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{{#Wiki_filter:KPNRC2209001 Enclosure 2 Response to NRC Request for Additional Information 350 (NonProprietary)
{{#Wiki_filter: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?


NRC Request for Additional Information RAI Package 350, Question 410 Section 50.34 of Title 10 of the Code of Federal Regulations (10 CFR 50.34), "Contents of applications; technical information," provides requirements for information to be provided in a Construction Permit (CP). 10 CFR 50.34(a)(4) states that a CP shall contain a preliminary analysis and evaluation of SSCs provided for mitigation of the consequences of accidents to determine margins of safety during normal operations and transient conditions during the life of the facility.
Section 3.1.1, "Design Criteria," of the Kairos Power (KP) Hermes Preliminary Safety Analysis Report (PSAR) references document KPTR003NPA, "Principal Design Criteria [PDC] for the Kairos Power FluorideSalt Cooled, High Temperature Reactor," Revision 1, to provide the PDC for the Hermes test reactor. KPFHR PDC 14, "Reactor coolant boundary," states that safety significant elements of the reactor coolant boundary shall have an extremely low probability of abnormal leakage, rapidly propagating failure, and gross rupture. KPFHR PDC 31, "Fracture prevention of the reactor coolant boundary," states that the reactor coolant boundary shall be designed to consider service degradation of material properties including effects of contaminants. KPFHR PDC 35, "Passive residual heat removal," states that a system shall be provided to remove residual heat during and after postulated accidents. KPFHR PDC 74, "Reactor vessel and reactor system structural design basis," states that the vessel and reactor system shall be designed to ensure integrity is maintained during postulated accidents to ensure the geometry for passive heat removal and allow for insertion of reactivity control elements.
Section 4.3 of the PSAR, "Reactor Vessel System," describes the components that form the natural circulation flow path needed to provide residual heat removal during and following postulated events. These include portions of the graphite reflector as well as metallic components such as the core barrel, reactor vessel, and fluidic diode. This section of the PSAR describes how these components are needed to meet PDCs 14, 31, 35, and 74.
Section 5.1.3 of the PSAR, "System Evaluation," states that "significant" air ingress into the primary heat transport system (PHTS) is excluded by design basis. In an event with postulated air ingress into the PHTS, the components that comprise the natural circulation flow path will need to perform their safety functions (i.e., maintain the natural circulation flow path) to meet the PDC listed above. The staff notes that air ingress into the PHTS can cause oxidation of the graphite reflector as well as corrosion of metallic components in the primary system, and such degradation could potentially challenge natural circulation flow. In order to evaluate effects of air ingress, the staff needs to understand the amount of air ingress that will be allowed and how the limitation of ingress will be achieved.
Therefore, the NRC staff requests the following information:
: 1. Define what constitutes "significant" air ingress into the PHTS and the basis for determining what is "significant."
: 2. Describe how component integrity is ensured if the duration of an air ingress event is longer than the duration covered by the materials qualification testing.
: 3. In an event such as a salt spill or heat radiator tube rupture, how is further air ingress prevented after a heat rejection blower trip?
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Kairos Power Response NRC Question 410, Item 1 Define what constitutes "significant" air ingress into the PHTS and the basis for determining what is "significant."
Kairos PowerResponse NRCQuestion410,Item1
The discussion in Preliminary Safety Analysis Report (PSAR) Section 5.1.3 is referring to limiting the amount of air ingress that is forced into the Flibe, not limiting the amount of air ingress to the reactor system as a whole. As cited in PSAR Section 5.1.3, the design evaluation of limiting significant air ingress demonstrates compliance with Principal Design Criteria (PDC) 33 and PDC 70. PDC 33 and PDC 70 are focused on details of the Flibe, not the gas space above the free surface of Flibe. This distinction is important to recognize for the responses provided to this RAI. The response to Item 2 below includes the consideration of oxidation effects for nonFlibewetted graphite above the free surface of Flibe.
 
As described in PSAR Section 5.1.3, significant air ingress into the Primary Heat Transport System (PHTS) refers to two scenarios:
Definewhatconstitutes"significant"airingressintothePHTS and thebasisfordetermining whatis "significant."
Significant air being entrained in the coolant during normal operation (to meet PDC 33)
 
Forced air ingress occurring during postulated system leakage events (to meet PDC 70)
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.
If air is entrained in the coolant during normal operation, operational controls are expected to monitor the quantity of air within the PHTS to prevent accumulating significant quantities with a technical specification, as discussed in PSAR Section 13.1.10.5. The limit for significant air ingress will prevent void accumulation and limit the total corrosion of Flibewetted components, as described in PSAR Table 14.11. Consistent with 10 CFR 50.34(a)(5), the PSAR identifies the variable expected to be subject to technical specification control, and PSAR Section 14.1 commits to providing the parameter limits with the application for an Operating License Application, consistent with 10 CFR 50.34(b)(6)(vi).
 
