOCAs, can

alsoleadtoovercoolingtransients.

The double-endedMSLBoccurringwhenthe core isinhotstandbyiscon-

sideredthe mostserioustransientofthisclass. Inthisaccident, the pressure

andtemperature ofthe reactorcoolantsystem(RCS) both begintodecrease

rapidly. For some core configurations, the reactor core can reach criticality

andreturntopower. Thisaccidentismitigatedinitially byanautomatichigh-


28/47

Ly m a n 60

pressure safety injection (SI) system, which injects borated water into the

core, andas the pressure continues todrop, bypassive accumulators, which

floodthe core withmore boratedwater.

Westinghouse analyzedthe MSLBaccidentfor bothpartialandfullWG-

MOXcoresinits 1996 report. Itfoundthat eventhoughreturntocriticality

andpeakcorepoweroccurredsoonerfortheMOXcore, safetylimitswerenot

exceeded.

However,Westinghouse didnotconsideraparticularlysevere outcome of

the MSLBaccidentthatislikelytocause core damage andcontainmentfail-

ure: rupture of the PWR reactor vessel asa result of pressurized thermal

shock(PTS). PTScanoccurifthetemperatureofthereactorvesselisreduced

below a threshold (known as the nil-ductility transition temperature, orNDTT)while the vesselispressurized, orifthe vesselisrepressurizedwhile

stillcold. The riskofPTSincreaseswiththe age ofthe reactorvessel, because

exposure of the vessel to the neutron flux in an operating reactor slowly

increasestheNDTT, aprocessknownasneutronembrittlement.

Topreventthe occurrence ofPTSfollowingaMSLB, twooperatoractions

are required: termination of the auxiliary feedwater system (AFS) to the

affectedloopandterminationofthe safetyinjectionsystem. The firstaction

willhaltthe decrease intemperature ofthe RCSandthe secondwillprevent

repressurizationtoapotentiallydangerouslevel.

Resultsofthe simulationofaMSLBinaMOX-fueledPWRconducted by

Westinghouse indicate that the rate of RCS temperature decrease is much

more rapidin bothpartialandfull-MOXcoresthaninan LEUcore. After 400

seconds, the RCSaverage temperature inthe fullMOXcore drops by 210F

(117C)to 350F(177C), whilethetemperatureinthe LEUcoredropsonly

byaround 120F(67C), to 440F(227C). AccordingtoNRC, finaltempera-

turesof350Fandlessare expectedto be potentialPTSinitiating events.70

Since the MOXRCStemperature entersthe regionofconcernsoonerthanthe

LEUcore, the operatormustterminate the AFSmore rapidlyinordertohalt

thedeclineintemperature. Thisdecreasesthelikelihoodthatoperatorswill

be able toactintime tosave the reactorvessel.

Inaddition, the rate ofRPV embrittlementis expectedto be greaterina

MOX core because a greater flux ofneutrons with energies greater than 1

MeVwill be produced.71 Therefore, the riskofPTSmay be greaterinaMOX

core for two reasons ---a greater frequencyof events that can overcool the

reactorvesseltoadangerouslylowtemperature, andahigherNDTT.

PTSisofspecialconcernforRussian-designedpressurized-waterreactors,

orVVERs, whichare especially prone to vessel embrittlement, according to

the InternationalAtomicEnergyAgency. Thisfactshouldnot be overlooked


29/47


30/47

Ly m a n 62

not bethecase. Concernswiththerobustnessofhigh-burnupMOXfuelinthe

eventofanaccidenthave led the Frenchnuclear safetyauthority DSIN to

restrict the average burnup of MOX fuel assemblies to about 41 GWD/t,

whereas LEUfuelispermittedtoreach 52 GWD/t. The U.S. shoulddolike-

wise andrestrictthe assembly-averagedMOX burnupto 41 GWD/tpending

resolutionofthesesafetyissues. Thiswill beacostlyinconvenienceforDCS,

whichintendsatthe beginningofthe programtoirradiate MOXfuelassem-

bliesto 45 GWD/t, andtoraise the burnuplimit eventually.

The U.S. planto encourage RussiatouseWG-MOXinRussianandUkrai-

nianVVER-1000sposes evengreaterrisksthanthe planforU.S. domesticuse

ofWG-MOX. VVER-1000sdonotmeetWesternsafetystandardsinsuchcriti-

calareasasfire protectionandinstrumentationandcontrolsystems. Inaddi-tion, the pressure vesselsofVVER-1000sare highly embrittled. The U.S. is

notrequiringthatthese plants be upgradedsothattheyfullymeetWestern

safety standards, which would cost on the order of $150 million per unit.

