ML22224A201

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Enclosure 1: Changes to Postulated Event Analysis Methodology Technical Report (KP-TR-018)
ML22224A201
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Issue date: 08/12/2022
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PostulatedEventAnalysisMethodology NonProprietary DocNumber Rev EffectiveDate KPTR018NP 0

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20of97 ForpipebreakscenariosinotherFlibecontainingSSCs(exceptthevessel)notconnectedtothereactor, thecoredoesnotexperienceatransientfromreactortrip.

Inordertoensurethatthedesignfeaturesmitigatingasaltspilleventaresufficienttokeepthe consequencesboundedbytheMHA,thefollowingkeyfiguresofmeritmustbeevaluated:

PeakTRISOtemperaturetolimitdiffusionofradionuclides PeakTRISOtemperatureTRISOfailureprobabilitytolimitincrementalTRISOlayerfailures PeakFlibecovergasinterfacialtemperaturetolimitevaporationmasstransferofradionuclides Peakvesselandcorebarreltemperaturestopreventvesselfailureandmaintainlongtermcooling AerosolsgeneratedbyreleasedFlibetolimitthematerialsatriskreleased VolatileproductsformedfromthechemicalreactionbetweenFlibeandair,Flibeandstainlesssteel, andFlibeandinsulationtolimitthematerialsatriskreleased Masslossofstructuralgraphiteduetooxidationtolimittritiumrelease Masslossofpebblecarbonmatrixduetooxidationtolimittritiumreleaseandpreventadditional releaseofmaterialsatrisk Peaktemperatureofstructuralgraphitetolimitthetritiumrelease Peaktemperatureofpebblecarbonmatrixtolimittheamountoftritiumrelease 3.2.2.2 InsertionofExcessReactivity Acontrolsystemerrororoperatorerrorcausesacontinuouswithdrawalofthehighestworthcontrol elementatmaximumreactivitycontrolandshutdownsystem(RCSS)drivespeed.Thereactivity insertionisdetectedbythereactorprotectionsystemwhichinitiatescontrolandshutdownelements insertion,fulfillingthereactivitycontrolfunction.Thereactordecayheatremovalsystemlimitsreactor temperatureandfulfillstheheatremovalfunction.

Asafestateisestablishedwhen:

Thecoreissubcriticalandlongtermreactivitycontrolisassured.

Thedecayheatisbeingremovedandlongtermcoolingisassured,wherefiguresofmerit temperaturesaresteadilydecreasingandFlibetemperatureremainsaboveFlibefreezing temperatureduringthemissiontimeofthedecayheatremovalsystem.

Thisnarrativecapturesthelimitingeventofthispostulatedeventcategory.Othereventsgroupedinthis categoryinclude:

Reactivityinsertioneventscausedbyfuelloadingerror(e.g.,errorsinrateoffreshfuelinjection, incorrectorderoffuelinsertion)

Reactivityinsertioneventswithconcurrentpumptrip Reactivityinsertioneventswithnormalheatrejectionavailable Localphenomenaleadingtorampinsertionofreactivity Changeinreactivityduetoshiftingofgraphitereflectorblocks

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21of97 Ventingofgasbubblesaccumulatedintheactivecore Localphenomenaleadingtostepinsertionofreactivity Localnegativereactivityanomaly(e.g.,inadvertentsingleelementinsertion,covergasinjection)

Reactivityinsertioneventsduringstartup Thecontrolelementwithdrawalatmaximumspeed,describedabove,isassumedtobethelimiting eventofthiscategory.However,theamountandrateofreactivityinsertionfromothergroupedevents underinsertionofexcessreactivity(e.g.,duringthepebbleloadingerrorevent,ventingofaccumulated gasbubblesintheactivecore)iscomparedwiththosefromthecontrolelementwithdrawalevents.

Additionally,thereactivityinsertionduetoIncreaseinHeatRemovaleventsanddesignbasisseismic event,respectively,iscomparedtothereactivityinsertionofcontrolelementwithdrawalevents.