For the scenarios where significant forced air ingress is prevented during postulated events involving a breach or break in the PHTS, significant refers to amounts of air that could be forced into the Flibe by the driving forces associated with the heat rejection blower or the primary salt pump. As described in PSAR Section 7.3.1, there are safetyrelated trips on the heat rejection blower and primary salt pump, which remove the mechanisms that could force air into the Flibe during a system leakage event to prevent significant forced air ingress.
AsdescribedinPSAR Section5.1.3,significantairingressintothePrimaryHeatTransportSystem (PHTS) referstotwoscenarios:
PSAR Sections 5.1.3, and 13.1.10.5 have been updated to clarify that forced air ingress into the PHTS is precluded by design.
Significantairbeingentrainedinthecoolantduringnormaloperation(tomeetPDC33)
NRC Question 410, Item 2 Describe how component integrity is ensured if the duration of an air ingress event is longer than the duration covered by the materials qualification testing.
Forcedairingressoccurringduringpostulatedsystemleakageevents(tomeetPDC70)
By maintaining the quantity of air within the technical specification limit during normal operation and removing the mechanisms to force air into the Flibe described in Item 1 of this RAI, the structural integrity of metallic and graphite components that remain Flibewetted is ensured to remain within conditions bounded by the materials qualification testing programs (References 1 and
 
: 2) for air ingress events up to seven days. The metallic materials qualification topical report includes
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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(Reference 1). The graphite material qualification topical report describes the assessment plan for the effects of air contamination in Flibe on ET10 graphite (Reference 2).
(Reference1).The graphitematerialqualificationtopicalreportdescribestheassessmentplanfor theeffectsofaircontaminationinFlibeonET10graphite (Reference2).
During normal operation, the argon in the gas space will be monitored for potential air ingress as described in the Kairos Power response to RCI02 (ML22231B230).
 
The graphite reflector blocks that are located above the free surface of the Flibe are subject to potential oxidation effects during a postulated air ingress event. Since the shutdown elements insert at the beginning of the event, this exposed graphite structure is not credited after insertion to perform a long term structural integrity safety function when oxidation could begin to affect the structural integrity. Additionally, if significant oxidation were to result in a loss of structural integrity of the exposed graphite, there is a layer of submerged (Flibewetted) graphite that mitigates debris from the exposed graphite from entering the natural circulation flow path.
Duringnormaloperation,the argoninthegasspacewillbemonitoredforpotentialairingressas describedintheKairosPowerresponseto RCI02(ML22231B230).
As shown in Figure 1, the secondary holddown structure is installed within the upper layers of the graphite reflector and extends below the minimum Flibe level for a PHTS break event. If significant oxidation were to result in a loss of structural integrity of the graphite above the minimum Flibe level, the secondary holddown structure will transfer loads from the submerged graphite to the top head, keeping the remaining reflector structure in place and submerged in Flibe. The effects of non forced air ingress on the integrity of components below the surface of Flibe will be bounded by the materials qualification testing programs for at least seven days following the initiation of the event.
 
Beyond seven days, defense in depth features include: implementing repairs on damaged SSCs, replenishing argon supply, or removal of fuel from the vessel. This ensures that the geometry of the core and the natural circulation flow paths are maintained. PSAR Section 4.3 has been updated to remove the statement the reactor vessel is designed to preclude air ingress and to reflect the secondary holddown structure design details described above. PSAR Section 13.1.10.5 has been updated to describe defense in depth features of the design available after the initial seven day period of a postulated air ingress event. A markup of changes to the graphite qualification topical report providing additional details of the of the assessment of air ingress on the integrity of components below the surface of Flibe is being provided with this response. A revision to the graphite qualification topical report will be submitted by separate letter.
Thegraphitereflectorblocksthatare locatedabovethefreesurfaceoftheFlibeare subjectto potentialoxidationeffectsduringapostulatedairingressevent.Sincetheshutdownelementsinsert atthe beginning oftheevent,thisexposedgraphitestructureisnotcreditedafterinsertionto performalongtermstructuralintegritysafety functionwhenoxidationcouldbegintoaffectthe structuralintegrity.Additionally,ifsignificantoxidationweretoresultinalossofstructuralintegrity oftheexposedgraphite,thereisalayerofsubmerged(Flibewetted)graphitethatmitigates debris fromtheexposedgraphitefromenteringthe natural circulationflowpath.
NRC Question 410, Item 3 In an event such as a salt spill or heat radiator tube rupture, how is further air ingress prevented after a heat rejection blower trip?
 
