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Kairos Power LLC, Changes to Postulated Event Analysis Methodology Technical Report (KP-TR-018)
	ML22244A248
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39of102 CompromisedTRISOdT+(1dIdSdOdT)xfT WheredI,dS,dO,anddTarethedefectivefractionsoftheIPyClayer,SiClayer,OPyClayer,andTRISO particle(i.e.,exposedkernel),respectively,whilefI(crackedIPyC),fIS(crackedIPyC+failedSiC),fS(failed SiC),andfT(failedTRISO)aretheinservicefailurefractionsfortheTRISOfuelfailuremodes.

Radionuclidereleaseiscalculatedforeachoftheintactandfivecompromisedstatesandtheoverall radionuclidereleasefromthepopulationofTRISOparticlesisobtainedbyweightingtheresulting releasefractionsbytheprobabilitiesofoccurrenceofthesestates.Disperseduraniumisassumedtobe fullyreleasedfromtheTRISOparticlesanditscontributionisaddedtothereleasefromtheintactand compromisedparticles.

TheverificationandvalidationplansfortheKPBISONcodearesummarizedinReference7.

4.3 NEUTRONICS TheSerpent2codeisusedforneutronicscalculations.TheStarCCM+codeisusedforbothdiscrete elementmodelingofthepebbleflowandporousmediaapproximationforthermalhydraulicsfeedback.

Thedescriptionofthesetoolsandmodelsalongwithvalidation,verification,anduncertaintiesare presentedinReference8.

4.4 STRUCTURALANALYSIS ThematerialsqualificationplanforhightemperaturemetallicmaterialsisprovidedinReference9.The materialsqualificationplanforgraphitematerialsisprovidedinReference11.Thesequalificationplans informthefiguresofmeritforthereactorvesselandinternalsdescribedinthisreport.Thestructural analysisofthematerialsunderpostulatedeventconditionswillbeperformedpriortosubmittalofan OperatingLicenseApplication.

4.5 EVENTSPECIFICMETHODS Thissectionprovidestheeventspecificmethodsthatusetheevaluationmodelswithconservative inputstoanalyzethetransientsdiscussedinSection3.Parameterrangesconsideredforalleventsare providedinTable44.SampleresultsforthepostulatedeventcategoriesareprovidedinAppendixAto illustratethetransientmethodologies.

4.5.1 SaltSpills ThesaltspilleventcategoryisdescribedinSection3.2.2.Theanalysisoftheboundingsaltspilleventis composedofthefollowingmodels:

Singlephasebreakflowmodel-themassflowratewithtimethroughthebreakandthefinalupper plenumfreesurfacelevelarethetwomajormodelingresults.Twophaseflowduetogas entrainmentispreventedthroughtheprimarypumpdesign.Twomodelingoptionsareavailable:(a)

KPSAMmodelbasedontheslightmodificationofthebaselineplantmodeltoincludethesingle phasebreakflowmodel;and(b)aconservativeanalyticalmodel Longtermperformanceofpassivedecayheatremovalmodel-thisissimilarasthemodelusedfor lossofforcedcirculationoverheatingboundingcasebutwithreducedfreesurfacelevel.

Radioactivesourcetermreleasemodelstoestimatetheboundingtotalreleasefromtheevent.Two majorsourcetermmodelsarerequired:

o Aerosolgenerationrateandamountduetosinglephasecoolantjet.

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43of102 MARreleaseassociatedwiththeaerosolgenerationisevaluatedthroughtheaerosolamountandthe concentrationofMARinthespilledFlibe.

EvaporativeReleasefromSpilledFlibe TheevaporativereleaseisthephasewhenthedischargeoftheFlibefromthevesselendsandthe spilledFlibecompletesspreadingonthereactorcellfloor.SmallamountofFlibeislikelytospreadonlya fractionofthereactorcellfloorareabeforeitiscompletelysolidified.ItisnotamajorconcernforMAR releaseforpartiallyspreadingFlibebecauseitfreezesquickly.Moreconcernislargeamountofspilled Flibewhichspreadstheentireareaofthereactorcellfloor.Inthiscase,aFlibepoolisexpectedtoform withadepthofmoltenFlibe.Thebottomofthepoolcontactswithsteellinerwhichisplacedtoprevent Flibeconcreteinteraction.Thetopofthepooltransfersheattoairthroughconvectionandto surroundingstructuresthroughradiation.NowaterandnowatersourcesarepresentwheretheFlibe spreads,andFlibewaterinteractionisexcluded.