Inordertoensurethatthedesignfeaturesmitigatingareactivityinsertioneventaresufficienttokeep theconsequencesboundedbytheMHA,thefollowingkeyfiguresofmeritmustbeevaluated:

PeakTRISOtemperaturetolimitdiffusionofradionuclides PeakTRISOtemperatureTRISOfailureprobabilitytolimitincrementalTRISOlayerfailures PeakFlibecovergasinterfacialtemperaturetolimitevaporationmasstransferofradionuclides Peakvesselandcorebarreltemperaturestopreventvesselfailureandmaintainlongtermcooling Peaktemperatureofstructuralgraphitetolimitthetritiumrelease Peaktemperatureofpebblecarbonmatrixtolimittheamountoftritiumrelease 3.2.2.3 IncreaseinHeatRemoval Theprimarycoolantpumpoverspeeds,causingasurgeinsertionofcoldFlibeintothecore.Theeventis detectedbythereactorprotectionsystem,whichinitiatescontrolandshutdownelementsinsertion, fulfillingthereactivitycontrolfunction.Thereactorprotectionsystemalsotripstheprimarycoolant pump.Thereactordecayheatremovalsystemlimitsreactortemperatureandfulfillstheheatremoval function.

Asafestateisestablishedwhen:

Thecoreissubcriticalandlongtermreactivitycontrolisassured.

Thedecayheatisbeingremovedandlongtermcoolingisassured,wherefigureofmerit temperaturesaresteadilydecreasingandFlibetemperatureremainsaboveFlibefreezing temperatureduringthemissiontimeofthedecayheatremovalsystem.

Thisnarrativecapturesthelimitingeventofthispostulatedeventcategory.Othereventsgroupedinthis categoryinclude:

Increaseinheatremovalduetooverspeedofintermediatesaltpump Increaseinheatremovalduringlowpoweroperation Theincreaseinheatremovaleventsaredemonstratedtobeboundedbytheinsertionofexcess reactivitypostulatedevent.

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22of97 Inordertoensurethatthedesignfeaturesmitigatinganincreaseinheatremovaleventaresufficientto keeptheconsequencesboundedbytheMHA,thefollowingkeyfiguresofmeritmustbeevaluated:

PeakTRISOtemperaturetolimitdiffusionofradionuclides PeakTRISOtemperatureTRISOfailureprobabilitytolimitincrementalTRISOlayerfailures PeakFlibecovergasinterfacialtemperaturetolimitevaporationmasstransferofradionuclides Peakvesselandcorebarreltemperaturestopreventvesselfailureandmaintainlongtermcooling Peaktemperatureofstructuralgraphitetolimitthetritiumrelease Peaktemperatureofpebblecarbonmatrixtolimittheamountoftritiumrelease 3.2.2.4 LossofForcedCirculation Thefailureoftheprimarysaltpumpresultsinthelossofforcedcirculation.Thereducedflowis detecteddirectlyorindirectlybythereactorprotectionsystem,whichinitiatescontrolandshutdown elementsinsertion,fulfillingthereactivitycontrolfunction.Thereactordecayheatremovalsystem limitsreactortemperatureandfulfillstheheatremovalfunction.

Asafestateisestablishedwhen:

Thecoreissubcriticalandlongtermreactivitycontrolisassured.

Thedecayheatisbeingremovedandlongtermcoolingisassured,wherefiguresofmerit temperaturesaresteadilydecreasingandFlibetemperatureremainsaboveFlibefreezing temperatureduringthemissiontimeofthedecayheatremovalsystem.

Thisnarrativecapturesthelimitingeventofthispostulatedeventcategory.Othereventsgroupedinthis categoryincludelossofforcedcirculationdueto:

Blockageofflowpathexternaltothereactorvesselintheprimaryheattransportsystem, Spuriouspumptripsignal Pumpseizure Shaftfracture Bearingfailure Pumpcontrolsystemerrors Supplybreakerspuriousopening Lossofnetpositivesuctionhead(e.g.,pumpoverspeed,lowlevel)

Lossofnormalelectricalpower Lossofnormalheatsink Therearetwoboundingeventswithinthiseventcategorytoevaluatethelongtermpassivecooling performance.Oneistoboundtheoverheatingconsequence,andanotheristoboundthedowncomer freezingconsequence.Twoscenariosareconsideredforthesetwoboundingevents:

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23of97 Thefirsteventscenario(overheating)considersthelimitingcasetoanalyzethepeakvesselandcore barreltemperaturestopreventvesselfailureandmaintaincoolablegeometry.Themostlimiting reactoroperationpowerandoperatinghistoryareassumed.

Thesecondscenario(longtermovercooling)aimatthereactorperformanceevaluationintermsof coolantfreezepreventionatdowncomer.Aspectrumofreactordecayheatlevelsandoperating powerlevelsareanalyzedforthispurpose.

