ML22251A400

Enclosure 2: Response to NRC Request for Additional Information 350

ML22251A400
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Site:	Hermes File:Kairos Power icon.png
Issue date:	09/01/2022
From:	Kairos Power
To:	Office of Nuclear Reactor Regulation
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ML22252A149	List: ML22244A256 ML22251A400
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KP‐NRC‐2209‐001
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KPNRC2209001

Enclosure2

ResponsetoNRCRequestforAdditionalInformation350

(NonProprietary)

NRCRequestforAdditionalInformation RAIPackage350,Question410

Section50.34ofTitle10oftheCodeofFederalRegulations(10 CFR50.34),"Contents ofapplications; technicalinformation,"providesrequirementsforinformationtobeprovided inaConstruction Permit(CP).10CFR50.34(a)(4)states thataCPshallcontainapreliminaryanalysisandevaluationof SSCs providedformitigationoftheconsequencesofaccidentstodeterminemarginsofsafetyduring normal operationsandtransient conditionsduringthelifeofthefacility.

Section3.1.1,"DesignCriteria,"oftheKairosPower(KP)Hermes PreliminarySafetyAnalysisReport (PSAR)referencesdocumentKP TR003 NPA,"PrincipalDesign Criteria [PDC]fortheKairosPower FluorideSaltCooled, HighTemperature Reactor,"Revision1,toprovidethePDCfortheHermestest reactor.KPFHRPDC 14, "Reactorcoolantboundary,"states thatsafetysignificantelementsofthe reactorcoolantboundaryshall haveanextremelylowprobabilityofabnormalleakage,rapidly propagating failure,andgrossrupture.KPFHRPDC31, "Fracturepreventionofthereactorcoolant boundary,"statesthatthereactorcoolantboundaryshall bedesignedtoconsiderservice degradationofmaterialpropertiesincludingeffectsofcontaminants.KPFHRPDC35,"Passive residualheatremoval,"statesthatasystemshallbeprovidedtoremoveresidualheatduringand afterpostulatedaccidents.KPFHRPDC 74,"Reactorvesselandreactorsystemstructuraldesign basis,"states thatthevesselandreactorsystemshallbedesignedtoensureintegrityismaintained duringpostulatedaccidentstoensurethegeometryforpassiveheatremovaland allowfor insertion ofreactivitycontrolelements.

Section4.3ofthePSAR,"Reactor VesselSystem,"describesthecomponentsthatformthenatural circulationflowpathneededtoprovideresidualheatremovalduringandfollowingpostulated events.These includeportionsofthegraphitereflectoraswellasmetalliccomponentssuchasthe core barrel,reactorvessel,andfluidicdiode.ThissectionofthePSARdescribeshowthese componentsareneededtomeetPDCs14,31, 35, and74.

Section5.1.3 ofthePSAR,"SystemEvaluation,"statesthat"significant"airingressintotheprimary heattransportsystem(PHTS)isexcludedbydesignbasis.Inaneventwithpostulatedairingressinto thePHTS,thecomponentsthatcomprisethenaturalcirculationflowpathwillneedtoperformtheir safetyfunctions(i.e.,maintainthenaturalcirculationflowpath)tomeetthePDClistedabove. The staffnotesthatairingressintothePHTS cancauseoxidationofthegraphitereflectoraswellas corrosionofmetalliccomponentsintheprimarysystem,and suchdegradationcouldpotentially challengenaturalcirculationflow.Inorder toevaluateeffects ofairingress, thestaffneedsto understand theamountofairingressthatwillbeallowedandhowthelimitationofingresswillbe achieved.

Therefore,theNRC staffrequeststhefollowinginformation:

1. Definewhatconstitutes"significant" airingressintothePHTS and thebasisfordetermining whatis"significant."

2. Describehowcomponentintegrityisensuredifthedurationofanairingresseventislonger thanthedurationcoveredbythematerialsqualificationtesting.

3. Inan eventsuchasasaltspillorheatradiatortuberupture,howisfurtherairingress preventedafter aheatrejectionblowertrip?

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Kairos PowerResponse NRCQuestion410,Item1

Definewhatconstitutes"significant"airingressintothePHTS and thebasisfordetermining whatis "significant."

