ML24073A319

Enclosure 1: Changes to Hermes 2 PSAR Chapter 13

ML24073A319
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Issue date:	03/13/2024
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KairosPowerHermes2,Units1and2 132 Revision0 13.1 INITIATINGEVENTSANDSCENARIOS Thissectionprovidestheeventspostulatedforthereactordesignbasis.Theeventsaregrouped accordingtotypeandcharacteristicsoftheevents.Theeventcategoriesare:

MHA InsertionofExcessReactivity SaltSpills LossofForcedCirculation(includesalossofnormalelectricpower)

MishandlingorMalfunctionofPebbleHandlingandStorageSystem RadioactiveReleasefromaSubsystemorComponent GeneralChallengestoNormalOperation InternalandExternalHazardEvents TheMHAisascenariothatboundsotherpostulatedeventgroups.TheanalysisoftheMHAin Section13.2demonstratesthesafetymarginsofthedesign.

Forpostulatedevents,figuresofmeritforeacheventcategoryprovidesurrogatemetricswhich demonstratethattheresultingdoseisboundedbythedoseconsequencesoftheMHAanalysisas describedinKPTR022P,PostulatedEventMethodologyTechnicalReport(Reference2).Acceptance criteriaforthesefiguresofmeritrepresentdesignlimitsthatensuretheMHAisbounding.The acceptancecriteriaforthepostulatedeventfiguresofmeritareprovidedinTable13.11.

Theconsequencesofpostulatedeventspresentedinthischapterwouldnormallybemitigatedbynon safetyrelatedSSCsforreactivitycontrolandheatremoval(andthebuildingforconfinementif radioactivematerialisreleased).However,consistentwiththeguidanceinNUREG1537,onlythe performanceofthesafetyrelatedstructures,systems,andcomponents(SSCs)arecreditedinthe postulatedevents.TheperformanceoftheSSCsalsoassumetheworstsinglefailureofanyactive components.Thisconservativeapproachtosafetyanalysisprovidesadditionalconfidencethatthe postulatedeventsareboundedbytheMHA.ThesafetyclassificationsofSSCsareprovidedin Section3.6.

ThediscussiononpreventingcertaineventsbydesignisprovidedinSection13.1.10.

ThepostulatedeventnarrativesandmethodspresentedinthischapterareinformedbytheHermes PostulatedEventMethodologytechnicalreport,KPTR018(Reference6).Thisisanappropriate precedent,giventhesimilaritiesbetweentheHermesandHermes2designs.Hermes2employsthe samefunctionalcontainmentstrategyasHermes,andincludeseverysafetyfeature(e.g.,DHRS)ofthe Hermesdesign.TodeveloptheHermes2postulatedeventmethodology(KPTR022),theevent narrativesandmethodsdescribedinKPTR018wereaugmentedbyassessingnewpotentialevent initiatorsidentifiedfortheHermes2design.

Hermes2includesanintermediateheattransportsystem(IHTS)andapowergenerationsystemas describedinPSARchapter5and9,respectively.AdditionalsafetyfeaturesinHermes2includetheIHTS rupturediscsdescribedinPSARSection5.2andareactorprotectionsystem(RPS)triponthe intermediatesaltpump(ISP)describedinPSARSection7.3.Limitsontheamountsofmaterialatriskin theIHTSandpowergenerationsystems,andlimitsontheallowableamountsofFlibeandwaterinthe IHTSaredescribedinChapter14.Theseadditionalsafetyfeaturesandproposedlimitsensurethat eventsresultingfrompostulatedfailuresoftheIHTSorpowergenerationsystemsareeitherprevented bydesign,orcanbegroupedwithintheeventcategoriesdescribedinKPTR018.Groupedevents specifictotheHermes2design(i.e.,IHTSspill,steamlinebreak,ISPfailure)donotintroducenew phenomenathatrequireadditionalfiguresofmerittobeevaluatedtoensurethattheconsequencesof

PreliminarySafetyAnalysisReport

AccidentAnalysis

KairosPowerHermes2,Units1and2 133 Revision0 postulatedeventsremainboundedbytheMHA.Therefore,thefiguresofmeritdescribedinKPTR018 areapplicabletotheHermes2postulatedeventmethods.AsdescribedinKPTR022,eventsare groupedbasedonsimilarcharacteristicsandmodelingapproaches,includingeventsspecifictothe Hermes2design.

Certaininitiatorsmayresultinatransientinbothunits(e.g.,turbinetrip).However,theunitsare independenttotheextentthattheeventprogressionandunitresponsecanbeevaluatedforeachunit separately.

13.1.1 MaximumHypotheticalAccident TheMHAisaneventwherehypothesizedconditionsresultinaconservativelyanalyzedreleaseof radionuclidesthatboundsapotentialreleasefromotherpostulatedevents.TheMHAanalysisis consistentwiththefissionproductreleaseaccidentanalysisrequiredforthe10CFR100.11 determinationofexclusionarea,lowpopulationzone,andpopulationcenterdistances.TheMHAisa boundingeventwithconservativeradionuclidetransportassumptionsthatchallengetheimportant radioactiveretentionfeaturesofthefunctionalcontainment.Thissectiondescribesthekeyassumptions andnonphysicalconditionsthatarehypothesizedtoensurethatthedoseconsequencesfromtheMHA analysisboundsthedoseconsequencesfrompostulatedeventsinthedesignbasis.Thedetails associatedwiththeseassumptions,aswellasthemethodsusedtocalculatethedoseconsequencesof theMHAareprovidedinSection13.2.1.