Fortheoverheatingboundingevent,thelossofforcedcirculationduetolossofnormalelectricalpower isboundedbytheprimarysaltpumpfailurescenario.Thelossofpowertothereactivitycontroland shutdownsystemmechanismsresultsinreleaseandinsertionofthecontrolandshutdownelements.As such,thereactorpowerisreducedfastercomparedtootherlossofforcedcirculationscenarioswhere thereactortripsonareactortripsignal.Forthelongtermovercoolingboundingevent,thelossof normalelectricalpowereventboundsotherlossofcirculationscenariossincethiseventhastheleast storedenergy.

Inordertoensurethatthedesignfeaturesmitigatingalossofforcedcirculationeventaresufficientto keeptheconsequencesboundedbytheMHA,thefollowingkeyfiguresofmeritmustbeevaluated:

PeakTRISOtemperaturetolimitdiffusionofradionuclides PeakTRISOtemperatureTRISOfailureprobabilitytolimitincrementalTRISOlayerfailures PeakFlibecovergasinterfacialtemperaturetolimitevaporationmasstransferofradionuclides Peakvesselandcorebarreltemperaturestopreventvesselfailureandmaintainlongtermcooling Peaktemperatureofstructuralgraphitetolimitthetritiumrelease Peaktemperatureofpebblecarbonmatrixtolimittheamountoftritiumrelease Theonlyfigureofmeritforthelongtermovercoolingscenariois:

Minimumreactorvesselinnersurfacetemperaturetopreventpartialfreezingwithindowncomer 3.2.2.5 PebbleHandlingandStorageSystemMalfunction Therearethreetypesofeventsinthiseventcategory:pebblehandlingandstoragesystem(PHSS) break,lossofPHSScooling,andgrindingofapebbleinthepebblehandlingmachine.However,theloss ofPHSScoolingisaneventmitigatedthroughdesignofpebblestoragesystem,andthegrindingof pebblemitigatedthroughthedesignofpebbleextractionmachine.Theconsequencesofthesetwo eventsareexpectedtobelimitedbythedesignspecificationswhichareboundedbyMHAconsequence.

Therefore,thePHSSbreakeventistheassumedlimitingeventtobeanalyzedforthiscategory.

Thepebblehandlingandstoragesystemtransferlinebreakswhenpebblesaregettingremovedfrom thecore,resultinginspillingofthepebbleswithinthetransferlineintothereactorcell.Thisconditionis detecteddirectlyorindirectlybythereactorprotectionsystem,whichtripsthepebblehandlingand storagesystemtostoppebblemovement.Forthespilledpebbles,thereactivitycontrolfunctionis fulfilledbythelowfissileinventoryofpebbles,whichprecludescriticalitysafetyconcerns,whileheat transfermechanismswithintheroomfulfillstheheatremovalfunction.Thestructuralintegrityofthe pebblesmaintainstheconfinementfunction.Forthepebblesremaininginthepebblehandlingand storagesystem,thereactivitycontrol,heatremovalandconfinementfunctionscontinuetobefulfilled bythesystemdesignresultinginasafeandstablestate.TheheatupofthepebblesinthePHSS

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24of97 mobilizestheFlibeaccumulatedonthepiping.AiringressintothePHSSandreactorcovergasregion occursthroughthebreak.

Asafestateisestablishedwhen:

Themovementofpebblesoutsideofthecorehasstoppedandcriticalitysafetyisassured.

Decayheatisbeingremovedfrompebblesoutsideofthecoreandlongtermcoolingisassured, wherefigureofmerittemperaturesaresteadilydecreasing.

ThisnarrativecapturesthelimitingPHSSbreakeventofthispostulatedeventcategory.OtherPHSS breakeventsgroupedinthiscategoryinclude:

Atransferlinebreakwhenpebblesaregettinginsertedintoemptycore Atransferlinebreakwhenpebblesaregettinginsertedintothecoreatpower Atransferlinebreakwhenpebblesaregettingtransferredtostoragecanisters Amishandlingoffueloutsidethereactor(e.g.,containmentbox,atthematerialbalanceareasand keymeasurepoints)

ThePHSSbreakeventwhenpebblesareextractedfromthecoreisconsideredboundingamongthe groupedeventsbecausethespilledpebbleshavehighertemperaturesandburnups,therefore,the highestdecayheatandMARloadingcomparedtoothereventsinthegroup.