ThediscussioninPreliminarySafetyAnalysisReport(PSAR)Section5.1.3 isreferringto limitingthe amount ofairingressthatisforcedinto theFlibe,not limitingthe amountofairingresstothe reactorsystemasawhole.AscitedinPSARSection5.1.3,the designevaluationoflimitingsignificant airingressdemonstratescompliancewithPrincipalDesignCriteria(PDC)33 andPDC70.PDC33and PDC70arefocusedondetailsoftheFlibe,notthegasspaceabovethefreesurface ofFlibe.This distinctionisimportanttorecognizefortheresponsesprovidedtothisRAI.Theresponseto Item2 belowincludestheconsiderationofoxidationeffectsfornonFlibewettedgraphite abovethefree surfaceofFlibe.

AsdescribedinPSAR Section5.1.3,significantairingressintothePrimaryHeatTransportSystem (PHTS) referstotwoscenarios:

Significantairbeingentrainedinthecoolantduringnormaloperation(tomeetPDC33)

Forcedairingressoccurringduringpostulatedsystemleakageevents(tomeetPDC70)

Ifairisentrainedinthecoolantduringnormaloperation,operationalcontrolsareexpectedto monitorthequantityofairwithinthePHTStopreventaccumulatingsignificantquantitieswith a technicalspecification,asdiscussedinPSARSection13.1.10.5. The limitforsignificantairingress willpreventvoidaccumulationandlimitthetotalcorrosionofFlibewettedcomponents,as describedinPSARTable14.11.Consistentwith10CFR50.34(a)(5),thePSARidentifiesthevariable expected tobesubjecttotechnicalspecificationcontrol,andPSAR Section14.1commitsto providing theparameterlimitswiththeapplicationforanOperatingLicenseApplication,consistent with10CFR50.34(b)(6)(vi).

Forthe scenarioswheresignificant forcedairingressispreventedduringpostulatedeventsinvolving abreachorbreakinthe PHTS,significantreferstoamountsofairthatcouldbeforced intothe Flibebythe drivingforcesassociatedwiththeheatrejectionblowerortheprimarysaltpump. As describedinPSARSection7.3.1,there aresafetyrelated tripsontheheatrejectionblowerand primarysaltpump,which removethemechanismsthatcouldforceairintothe Flibeduringasystem leakageeventtopreventsignificantforcedairingress.

PSARSections5.1.3,and13.1.10.5havebeenupdatedto clarifythatforcedairingressintothe PHTS isprecludedbydesign.

NRCQuestion410,Item2

Describehowcomponentintegrityisensuredifthedurationofan airingresseventislongerthanthe durationcoveredbythematerialsqualificationtesting.

Bymaintainingthe quantityofairwithinthetechnicalspecificationlimitduringnormaloperation andremovingthe mechanismstoforceairintotheFlibedescribed inItem1ofthisRAI,the structuralintegrityofmetallicandgraphitecomponentsthatremain Flibewettedisensured to remain withinconditionsboundedbythematerialsqualificationtestingprograms(References1and 2)forairingresseventsuptosevendays. Themetallic materials qualificationtopicalreportincludes

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(Reference1).The graphitematerialqualificationtopicalreportdescribestheassessmentplanfor theeffectsofaircontaminationinFlibeonET10graphite (Reference2).

Duringnormaloperation,the argoninthegasspacewillbemonitoredforpotentialairingressas describedintheKairosPowerresponseto RCI02(ML22231B230).

Thegraphitereflectorblocksthatare locatedabovethefreesurfaceoftheFlibeare subjectto potentialoxidationeffectsduringapostulatedairingressevent.Sincetheshutdownelementsinsert atthe beginning oftheevent,thisexposedgraphitestructureisnotcreditedafterinsertionto performalongtermstructuralintegritysafety functionwhenoxidationcouldbegintoaffectthe structuralintegrity.Additionally,ifsignificantoxidationweretoresultinalossofstructuralintegrity oftheexposedgraphite,thereisalayerofsubmerged(Flibewetted)graphitethatmitigates debris fromtheexposedgraphitefromenteringthe natural circulationflowpath.

AsshowninFigure1,thesecondaryholddownstructureisinstalledwithintheupperlayersofthe graphitereflectorandextendsbelowtheminimumFlibelevel foraPHTSbreakevent.Ifsignificant oxidationweretoresultinalossofstructural integrityofthegraphiteabovethe minimumFlibe level,thesecondary holddownstructurewilltransferloadsfromthesubmergedgraphiteto thetop head,keepingthe remaining reflectorstructureinplaceandsubmergedinFlibe.Theeffectsofnon forcedairingressontheintegrityofcomponentsbelowthesurfaceofFlibewillbeboundedbythe materialsqualificationtestingprogramsforatleastsevendaysfollowingtheinitiationoftheevent.