MARreleasefromtheFlibepoolisdominatedbyevaporationoverthetopsurfaceofthepool.It continuesuntilthetopsurfaceissolidified.ToevaluatetheamountofMARreleased,Flibe temperaturesareevaluatedfirst.TheFlibetemperatureisbasedonenergybalanceofthepool.Forthe downwardheattransfer,alayerofsolidifiedFlibeisexpectedbetweentheliquidFlibeandtheliner.A 1Dmovingboundaryequationneedstobesolvedforthetemperatureprofilewithinthesolidifiedlayer, andgrowth(orshrinkage)ofthelayer.TheboundaryconditionattheinterfacebetweentheliquidFlibe andthesolidifiedlayerisdeterminedbyGlobeDropkincorrelation(Reference20).Theboundary conditionattheinterfacebetweenthesolidifiedlayerandtheunderneathlinerisgivenbygap conductancebetweenthesolidifiedlayerandtheliner,orthroughcontinuityconditionsoftemperature andheatfluxifnogapisassumed.TheheattransferbetweentheliquidFlibetothetopsurfaceis determinedbyGlobeDropkincorrelationagain,andtheheattransferontheairsideisbasedon McAdamscorrelation(Reference21)fornaturalconvectionandradiationwithalowtemperatureheat structure.TheseheattransfertermsarecombinedtodeterminetheenergychangeoftheliquidFlibe duetoheattransferandsolidificationatthebottom,andeventuallythetemperaturesoftheliquidFlibe andatthetopsurface.

Oncethetemperaturesaredetermined,evaporationratesareassessedwiththesamemethodasthe MHAforMAR.Theevaporationrateandintegralreleaseamountareevaluateduntilthetemperatureof thetopsurfaceislowerthantheFlibemeltingtemperature.

4.5.2 InsertionofExcessReactivity ThelimitinginsertionofexcessreactivityisdescribedinSection3.2.2.Theanalysisofthelimitingevent inthiscategory(acontrolelementwithdrawal)includesasystemsanalysiswithconservativeneutronics andfuelperformanceinput.

4.5.2.1 InitialConditions Theinitialconditionsofthetransientarebiasedtoensureaconservativeevaluationofthefiguresof merit.Thelimitingcontrolrodwithdrawalscenarioisassumedtoinitiatefromthehighestpossible reactorpowerbecausethehigherpowerprovidesthehighestheatinputtochallengetheidentified figuresofmerit.However,sensitivitiesmustbeperformedtoensurethatreactivityinsertionsfrom lowerpowerlevelsdonotunexpectedlychallengeafigureofmerit.Apoweruncertaintyisappliedto reactorpowertobiasthepowerhigh.coveruncertaintiesassociatedwithdetectionandsignaldelays.

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44of102 Sincethereactorpowerisbiasedhighintheassumedlimitingreactivityinsertionevent,theinitial reactorpowerismodeledat102%power.AdditionalinitialconditionvaluesareprovidedinTable44.

4.5.2.2 TransientAnalysisMethods Thereactivityinsertiontransientinvolvesachangeincorereactivitythataddsheattoprimarysystem.

Therefore,theeventanalysisrequiresinformationfromthesystemscode,fuelperformance,and neutronicsEMs.Thesystemscode,KPSAManalyzestheeventprogressionwithinputsfromthe neutronicsEMandprovidesinputstothefuelperformanceEM.

TheKPSAMbasemodelinSection4.1isusedwithmodificationstothereactorcoremodel.Thenuclear fissionpowerprofilewithinthepebblebedisaffectedbytheneutronfluxdistributioninthecoreregion andthefuelburnupstatusofthepebbles.Withasinglechannelmodelingofthecorezone,theaxial powerprofilecanbedefinedbyprovidingthepowershapefunctionintheKPSAMcodeinputdeck.

Theradialpowerprofileanditseffectonthecoolantandfueltemperaturesarenotexplicitlymodeled, however,becausethesinglechannelmodelusestheaveragepowerateachaxiallevel.Inorderto addresstheradialpowerdistributionandmodelitseffectsonthecoolantandfueltemperature, especiallytocapturetheirmaximumvalues,aseparatecorechannelrepresentinghighradialpoweris analyzedasahotchannel.Consequently,thecoreismodeledastwochannels,i.e.,anaveragechannel andahotchannel.Thehotchannelmodelassumescompletethermalisolationfromtheadjacent averagechannel.Inreality,however,sincethereisnophysicaldistinctionbetweenthetwochannels, somethermalhydraulicinteractionsareexpected.Theisolationassumption,therefore,wouldpredict higherfuelandcoolanttemperaturesinthehotchannel,resultinginmoreconservativepredictions.