InordertoensurethatthedesignfeaturesmitigatingaPHSSbreakeventaresufficienttokeepthe consequencesboundedbytheMHA,thefollowingkeyfiguresofmeritmustbeevaluated:

PeakTRISOtemperatureexvesseltolimitdiffusionofradionuclides MobilizedFlibeandgraphitedustreleased PeakTRISOtemperatureinvesseltolimitdiffusionofradionuclides PeakTRISOtemperatureTRISOfailureprobabilitytolimitincrementalTRISOlayerfailures PeakFlibecovergasinterfacialtemperaturetolimitevaporationmasstransferofradionuclides Peakvesselandcorebarreltemperaturestopreventvesselfailureandmaintainlongtermcooling Masslossofpebblecarbonmatrixduetooxidationtolimittritiumreleaseandpreventadditional releaseofmaterialsatrisk Masslossofstructuralgraphiteduetooxidationtolimittritiumrelease Peaktemperatureofstructuralgraphitetolimitthetritiumrelease Peaktemperatureofpebblecarbonmatrixtolimittheamountoftritiumrelease 3.2.2.6 RadioactiveReleasefromaSubsystemorComponent Anexternalhazardeventcausesafailureofcomponentsnotprotectedfromthehazardtofailand releaseMARstoredinthesesystems.Thesesystemsinclude:

Tritiummanagementsystem Inertgassystem Chemistrycontrolsystem

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29of97 Figuresofmeritmethodcansignificantlyreduceanalysiscostsincethedoseofthesamerelease pathwaycanbeboundedbyoneboundingcase.

Figuresofmeritforthepostulatedeventmustbedemonstratedtomeetacceptancecriteriaderived fromtheMHAconditions.Thefiguresofmeritforeachpostulatedeventaredevelopedbasedonthe releasepathwaysofradionuclidesduringtheevent.Theacceptancecriteriaforfiguresofmeritare developedtoensuretheradionuclidereleasesfromthepostulatedeventsthroughthesamepathways astheMHAarelessthanthosefromtheMHA.Therefore,iftheacceptancecriteriaforallfiguresof meritforapostulatedeventaremet,thedoseofthepostulatedeventisboundedbytheMHA.

ForthepostulatedeventswithadditionalreleasepathwaysthatdonotexistintheMHA,thethird methodisused.Thismethodhasthreesteps:

1.

Boundingdosesarecalculatedforeachreleasepathway;boundingdoseforeachrelease pathwayisthenusedtoderiveacceptancecriteriaforfiguresofmeritaccordingtothe boundingreleasepathwayconditionsforthepostulatedevent.

2.

Foreachspecificpostulatedevent,iffiguresofmeritfortheinvolvedreleasepathways meetacceptancecriteria,thecorrespondingboundingdosevaluesforthepathwayscanbe usedinsteadofdirectdoseanalysis.Directdoseanalysisforcertainreleasepathwayscan alsobeperformed.

3.

Allthedosevaluesforeachreleasepathwayforthepostulatedeventaresummedto comparewiththeMHAtotalreleasedose.Thetotaldoseforthepostulatedeventmustbe lowerthantheMHAdose.

Asanexample,forthefiguresofmeritmethod(i.e.,secondmethod),duringacoretransient, radionuclidesdiffusethroughtheTRISOfuellayersasafunctionoftemperature.Radionuclides inFlibeevaporatefromtheFlibecovergasinterfaceasafunctionoftemperature.Tritiumdesorbsfrom thegraphiteandpebblecarbonmatrix.Therefore,thepeakTRISOtemperaturetime,peakFlibecover gasinterfacialtemperature,peakgraphitetemperatureandpeakpebblecarbonmatrixtemperature profilesduringtheeventarefiguresofmeritforapostulatedeventthatinvolvesthecore.Additionally, peakTRISOtemperatureTRISOfailureprobabilityisalsoafigureofmerittolimitincrementalfuelfailure toanegligiblelevelduringthetransient;peakvesselandcorebarreltemperaturesarekeyfigureof merittoensurethereactorvesselperformsitssafetyfunction.