Beyondsevendays,defenseindepthfeaturesinclude:implementingrepairs ondamagedSSCs, replenishingargonsupply,orremovaloffuelfromthe vessel.This ensuresthatthegeometryofthe core andthenaturalcirculationflowpathsaremaintained.PSARSection4.3hasbeen updatedto removethe statementthe reactorvesselisdesignedtoprecludeairingressandto reflectthe secondaryholddown structuredesigndetailsdescribedabove.PSARSection13.1.10.5 hasbeen updatedto describedefenseindepthfeaturesofthedesignavailableaftertheinitialsevenday period ofapostulatedairingressevent.Amarkupofchangestothegraphitequalificationtopical reportprovidingadditionaldetailsoftheoftheassessmentofairingressonthe integrityof componentsbelowthe surfaceofFlibeisbeingprovidedwiththisresponse.Arevisiontothe graphitequalificationtopicalreportwillbesubmittedbyseparateletter.

NRCQuestion410,Item3

Inan eventsuchasasaltspillorheatradiatortuberupture, how isfurtherairingresspreventedafter aheatrejectionblower trip?

AsdescribedinItem 1,safetyrelatedtripsontheheatrejectionblowerandprimarysaltpump removethe mechanismsthatcouldforceairinto theFlibeduringasystemleakageevent. The Hermesdesigndoesnotcredit anymeansoflimitingfurthernonforcedairingressintothePHTSin theevent ofasaltspillorradiatortuberupture.SeeresponsetoItem2fordiscussionoftheimpacts ofnonforcedairingressonvesselinternals.

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References:

1. KairosPowerLLC,MetallicMaterialsQualificationfortheKairosPowerFluorideSalt CooledHighTemperatureReactor,KPTR013 P,Revision3.

2. KairosPowerLLC,GraphiteMaterialQualificationfortheKairosPowerFluorideSalt CooledHighTemperatureReactor,KPTR014 P,Revision3.

Impact onLicensingDocument:

ThisresponseimpactsSections4.3,5.1.3,and13.1.10.5oftheKairosPowerPreliminarySafety AnalysisReportandSection5.3ofGraphiteMaterials QualificationfortheKairosPowerFluoride SaltCooledHighTemperatureReactor. Markupsoftheaffectedsectionsareprovidedwith this response.

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Figure1

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Preliminary Safety AnalysisReport ReactorDescription

coolantlevel.Thedesign ofthereactorvesselallowsfor onlinemonitoring,inservice inspection, and maintenance.

4.3.1.1.1 VesselTopHead Thereactorvesseltophead (see Figure 4.32) isaflat316HSSdiscboltedandflangedtothevessel shell. Thisinterfaceisdesignedfor leaktigh tnessbutisnotcreditedas beingleaktightin safety analyses.Thevesseltopheadcontrolstheradialandcircumferentialpositionsofthereflectorblocksto ensureastable coreconfigurationfor allconditions(e.g., reactortrip andcoremotion).Thetophead containspenetrations,as shownin Figure 4.32 andTable4.31, intoandoutofthevesselandprovides for theattachmentofsupportingequipmentandcomponents(e.g.,reactivitycontrolelements,pebble handlingandstoragesystemcomponents,materialsamplingport,neutrondetectors,thermocouples, etc.).Thetopheadsupportsthevesselmaterialsurveillancesystem(MSS)whichprovidesaremote meanstoinsertandremovematerialandfueltestspecimensintoandfromthereactortosupport testing. Aholddown structuresubassembly isweldedunderneaththevesseltophead.Thisstructure contactswiththetopsurfaceofthegraphitereflectorandprovidesstructuralsupportagainstupward loadsduringnormaloperationandmostpostulatedevents.Asecondaryholddownstructureisinstalled throughtheuppergraphitelayers, extendingfromthereflector topintosubmerged graphitelayers to transferupwardloadsfromsubmergedgraphitetothevesseltopheadduringpostulatedair ingress events.Thesecondary holddownstructureextendstobelowtheminimumreactorvesselcoolantlevel thatcouldresultfrompostulated saltspillevents.

4.3.1.1.2 VesselShell Thereactorvesselisa316HSScylindrical shell that,alongwiththevesselbottomhead,servestoform thesafetyrelated reactorcoolantboundarywithinthereactorvessel.Itcontainsandmaintainsthe inventory ofreactorcoolantinsidethevessel.Theshellprovidesthegeometryfor coolantinletand vesselsurfacefor theDHRSwhichtransfersheatfromthereactorvesselduringpostulated events.The insideoftheshell uses316HSStabstomaintainthecorebarrelinacylindricalgeometryandhasa weldedconnectionatthetopofthecorebarrel.

