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Kairos Power LLC, Changes to Postulated Event Analysis Methodology Technical Report (KP-TR-018)
	ML22244A248
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Issue date:	09/30/2022
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Enclosure1 ChangestoPostulatedEventAnalysisMethodologyTechnicalReport(KPTR018)

(NonProprietary)

PostulatedEventAnalysisMethodology

NonProprietary DocNumber Rev EffectiveDate KPTR018NP 0 September2021

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

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

Minimumperformanceassumes MaximumDHRSperformanceis lossofatrainofDHRSand expectedtobeboundingfor minimumperformance overcoolingevents requirements

Maximumperformance assumesfullcapacityofDHRS plusuncertainty Decayheat Minimumandmaximumvalues Maximizingdecayheatis assumedonaneventspecific expectedtobeboundingfor basis heatupevents

Minimizingdecayheatis expectedtobeboundingfor overcoolingevents Materialproperties Rangedwithinuncertainties Uncertaintyinmaterial propertiesforcoolantand

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structurestreatedonanevent specificbasis ReactorProtectionSystem Actuationon: Analyticallimitsprovidemargin analyticallimits HighReactorPower tosafetylimits HighFluxRate HighCoolantTemperature Measurementuncertainty LowLevel appliedtosetpointsarederived fromanalyticallimits ReactorProtectionSystem Conservativedelaytimes Delayreactortrip actuationdelay applied PlantControlSystems Potentialeventmitigation Plantcontrolsystemsarenot capabilitiesoftheplantcontrol safetyrelated systemsarenotcredited Potentiallyadverse Suitablyconservativetreatment performanceofplantcontrol ofrelevantplantcontrol systemsneedstobeconsidered featuresisappliedinthesafety analysis

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Table44:InitialconditionsforInsertionofExcessReactivity Parameter InitialCondition Rationale Note ReactorInitial [102%] Potentialpowermeter Modeledexplicitly power uncertainty Coolantaverage Nominal+3%°C Controllerdeadbandand Modeledexplicitly temperature measurement uncertainties Systempressure Nominal Theeffectofthesystem Notmodeled pressureisinsignificant Powerdistribution Axial+radialpower Mostlimingpower Theaxialradially distributionforpeaking distributionisconsidered averagedpowerprofile factor ismodeledexplicitlyin KPSAM.Radialpeaking Bothfreshcore,and anduncertaintiesare equilibriumcoreare handledviahotchannel consideredaslimiting factors conditions

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Table45:InitialconditionsforLossofForcedCirculationOverheatingBoundingEvent Parameter InitialCondition Rationale Note ReactorInitial 102% Potentialpowermeter Modeledexplicitly power uncertainty Coolantaverage Nominal+3%°C Controllerdeadbandand Modeledexplicitly temperature measurement uncertainties Systempressure Nominal Theeffectofthesystem Notmodeled pressureisinsignificant Powerdistribution Axial+radialpower Mostlimingpower Theaxialradially distributionfor distributionare averagedpowerprofileis peakingfactor considered modeledexplicitlyinKP SAM.Radialpeakingand Bothfreshcore,and uncertaintiesarehandled equilibriumcoreare viahotchannelfactors consideredaslimiting conditions DHRScapacity 75% AssumeoneDHRStrainis Modeledexplicitlyby outofoperation reducingradiationview factor Heatstructureheat 75% Accountforany Modeledexplicitlyby capacity uncertaintyrelatedtothe applyingascalefactorto heatcapacityofsolid solidmaterialheat materialsinthemodel capacities Flibeheatcapacity 95% Accountforuncertaintyin Modeledexplicitlyby theheatcapacityofFlibe applyingascalefactorto Flibeheatcapacity Reactivity 75% Reducedtoconservatively Modeledexplicitly coefficient biastheimpactof magnitude reactivityfeedbackprior toreactortrip

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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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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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TableA11:InitialconditionsforInsertionofExcessReactivityAssumedBoundingEvent

Parameter InitialCondition Rationale Reactorinitial 102% Assumedpowermeasurementuncertainty power Coolantaverage Nominal+3%°C Controllerdeadbandandmeasurement temperature uncertainties Systempressure Nominal Theeffectofthesystempressureisinsignificant Powerdistribution Axial+radialpower Mostlimitingpowerdistributionisconsidered distributionforpeaking factor

Bothfreshcore,and equilibriumcoreare consideredaslimiting conditions

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TableA121:95%ConfidenceLevelUpperLimitonInServiceFailureFractionsforNormalOperationand ReactivityInsertionPostulatedEvent

FailureProbability Normal NormalOperation+

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

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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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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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(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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TableA41:InitialConditionsforLossofForcedCirculationOverheatingAssumedBoundingEvent

Parameter InitialCondition Rationale Reactorinitial 102% Assumedpowermeasurementuncertainty power Coolantaverage Nominal+3%°C Controllerdeadbandandmeasurementuncertainties temperature Systempressure Nominal Theeffectofthesystempressureisinsignificant Powerdistribution Axial+radialpower Mostlimitingpowerdistributionisconsidered distributionfor peakingfactor

Bothfreshcore,and equilibriumcoreare consideredaslimiting conditions DHRScapacity 75% AssumeoneDHRStrainisoutofoperation Heatstructureheat 75% Accountforanyuncertaintyrelatedtotheheat capacity capacityofsolidmaterialsinthemodel Flibeheatcapacity 95% AccountforuncertaintyintheheatcapacityofFlibe Reactivity 75% Reducedtoconservativelybiastheimpactofreactivity coefficient feedbackpriortoreactortrip magnitude

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TableA42:InitialConditionsforLossofForcedCirculationOvercoolingAssumedBoundingEvent

Parameter InitialCondition Rationale Reactorinitial 98% Assumedpowermeasurementuncertainty power Minimizedstoredenergy

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

Bothfreshcore,and equilibriumcoreare consideredaslimiting conditions DHRScapacity 100% FullcapacityofDHRS Heatstructureheat 75% Accountforanyuncertaintyrelatedtotheheat capacity capacityofsolidmaterialsinthemodel Minimizesstoredenergyandacceleratescooldown Flibeheatcapacity 95% AccountforuncertaintyintheheatcapacityofFlibe Minimizesstoredenergyandacceleratescooldown Reactivity Nominal Reactortripinitiatedimmediatelyfollowingevent coefficient initiation magnitude

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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

NonProprietary DocNumber Rev EffectiveDate KPTR018NP 0 September2021

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 0.01 Primarysaltpumphalvingtime(s) 2 seizureapproximation)(s)

Intermediatevelocityhalvingtime(s) 1 Intermediatevelocityhalvingtime(s) 1 DHRScapacity(%) 75 DHRScapacity(%) 100

NonProprietary DocNumber Rev EffectiveDate KPTR018NP 0 September2021

TableA453:95%ConfidenceLevelUpperLimitsonInServiceFailureFractionsforNormalOperation andLossofForcedCirculationPostulatedEvents

FailureProbability NormalOperation NormalOperation+

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

NonProprietary DocNumber Rev EffectiveDate KPTR018NP 0 September2021

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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