DesignandmodificationofsteamgeneratorsafetysystemofFBRMONJUOriginalResearchArticle

MONJUisaprototypefastbreederreactor(FBR)inJapan.Thesodium–waterreactioninthesteamgenerator(SG)isoneoftheimportantsafetyassessmentitemsforasodiumcooledreactorlikeMONJU.MONJUisequippedwithhydrogengasdetectorsforthesmallwaterleakdetection,gaspressuregaugesforthemediumleakandsensorsofrupturediscsforthelargeleak.Asadesignbasisaccident,onetubefailurethenfailurepropagationofneighboringthreetubesisassessedtoverifythestructuralintegrityofthesecondarycomponents.AlatestevaluationmethodonthedesignmarginagainsttheoverheatingtuberuptureshowedthatthepresentSGsystemhadnotanenoughmarginintheworstcase.Forimprovingthemargin,itneedstoshortenthetimeofthesodium–waterreactionbyearlierwaterleakdetectionintheSGandsoonerwaterejectionfromtheSGtubes.Therefore,MONJUisnowcarryingoutthefollowingmodificationworks:(1)additionofsteamreliefvalves,(2)additionofagaspressuregaugewithchangingtheinterlocklogicandloweringthetriggerlevel,(3)reducingtheopeningofthevalvesontheSGgasflowlinetothedumptankbecauseofearlierdetectionforthepressurerise.Afterthismodification,thedesignmarginoftheSGsystemwillbesufficientlyimproved.