Thehotchannelflowareaissettobesmallenoughtorepresenttheradialhighpowerzone.Acoreflow ratecorrespondingtotheareaisassignedtothehotchannel.

Inordertoensureaconservativeevaluationofthelimitingreactivityinsertionevent,thefollowing conservatismsareappliedtomodelinputs:

Highestworthcontrolelementisassumedtobewithdrawn.

o Thelimitingreactivityinsertionrateisdeterminedfromthelimitingreactivityrodworthper lengthfromneutronicsEM,combinedwiththemaximumcontrolelementwithdrawalspeed.

o Arangeofreactivityinsertionrates,uptoandincludingthemaximumreactivityinsertionrate, dependingonthecontrolelementcontroldesign,isareanalyzedinthefinalsafetyanalysis.to ensurethatthehighestreactivityinsertionrateisidentifiedthatboundsthereactivityinsertion ratespossibleforothereventsinthecategory.

o Atfullpowerandhotzeropower,theinitialcontrolelementpositionisassumedtobefully insertedinthereactorcore.

o Aconservativetreatmentisappliedtoaddresstheimpactofadynamicchangeinpowershape associatedwiththecontrolelementmovement.

Leastnegativereactivityfeedbackcoefficientsareusedtominimizethepowersuppressioneffectby thenegativereactivityfeedbackinpreliminarysafetyanalysis.

Mostnegativereactivityfeedbackcoefficientsarealsobeappliedandanalyzedtoinvestigatethe effectofdelayedreactortripinthefinalsafetyanalysis.

Thiseventisalsoidentifiedasoneoftheboundingfuelperformancecasesandmustbeanalyzedwith theKPBISONusingthemethodologydescribedinSection4.2.

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45of102 4.5.3 LossofForcedCirculation ThelimitinglossofforcedcirculationscenarioisdescribedinSection3.2.2.Theanalysisofthelimiting eventinthiscategoryincludesasystemsanalysiswithconservativeneutronicsinput.

4.5.3.1 InitialConditions Theinitialconditionsofthetransientarebiasedtoensureaconservativeevaluationofthefiguresof merit.Thelimitinglossofforcedcirculationscenarioisassumedtoinitiatefromthehighestpossible reactorpowerbecausethehigherpowerprovidesthehighestheatinputtochallengetheidentified figuresofmerit.However,sensitivitiesmustbeperformedtoensurethatlossofforcedcirculation eventsfromlowerpowerlevelsdonotunexpectedlychallengeafigureofmerit.Initialconditionvalues areprovidedinTable45.

4.5.3.2 TransientAnalysisMethods Theimportantthermalandhydraulicphenomenaduringthetransientincludetheflowfriction(negative head)atthepump,heattransferbetweenthecoolantandvariousinterfacingstructuressuchaspebble, reactorvesselwallandinternals.Becausetheforcedcirculationislost,thefluidfrictionthroughthe coolantloop,includingthereactorcore,ismoreimportantthanothereventswhereforcedflowis maintained.

KPSAMisusedtoanalyzetheeventprogressionwithinputsfromtheneutronicsEMandprovides inputstothestructuralintegrityEM.Uponalossofforcedcirculation,thereactorexperiencesan immediateincreaseinthefuel(pebble)temperaturebecauseofthereducedheattransfertothe coolant.ThecoolanttemperaturealsorisesbecauseheatremovalfromthereactorcoretothePHXis reducedandeventuallystops.Theincreasedtemperatureofthecoolantcouldchallengetheintegrityof reactorvesselandcorebarrelstructures.

Thenuclearfissionpowerprofilewithinthepebblebedisaffectedbytheneutronfluxdistributioninthe coreregionandthefuelburnupstatusofthepebbles.Thecurrentapproachtomodelingcorepower densityisanaxiallyresolvedradiallyaveragedmethodanddoesnotexplicitlyaccountforradialpower peakinginthecore.Theradialpowerprofileanditseffectonthecoolantandfueltemperaturesarenot explicitlymodeled;therefore,localpeakcoolantandfueltemperaturesarenotfullyresolved.Thehot channelfactormethodologydescribedinSection4.1accountsforbothpowerpeakingandthe possibilityofflowbeingpoorlydistributedinthecore.