Given that theuse of MOX will increase risk even in plants that do meet

Westernstandards, encouragingRussiatouse MOXinVVER-1000swithout

ensuringmaximaladherence tosafetyisunwise.

APPENDIX A : C alculation ofLEU and WG-MOX Core Inventories

This appendix contains additional information necessary to carry out the

SCALE 4.3 inventorycalculationsdescribedabove andthe resultsofthe cal-culations for 58 radionuclides. Table A.1 shows the dimensions of the PWR

fuelassembliesusedinthecalculations.

Table A .1:Fu e l a sse m b ly sp e c ific a tio ns

Fu e l Pa ra m e t e r Va lu e

M ass o f a sse m b ly (TH M) 0.423

N u m b e r o ffu e lro ds p e r asse m b ly 264

A c tiv e le n g th ( c m) 365.76

Fu e

lro d p

it c h ( c m) 1.2598

Fu e lro d c l a d o ute r d i a m e t e r ( c m) 0.9144

Fu e lro d c l a d inn e r d i a m e t e r ( c m) 0.8001

Fu e lrod ou te r d i a m e t e r ( c m) 0.7844


31/47


Thefuelassemblyspecificpowersarealsoneededforeachfuel batch. Forthe

EOC calculation, specific powers for each batch were approximated by the

core-averaged assembly power near the midpoint of the equilibrium cycle

(Table A.2). The differentrate ofchange ofreactivitywith burnupofthe two

typesof fuelresults insomewhatdifferentassemblypowersand irradiation

histories. BOCcalculationswerecarriedoutforanexposuretimeof3.4 days,

correspondingtoareloadfuel burnupofabout 0.15 GWD/t.

Full-MOX Results

TableA.3 providestheresultsofthecalculationofradionuclideinventoriesat

EOCforLEUandfullWG-MOXcores. Fissionproductinventoriesrarelydif-

fer bymore thanafactoroftwo betweenthe twocores. However, some radio-

logicallyimportantradionuclidesare increasedintheWG-MOXcore, suchas

the semi-volatile isotopes of ruthenium (Ru), antimony (Sb) and tellurium

(Te), while thosewithreduced inventories includethe lower-volatilityradio-

nuclidessuchasyttrium(Y), strontium(Sr)andcerium(Ce).Table A.4 liststhe actinide inventoriesforLEUandfullWG-MOXcoresat

the beginning-of-cycle (BOC). Because the inventoriesoftransuranic(TRU)

elements(Np, Pu, Am, Cm)aremuchsmalleratBOCinthe LEUcore, while

theplutoniumconcentrationisnearitsmaximumintheMOXcore, theMOX/

LEUratiosformosthigheractinidesare greateratBOCthanatEOC.

Table A .2:C ore irra d i a t io n h istori es

Ba t c h N o . asse m b li es

Av e ra g e p o w e r

a t m id-cyc le(MW/t)

Av e ra ge burnu p(M WD / t )

LEUFe e dO n c e -burn e dTw i c e-burn e d

888817

53 .938 .915 .4

24 ,79042 ,69049 ,770

C ore - a v e ra g e d b u rnup a t end o f c y c le 35 ,150

WG-PuFe e dO n c e -burn e dTw i c e-burn e d

92929

49 .639 .625 .0

23 ,86042 ,91054 ,930

C ore - a v e ra g e d b u rn up a t end o f c y c l e34 ,390


32/47

Ly m a n 64

TableA.3 showstheEOC TRUinventoriesfortheDCScore(a 40%WG-

MOX core with plutonium polishing). The significantly smaller actinide

inventoriesare apparent.

Extra polation to Partia l MOX Cores

The radionuclide inventoriesforpartialMOXcorescan be estimatedfromthe

full-core inventories by linearly scaling the result with respect to the MOX

core fraction. Forinstance, fora 40%MOXcore (78 MOXassemblies), agiven

partialMOXcoreinventoryvaluecan beexpressedintermsofthe LEUand

fu

ll MOX

va

lu

es

fr

omth

e equat

ion

I0.4= 55(U0+U1) + 37(M0+M1) + 5 U2 +4M2, where U0isthe inventoryoffirst-cycle LEUfuel, etc. Here itisassumed

thatthe average plutonium enrichmentsinMOXfuelreloadsare the same for

bothfullandpartialcores.