4.3.1.1.3 VesselBottomHead Thereactor vesselbottomheadisaflat316HSSdiscthatisweldedtothevesselshell.Itcontainsand maintainstheinventory ofthereactorcoolantinsidethevessel,supportsthevesselinternals,maintains thereactorcoolantboundaryandprovidesflowgeometryfor lowpressurereactorcoolantinlettothe core.Hydrostatic,seismicandgravityloadsonthevesselandvesselinternalsaretransferredtothe bottomheadandaretransferredtotheRVSS.

4.3.1.2 ReactorVesselInternals Thereactorvesselinternalstructuresincludethegraphitereflectorblocks,corebarrelandreflector supportstructure.Thevessel internalstructuresdefine theflowpaths ofthefuelandreactorcoolant, provideaheatsink,apathwayfor instrumentation insertion, control andshutdownelementinsertion, as well as provide neutron shieldingandmoderationsurroundingthecore.Thedesignofthestructures supportinspectionandmaintenanceactivitiesas wellas monitoring ofthereactorvesselsystem.

4.3.1.2.1 ReflectorBlocks ThereflectorblocksareconstructedofgradeETU10 graphite.Thereflector blocksprovideaheatsink for thecoreandarerestrainedensuringalignmentofthepenetrationstoinsertandwithdrawcontrol elements.Thereflector blocksarebuoyantinthereactorcoolant.Thetopsurfaceofthereflectorblocks contactsthevesseltopheadholddown structuresubassembly whichprovidesstructuralsupport

Kairos PowerHermesReactor Revision0429 Preliminary Safety AnalysisReport ReactorDescription

againstupward loadsduring normaloperationandmostpostulatedevents.A secondary holddown structureisinstalledthroughtheupperreflectorlayerstotransferupwardloadsfromsubmerged graphitetothevesseltopheadduringpostulatedairingressevents.Thebottomreflector blocksare machined withcoolantinlet channelsfor distributionofcoolantinlet flowintothecore.Thetop reflectorblocksaremachined withcoolantoutletchannelstodirectthecoolantexitingfromthecore intotheupperplenum,fromwhichthePSPdrawssuction.Thetopreflector blocksalsoformapebble defuelingchute,asshowninFigure4.31, todirectthepebblesfromthecoretothepebbleextraction machine(PEM),allowingonlinedefuelingofthereactor (see Section9.3).Thereflectorblocksalso providemachinedchannelsfor insertion andwithdrawalofthereactivitycontrol andshutdown elementsdescribedinSection4.2.2.

Thereflectorblocksformanupperplenumandafluidic diode,whichisastainlesssteel passivedevice thatconnectstheupperplenumtothetopofthedowncomeras showninFigure4.31. Thediode introducesahigherflowresistanceinonedirection,whilehavingalower flowresistanceintheother direction.Thedioderestrictsflowfromthehigherpressure downcomer intotheupperplenumduring conditionswithforcedcirculation.Theflowpassesin thelowresistance direction ofthediodefromthe upperplenumtothetopofthedowncomer drivenbynatural circulation.

Thegraphitereflectorblocksreflectneutronsbackinto thecore, increasingthefuelutilizationwhile protectingthereactorvesselfromfluencebasedforms ofdegradation.Furtherdiscussionofthe reflectorsneutroniccharacteristicsaredetailedinSection4.5.

4.3.1.2.2 CoreBarrel The316HSScorebarrelcreatesanannularspacebetweenitselfandthereactorvesselanddefinesthe downcomerflowpathfor thecoolant.Thecorebarrelhasaflangedtopwhichisweldedtotheinner wallofthevesselshell. Thebarreliskeptconcentrictotheshellbyradialtabswhichallowfor differentialthermalexpansion.

4.3.1.2.3 ReflectorSupportStructure The316HSSreflectorsupportstructure,asshowninFigure4.31, definestheflowpathfromthe downcomerannulusintothecoreas wellas providessupporttothegraphitereflectorblocks.The reflectorsupportstructureensuresastable coreconfigurationforall conditions (e.g.,reactortripand coremotion)bycontrollingtheradialandcircumferentialpositionsofthereflector blocks.

4.3.2 Design Basis ConsistentwithPDC 1, thesafetyrelated portionsofthereactorvesselandreactorvesselinternalsare fabricatedandtestedin accordancewithgenerallyrecognizedcodesandstandards.