Thefiguresofmeritusedforsystemscodeanalysis(KPSAM)areasurrogatefordemonstratingthat consequencesareboundedbyMHAdoses,orformaintainingacoolablegeometry.However,ifdoseis thefigureofmeritforanevent(i.e.,adoseanalysisisperformedfortheevent),thenthosesurrogate figuresofmeritfordosedonotneedtomeetacceptancecriteria,becausethedoseacceptancecriterion isbeingexplicitlyevaluated.Likewise,whenafigureofmerithasbeenanalyzedseparatelyforbounding conditions(e.g.,astructuralanalysisofthevesselisperformedseparatelyfromthesystemsanalysis) thenthatfigureofmeritdoesnotneedtobeanalyzedinthesystemscodetomeetanacceptance criterion.

Thefiguresofmeritderivedforeachpostulatedeventandtheassociatedacceptancecriteriaare providedinTable32.

3.4.2.1 PeakTRISOTemperatureTime Thereleasepathwayforfuelisdiffusionalreleaseasafunctionoftemperature.Duringapostulated event,peakTRISOtemperatureisboundedbytemperaturetimecurvederivedfromtheassumedMHA

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30of97 fueltemperaturetimecurvetolimitdiffusionofradionuclidestolessthantheamountduringtheMHA.

BoundingtemperaturetimecurvederivedfromtheassumedMHAtemperaturetimecurvecanbe basedonintegratedeffectsondose.

3.4.2.2 PeakTRISOFailureProbabilitytemperature BasedonTRISOfuelqualificationeffortsasdescribedin(Reference26),itisexpectedthatdDuringa postulatedevent,incrementalfailureofTRISOfuelislimitedtoanegligiblelevelifthepeaktemperature isbelow1600°C.FailureprobabilityofTRISOfuelcanincreaseduetooverpressureintheTRISO particles,whichisafunctiontemperature.ThefailureprobabilityofTRISOfuelisevaluatedusingthe methodologydescribedinSection4.2.Incrementalfailureisdemonstratedtobenegligibleforpeakfuel temperaturesupto1600°C.

Alternatively,usingthemethodologydescribedinSection4.2,theincrementalfuelfailureduringthe postulatedeventiscalculatedusingtheassumedMHAfueltemperatureprofilesandisdemonstratedto benegligible.IfthepeakTRISOtemperatureduringapostulatedeventisboundedbytheassumedMHA fueltemperaturetimecurve,incrementalfailurefuelfailureisdemonstratedtobenegligible.

3.4.2.3 PeakFlibecovergasinterfacialtemperatures RadionuclidereleasefromFlibeisthroughevaporation.Duringapostulatedevent,peakFlibecovergas interfacialtemperatureisboundedbytemperaturetimecurvederivedfromtheassumedMHAFlibe covergasinterfacialtemperaturetimecurvetolimitevaporationmasstransferofradionuclidestoless thantheamountduringtheMHA.BoundingtemperaturetimecurvederivedfromtheassumedMHA temperaturetimecurvecanbebasedonintegratedeffectsondose.

3.4.2.4 Peakvesselandcorebarreltemperature Topreventvesselfailureandmaintainlongtermcoolingduringapostulatedevent,thepeakvesseland corebarreltemperaturesmustbelessthanboth(a)amaximumallowabletemperaturederivedtolimit excessivecreepdeformationanddamageaccumulationand(b)816°C.Themaximumallowable temperatureiscalculatedsothatthecreepstraininducedbyprimarymembranestresseswithinthe vesselandthecorebarreldoesnotexceed1%attheendofreactorlife.Itsderivationreliesonthe followingassumptions:

AllregionsofthevesselandcorebarrelincontactwithFlibeareexposedtotemperatureslower thanorequalto650°Cforthehotoperatingtimeofthevesselandtemperatureslowerthanor equaltothevesselandcorebarrelpeaktemperaturesforamaximumdurationof360hours(15 days).

Themaximumprimarystressesundergonebythevesselandcorebarrelcanbeboundedbya maximumstressvaluederivedasdescribedintheevaluationmodelforstructuralintegrity.

3.4.2.5 Minimumreactorvesselinnersurfacetemperature Duringthelongtermcoolingphaseofapostulatedeventwhendecayheatisbeingremovedpassively bytheDHRS,freezingmustbeavoidedwithinthedowncomertopreservethenaturalcirculation characteristicsinthevesselthatallowforuninterrupteddecayheatremoval.Aconservativetreatment istolimitthereactorvesselinnersurfacetemperaturetoalwayshigherthantheFlibefreezing temperatureof459°C.Ifthisconditionismet,nofreezinghappenswithinthedowncomer.