Thisestimatedependsontheassumptionthattheradionuclideinventory

inaMOXassemblyisinsensitive tothe differencesinthe neutronspectraof

fullandpartialMOXcores. The validityofthisassumption is evidentfrom

graphspresentedinthe 1996 Westinghouse studyforplutonium inventories

versus burnup of MOX assemblies with nearly identical fissile plutonium

enrichmentsof4.5%and 4.515%thatareirradiatedinpartialandfullMOX

cores, respectively. Comparisonofthe twographsdoesnotrevealanyvisible

differencesinthe plutoniumisotopiccontentofthe twoassemblies, implying

thatsuchdifferencesare well below 1%forall burnups. Inventoriesofotheractinidesshouldhave asimilardependence onneutronspectrum, andinven-

toriesoffissionproductsshould beevenmoreweaklydependent. Thusitcan

be concludedthatthe linearapproximationforpartialMOXcore inventories

isreasonable.


33/47


Table A .3:R a d io n u c li d e c ore inven tori es at E O C , DEIS C ore

LEU C ore Inven tory(M C i)

W G-M OX C ore Inven tory(M C i)

M O X / LEUR a t io

Fissio n pro d u c ts

Kr-85Kr-85mKr-87Kr-88Rb-86Sr-89Sr-90

Sr-91Sr-92Y-90Y-91Y-92Y-93Zr-95Zr-97Nb-95Mo-99Tc-99mRu-103Ru-105Ru-106Rh-105

Sb-127Sb-129Te-127Te-127mTe-129Te-129mTe-131mTe-132I-131I-132I-133I-134I-135

0.827724 .3949 .0468 .260.133897 .497.110

118.9125.77.409125.6126.795 .93165.6153.9166.1177.2155.7143.895 .9842 .5790 .07

7.81229 .677.6621.24028 .215.70618 .20135.193 .95137.4194.7216.4185.6

0.452714 .7928 .4237 .960.0727850 .213.248

71 .4884 .523.34673 .0784 .9971 .40136.2142.0136.1172.9151.8190.0149.990 .60143.0

11 .1335 .1911 .111.97433 .687.05423 .66139.199 .80142.9191.3205.1183.9

0.550.610.580.560.540.520.46

0.600.670.450.580.670.740.820.920.820.980.971.321.562.131.59

1.421.191.451.591.191.241.301.031.061.040.980.950.99


34/47

Ly m a n 66

Table A .3 (Continued):R a d io n u c li d e c ore inven tori es at E O C , DEIS C ore


W G-MO X C ore Inven tory(M C i)

M O X / LEUR a tio

Fissio n Pro d u c ts

Xe-133Xe-135C s-134C s-136C s-137Ba-139

Ba-140La-140La-141La-142C e-141C e-143C e-144Pr-143Nd-147

194.845 .8712 .473.7229.422172.5

174.0179.8158.0154.5159.9148.5115.3145.763 .73

191.777 .0211 .996.2139.331159.4

161.7165.6145.5139.1147.9126.490 .35124.060 .33

0.981.680.961.670.990.92

0.930.920.920.900.920.850.780.850.95

A c tin id es

Np-239Pu-238Pu-239Pu-240

Pu-241Am-241C m-242C m-244

17550.21520.026670.03479

10 .620.009732.9650.1757

16920.29460.088290.1534

38 .010.0663321 .370.5386

0.961.373.314.41

3.586.847.213.07


35/47


APPENDIX B: Severe Accident Consequences atthe End-of-Cycle

(EOC) and Beginning-of-Cycle (BO C)

TheresultsofMACCS2 calculationsforsevereaccidentsatEOCandBOCare

pr

esent

ed in

Table

s B.1

throu

gh

B.4. Th

e infor

mat

ion

in

th

ese

table

s w

as

usedtocalculate the consequencesaveragedoveracycle. TablesB.1 andB.2

containthe dataforthe three severe accidentsource termsin Table 3, while

TablesB.3 andB.4 showthe resultsfordifferentactinide release fractions.