ConsistentwithPDC 2, thereactor vesselandreactor vesselinternalsperformtheir safetyfunctionsin theeventofasafeshutdown earthquakeandothernatural phenomenahazards.

ConsistentwithPDC 4, thereactor vesselandreactorvesselinternalsaccommodatetheenvironmental conditionsassociated withnormaloperation,maintenance,testing,andpostulatedevents.

ConsistentwithPDC 10, thereactorvessel andinternals maintainageometryandcoolantflowpathto ensurethatthespecifiedacceptablesystemradionuclidereleasedesignlimits(SARRDLs)willnotbe exceededduring normaloperation includingpostulatedevents.

ConsistentwithPDC 14, thereactorvesselisfabricated andtested tohaveanextremely lowprobability ofabnormalleakage orsuddenfailure ofthereactorcoolantboundarybygrossrupture.

Kairos PowerHermesReactor Revision0430 Preliminary Safety AnalysisReport ReactorDescription

factorsuptoatemperatureof650°Cfor ER168 2 weldmetal with316Hbasemetal.Testingprovides stress rupture factorsupto816°Cfor weldmaterialwith316Hbasemetal(Reference3).Theplant controlsystemwilldetectleakagefromthereactorvesselandcatch basinsareused todetectleaksin nearbycoolantcarrying systems.ThesefeaturesdemonstratecompliancewithPDC 30.

Reactorvesselstress rupturefactorsaredeterminedupto816°Ctoencompasstransientconditions.

Thestress rupturefactorsaredeterminedbyacreeprupture testonthevesselbasematerialwithweld metal underthegastungstenarcwelding process.Thevesselprecludesmaterialcreep, fatigue,thermal, mechanical,andhydraulicstresses.Theleaktightdesignofthereactorvesselheadminimizesair ingress intothecovergasandprecludescorrosionoftheinternals.Thehightemperature, highcarbongrade 316HSSofthecorebarrel andreflector supportstructurehavehighcreepstrength andareresistantto radiation damage,corrosionmechanisms,thermal aging,yielding,andexcessiveneutronabsorption.

Vesselfluencecalculations,as describedin Section 4.5, confirm adequatemarginrelativetotheeffects ofirradiation.Thefastneutronfluence receivedbythereactorvesselfromthereactorcoreandpebble insertion andextractionlinesisattenuatedbythecorebarrel,thereflector,andthereactorcoolant.

Coolantpuritydesignlimitsarealsoestablished inconsiderationoftheeffectsofchemicalattackand foulingofthereactor vessel.Thesefeaturesdemonstrate conformancewithPDC31.

TheMSSutilizescouponsandcomponentmonitoring toconfirmthatirradiationaffected corrosionis nonexistent ormanageable.The316HSSreactorvesselandER168 2 weldmaterial,as apartofthe reactor coolantboundary,willbeinspected for structuralintegrityandleaktightness.Asdetailed in Reference3, fracturetoughnessissufficientlyhigh in316HSSunderreactor operatingconditionsthat additionaltensileorfracturetoughnessmonitoringandtestingprogramsareunnecessary.These featuresdemonstrateconformancetoPDC32.

Fluidicdiodesareusedtoestablish aflowpathfor continuousnatural circulationofcoolantinthecore duringpostulatedeventstoremoveresidualheatfromthereactorcoretothevesselwall. During and followingapostulated event,thehotcoolantfromthecoreflowsfromtheupperplenumthroughthe low flowresistancedirectionofthefluidicdiode tothecoolerdowncomervianatural circulation, therebycooling thecorepassively.Continuouscoolantflowthroughthereactorcorepreventspotential damage tothevesselinternalsduetooverheatingtherebyensuringthecoolablegeometryofthecoreis maintained.Theantisiphon featurealsolimitsthelossofreactor coolantinventoryfrominsidethe reactor vesselin theeventofaPHTSbreach. Thesefeaturesdemonstrate compliance withPDC 35.