As described in Item 1, safetyrelated trips on the heat rejection blower and primary salt pump remove the mechanisms that could force air into the Flibe during a system leakage event. The Hermes design does not credit any means of limiting further nonforced air ingress into the PHTS in the event of a salt spill or radiator tube rupture. See response to Item 2 for discussion of the impacts of nonforced air ingress on vessel internals.
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:==
==References:==
: 1. Kairos Power LLC, Metallic Materials Qualification for the Kairos Power Fluoride Salt Cooled HighTemperature Reactor, KPTR013P, Revision 3.
: 1. KairosPowerLLC,MetallicMaterialsQualificationfortheKairosPowerFluorideSalt CooledHighTemperatureReactor,KPTR013 P,Revision3.
: 2. Kairos Power LLC, Graphite Material Qualification for the Kairos Power Fluoride Salt Cooled HighTemperature Reactor, KPTR014P, Revision 3.
: 2. KairosPowerLLC,GraphiteMaterialQualificationfortheKairosPowerFluorideSalt CooledHighTemperatureReactor,KPTR014 P,Revision3.
Impact on Licensing Document:
 
This response impacts Sections 4.3, 5.1.3, and 13.1.10.5 of the Kairos Power Preliminary Safety Analysis Report and Section 5.3 of Graphite Materials Qualification for the Kairos Power Fluoride SaltCooled HighTemperature Reactor. Markups of the affected sections are provided with this response.
Impact onLicensingDocument:
ThisresponseimpactsSections4.3,5.1.3,and13.1.10.5oftheKairosPowerPreliminarySafety AnalysisReportandSection5.3ofGraphiteMaterials QualificationfortheKairosPowerFluoride SaltCooledHighTemperatureReactor. Markupsoftheaffectedsectionsareprovidedwith this response.
 
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Figure 1 Page 5 of 5
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


Preliminary Safety Analysis Report                                                        Reactor Description coolant level. The design of the reactor vessel allows for online monitoring, inservice inspection, and maintenance.
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.
4.3.1.1.1        Vessel Top Head The reactor vessel top head (see Figure 4.32) is a flat 316H SS disc bolted and flanged to the vessel shell. This interface is designed for leaktightness but is not credited as being leak tight in safety analyses. The vessel top head controls the radial and circumferential positions of the reflector blocks to ensure a stable core configuration for all conditions (e.g., reactor trip and core motion). The top head contains penetrations, as shown in Figure 4.32 and Table 4.31, into and out of the vessel and provides for the attachment of supporting equipment and components (e.g., reactivity control elements, pebble handling and storage system components, material sampling port, neutron detectors, thermocouples, etc.). The top head supports the vessel material surveillance system (MSS) which provides a remote means to insert and remove material and fuel test specimens into and from the reactor to support testing. A holddown structure subassembly is welded underneath the vessel top head. This structure contacts with the top surface of the graphite reflector and provides structural support against upward loads during normal operation and most postulated events. A secondary holddown structure is installed through the upper graphite layers, extending from the reflector top into submerged graphite layers to transfer upward loads from submerged graphite to the vessel top head during postulated air ingress events. The secondary hold down structure extends to below the minimum reactor vessel coolant level that could result from postulated salt spill events.
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.
4.3.1.1.2        Vessel Shell The reactor vessel is a 316H SS cylindrical shell that, along with the vessel bottom head, serves to form the safetyrelated reactor coolant boundary within the reactor vessel. It contains and maintains the inventory of reactor coolant inside the vessel. The shell provides the geometry for coolant inlet and vessel surface for the DHRS which transfers heat from the reactor vessel during postulated events. The inside of the shell uses 316H SS tabs to maintain the core barrel in a cylindrical geometry and has a welded connection at the top of the core barrel.
Thedesignofthereactorcoolant,in part, ensuresthat power oscillationscannotresultin conditions exceedingSARRDLs.Thereactoriskeptnearambient pressureandthereactorcoolantin thePHTSdoes notexperiencetwophaseflow.Thecoolanthasahighthermalinertiamakingthereactorresilientto thermal hydraulic instabilityevents.These features, in part,demonstrateconformancewiththe requirementsin PDC 12.
4.3.1.1.3        Vessel Bottom Head The reactor vessel bottom head is a flat 316H SS disc that is welded to the vessel shell. It contains and maintains the inventory of the reactor coolant inside the vessel, supports the vessel internals, maintains the reactor coolant boundary and provides flow geometry for low pressure reactor coolant inlet to the core. Hydrostatic, seismic and gravity loads on the vessel and vessel internals are transferred to the bottom head and are transferred to the RVSS.
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.
4.3.1.2          Reactor Vessel Internals The reactor vessel internal structures include the graphite reflector blocks, core barrel and reflector support structure. The vessel internal structures define the flow paths of the fuel and reactor coolant, provide a heat sink, a pathway for instrumentation insertion, control and shutdown element insertion, as well as provide neutron shielding and moderation surrounding the core. The design of the structures support inspection and maintenance activities as well as monitoring of the reactor vessel system.
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.
4.3.1.2.1        Reflector Blocks The reflector blocks are constructed of grade ETU10 graphite. The reflector blocks provide a heat sink for the core and are restrained ensuring alignment of the penetrations to insert and withdraw control elements. The reflector blocks are buoyant in the reactor coolant. The top surface of the reflector blocks contacts the vessel top head holddown structure subassembly which provides structural support Kairos Power Hermes Reactor                          429                                            Revision 0
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