TheKPSAMbasemodeldescribedinSection4.1isusedtoanalyzealossofforcedcirculationevent withthefollowingmodifications:

Typically,theinteractionbetweenthefluidsystemandpump,duringthetransient,ismodeledusing headandtorquecurvesofthepump.Forthelossofforcedcirculationanalysis,thecoolantflow responseismodeledwithoutthedetailedpumpcharacteristics,byconservativelyassumingthe pumpheadafterthetransientstarts.Sincethepumprotorisassumedtostopinstantly,thepump torqueinformationisnotneeded.

Thereactivityfeedbackeffectonpowerisminimizedforconservativecalculationbyusingleast negativereactivitycoefficientvaluestominimizetheeffectofpowerreductionfromtheinitial temperatureincreasebythereducedcoolantflow.

TheuncertaintiesinmaterialpropertiesoftheFlibecoolantandvesselstructuresareaddressed conservatively.Thethermalmassofthematerialisreducedsuchthatthetemperaturesoffueland

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63of102 Table44:InputParametersConsideredforPostulatedEvents Parameter Value Rationale Reactorinitialpower Rangeofvaluesuptoand includingmaximumpowerlevel includinguncertainty Rangesofpowerlevelsanalyzed Coolantaveragetemperature Rangeovercontrollerdeadband andmeasurementuncertainty Limitingvaluemaybeevent dependent Systempressure Nominalforalleventsexcept forsaltspill Theeffectofthesystem pressureisinsignificantforall eventsexceptforsaltspill events Powerdistribution Axial+radialpowerdistribution forpeakingfactor

Bothfreshcoreandequilibrium coreareconsideredaslimiting conditions Mostlimitingpowerdistribution isconsidered Shutdownmargin Considersmostreactive shutdownrodisunavailable Providemarginformalfunctions Shutdownrodinsertiontime Conservativeshutdownrod insertiontimesassumed Delaystheshutdownofthe reactor Reactivitycoefficients Valuesassumedonanevent specificbasisandaccountfor uncertainty Limitingvaluesmaybeevent dependent DHRSCapacity Minimumandmaximum performanceassumedonan eventspecificbasis

Minimumperformanceassumes lossofatrainofDHRSand minimumperformance requirements

Maximumperformance assumesfullcapacityofDHRS plusuncertainty MinimumDHRSperformanceis expectedtobeboundingfor heatupevents

MaximumDHRSperformanceis expectedtobeboundingfor overcoolingevents Decayheat Minimumandmaximumvalues assumedonaneventspecific basis Maximizingdecayheatis expectedtobeboundingfor heatupevents

Minimizingdecayheatis expectedtobeboundingfor overcoolingevents Materialproperties Rangedwithinuncertainties Uncertaintyinmaterial propertiesforcoolantand

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64of102 structurestreatedonanevent specificbasis ReactorProtectionSystem analyticallimits Actuationon:

HighReactorPower HighFluxRate HighCoolantTemperature LowLevel Analyticallimitsprovidemargin tosafetylimits

Measurementuncertainty appliedtosetpointsarederived fromanalyticallimits ReactorProtectionSystem actuationdelay Conservativedelaytimes applied Delayreactortrip PlantControlSystems Potentialeventmitigation capabilitiesoftheplantcontrol systemsarenotcredited

Suitablyconservativetreatment ofrelevantplantcontrol featuresisappliedinthesafety analysis Plantcontrolsystemsarenot safetyrelated

Potentiallyadverse performanceofplantcontrol systemsneedstobeconsidered

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65of102 Table44:InitialconditionsforInsertionofExcessReactivity Parameter InitialCondition Rationale Note ReactorInitial power

[102%]

Potentialpowermeter uncertainty Modeledexplicitly Coolantaverage temperature Nominal+3%°C Controllerdeadbandand measurement uncertainties Modeledexplicitly Systempressure Nominal Theeffectofthesystem pressureisinsignificant Notmodeled Powerdistribution Axial+radialpower distributionforpeaking factor

Bothfreshcore,and equilibriumcoreare consideredaslimiting conditions Mostlimingpower distributionisconsidered Theaxialradially averagedpowerprofile ismodeledexplicitlyin KPSAM.Radialpeaking anduncertaintiesare handledviahotchannel factors

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66of102 Table45:InitialconditionsforLossofForcedCirculationOverheatingBoundingEvent Parameter InitialCondition Rationale Note ReactorInitial power 102%