Table A .4:A c tin id e inven tori es at BO C , DEIS C ore


W G-M OX C ore Inven tory(M C i)

M O X-LEUR a t io

A c tin id es

Np-239Pu-238Pu-239Pu-240Pu-241Am-241

C m-242C m-244

984.60.064940.014430.015164.6640.00365

0.69980.03244

966.50.12270.15770.104918 .430.07942

8.8070.09869

0.981.8910 .96.923.9521 .8

12 .63.04

Table A .5:A c tin id e inven tori es at E O C , D CS C ore


D CS W G-MO X C oreInven tory (M C i)

D CS-M O X/ LEU R a t io

A c tin id es

Np-239Pu-238

Pu-239Pu-240Pu-241Am-241C m-242C m-244

17550.2152

0.026670.0347910 .620.009732.9650.1757

17350.1926

0.052160.0828921 .7410.02456.4580.3007

0.990.89

1.962.382.052.522.181.71


36/47

Ly m a n 68

Table B.1:C on se q u e n c es o fse v e re re a ct or a c cid e n ts at E O C



ST-1Lat ent c an c e rfa t a lit iesPro m p t fa t a liti es

11 ,00086

18 ,800129

13 ,60091

1.711.50

1.241.06


3,62030

6,69040

4,38031

1.851.33

1.211.03


2,41035

4,28034

2,90033

1.780.97

1.200.94

Table B.2:C onse q u e n c es o fse v e re re a ct or a c cid e n ts at B O C



ST-1

Lat ent c an c e rfa t a lit iesPro m p t fa t a liti es

4,13037

8,66047

5,24036

2.101.27

1.270.97


1,41010

3,15015

1,78010

2.231.50

1.261.00


1,06030

2,07021

1,27025

1.950.70

1.200.83


37/47


Table B.3:Se nsitiv ity o f E O C a c c id e n t c o nse q u e n c es to a ct in id e re le ase fra ct io n



ST-M (La R.F.= O ,015)Late nt c an c e rfa t a litiesPro m p t fa t a lities

11 ,00086

18 ,800129

13 ,60091

1.711.50

1.241.06

ST-H (La R.F.=0 .06)Late nt c an c e rfa t a litiesPro m p t fa t a lities

20 ,300359

44 ,500625

26 ,100391

2.191.74

1.291.09

ST-L (La R.F.=0.003)Late nt c an c e rfa t a litiesPro m p t fa t a lities

6,41038

8,68044

7,14039

1.351.16

1.111.03

Table B.4: Se nsit iv ity o f BO C a c c id e n t c o nse q u e n c es to a ct in id e re le ase fra c tio n



ST-M (La R.F.=0 .015)Late nt c an c e rfa t a litiesPro m p t fa t a lities

4,13037

8,66047

5,24036

2.101.27

1.270.97

ST-H (La R.F.=0 .06)Late nt c an c e rfa t a lit esPro m p t fa t a lities

7,530108

19 ,700188

10 ,100105

2.621.74

1.340.97

ST-L (La R.F.=0.003)Late nt c an c e rfa t a litiesPro m p t fa t a lities

2,55014

3,74020

2,84015

1.471.43

1.111.07


38/47

Ly m a n 70

APPENDIX C : Consequences ofReactor-Grade MOX Use

The use ofreactor-grade MOX (RG-MOX) inLWRsposes evenmore serious

health consequences than use ofWG-MOX. Because RG-MOX contains a

muchhigherpercentage ofthe higher-massisotopesPu-240, Pu-241 andPu-

242, concentrationsofradiologicallysignificantactinidessuchasAmandCm

increasemorerapidlyduringirradiationofRG-MOX. Moreover, becausePu-

240 andPu-242 degrade the reactivitypropertiesofMOXfuel, the concentra-

tionoftotalRG-Pumust be greaterthanthatofWG-Putoachieve comparable

fuel energycontent.

To estimate the impactofthese effects, the calculationsofthe preceding

sectionswererepeatedforafullcoreofMOXcontainingplutoniumfromspentLWRfuelof44 GWD/t burnup(see Table 2 forthe initialisotopiccomposition

ofthe RG-Pu). JapanisplanningtoconstructanLWRthatwilluse afullRG-

MOXCore. The spentfuelwasassumedtohave beencooledfor 9 years before

reprocessing, andthe freshRG-MOXfuelwasassumedtohave beenstoredfor

threeyears before beingloadedintoareactor. ThetotalRG-Puconcentration

of the fresh fuel necessary to achieve equivalent performance to 4.25%-

enrichedLEUwascalculatedto be 8.3%, nearlytwice asgreatastheWG-Pu

concentration. Otherwise, all other parameters were unchanged from the

WG-MOXcalculation. While some ofthese parameters, suchasthe average

fuelassemblypowers for each batch, would be different for thecaseofRG-

MOXfuel, the differenceswouldnot be expectedtohave alarge effectonthe

calculatedcore inventories.