Preliminary Safety Analysis Report                                                        Reactor Description against upward loads during normal operation and most postulated events. A secondary holddown structure is installed through the upper reflector layers to transfer upward loads from submerged graphite to the vessel top head during postulated air ingress events. The bottom reflector blocks are machined with coolant inlet channels for distribution of coolant inlet flow into the core. The top reflector blocks are machined with coolant outlet channels to direct the coolant exiting from the core into the upper plenum, from which the PSP draws suction. The top reflector blocks also form a pebble defueling chute, as shown in Figure 4.31, to direct the pebbles from the core to the pebble extraction machine (PEM), allowing online defueling of the reactor (see Section 9.3). The reflector blocks also provide machined channels for insertion and withdrawal of the reactivity control and shutdown elements described in Section 4.2.2.
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The reflector blocks form an upper plenum and a fluidic diode, which is a stainlesssteel passive device that connects the upper plenum to the top of the downcomer as shown in Figure 4.31. The diode introduces a higher flow resistance in one direction, while having a lower flow resistance in the other direction. The diode restricts flow from the higherpressure downcomer into the upper plenum during conditions with forced circulation. The flow passes in the lowresistance direction of the diode from the upper plenum to the top of the downcomer driven by natural circulation.
The graphite reflector blocks reflect neutrons back into the core, increasing the fuel utilization while protecting the reactor vessel from fluence based forms of degradation. Further discussion of the reflectors neutronic characteristics are detailed in Section 4.5.
4.3.1.2.2        Core Barrel The 316H SS core barrel creates an annular space between itself and the reactor vessel and defines the downcomer flow path for the coolant. The core barrel has a flanged top which is welded to the inner wall of the vessel shell. The barrel is kept concentric to the shell by radial tabs which allow for differential thermal expansion.
4.3.1.2.3        Reflector Support Structure The 316H SS reflector support structure, as shown in Figure 4.31, defines the flow path from the downcomer annulus into the core as well as provides support to the graphite reflector blocks. The reflector support structure ensures a stable core configuration for all conditions (e.g., reactor trip and core motion) by controlling the radial and circumferential positions of the reflector blocks.
4.3.2            Design Basis Consistent with PDC 1, the safetyrelated portions of the reactor vessel and reactor vessel internals are fabricated and tested in accordance with generally recognized codes and standards.
Consistent with PDC 2, the reactor vessel and reactor vessel internals perform their safety functions in the event of a safeshutdown earthquake and other natural phenomena hazards.
Consistent with PDC 4, the reactor vessel and reactor vessel internals accommodate the environmental conditions associated with normal operation, maintenance, testing, and postulated events.
Consistent with PDC 10, the reactor vessel and internals maintain a geometry and coolant flow path to ensure that the specified acceptable system radionuclide release design limits (SARRDLs) will not be exceeded during normal operation including postulated events.
Consistent with PDC 14, the reactor vessel is fabricated and tested to have an extremely low probability of abnormal leakage or sudden failure of the reactor coolant boundary by gross rupture.
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Preliminary Safety Analysis Report                                                      Reactor Description factors up to a temperature of 650°C for ER1682 weld metal with 316H base metal. Testing provides stress rupture factors up to 816°C for weld material with 316H base metal (Reference 3). The plant control system will detect leakage from the reactor vessel and catch basins are used to detect leaks in nearby coolantcarrying systems. These features demonstrate compliance with PDC 30.
ensurethereisnorecriticalityaftertheRCSShasinitiatedshutdown,as describedinSection4.5.
Reactor vessel stress rupture factors are determined up to 816°C to encompass transient conditions.
Additionally,thegraphitereflectorblocksaredesignedtomaintainstructuralintegrity andensure misalignments donotpreventtheinsertion pathoftheshutdown elements, as discussedinSection4.3.