3.4.2.6 Airbornereleasefractionofspilled/splashedFlibe

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31of97 Duringasaltspillevent,aerosolscanbegeneratedthroughjetbreakup,andspillingandsplashing.The airbornereleasefractionsduetoaerosolizationmustbelimitedsothatthedoseconsequencesofthe saltspilleventsareboundedbytheMHA.

3.4.2.7 VolatileproductsfromFlibechemicalreactions Flibecouldbeexposedtoairduringasaltspillevent.ThekeyreleasepathwayofradionuclidefromFlibe isthroughevaporation,whichisafunctionofvaporpressureoftheradionuclidespecies.WhenFlibeis exposedtoair,theFlibeairchemicalreactiondoesnotresultinexcessivereactivevaporizationwhich wouldformradionuclidechemicalspeciesthathaveahighervaporpressurethanthosealreadyexistsin Flibecirculatingactivity.ItisexpectedthatafewspecificRNchemicalspecieswillhaveahighervapor pressureafterreactingwithairthanthoseinthecirculatingactivity.However,thosespeciesare expectedtobepresentatverylowconcentrationsandtheresultingdifferenceinevaporationratewill beofminimalsignificance.Forexample,CsFdissolvedinFlibedoesnotreactwithairtoformahighly volatilecesiumhydroxide.Assuch,Flibeairreactiondoesnotresultinsignificantadditionalreleaseof radionuclidesfromFlibethroughevaporation.

ThereactorcellfloorisassumedtobedesignedtoprecludeFlibeconcretereaction.WhenFlibeis spilled,ithasthepotentialtocomeincontactwithstainlesssteelandinsulationmaterial.Flibe interactionswithstainlesssteelandinsulationdonotresultinformationofradionuclidechemical speciesthathaveahighervaporpressurethanthosealreadyexistsinFlibecirculatingactivity.

Therefore,FlibestainlesssteelandFlibeinsulationreactionsintheHermesdesignbasisdonotresultin additionalreleaseofradionuclidesfromFlibethroughevaporation.

Duringasaltspillevent,Flibeisnotexposedtowater,andthereforenoFlibewaterreactionneedtobe considered.However,ifacommoncausefailure(e.g.,seismic)causesawatercontainingSSCandFlibe containingSSCtofailconcurrently,theamountofwaterthatFlibecouldbeexposedtoisassumedtobe limitedtoanupperboundlimitbydesign.Wheninteractingwiththisupperboundamountofwater, Fliberedoxpotentialisstillmaintainedwithintheboundsofsaltchemistryconditionsdefinedforthe evaporationmodel;therefore,doesnotresultinadditionalreleaseofradionuclidesfromFlibethrough evaporation.

Duringapostulatedeventthatinvolvestheprimaryheatexchanger(PHX),Flibecouldmixwithnitrate saltandreactchemically.ThevolatileproductsformedfromFlibeandnitratemixingareaddressedin Section4.5.

3.4.2.8 Masslossofstructuralgraphiteandpebblecarbonmatrix PebblesandstructuralgraphitenotsubmergedinFlibecanoxidizewhenexposedtoair.Ifthemassloss ofthepebblecarbonmatrixdoesnotextendtothefueledzone,tritiumreleaseistheonlyadditional MARreleasepathwaytobeconsideredwhenfuelpebbleoxidizes.Tritiumispuffreleasedfromoxidized pebblecarbonmatrixandoxidizedstructuralgraphite.IntheMHAanalysis,theassumedtemperature forpebblecarbonmatrixissohighthatallavailabletritiumiseffectivelypuffreleasedfromthepebble carbonmatrix.TheportionofstructuralgraphiteunsubmergedinFlibeissmall.Theinventoryoftritium puffreleased(insteadofasafunctionoftemperature)fromoxidizationofstructuralgraphitenot submergedinFlibeisaccommodatedbythefollowinginherentconservatisminthetreatmentoftritium intheMHA:

Conservativeinventoryoftritiumavailableforrelease Conservativelyhighassumedtemperatureofpebbles

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35of97 thevolumebranch,valve,pump,tank,pointkineticmodels,thermalradiation,andgapconductance models.