Thereactorvesselreflector blockspermitinsertion ofthereactivitycontrolandshutdownelements.The ETU10 gradegraphiteofthereflector blocks iscompatiblewiththereactorcoolantchemistryandwill notdegrade due tomechanicalwear, thermalstresses andirradiation impactsduringthereflectorblock lifetime. Thegraphitereflector materialisqualifiedas describedin theKairos Powertopicalreport GraphiteMaterialQualificationfor theKairos PowerFluorideSaltCooledHighTemperatureReactor, KPTR 014 (Reference4).Toprecludedamagetothereflector duetoentrainedmoistureinthegraphite, thereflectorblocksarebaked(i.e.,heateduniformly)priortocomingintocontactwithcoolantand thereactorvesselisdesign toprecludeair ingress.Thereflectors, whichactas aheatsink inthecore, arespacedtoaccommodatethermalexpansion andhydraulicforcesduring normaloperation and postulatedevents.Thegapsbetweenthegraphiteblocksalsoallowfor coolanttoprovidecooling tothe reflectorblocks.Thereactorvesselpermitstheinsertion ofthereactivitycontrolandshutdown elementsas well. Thevessel isclassifiedas SDC3 perASCE4319 andwillmaintainitsgeometryto ensuretheRCSSelementscanbeinserted duringpostulated eventsincludingadesignbasisearthquake.

Thesefeaturesdemonstratecompliance withPDC 74.

Kairos PowerHermesReactor Revision0433 Preliminary Safety AnalysisReport ReactorDescription

Figure4.33:The ReactorVesselSystemSecondaryHoldDownStructure

Kairos PowerHermesReactor Revision0439 Preliminary Safety AnalysisReport HeatTransportSystems

5.1.3 SystemEvaluation Thedesignofthenonsafetyrelated PHTSissuchthatafailure ofcomponentsofthePHTSdoesnot affect theperformanceofsafetyrelated SSCsduetoadesignbasisearthquake.Inaddition toprotective barriers,thePHTSpipeconnectionstothereactorvesselnozzleshavesufficientlysmallwallthickness, suchthatifloadedbeyondelasticlimits,inelasticresponseoccursinthePHTSpipingwhichisnonsafety related.These features, alongwiththeseismicdesigndescribedinSection3.5,demonstrate conformancewiththerequirementsinPDC 2for thePHTS.

While thePHTSisaclosed system,there areconceivablescenariosthatmay resultinthereleaseof radioactive effluents.Thefueldesignlocatesthefuelparticlesneartheperiphery ofthefuelpebble, enhancingtheabilityofthefueltotransfer heattothecoolant.Thethermalhydraulicanalysisofthe core(see Section4.6)ensuresthatadequatecoolantflowismaintainedtoensurethatSARRDLs,as discussed inSection6.2, arenotexceeded.Thesefeaturesdemonstrateconformancewiththe requirementsin PDC 10.

Thedesignofthereactorcoolant,in part, ensuresthat power oscillationscannotresultin conditions exceedingSARRDLs.Thereactoriskeptnearambient pressureandthereactorcoolantin thePHTSdoes notexperiencetwophaseflow.Thecoolanthasahighthermalinertiamakingthereactorresilientto thermal hydraulic instabilityevents.These features, in part,demonstrateconformancewiththe requirementsin PDC 12.

Thefunctionalcontainment isdescribed inSection6.2.Thedesign reliesprimarilyonthemultiple barrierswithintheTRISOfuelparticlestoensurethattheradiological doseattheexclusionarea boundaryas aconsequenceofpostulatedeventsmeetsregulatorylimits.However,thereactorcoolant alsoservesasadistinctphysicalbarrier for fuelsubmergedinFlibebyproviding retentionoffission productsthatescapethefuel.Thedesign ofthereactorcoolantcompositionprovides,in part, ameans tocontroltheaccidentalreleaseofradioactive materialsduringnormal reactoroperation and postulatedevents(PDC 60),andsupports,in part,demonstrationofthefunctionalcontainment aspects.

ThedesignaspectsofthereactorcoolantarediscussedinReference5.1.51. TheFlibealsoaccumulates radionuclidesfromfissionproducts,andtransmutation productsfromtheFlibeandFlibeimpurities.The retentionpropertiesoftheFlibearecreditedinthesafetyanalysisas abarrier toreleaseof radionuclidesaccumulatedin thecoolant,andradionuclidecon Highlightedtextwas previously specifications.ThetransportofradionuclidesthroughFlibeisb changed.Submitted 218 22 justifiedintheapplicationfor anOperating License.Thesefeat (ML22049B556) requirementsin PDC 16.

Significantforced airingressintothePHTSisexcludedbydesignbasis.Airingresscould affectthe inventory ofreactorcoolantinthereactorvesselas well as affectthepurityofthereactorcoolant.

Design featuresoftheheatrejectionsubsystemandthereactortrip systemwilllimit thequantitiesof air ingressduring systemleakageeventsbytrippingtheheatrejectionblowersandtrippingthePSP.