Potentialpowermeter uncertainty Modeledexplicitly Coolantaverage temperature Nominal+3%°C Controllerdeadbandand measurement uncertainties Modeledexplicitly Systempressure Nominal Theeffectofthesystem pressureisinsignificant Notmodeled Powerdistribution Axial+radialpower distributionfor peakingfactor

Bothfreshcore,and equilibriumcoreare consideredaslimiting conditions Mostlimingpower distributionare considered Theaxialradially averagedpowerprofileis modeledexplicitlyinKP SAM.Radialpeakingand uncertaintiesarehandled viahotchannelfactors DHRScapacity 75%

AssumeoneDHRStrainis outofoperation Modeledexplicitlyby reducingradiationview factor Heatstructureheat capacity 75%

Accountforany uncertaintyrelatedtothe heatcapacityofsolid materialsinthemodel Modeledexplicitlyby applyingascalefactorto solidmaterialheat capacities Flibeheatcapacity 95%

Accountforuncertaintyin theheatcapacityofFlibe Modeledexplicitlyby applyingascalefactorto Flibeheatcapacity Reactivity coefficient magnitude 75%

Reducedtoconservatively biastheimpactof reactivityfeedbackprior toreactortrip Modeledexplicitly

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71of102 APPENDIXA.

SAMPLETRANSIENTRESULTS A.1 InsertionofExcessReactivity EventDescription Acontrolelementwith3.02$reactivityworthisassumedtobewithdrawncompletelyover100seconds.

Therateofreactivityinsertiondependsonaworthcurveandtheprogressionoftherodwithdrawal.

Whenthepowerlevelexceedsthetripsetpoint,16.8$ofreactivityisinsertedtothecoreover10 secondsaccordingtoanelementworthcurve.After10seconds,thisreactivityismaintained,simulating thetotalassumedelementworth.Theassumptionsmadearesummarizedasbelowandinitial conditionsareprovidedinTableA11.

Powertripsetpoint=120%

Upperplenumtemperaturetripsetpoint=958.1K(665°C+3%)

Powertripdelaytime=2s Temperaturetripdelaytime=2s Elementinsertiondelayaftertrip=2s Timetofullyinsertrodsaftertrip=10s Elementworth=16.8$

Primarysaltpumphalvingtime=2s Intermediatevelocityhalvingtime=1s KPSAManalysisresults Thetransientisinitiatedat0secondswiththestartofreactivityinsertion.Priortoareactortrip,this positivereactivityinsertioniscounteractedinpartbynegativeDoppler,moderator,andcoolant feedbackrespectivelyinorderofmagnitude.Soonafterreactortripisinitiated,thetotalchangein reactivityofthesystembecomesnegativeandremainssodespitethecontinuationofthereactivity insertion,asshowninFigureA11 Whenthereactortripisinitiated,thePSPistrippedaswell,causingadecreaseinflowratethroughout thesystem.Thishasnotableimpactsonheattransferthroughoutthesystemduringtheentire simulation,asthiswillcharacterizesflowbehaviorinthecoreduringearlierstagesofthetransientand facilitatethetransitiontonaturalcirculationinthelongterm.

KPSAMConclusions Areactivityinsertionof3.02$over100secondswasassumedtosimulateanuncontrolledcontrol elementwithdrawal.Thereactoristrippedbyahighfluxprotectionsignal(120%)at9secondsafterthe

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72of102 eventinitiation.FigureA12showskeypredictedtemperaturesrelativetothetemperatureusedinthe MHAanalysis.ThetemperaturerisesintheTRISOandfuelmatrixwereobserved,withverylittlechange intheFlibetemperature.Theresultingtemperatures,withtheexceptionofreflectortemperatures,are withintheacceptancelevel,withsignificantmargins.Theshortdeviation(i.e.,ontheorderofafew minutes)ofthereflectortemperatureslightlyabovetheMHAtemperatureisacceptableduetothe timeattemperaturenatureofdiffusionoftritiumoutofgraphitegrains.

FuelPerformanceAnalysis ThepowerandtemperatureprofileswereusedasinputstoKPBISON.Thetransientismodeledatthe endofanormaloperationphasethatprovidestheadequatestateoftheTRISOfuelparticles(e.g.,

failurefractions,fissionproductdistribution,fissiongasinventory,etc.).

ThenormaloperationphaseismodeledusingtheirradiationconditionsshowninTableA121.

