HELIOcodepredictionofhydrogenaccumulationinnon-ventedsteampipelinesOriginalResearchArticle

Thethermal-hydraulicandphysico-chemicalcodeHELIOisdevelopedfornumericalsimulationsandanalysesofradiolyticgases(hydrogenandoxygen)accumulationinnon-ventedsteampipelinesofnuclearpowerplantsystems.TheHELIOcodesimulatesallrelevantprocess,suchasthesteam–radiolyticgasesmixtureconvection,thecondensatedrainage,thebuoyancyforcesinducedbyconcentrationandrelatedgasmixturedensitydifferences,thegasmixtureandcondensatefilminterfacialshear,thesteamcondensationinthepresenceofnon-condensables,thenon-condensablesdiffusion,andtheradiolyticgasesabsorptionanddegassingontheliquidfilmsurface.ThispaperpresentsHELIOcodemodels,numericalsolvingprocedureandthreecasesofresultsofthree-dimensionalradiolyticgasesaccumulation:(a)inanon-ventedsteampipelineoftheResidualHeatRemovalSystemintheHamaokaNuclearPowerPlant,(b)inanon-ventedhorizontalpipeconnectedwithanelbowandaverticalpipe,and(c)inaslightlyinclined,nearlyhorizontalpipe.Theradiolyticgasesaccumulationinthemixturewithsteamisaveryslowandlongdurationtransientthatisinducedbythesteamcondensationduetoheatlossestotheatmosphere.Thecondensateisdrained,whiletheconcentrationofremainingnon-condensableradiolyticgasesincreases;theprocessthatcouldleadtotheformationofacriticalhydrogenandoxygenconcentrationandsubsequentexplosion,asincaseoftheHamaokaplantincident.IncaseoftheHamaokasteampipelinegeometry,presentedresultsshowanaccumulationoftheradiolyticgasesfromtheclosedtopendofthepipeline,aformationoftheconcentrationfrontanditspropagationtowardstheopenend.Inthesecondanalyzedcase,thegasmixturecirculatesintheelbowandhorizontalpipeduetobuoyancyforcesinducedbyconcentrationandrelateddensitydifferences.Thecirculationflowpreventstheformationofradiolyticgasesconcentrationfrontandanaccumulatedradiolyticgasesconcentrationislow.Thethirdcaseshowsthatunderlowerheatlossratesorhigherpipediametersthehydrogen–oxygenaccumulationdoesnotoccurduetothebuoyancydrivencirculationofgasmixture;thecirculationexpelsthegasmixturestreamwithahigherradiolyticgasesconcentrationoutofthepipe.Obtainedresultsclarifymechanismsoftheradiolyticgasesaccumulationandtheyareasupporttotheprescriptionofsafetymeasuresthatshouldbeappliedinordertopreventanaccumulationandeventualexplosion.