Thesedesignfeaturessatisfy PDC 33 andPDC 70.Theeffectsofnonforced air ingressintothePHTSon safetyrelated Hermescomponentsareboundedbytheresultsofmaterialsqualificationprogramsas described inSection4.3.

ThedesignofthePHTScontrolsthereleaseofradioactive materialsin gaseous andliquideffluentsin theeventthePHTSworkingfluidisinadvertentlyreleasedtotheatmospherevialeaksinthepiping system.ThePHTSSSCsthatarepartofthereactorcoolantboundaryaredesigned totheASMEB31.3 Code (forthepiping)andSectionVIII(forthePHX)suchthatleaksareunlikely.Meansareprovidedfor detecting and,totheextentpractical,identifyingthelocationofthesourceofreactor coolantleakagein thePHTSSSCs. A postulated eventinthePHTSwouldbe aPHXtubefailure.ThiseventwouldcauseFlibe

Kairos PowerHermesReactor 54 Revision0 Preliminary Safety AnalysisReport AccidentAnalysis

ensurethereisnorecriticalityaftertheRCSShasinitiatedshutdown,as describedinSection4.5.

Additionally,thegraphitereflectorblocksaredesignedtomaintainstructuralintegrity andensure misalignments donotpreventtheinsertion pathoftheshutdown elements, as discussedinSection4.3.

13.1.10.2 DegradedHeatRemovalorUncooledEvents Inpostulatedeventswherethenormalheatrejectionisnotavailable,natural circulationin thereactor vesselandtheheatremovalfunctionoftheDHRSarereliedupontoremoveheatfromthereactor core.

Degradedheatremovaloruncooledeventsareexcludedfromthedesign basis.Theinitiationof natural circulationiscompletelypassive, andthedesign features,includingthestructuralintegrityofthereactor vesselinternals, thatensureacontinued natural circulationflowpatharediscussedinSection4.6.The DHRSisalignedandoperatingwhenthereactorpowerisaboveathresholdpowerandremainsinthis stateas described inSection6.3, precludingtheneed for anactuationtooccur fortheDHRStoremove heatduringapostulated event.TheDHRSdesignincludes sufficientredundancytoperformitssafety functionassumingthelossofasingletrain,as discussedinSection 6.3.

13.1.10.3 FlibeSpillBeyond MaximumVolumeAssumedinPostulatedSaltSpills In thesaltspill postulatedeventcategory,anupperboundvolumeofFlibe isassumedtospilloutofthe PHTSontothefloor.A volumeofFlibespillingoutofthesystembeyondtheamountassumedinthe boundingsaltspilleventisexcludedfromthedesignbasis.Thereareseveraldesignfeaturesensuring theamountofFlibeavailable tospillislimitedtoanupperboundvalue.Thereactorvesselisdesigned withantisiphon featuresdiscussedinSection4.3.These featuresaredesignedtopassivelybreakthe siphonintheeventofabreak.ThePSPalsotripstoallowtheprimarysystemtodepressurize. The reliabilityoftheRPS,whichtripsthePSPandISPin theeventofasaltspill,isdiscussedinSection7.3.

Thereactorvesselshell alsomaintainsintegrityinpostulatedeventstoensurethefuelinthecore remainscoveredwithFlibe.Thereactorvesselshell designfeaturesthatpreventleakagearediscussed in Section4.3.

13.1.10.4 InService TRISOFailureRatesandBurnupsAboveAssumptionsinPostulated Events Theinservice fuelfailure ratesandtheburnupofpebblesassumedinthepostulatedevents arebased onthefuelqualificationspecificationsinSection4.2.1.Inservice TRISO failureratesabovetherate assumedinpostulated eventsareexcludedfromthedesignbasis.Theinsertion ofpebbleswitha burnuphigherthanthefuelqualificationenvelopeisexcluded fromthedesign basis.Asdescribedin Section7.3, theRPSincludesafunctiontostopthepebbleinsertion andextractionfunctionstoensure pebblesarenotdamaged infaultsoccurring afteraneventinitiation.Thefuelqualificationprogram includestesting,inspection,andsurveillancetoensurethefueloperatingenvelopeiswithinthefuel qualificationenvelope.InspectionandsurveillanceofthefuelinserviceisperformedinthePHSS as discussed inSection9.3. Highlightedtextwas previously changed.Submitted 218 22 (ML22049B556)

13.1.10.5 SignificantIntermediateCoolantAirIngressIntoPHTS Eventswheresignificantquantitiesofairareentrained inthePHTScoolantduring normaloperationare excludedfromthedesignbasis.Operationalcontrolsareexpectedtomonitorthequantityofair within thePHTStopreventaccumulatingsignificantquantities.Chapter14discussestheexpectedcoolant systemstechnicalspecificationsthatmonitorsignificantair ingress.

