ML23205A228

Response to Sdaa Audit Question A-15.0.3.7-1

ML23205A228
Person / Time
Site:	99902078, 05200050
Issue date:	07/24/2023
From:	NuScale
To:	Office of Nuclear Reactor Regulation
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ResponsetoSDAAAuditQuestion

QuestionNumber:A-15.0.3.7-1

ReceiptDate:04/17/2023

Question:

Section15.0.3.7.1andSection15.0.3.7.2assumethecontainmentandsecondarysystemsare isolatedat30minutes.Providethebasisforthisassumption.

Response

ThedoseconsequenceanalysesinSections15.0.3.7.1and15.0.3.7.2areforthefailureof smalllinescarryingprimarycoolantoutsidecontainmentandsteamgeneratortubefailure events,respectively.Bothoftheseeventsassumeaboundingmassreleaseofprimarycoolant fromtheRPVdirectlytotheenvironment.Isolationofthereleaseisassumedat30minutes.

Beyond30minutes,furtherreleasesarelimitedtothedesign-basislimitforcontainmentleak rate.Regardingtheassumptionoftheisolationtiming,therearetwoconsiderations:thetotal massreleaseandtreatmentofiodinespiking.

Forbothevents,aboundingmassof23,000lbmofprimarycoolantisassumedtobereleased.

Thismassiscalculatedbasedonthemaximumpossibledecreaseinpressurizerlevelbefore thecontainmentisolationsetpointisreachedasdescribedinSections15.0.3.7.1and15.0.3.7.2.

BecausethemassreleaseamountisassociatedwithreachingthelowpressurizerlevelMPS setpointinTable15.0-7,automaticcontainmentisolationandsecondarysystemisolationare assured.Thetotalmassreleaseisthereforecalculatedindependentlyofthetimeofisolation.

Usinganalternateisolationtime,eitherslowerorfasterthan30minutes,wouldnotalterthe totalmassrelease.

Theiodinespikingassumptionsassociatedwiththesetwoeventsareconsistentwiththe NuScaleaccidentsourcetermmethodologytopicalreport(Reference15.0-6intheFSAR).The failureofsmalllineseventassumesacoincidentiodinespike.Thesteamgeneratortubefailure eventassumesbothapre-incidentandcoincidentiodinespike.Forpre-incidentiodinespiking,

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theisolationtimeisnotrelevant.Forcoincidentiodinespiking,theiodinetransportfromthefuel tothecoolantincreasesovertimeafterthechangeinpowercausedbyreactortrip.Theearlier thatisolationhappensfollowingreactortrip,thelessimpactoftheiodinespiking.IntheNuScale US460design,manyoftheMPSsignalsthatcausereactortripfortheseeventsalsocause isolation.Asaresult,thetimeperiodforiodinespikingpriortoisolationisexpectedtobe minimal.Theassumptionof30minutesforisolationisthereforeconservativeforiodinespiking forthebreakreleaseasiteffectivelymodelsa30minutedelaybetweenreactortripand isolation.Forcontainmentleakagereleasesbeyondthebreakrelease,aneighthourperiodof coincidentiodinespikingisalsoevaluated.

ThedosecalculationsinSections15.0.3.7.1and15.0.3.7.2wereperformedinabounding mannerindependentofevent-specifictransientinputs.Theassumptions,includingtheisolation timeof30minutes,areconservativeforthereasonsdescribedabove.Theboundingnatureof thecalculationsinSections15.0.3.7.1and15.0.3.7.2canfurtherbeverifiedbycomparingthe assumptionstotheevent-specificvalues.Table1showsthecomparisonforthefailureofsmall linescarryingprimarycoolantoutsidecontainment.Table2showsthecomparisonforthesteam generatortubefailureevent.Notethatfortheevent-specificanalysisresultsinbothtables,the valuesrepresentthelimitingvalueforthatparameterfromtheanalyzedcasesandmaynotbe fromthesamecase.Asdemonstratedinthetables,theassumptionsinSections15.0.3.7.1and 15.0.3.7.2areclearlybounding.

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Table1:

FailureofSmallLinesCarryingPrimaryCoolantOutsideContainment

BoundingAssumptionin Event-SpecificAnalysis 15.0.3.7.1 (15.6.2)Result IsolationTime 30min ((2(a),(c) (fromeventinitiation)

BreakReleaseMass 23,000lbm (( }}2(a),(c) (eventinitiationtoisolation)

BreakReleaseIodine

SpikingTime 30min (( }} 2(a),(c)

(reactortriptoisolation)

Table2:

SteamGeneratorTubeFailure

BoundingAssumptionin Event-SpecificAnalysis

15.0.3.7.2 (15.6.3)Result

IsolationTime 30min (( }} 2(a),(c) (fromeventinitiation)

BreakReleaseMass 23,000lbm (( }}2(a),(c) (eventinitiationtoisolation)

BreakReleaseIodine

SpikingTime 30min (( }} 2(a),(c)

(reactortriptoisolation)

NochangestotheSDAAarenecessary.

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