The stress rupture factors are determined by a creeprupture test on the vessel base material with weld metal under the gas tungsten arc welding process. The vessel precludes material creep, fatigue, thermal, mechanical, and hydraulic stresses. The leak tight design of the reactor vessel head minimizes air ingress into the cover gas and precludes corrosion of the internals. The high temperature, high carbon grade 316H SS of the core barrel and reflector support structure have high creep strength and are resistant to radiation damage, corrosion mechanisms, thermal aging, yielding, and excessive neutron absorption.
13.1.10.2 DegradedHeatRemovalorUncooledEvents Inpostulatedeventswherethenormalheatrejectionisnotavailable,natural circulationin thereactor vesselandtheheatremovalfunctionoftheDHRSarereliedupontoremoveheatfromthereactor core.
Vessel fluence calculations, as described in Section 4.5, confirm adequate margin relative to the effects of irradiation. The fast neutron fluence received by the reactor vessel from the reactor core and pebble insertion and extraction lines is attenuated by the core barrel, the reflector, and the reactor coolant.
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.
Coolant purity design limits are also established in consideration of the effects of chemical attack and fouling of the reactor vessel. These features demonstrate conformance with PDC 31.
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.
The MSS utilizes coupons and component monitoring to confirm that irradiationaffected corrosion is nonexistent or manageable. The 316H SS reactor vessel and ER1682 weld material, as a part of the reactor coolant boundary, will be inspected for structural integrity and leaktightness. As detailed in Reference 3, fracture toughness is sufficiently high in 316H SS under reactor operating conditions that additional tensile or fracture toughness monitoring and testing programs are unnecessary. These features demonstrate conformance to PDC 32.
Thereactorvesselshell alsomaintainsintegrityinpostulatedeventstoensurethefuelinthecore remainscoveredwithFlibe.Thereactorvesselshell designfeaturesthatpreventleakagearediscussed in Section4.3.
Fluidic diodes are used to establish a flow path for continuous natural circulation of coolant in the core during postulated events to remove residual heat from the reactor core to the vessel wall. During and following a postulated event, the hot coolant from the core flows from the upper plenum through the low flow resistance direction of the fluidic diode to the cooler downcomer via natural circulation, thereby cooling the core passively. Continuous coolant flow through the reactor core prevents potential damage to the vessel internals due to overheating thereby ensuring the coolable geometry of the core is maintained. The antisiphon feature also limits the loss of reactor coolant inventory from inside the reactor vessel in the event of a PHTS breach. These features demonstrate compliance with PDC 35.
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)
The reactor vessel reflector blocks permit insertion of the reactivity control and shutdown elements. The ETU10 grade graphite of the reflector blocks is compatible with the reactor coolant chemistry and will not degrade due to mechanical wear, thermal stresses and irradiation impacts during the reflector block lifetime. The graphite reflector material is qualified as described in the Kairos Power topical report Graphite Material Qualification for the Kairos Power Fluoride SaltCooled HighTemperature Reactor, KPTR014 (Reference 4). To preclude damage to the reflector due to entrained moisture in the graphite, the reflector blocks are baked (i.e., heated uniformly) prior to coming into contact with coolant and the reactor vessel is design to preclude air ingress. The reflectors, which act as a heat sink in the core, are spaced to accommodate thermal expansion and hydraulic forces during normal operation and postulated events. The gaps between the graphite blocks also allow for coolant to provide cooling to the reflector blocks. The reactor vessel permits the insertion of the reactivity control and shutdown elements as well. The vessel is classified as SDC3 per ASCE 4319 and will maintain its geometry to ensure the RCSS elements can be inserted during postulated events including a design basis earthquake.
These features demonstrate compliance with PDC 74.
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Preliminary Safety Analysis Report                                    Reactor Description Figure 4.33: The Reactor Vessel System Secondary HoldDown Structure Kairos Power Hermes Reactor                    439                          Revision 0
13.1.10.5 SignificantIntermediateCoolantAirIngressIntoPHTS Eventswheresignificantquantitiesofairareentrained inthePHTScoolantduring normaloperationare excludedfromthedesignbasis.Operationalcontrolsareexpectedtomonitorthequantityofair within thePHTStopreventaccumulatingsignificantquantities.Chapter14discussestheexpectedcoolant systemstechnicalspecificationsthatmonitorsignificantair ingress.
Eventswheresignificantquantitiesofforcedair enter thePHTSfollowingpostulatedHRRtube break eventsarealsoexcludedfromthedesignbasis.Chapter 5discussesthedesignfeaturesoftheHRRthat