TableA132showsthefailureprobabilitiescalculatedbyKPBISONwithintheMonteCarlocalculation schemefortheTRISOfailuremodesfornormaloperationandreactivityinsertionevent.Theresultsin TableA132indicatethatthetemperatureduringnormaloperationandtransientisnothighenoughto challengetheTRISOfuelwithoverpressureorPdattack.Furthermore,TableA132showsthatthe reactivityinsertioneventdoesnotleadtoanysignificantincrementalfailure.

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73of102 TableA11:InitialconditionsforInsertionofExcessReactivityAssumedBoundingEvent

Parameter InitialCondition Rationale Reactorinitial power 102%

Assumedpowermeasurementuncertainty Coolantaverage temperature Nominal+3%°C Controllerdeadbandandmeasurement uncertainties Systempressure Nominal Theeffectofthesystempressureisinsignificant Powerdistribution Axial+radialpower distributionforpeaking factor

Bothfreshcore,and equilibriumcoreare consideredaslimiting conditions Mostlimitingpowerdistributionisconsidered

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74of102 TableA121:95%ConfidenceLevelUpperLimitonInServiceFailureFractionsforNormalOperationand ReactivityInsertionPostulatedEvent

FailureProbability Normal Operation NormalOperation+

ReactivityInsertion ProbabilityofIPyCcracking 9.75x101 9.75x101 ProbabilityofSiCfailure Contributionduetopalladiumpenetration ContributionduetoIPyCcracking 2.26x103 3.00x106 2.26x103 2.26x103 3.00x106 2.26x103 ProbabilityofTRISOfailure 3.00x106 3.00x106

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75of102 TableA132:CompromisedFractionsforNormalOperationandReactivityInsertionPostulatedEvent

ReleaseFraction NormalOperation NormalOperation+

ReactivityInsertion Intact 2.25x102 2.25x102 CompromisedIPyC 9.65x101 9.65x101 CompromisedIPyC+SiC 2.24x103 2.24x103 CompromisedSiC 1.03x104 1.03x104 CompromisedOPyC 1.00x102 1.00x102 CompromisedIPyC+SiC+OPyC 5.30x105 5.30x105

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85of102 A.4 LossofForcedCirculation EventDescription Thepurposeofthiseventistodetermineifthereactorisadequatelydesignedforlongtermheatup events.Assuch,oneofthekeyassumptionsisthatonly75%ofDHRScapacityisavailable.Thelossof forcedcirculationoverheatingboundingeventappliedtotheplantmodelisinitiatedbymanually trippingthepumpandreducingtheheadtozeronearlyinstantaneously.Thecompletelossofflow definesthebeginningofthetransientandoccursconcurrentlywithalossofintermediatecoolantflow.

Intermediatecoolantflowisnotlikelytobelostduringalossofforcedcirculationeventbutisimposed inthisanalysistodemonstratethatintermediatecoolantflowisnotneededtoprotecttheplantduring alossofforcedcirculationevent.Duringthistransient,itisexpectedthatthelargereductionincoolant flowthroughthecoreregionresultsinasignificantriseintemperatureacrossthecore.Therisein temperatureeventuallycausesthereactortotrip,leadingtoalongtermcoolingtransientandthesafe shutdownconditionofthereactor.Initialconditionsfortheoverheatinglossofforcedcirculation assumedboundingeventareprovidedinTableA41.Asetofassumptionskeytothisanalysisarelisted inTableA442.

Thelossofforcedcirculationtransientwasrunoverthecourseof72hoursofsimulationtime.During thetransient,theupperplenumtemperatureexceedsthetripsetpointafter23seconds,withrod insertionfollowingatripdelay.Priortotherodinsertion,powerisreducedbyreactivityfeedbackasthe coreheatsup,afterwardsthestronginsertionofnegativereactivityfromtherodinsertionbringsthe reactorpowerdowntodecayheatlevels.FigureA41showskeypredictedtemperaturesrelativetothe temperatureusedintheMHAanalysis.

Thecompromisedfractionsforthesixstatesareobtainedfromthedefectandinservicefailure fractionsinTable43(seeSection4.2)andTableA453.TheseareshowninTableA464,assumingthe upperspecificationorboundingvalues.

LossofForcedCirculationOverheating AlossofforcedcirculationtransientbiasedforoverheatingwasperformedusingKPSAM.Inthis simulation,itwasdemonstratedthatdecayheatremovalthroughtheDHRScancompensatefortheloss oftheintermediatesaltflowtoachievestablecoolingafterthefaststageofthetransient.