Eventswheresignificantquantitiesofforcedair enter thePHTSfollowingpostulatedHRRtube break eventsarealsoexcludedfromthedesignbasis.Chapter 5discussesthedesignfeaturesoftheHRRthat

Kairos PowerHermesReactor 1315 Revision0 PreliminarySafetyAnalysisReport AccidentAnalysisHighlightedtextwaspreviously

changed.Submitted21822 limitsthequantitiesofforcedairingressduringsaltspilltransients.Thepostulatedeventsassumea(ML22049B556) positivepressuredifferentialbetweentheprimaryandintermediatecoolantsystems.Eventswhere significantquantitiesofintermediatecoolantenterthePHTSareexcludedfromthedesignbasis.

Chapter5discussesthedesignfeaturesofthePHTSandPHRSthatmaintainapositivepressure differential.

Theeffectsofnonforcedairingressonreactorvesselandvesselinternalcomponentswillremain boundedbythematerialsqualificationtestingprogramsforatleastsevendaysduringairingressevents asdescribedinSection4.3.Beyondsevendays,defenseindepthstrategiesinclude:implementing repairsondamagedSSCs,replenishingtheargonsupply,andremovaloffuelfromthevessel(fuelcore offloadcapabilitydiscussedinSection9.3.1.8.3).

13.1.10.6 DHRSReactorCavityFlooding TheDHRSisawaterbasedsystemthatremovesheatfromthereactorvesselshell.Eventswherethe waterfromtheDHRSleaksintothereactorcavityinquantitiessignificantenoughtowetthereactor vesselareexcludedfromthedesignbasis.Leakprevention,includingdoublewalledcomponentsand leakdetection,fortheDHRSisdescribedinSection6.3.

13.1.10.7 InsertionofExcessReactivityBeyondRateAssumedinPostulatedEvents Theinsertionofexcessreactivitypostulatedeventcategoryincludesalimitingreactivityinsertionrate basedonthemaximumcontrolelementdrivewithdrawalrate.Multiplecontrolelementsmoving simultaneouslyisexcludedfromthedesignbasis.Controlelementmovementislimitedtooneelement atatime,asdescribedinSection7.2.Acontrolelementwithdrawingfasterthanthelimitisexcluded fromthedesignbasis.Themaximumdrivewithdrawalspeedislimitedbythedrivehardware,as describedinSection4.2.2.Arapidcontrolelementejectionisexcludedfromthedesignbasisbecause thereactoroperatesatlowpressures.

Theinsertionofreactivityduetoanovercoolingeventisalsoboundedbythelimitingreactivityinsertion rate.CorecoolingduetopumpoverspeedfromthePSP,ISP,orPHRSblowerarelimitedtoamaximum limitwithintheprogrammednormaloperatingrangediscussedinSection7.2.

13.1.10.8 CriticalityOccurrenceExternaltoReactorCore PebblesoutsideofthereactorcorearecontainedinthePHSS.ThePHSSincludespebblesintransit duringhandling,instorage,andinatransportconfiguration.ThePHSSisdesignedtoprecludecriticality assumingpostulatedeventconditionsusingdesignfeaturesthatmaintainanoncriticalgeometryof pebblesineachoftheseareas.ThedesignfeaturesofPHSSpreventingcriticalityaredescribedin Section9.3.

13.1.10.9 ExcessiveRadionuclideReleasefromFlibe ThepostulatedeventsassumeareleaseofradionuclidesfromthefreesurfacesofFlibe.Theassumed releaseofradionuclidesfromFlibecouldbeaffectedbythecharacteristicsofthecovergassuchasa higherpressureaffectingthecovergasfloworthepurityofthecovergasaffectingtheradionuclides availableforrelease.Thecovergasismaintainedbytheinertgassystem,describedinSection9.1.2.

13.1.10.10 InternalorExternalEventsInterferingwithSSCs SSCsthatperformsafetyfunctionsarelocatedinaportionofthereactorbuildingdesignedtopreclude damagefrombothinternalandexternalhazardsthatcouldinterferewiththosefunctions.Additionally, SSCscontainingFlibeareprotectedfrominternalfloodstoprecludethepotentialforFlibe-water interactions.Thefailureofsafetyfunctionsduetointernalorexternalhazardsisexcludedfromthe

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