Preliminary Safety Analysis Report                                                  Heat Transport Systems 5.1.3            System Evaluation The design of the nonsafetyrelated PHTS is such that a failure of components of the PHTS does not affect the performance of safetyrelated SSCs due to a design basis earthquake. In addition to protective barriers, the PHTS pipe connections to the reactor vessel nozzles have sufficiently small wall thickness, such that if loaded beyond elastic limits, inelastic response occurs in the PHTS piping which is nonsafety related. These features, along with the seismic design described in Section 3.5, demonstrate conformance with the requirements in PDC 2 for the PHTS.
Kairos PowerHermesReactor 1315 Revision0 PreliminarySafetyAnalysisReport AccidentAnalysisHighlightedtextwaspreviously
While the PHTS is a closed system, there are conceivable scenarios that may result in the release of radioactive effluents. The fuel design locates the fuel particles near the periphery of the fuel pebble, enhancing the ability of the fuel to transfer heat to the coolant. The thermal hydraulic analysis of the core (see Section 4.6) ensures that adequate coolant flow is maintained to ensure that SARRDLs, as discussed in Section 6.2, are not exceeded. These features demonstrate conformance with the requirements in PDC 10.
The design of the reactor coolant, in part, ensures that power oscillations cannot result in conditions exceeding SARRDLs. The reactor is kept near ambient pressure and the reactor coolant in the PHTS does not experience two phase flow. The coolant has a high thermal inertia making the reactor resilient to thermalhydraulic instability events. These features, in part, demonstrate conformance with the requirements in PDC 12.
The functional containment is described in Section 6.2. The design relies primarily on the multiple barriers within the TRISO fuel particles to ensure that the radiological dose at the exclusion area boundary as a consequence of postulated events meets regulatory limits. However, the reactor coolant also serves as a distinct physical barrier for fuel submerged in Flibe by providing retention of fission products that escape the fuel. The design of the reactor coolant composition provides, in part, a means to control the accidental release of radioactive materials during normal reactor operation and postulated events (PDC 60), and supports, in part, demonstration of the functional containment aspects.
The design aspects of the reactor coolant are discussed in Reference 5.1.51. The Flibe also accumulates radionuclides from fission products, and transmutation products from the Flibe and Flibe impurities. The retention properties of the Flibe are credited in the safety analysis as a barrier to release of radionuclides accumulated in the coolant, and radionuclide con Highlighted text was previously specifications. The transport of radionuclides through Flibe is b changed. Submitted 21822 justified in the application for an Operating License. These feat (ML22049B556) requirements in PDC 16.
Significant forced air ingress into the PHTS is excluded by design basis. Air ingress could affect the inventory of reactor coolant in the reactor vessel as well as affect the purity of the reactor coolant.
Design features of the heat rejection subsystem and the reactor trip system will limit the quantities of air ingress during system leakage events by tripping the heat rejection blowers and tripping the PSP.
These design features satisfy PDC 33 and PDC 70. The effects of nonforced air ingress into the PHTS on safetyrelated Hermes components are bounded by the results of materials qualification programs as described in Section 4.3.
The design of the PHTS controls the release of radioactive materials in gaseous and liquid effluents in the event the PHTS working fluid is inadvertently released to the atmosphere via leaks in the piping system. The PHTS SSCs that are part of the reactor coolant boundary are designed to the ASME B31.3 Code (for the piping) and Section VIII (for the PHX) such that leaks are unlikely. Means are provided for detecting and, to the extent practical, identifying the location of the source of reactor coolant leakage in the PHTS SSCs. A postulated event in the PHTS would be a PHX tube failure. This event would cause Flibe Kairos Power Hermes Reactor                            54                                          Revision 0