Thewidelyusedpointkineticsequationsmodelformultiplegroupsofdelayedneutronprecursorswas implementedinKPSAMwithfullyimplicittimeschemeoptionsavailableupto5thorderaccuracy.The decayheatpowercanbecalculatedfromtheuserprovideddecaycurve,ortheANSI/ANS5.12005 standardmethod.Whichevermethodisused,uncertaintyfactorswillbeappliedtoensureitis conservative.orfromthedecayheatmodelbasedonastandarddecayheatcurve.Forthepredictive decayheatmodel,thefissilematerialfissionfractionsincludeU235,U238,Pu239,andPu241andare providedbythereactorcoredesigncalculation.Thefissionratiosoffissilematerialsareprovidedfor variousstagesofoperation(buildup).Asensitivityfactorcanalsobeappliedtothedecayheatfraction inordertoconservativelyaccountforuncertaintiesindecayheat.

Closurerelationsarecorrelationsandequationsthathelptomodelthetermsinthefieldequationsby providingcodecapabilitytomodelandscaleparticularprocesses.Typicalclosuremodelsincludewall frictionfactorandformlossmodelsfordifferentflowgeometries,convectiveheattransfercorrelations fordifferentheattransfersurfacesandpumpperformancecurves.Fluidandsolidproperties,including equationsofstatearealsoneededtoclosethefieldequations.ThefluidstobesimulatedincludeFlibe, intermediatesalt,water,simulantoil,air,andargongas.

Table41summarizesthemodelsandthefieldequationsusedbyKPSAM.

4.1.1.2 ControlSystemDescription TheSAMcontrolsystemisusedtoperformtheevaluationofalgebraicandsimpleordinarydifferential equations;thetripsystemisusedtoperformtheevaluationoflogicalstatements.Thefundamental approximationmadeinthedesignofcontrol/tripsystemisthattheexecutionofcontrol/tripsystemis decoupledfromtheotherpartsofthehydraulicsystems.Themainexecutionofindividualcontrol/trip unitsissetattheendofeachtimestep.

4.1.1.3 NumericalMethods SAMusesacontinuousfiniteelementmethodsformulationforthespatialdiscretizationofthe1Dor2 Dfieldequations.ThedetaileddiscretizationforbothtimeandspaceismanagedbyMOOSE,withthe codeformulatedsuchthatthenumericalmethodordersarecontrolledthroughuserinputs.Forfluid models,aspatialstabilizationmethodisrequiredtosuppresscheckerboardtypespatialoscillationsthat manifestwhensolvingadvectiondominatedproblemsusingcontinuousfiniteelementmethods.The StreamLineUpwind/PetrovGalerkinandthePressureStabilizing/PetrovGalerkinschemeare implementedinSAMtoresolvethenumericalinstabilityissues(Reference10).

ThephysicsinSAMisintegratedintoasinglefullycouplednonlinearequationsystem.Thediscretized nonlinearequationsystemissolvedusingapreconditionedJacobianFreeNewtonKrylovmethod.The combinationoftheJacobianFreeNewtonKrylovnonlinearsolverandhighordernumericalmethodsfor bothtimeandspaceenablesthecapabilitytominimizenumericalerrors.

4.1.1.4 QualityAssuranceandConfigurationControl Thesoftwarequalityassuranceplanisdesignedtoprovideaframeworkforsolvingcomputational engineeringproblems.Thesoftwarequalityassuranceplanincludesrolesandresponsibilitiesforthe softwaredeveloper,reviewer,tester,anduseraswellasdocumentationandsoftwarereview requirements.Thesoftwarequalityassuranceplanalsodescribesconfigurationmanagement,change control,auditrequirements,softwareengineeringmethods,standards,practices,conventions,and

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39of97 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.Keymodeluncertaintiesandinitialconditionsare conservativelyappliedtothemethodstoensurefiguresofmeritareconservativelypredicted.Sample resultsforthepostulatedeventcategoriesareprovidedinAppendixAtoillustratethetransient methodologies.

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:

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43of97 whereEisanentrainmentcoefficient.AconservativelyhighlowvalueofEis2.1E6(Reference19).The aerosolgenerationduetothespillingandsplashingisthenobtainedthroughtheFlibespillingrateand Equations16and17.

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

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44of97 lowerpowerlevelsdonotunexpectedlychallengeafigureofmerit.Apoweruncertaintyisappliedto reactorpowertocoveruncertaintiesassociatedwithdetectionandsignaldelays.Sincethereactor powerisbiasedhighintheassumedlimitingreactivityinsertionevent,theinitialreactorpoweris modeledat102%power.AdditionalinitialconditionvaluesareprovidedinTable44.