Table C.1 compares the calculated actinide inventories of the RG-MOX

and LEUcores.


39/47


Using the calculated radionuclide inventories for the end-of-cycle RG-

MOXcore, theMACCS2 calculationsoftheprevioussectionswererepeated.

Resultsofthe consequence calculationsare presentedinTable C.2. Compar-

ingTable C.2 andTable B.3, one findsthatthe percentincreasesinLCFsare

aboutfourtimesgreaterforRG-PuthanforWG-Pu.

Table C .1:RG-M OX a ctin id e c ore inven tory a t EO C


RG-M OX C ore Inven tory(M C i)

M O X / LEU R a tio

A c tin id es

Np-239Pu-238Pu-239Pu-240Pu-241Am-241C m-242

C m-244

17550.21520.026670.0347910 .620.009732.965

0.1757

14432.6670.13680.353286 .510.260058 .29

3.801

0.8212 .45.1310 .28.1526 .719 .7

21 .6

Table C .2: C onse q u e n c es o fSe v e re Ac c id e n tsInvo lv in g RG-M OX C ores at E O C

RG - M O X RG -M O X/ LEU R a tio

So u

rc e te

rm:

ST-MLate nt c an c e rfa t a litiesPro m p t fa t a lities

43 ,600299

3.963.48

ST-HLate nt c an c e rfa t a litiesPro m p t fa t a lities

119,0001,590

5.864.42

ST-LLate nt c an c e rfa t a litiesPro m p t fa t a lities

14 ,30059

2.231.55


40/47

Ly m a n 72

N OTES A ND REFERE N C ES

1. NationalAcademy of Sciences, Committee on International Security andArmsControl, ManagementandDispositionofExcessPlutonium(Washington, DC: NationalAcademyPress, 1994). Some observershave questionedthe proliferationresistance ofthe proposedCICwaste form. Othershave remarkedthatspentMOXfuelitselfmaynotmeetthe spentfuelstandard. The NASiscurrentlyundertakingastudyto exam-ine these issues. ItislikelythatavariantofCICwill be foundthatclearlymeetsthespentfuelstandard.

2. StoneandWebster filedforChapter 11 bankruptcyinMay 2000 andwasacquiredbythe ShawGroup laterthatyear.

3. Glenn T. Seaborg, APlutoniumWarningFromItsDiscoverer,WashingtonPost,August 3, 1997.

4. At present, the greatest risk to the plutonium immobilization program is theDWPF itself. The processthatwas beingdevelopedtopretreatandconcentrate thehigh-level liquid waste in the SRS tanks, in-tank precipitation, wasabandoned byDOEin 1998 because itwasunable tosolve the problemofexcessive benzene genera-tion. The developmentofasubstitute technologyislikelytocause substantialdelaysandadditionalcosts.

5. U.S. NuclearRegulatoryCommission(NRC), Office ofPublicAffairs, ProbabilisticRiskAssessment, TechnicalIssuesPaper 12A(Washington, DC, 1999).

6. U.S. NRC, Individual PlantExamination Program, NUREG-1560 (Washington,DC, 1996).

7. IntheintroductiontoNRC'sIPEdatabase, itstatesTheinformationinthedata-base hasNOT beenverifiedorvalidated.

8. U.S. NRC, Severe Accident Risks: An Assessment for Five U.S. Nuclear PowerPlant

s, NURE

G-1150 (Wash

in

gton

, DC, 1990).9. U.S. NRC, Executive Director for Operations, Mixed-Oxide Fuel in Light-WaterReactors, CommissionMemorandum(Washington, DC, April 14, 1999), 11.

10. Ibid., 12-13.

11. Thisassumptionmayresultinanunderestimate ofthe additionalconsequencesofusingMOXfuelresultingfromcertainaccidents, inviewofrecentresearchsuggestingthathigh burnupMOXfuelmay be prone togreaterreleasesofbothvolatile andlow-volatile radionuclidesunderaccidentconditions(see notes 23-27 below).

12. Onanactivity basis, the actinide inventoryisactuallysmallerforMOXcoresthanforLEUcores, primarily because ofalowerinventoryofNp-239 (see Table A.1). How-ever, the radiologicalhazardisnotproportionaltothe totalinventoryofreleasedradio-nuclides. Everyradionuclidehasaunique biologicaleffectthatdependsonthetypeand energyofthe radiationit emits, aswellasonth

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Public Health Risks of Substituting Mixed-Oxide For Uranium Fuel in Pressurized-Water Reactors