AssessmentofRELAP5/MOD3.2.2GammaagainstABBAtom3×3-RodBundleRefloodingTestsOriginalResearchArticle

Abstract TheresultsoftheABBAtom3×3-RodBundleRefloodingTestswereusedforassessmentoftherefloodingmodelusedinRELAP5/MOD3.2.2Gammaversion.Theassessmentcalculationswereperformedusingthedefaultcalculationmodeloptionsimplementedinthecode. Thetestswereperformedtoinvestigatetheeffectsofdifferentspacergriddesignsonheattransferduringtherefloodingperiodofapressurizedwaterreactorloss-of-coolantaccident(LOCA).Thetestswereconductedunderlow-pressureandlow-flow(LPLF)conditionsusingaPWR-type3×3-rodbundlewithfull-lengthindirectlyelectricallyheated,steppedcosineaxialpower-shapedheaterrods.Threedifferentspacergridconfigurationswerestudied:spacergridswithoutmixingvanes,mixingvanespacergrids,andmixingvanespacergridstogetherwithintermediateflowmixers(IFM). Atotalof36testswithdifferentspacergridconfigurationswerecalculated.Fortwoselectedbasictestswithnon-mixingspacergridsanextendedcomparisonofcalculatedandmeasuredparametersispresented.Thecomparisonofthepredictedandmeasuredmaximalcladdingtemperaturesandquenchtimes,whicharethemostimportantparametersinlicensingcalculations,ispresentedforalltheperformedtests. Theassessmentcalculationswereprecededbynodalization,timestep,andmovingmeshstudies. TheRELAP5/MOD3.2.2Gammacodewasfoundtostillhaveseveraldeficienciesintherefloodmodel.Thecalculationresultsshowasatisfactoryagreementwithexperimentalinnerpeakcladdingtemperature,howeverthepredictedtemperatureturn-aroundtimesandquenchtimesaresignificantlytooshort.Theresultsalsoshowasignificantover-predictionoftherefloodheattransferandthevapourtemperatures.Thevoidprofiledownstreamthequenchfrontisnotcorrectlypredictedeither.Finally,thepresentrefloodmodeldoesnotproperlyreflecttheeffectsofspacergridsontherefloodheattransfer. InspiteofthesedeficienciestheimprovementsincorporatedintoRELAP5/MOD3.2bythePaulScherrerInstitute(PSI)eliminatedtheunphysicalbehaviorssuchascontinuouscoolingwithoutclearturn-aroundtemperatureandnovisiblequenchingphenomena,whichwereshownintherefloodcalculationsbymeansoftheRELAP5/MOD3.1code.