TheTRISOtemperatureprofileisboundedbytheMHAcurve,whichdemonstratesthatthediffusional releaseofradionuclidesfromfuelisboundedbytheMHA.TheFlibecovergasinterfacialtemperature profileisboundedbytheMHAcurve,whichdemonstratesthatthereleasefromFlibethrough evaporationisalsoboundedbytheMHA.

ThegraphitereflectorandfuelpebbletemperatureprofilesareboundedbytheMHAcurves,which demonstratesthatthetritiumreleaseisboundedbytheMHA.Itisshownthattemperaturesstaybelow thosedefinedbytheMHAexceptfortheupperplenumandreflector/graphitetemperatures.TheMHA releaseanalysisisconservative.TheMHAmarginismaintainedsincedeviationsareminimalandof shortduration(asscaledrelativetothecorrespondingX/Qwindowassociatedwiththedeviation)due totheconservativeevaporativeboundaryconditionsintheMHA(i.e.,aggressivetemperaturegradients drivingnaturalcirculation)andtimesassociatedwiththosetemperaturescorrespondingintheMHA

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86of102 (i.e.,evaporationanddiffusionaretimeattemperaturereleasemechanisms).Freezingdoesnotoccur inthiseventandthatthevesselremainsbelowthedefinedtemperaturelimit.

ThepowerandtemperatureprofileswereusedasinputstoKPBISON.Thetransientismodeledatthe endofanormaloperationphasethatprovidestheadequatestateoftheTRISOfuelparticles(e.g.,

failurefractions,fissionproductdistribution,fissiongasinventory,etc.).Thenormaloperationphaseis modeledusingtheirradiationconditionsshowninTableA431.

ThefailureprobabilitiesassociatedwiththepotentialfailuremodeslistedinSection4.2wereobtained byaMonteCarlosimulationof106samples.Note:thesamplesizewaschosentooptimizecomputing time.FromtheMonteCarlosimulationresults,upperlimitsonthefailureprobabilitiesassociatedwith eachfailuremodesareobtainedata95%confidencelevelusingtheCopperPearsonexactmethod.

TheselimitsarereportedinTableA453forthenormaloperationandlossofforcedcirculation postulatedevent.

TheresultsinTableA453indicatethatthetemperaturesduringnormaloperationandthetransientare nothighenoughtochallengetheTRISOfuelwithoverpressureorPdattack.Inparticular,theupperlimit onTRISOfailurebyoverpressureisonlyafewpercent(6%)oftheasmanufacturedexposedkernel fractionof5.0x105.Furthermore,TableA453showsthattheTRISOfuelismorelikelytofailduring normaloperationandthatthelossofforcedcirculationeventdoesnotleadtoanysignificant incrementalfailure.Becauseoftheconservativeassumptionsusedtosetupthelowandhigh temperaturetrajectories,thecalculatedfailureprobabilitiesarealsoconservativeandrepresentupper limitsforexpectedfailureprobabilities.

LossofForcedCirculationOvercooling Whiletheoverheatingversionofthiseventisdesignedtochallengethemargintomaximum temperatures,theovercoolingscenarioisdesignedtochallengethemargintominimumtemperatures.

InthiscasethelimitingminimumtemperatureistakenasthepointatwhichFlibefreezes.Inorderto conservativelyprecludefreezing,theminimumvesselinnersurfacetemperatureistakenasabounding surrogatefortheminimumFlibetemperature.Theeventisinitiatedbymanuallyinitiatingacontrolrod insertion,primarypumptripandintermediateflowtripatt=0.Theprimarypumpandintermediate flowareallowedtocoastdownnormally.Additionally,theDHRSismodeledat100%capacity.Initial conditionsforthelossofforcedcirculationovercoolingeventareprovidedinTableA42.Asetofkey assumptionsforTheinputparametersassumedintheexamplecalculationisareprovidedinTableA4 42.

Theexamplecalculationofacooldownbiasedlossofforcedcirculationtransientwasrunoverthe courseof72hoursofsimulationtime.FigureA42showskeypredictedtemperaturesrelativetothe temperatureusedintheMHAanalysis.TemperaturespredictedbytheKPSAMmodelarebelowthe temperaturesdefinedbytheMHAandfreezingdoesnotoccurwithin72hours.