Preliminary Safety Analysis Report                                                            Accident Analysis ensure there is no recriticality after the RCSS has initiated shutdown, as described in Section 4.5.
changed.Submitted21822 limitsthequantitiesofforcedairingressduringsaltspilltransients.Thepostulatedeventsassumea(ML22049B556) positivepressuredifferentialbetweentheprimaryandintermediatecoolantsystems.Eventswhere significantquantitiesofintermediatecoolantenterthePHTSareexcludedfromthedesignbasis.
Additionally, the graphite reflector blocks are designed to maintain structural integrity and ensure misalignments do not prevent the insertion path of the shutdown elements, as discussed in Section 4.3.
Chapter5discussesthedesignfeaturesofthePHTSandPHRSthatmaintainapositivepressure differential.
13.1.10.2          Degraded Heat Removal or Uncooled Events In postulated events where the normal heat rejection is not available, natural circulation in the reactor vessel and the heat removal function of the DHRS are relied upon to remove heat from the reactor core.
Theeffectsofnonforcedairingressonreactorvesselandvesselinternalcomponentswillremain boundedbythematerialsqualificationtestingprogramsforatleastsevendaysduringairingressevents asdescribedinSection4.3.Beyondsevendays,defenseindepthstrategiesinclude:implementing repairsondamagedSSCs,replenishingtheargonsupply,andremovaloffuelfromthevessel(fuelcore offloadcapabilitydiscussedinSection9.3.1.8.3).
Degraded heat removal or uncooled events are excluded from the design basis. The initiation of natural circulation is completely passive, and the design features, including the structural integrity of the reactor vessel internals, that ensure a continued natural circulation flow path are discussed in Section 4.6. The DHRS is aligned and operating when the reactor power is above a threshold power and remains in this state as described in Section 6.3, precluding the need for an actuation to occur for the DHRS to remove heat during a postulated event. The DHRS design includes sufficient redundancy to perform its safety function assuming the loss of a single train, as discussed in Section 6.3.
13.1.10.6 DHRSReactorCavityFlooding TheDHRSisawaterbasedsystemthatremovesheatfromthereactorvesselshell.Eventswherethe waterfromtheDHRSleaksintothereactorcavityinquantitiessignificantenoughtowetthereactor vesselareexcludedfromthedesignbasis.Leakprevention,includingdoublewalledcomponentsand leakdetection,fortheDHRSisdescribedinSection6.3.
13.1.10.3          Flibe Spill Beyond Maximum Volume Assumed in Postulated Salt Spills In the salt spill postulated event category, an upper bound volume of Flibe is assumed to spill out of the PHTS onto the floor. A volume of Flibe spilling out of the system beyond the amount assumed in the bounding salt spill event is excluded from the design basis. There are several design features ensuring the amount of Flibe available to spill is limited to an upper bound value. The reactor vessel is designed with antisiphon features discussed in Section 4.3. These features are designed to passively break the siphon in the event of a break. The PSP also trips to allow the primary system to depressurize. The reliability of the RPS, which trips the PSP and ISP in the event of a salt spill, is discussed in Section 7.3.
13.1.10.7 InsertionofExcessReactivityBeyondRateAssumedinPostulatedEvents Theinsertionofexcessreactivitypostulatedeventcategoryincludesalimitingreactivityinsertionrate basedonthemaximumcontrolelementdrivewithdrawalrate.Multiplecontrolelementsmoving simultaneouslyisexcludedfromthedesignbasis.Controlelementmovementislimitedtooneelement atatime,asdescribedinSection7.2.Acontrolelementwithdrawingfasterthanthelimitisexcluded fromthedesignbasis.Themaximumdrivewithdrawalspeedislimitedbythedrivehardware,as describedinSection4.2.2.Arapidcontrolelementejectionisexcludedfromthedesignbasisbecause thereactoroperatesatlowpressures.
The reactor vessel shell also maintains integrity in postulated events to ensure the fuel in the core remains covered with Flibe. The reactor vessel shell design features that prevent leakage are discussed in Section 4.3.
Theinsertionofreactivityduetoanovercoolingeventisalsoboundedbythelimitingreactivityinsertion rate.CorecoolingduetopumpoverspeedfromthePSP,ISP,orPHRSblowerarelimitedtoamaximum limitwithintheprogrammednormaloperatingrangediscussedinSection7.2.
13.1.10.4          InService TRISO Failure Rates and Burnups Above Assumptions in Postulated Events The inservice fuel failure rates and the burnup of pebbles assumed in the postulated events are based on the fuel qualification specifications in Section 4.2.1. Inservice TRISO failure rates above the rate assumed in postulated events are excluded from the design basis. The insertion of pebbles with a burnup higher than the fuel qualification envelope is excluded from the design basis. As described in Section 7.3, the RPS includes a function to stop the pebble insertion and extraction functions to ensure pebbles are not damaged in faults occurring after an event initiation. The fuel qualification program includes testing, inspection, and surveillance to ensure the fuel operating envelope is within the fuel qualification envelope. Inspection and surveillance of the fuel in service is performed in the PHSS as discussed in Section 9.3.                                                       Highlighted text was previously changed. Submitted 21822 (ML22049B556) 13.1.10.5          Significant Intermediate Coolant Air Ingress Into PHTS Events where significant quantities of air are entrained in the PHTS coolant during normal operation are excluded from the design basis. Operational controls are expected to monitor the quantity of air within the PHTS to prevent accumulating significant quantities. Chapter 14 discusses the expected coolant systems technical specifications that monitor significant air ingress.
13.1.10.8 CriticalityOccurrenceExternaltoReactorCore PebblesoutsideofthereactorcorearecontainedinthePHSS.ThePHSSincludespebblesintransit duringhandling,instorage,andinatransportconfiguration.ThePHSSisdesignedtoprecludecriticality assumingpostulatedeventconditionsusingdesignfeaturesthatmaintainanoncriticalgeometryof pebblesineachoftheseareas.ThedesignfeaturesofPHSSpreventingcriticalityaredescribedin Section9.3.
Events where significant quantities of forced air enter the PHTS following postulated HRR tube break events are also excluded from the design basis. Chapter 5 discusses the design features of the HRR that Kairos Power Hermes Reactor                          1315                                            Revision 0
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