4.5.2.2 TransientAnalysisMethods Thereactivityinsertiontransientinvolvesachangeincorereactivitythataddsheattoprimarysystem.

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

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

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,dependingonthecontrolelementcontroldesign,is analyzedinthefinalsafetyanalysistoensurethatthehighestreactivityinsertionrateis identifiedthatboundsthereactivityinsertionratespossibleforothereventsinthecategory.

o Atfullpowerandhotzeropower,theinitialcontrolelementpositionisassumedtobefully insertedinthereactorcore.

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19. M.Epstein,andM.Plys,PredictionofAerosolSourceTermsforDoseSiteFacilityApplications, ProceedingsofEFCOGSafetyAnalysisWorkingGroup,AprilMay2006.
20. S.Globe,andD.Dropkin,NaturalConvectionHeatTransferinLiquidsConfinedbyTwoHorizontal PlatesandHeatedfromBelow,J.HeatTransfer,Vol81,No.1,pp.2428,February1959.
21. W.McAdams,HeatTransmission,3rdEdition,McGrawHill,NewYork,p.180,1954.
22. K.Jafarpur,Yovanovich,M.M.,LaminarFreeConvectiveHeatTransferfromIsothermalSpheres:a NewAnalyticalMethod,Int.J.HeatMassTransfer,Vol35,No.9,pp.21952201,1992.
23. M.F.Young,LiftoffModelforMELCOR,SAND20156119,SandiaNationalLab,2015.
24. S.Levy,TwoPhaseFlowinComplexSystems,1stEdition,WileyInterscience,August1999.
25. S.K.Friedlander,Smoke,Dust,andHaze:FundamentalsofAerosolDynamics,2ndEd.,Oxford UniversityPress,2000.
26. ElectricPowerResearchInstitute,UraniumOxycarbide(UCO)TristructuralIsotropic(TRISO)Coated ParticleFuelPerformance,TopicalReportEPRIAR(NP)A,3002019978,November2020.

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58of97 Table32:DerivedFiguresofMeritandAcceptanceCriteriaforPostulatedEvents FigureofMerit AcceptanceCriterion ApplicableEvents PeakTRISOtemperaturetime Generallyboundedbytemperature timecurvesderivedfromthe assumedMHAfueltemperature timecurve SaltSpills,Reactivity Insertion,IncreaseinHeat Removal,LossofForced Circulation,PHSSbreak, Seismic,PHXTubeBreak PeakTRISOtemperature timeTRISOfailureprobability BelowincrementalNegligibleTRISO fuelfailuretemperatureprobability SaltSpills,Reactivity Insertion,IncreaseinHeat Removal,LossofForced Circulation,PHSSbreak, PHXTubeBreak PeakFlibecovergasinterfacial temperature Generallyboundedbytemperature timecurvesderivedfromthe assumedMHAFlibecovergas interfacialtemperaturetimecurve

SaltSpills,Reactivity Insertion,IncreaseinHeat Removal,LossofForced Circulation,PHSSbreak, PHXTubeBreak Peakvesselandcorebarrel temperatures Boundedbyboththemaximum allowabletemperaturederivedto limitexcessivecreepdeformation anddamageaccumulationandby 816°C(highesttemperature consideredbyASMESectionIII Division5for316H)

SaltSpills,Reactivity Insertion,IncreaseinHeat Removal,LossofForced Circulation,PHSSbreak, PHXTubeBreak Minimumreactorvesselinner surfacetemperature AboveFlibemeltingtemperature LossofForcedCirculation Airbornereleasefractionof spilled/splashedFlibe Belowairbornereleasefraction limitderivedtoboundtotalreleases ofthepostulatedeventtolessthan theMHA SaltSpills,Seismic,PHX TubeBreak Volatileproductformationfrom Flibeairreaction Negligibleamountofadditional volatileproductsformed SaltSpills,PHSSbreak, PHXTubeBreak Volatileproductformation fromFlibechemicalreactionwith water,concrete,and/or constructionmaterials(e.g.,

insulation,steel)

Negligibleamountofadditional volatileproductsformed SaltSpill Volatileproductformationfrom Flibechemicalreactionwith nitrate Negligibleamountofadditional volatileproductsformed PHXTubeBreak Masslossofpebblecarbon matrixduetooxidation Masslossdoesnotextendintothe fueledzone SaltSpills,PHSSbreak Masslossofstructuralgraphite duetooxidation BoundedbytheMHArelease SaltSpills,PHSSbreak

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