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87of102 TableA41:InitialConditionsforLossofForcedCirculationOverheatingAssumedBoundingEvent

Parameter InitialCondition Rationale Reactorinitial power 102%

Assumedpowermeasurementuncertainty Coolantaverage temperature Nominal+3%°C Controllerdeadbandandmeasurementuncertainties Systempressure Nominal Theeffectofthesystempressureisinsignificant Powerdistribution Axial+radialpower distributionfor peakingfactor

Bothfreshcore,and equilibriumcoreare consideredaslimiting conditions Mostlimitingpowerdistributionisconsidered DHRScapacity 75%

AssumeoneDHRStrainisoutofoperation Heatstructureheat capacity 75%

Accountforanyuncertaintyrelatedtotheheat capacityofsolidmaterialsinthemodel Flibeheatcapacity 95%

AccountforuncertaintyintheheatcapacityofFlibe Reactivity coefficient magnitude 75%

Reducedtoconservativelybiastheimpactofreactivity feedbackpriortoreactortrip

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88of102 TableA42:InitialConditionsforLossofForcedCirculationOvercoolingAssumedBoundingEvent

Parameter InitialCondition Rationale Reactorinitial power 98%

Assumedpowermeasurementuncertainty Minimizedstoredenergy

Coolantaverage temperature Nominal3%°C Controllerdeadbandandmeasurementuncertainties Systempressure Nominal Theeffectofthesystempressureisinsignificant Powerdistribution Axial+radialpower distributionfor peakingfactor

Bothfreshcore,and equilibriumcoreare consideredaslimiting conditions Mostlimitingpowerdistributionisconsidered DHRScapacity 100%

FullcapacityofDHRS Heatstructureheat capacity 75%

Accountforanyuncertaintyrelatedtotheheat capacityofsolidmaterialsinthemodel Minimizesstoredenergyandacceleratescooldown Flibeheatcapacity 95%

AccountforuncertaintyintheheatcapacityofFlibe Minimizesstoredenergyandacceleratescooldown Reactivity coefficient magnitude Nominal Reactortripinitiatedimmediatelyfollowingevent initiation

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September2021

89of102 TableA431:IrradiationConditionsforSimulatedNormalOperationofHermes

Parameter Value Irradiationlength(EFPD) 300 Powerdensity(fission/m3s 5.7x1019 Burnup(%FIMA) 6.0 Fastflux(n/m2s,E>0.1MeV) 7.7x1017 Fastfluence(n/m2s,E>0.1MeV) 2.0x1025

PostulatedEventAnalysisMethodology NonProprietary DocNumber Rev EffectiveDate KPTR018NP 0

September2021

90of102 TableA442:InputsforLossofForcedCirculationPostulatedEvents

LossofForcedCirculation-Overheating LossofForcedCirculationOvercooling Parameter Value Parameter Value Temperaturetripdelaytime(s) 2 Timetofullyinsertrodsaftertrip(s) 10 Elementinsertiondelayaftertrip(s) 2

Timetofullyinsertrodsaftertrip(s) 10 Tripdelayaftereventinitiation(µs) 20 Tripworth($ofreactivity) 16.8 Tripworth($ofreactivity) 16.8 Primarysaltpumphalvingtime(pump seizureapproximation)(s) 0.01 Primarysaltpumphalvingtime(s) 2 Intermediatevelocityhalvingtime(s) 1 Intermediatevelocityhalvingtime(s) 1 DHRScapacity(%)

75 DHRScapacity(%)

100

PostulatedEventAnalysisMethodology NonProprietary DocNumber Rev EffectiveDate KPTR018NP 0

September2021

91of102 TableA453:95%ConfidenceLevelUpperLimitsonInServiceFailureFractionsforNormalOperation andLossofForcedCirculationPostulatedEvents

FailureProbability NormalOperation NormalOperation+

LossofForced Circulation IPyCCracking 9.75x101 9.75x101 SiCFailure Contributionduetopalladiumpenetration ContributionduetoIPyCcracking 2.26x103 3.00x106 2.26x103 2.26x103 3.00x106 2.26x103 TRISOFailure 3.00x106 3.00x106

PostulatedEventAnalysisMethodology NonProprietary DocNumber Rev EffectiveDate KPTR018NP 0

September2021

92of102 TableA464:CompromisedFractionsforNormalOperationandLossofForcedCirculationPostulated Event

ReleaseFraction NormalOperation NormalOperation+Lossof ForcedCirculation Intact 2.25x102 2.25x102 CompromisedIPyC 9.65x101 9.65x101 CompromisedIPyC+SiC 2.24x103 2.24x103 CompromisedSiC 1.03x104 1.03x104 CompromisedOPyC 1.00x102 1.00x102 CompromisedIPyC+SiC+OPyC 5.30x105 5.30x105

ML22244A248

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