Preliminary Safety Analysis Report                                                          Accident Analysis Highlighted text was previously changed. Submitted 21822 (ML22049B556) limits the quantities of forced air ingress during salt spill transients.The postulated  events assume a positive pressure differential between the primary and intermediate coolant systems. Events where significant quantities of intermediate coolant enter the PHTS are excluded from the design basis.
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Chapter 5 discusses the design features of the PHTS and PHRS that maintain a positive pressure differential.
The effects of nonforced air ingress on reactor vessel and vessel internal components will remain bounded by the materials qualification testing programs for at least seven days during air ingress events as described in Section 4.3. Beyond seven days, defense in depth strategies include: implementing repairs on damaged SSCs, replenishing the argon supply, and removal of fuel from the vessel (fuel core offload capability discussed in Section 9.3.1.8.3).
13.1.10.6        DHRS Reactor Cavity Flooding The DHRS is a waterbased system that removes heat from the reactor vessel shell. Events where the water from the DHRS leaks into the reactor cavity in quantities significant enough to wet the reactor vessel are excluded from the design basis. Leak prevention, including double walled components and leak detection, for the DHRS is described in Section 6.3.
13.1.10.7        Insertion of Excess Reactivity Beyond Rate Assumed in Postulated Events The insertion of excess reactivity postulated event category includes a limiting reactivity insertion rate based on the maximum control element drive withdrawal rate. Multiple control elements moving simultaneously is excluded from the design basis. Control element movement is limited to one element at a time, as described in Section 7.2. A control element withdrawing faster than the limit is excluded from the design basis. The maximum drive withdrawal speed is limited by the drive hardware, as described in Section 4.2.2. A rapid control element ejection is excluded from the design basis because the reactor operates at low pressures.
The insertion of reactivity due to an overcooling event is also bounded by the limiting reactivity insertion rate. Core cooling due to pump overspeed from the PSP, ISP, or PHRS blower are limited to a maximum limit within the programmed normal operating range discussed in Section 7.2.
13.1.10.8        Criticality Occurrence External to Reactor Core Pebbles outside of the reactor core are contained in the PHSS. The PHSS includes pebbles in transit during handling, in storage, and in a transport configuration. The PHSS is designed to preclude criticality assuming postulated event conditions using design features that maintain a noncritical geometry of pebbles in each of these areas. The design features of PHSS preventing criticality are described in Section 9.3.
13.1.10.9        Excessive Radionuclide Release from Flibe The postulated events assume a release of radionuclides from the free surfaces of Flibe. The assumed release of radionuclides from Flibe could be affected by the characteristics of the cover gas such as a higher pressure affecting the cover gas flow or the purity of the cover gas affecting the radionuclides available for release. The cover gas is maintained by the inert gas system, described in Section 9.1.2.
13.1.10.10      Internal or External Events Interfering with SSCs SSCs that perform safety functions are located in a portion of the reactor building designed to preclude damage from both internal and external hazards that could interfere with those functions. Additionally, SSCs containing Flibe are protected from internal floods to preclude the potential for Flibe - water interactions. The failure of safety functions due to internal or external hazards is excluded from the Kairos Power Hermes Reactor                          1316                                         Revision 0}}

Latest revision as of 04:11, 16 November 2024

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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?

Page 1 of 5

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.

Page 3 of 5

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

KairosPowerHermesReactor 1316 Revision0

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