13.1.1.1 InitialConditionsAssumptions NormaloperatingparametersarediscussedinSection4.1.Conservativeinitialvaluesareassumedfor eachoperatingparametertomaximizethereleaseofradionuclidesintheMHA.

TheradioactivematerialthatisatriskforreleasefortheMHAincludesradionuclidescontainedinthe fuel,theradionuclidescirculatingintheFlibe,andtheradioactivematerialatriskforrelease(MAR) distributedwithintheprimarysystem(i.e.,steelstructuresandgraphite).Althoughradionuclidescould havediffusedawayfromthetristructuralisotropic(TRISO)fuelparticles,theinitialinventoryofthe smallfractionoffuelthatisdefectiveattheinitiationofthetransientassumesthatnodiffusionhas occurred.Thishypotheticalconditionaddsaboundingconservatismtotheradionuclidereleasefrom thefuelandFlibe.

TheTRISOfuelformandthebasisforitsradionuclideretentionperformanceisdiscussedinSection 4.2.1.Themethodologyfordeterminingtheradionuclidebehaviorandretentionpropertiesofthefuelis providedinSection3ofKPTR012,KPFHRMechanisticSourceTermMethodologyTopicalReport, (Reference1).Fuelmanufacturingandinserviceperformancespecificationsarediscussedin Section4.2.1.

TheFlibedesignisdiscussedinSection5.1.Themethodologyfordeterminingtheradionuclidebehavior andretentionpropertiesoftheFlibeisprovidedinSection4ofReference1.AboundingvalueforFlibe circulatingactivityisassumedastheinitialcondition.

Aboundingvalueofretainedtritiumandactivatedargonavailableforreleaseisassumedtoencompass availablevolumeandgeometryoftritiumabsorbingmaterialsinthesystem.

13.1.1.2 Structures,SystemsandComponentsMitigationAssumptions Thissectiondescribesthestructures,systems,andcomponents(SSCs)thatperformafunctionto mitigatethedoseconsequencesoftheMHA.

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KairosPowerHermes2,Units1and2 1321 Revision0 TheradionuclidesevaporatedfromtheFlibefreesurfaceareseparatedintothefollowingrelease inventories:

1)

PuffreleaseofdissolvednoblegasesandbubbleburstFlibeaerosolsatthebeginningofthe transient;

2)

Onelinearreleaseforevaporationofradionuclidesoverthefirst10mintemperatureinterval correspondingtoprescrammedfueltemperature

3)

Onelinearreleaseforevaporationofradionuclidesoverthenext110mintemperatureinterval;

4)

Onelinearreleaseforevaporationofradionuclidesoverthenext70hourreleaseinterval;

5)

Onelinearreleaseperdayforthenextsevendaysforthereactorcooldownperiod;and

6)

Onefinallinearreleaseovertheremaining20days.

GasSpace Thegasspacetransportevaluationisdividedintotwomodels:buildingtransportandatmospheric dispersion.ThemethodologyforDesignBasisAccidentsinReference1isusedtoevaluatethegasspace transport.The

valuesarecalculatedasdescribedinSection2.3.4foranassumedexclusionarea boundary(EAB)distanceof250mandalowpopulationzone(LPZ)distanceof800masprovidedin Section2.1.1.2.TheX/QvaluesareprovidedinTable13.21.

13.2.1.2 Results ThedoseconsequencesoftheMHAareprovidedinTable13.22.Thedoseconsequenceresultsmeet thesitedoselimitsin10CFR100.11(a)(12)attheEABandLPZwithsignificantmargin.

13.2.2 PostulatedEventMethodologyandSampleResults Theevaluationmodelsandmethodologiesusedtoanalyzethepostulatedeventsdescribedin Section13.1aredetailedinthePostulatedEventMethodologyTechnicalReport(Reference2).The methodologiesincludetherationaleforthefiguresormeritandtheassociatedacceptancecriteria (providedinTable13.11)foreachpostulatedeventcategory.Thefiguresofmeritarefiguresofmerit thatwhenanalyzedagainsttheacceptancecriteriaensurethatthepostulatedeventsresultindoses boundedbytheMHA.

13.3 REFERENCES

1. KairosPowerLLC,KPFHRMechanisticSourceTermMethodologyTopicalReport,KPTR012PA.

May2022.

2. KairosPowerLLC,Hermes2PostulatedEventMethodologyTechnicalReport,KPTR022P, Revision0.June2023.

3. MELCORFissionProductReleaseModelforHTGRs,SandiaNationalLaboratories,2010.

4. B.J.Lewis,D.Evens,F.C.Iglesias,andY.Liu,ModellingofShortLivedFissionProductRelease BehaviorDuringAnnealingConditions,JournalofNuclearMaterials,vol.238,pp.183-188,1996.

5. HighTemperatureGasCooledReactorFuelsandMaterials,INTERNATIONALATOMICENERGY AGENCY,Vienna,IAEATECDOC1645,2010.

5.6. KairosPowerLLC,PostulatedEventMethodologyTechnicalReport,KPTR018P,Revision2.

February2023.