EvaluationoffluidflowinthelowercoreofaPWRwithCode_SaturneOriginalResearchArticle

Inordertobetterunderstandthestressestowhichfuelrodsaresubjected,weneedtoimproveourknowledgeofthefluidflowinsidethecore.Thefirstspacergridregionisofparticularinterest,asfuelrodfrettinghassometimesbeenobservedatthatlevel.Entryconditionsdependonthegeometryofthelowercoreplateandoftheassemblynozzles.Distributionofflowinthedowncomerandlowerplenumisalsoafactor.AseriesofcalculationsarethusrunwiththeincompressibleNavier–Stokessolver,Code_Saturne,usingaclassicalRANSturbulencemodel.Thefirstcalculationsinvolveaglobalgeometry,includingpartofthecoldlegs,downcomer,lowerplenum,andlowercoreofapressurizedwaterreactor.Thelevelofdetailincludesmostobstaclesbelowthecore.Thelowercoreplateandthefuelrodassembliesaboveitcannotbewellrepresentedwithinapracticalmeshsize,sothataheadlossmodelisused.Differenttypesofassembliescanberepresentedthroughdifferentheadlosscoefficients.WemakefulluseofCode_Saturne’snon-conformingmeshpossibilitiestorepresentacomplexgeometry,beingcarefultoretainagoodmeshquality.Steady-stateornearsteady-stateresultsareobtained,whichmaybeusedasrealisticentryconditionsforfullcorecalculationsatassemblywidthresolution,andbeyondthose,mechanicalstraincalculations.Weareespeciallyinterestedinmoredetailedflowconditionsandinthelowercorearea,soasinthefuturetoquantifyvibrationalinput.Thisrequiresamuchhigherresolution,whichislimitedtoascaleofafewassembliesforpracticalreasons.Atthisscale,mostofthefeaturesofthefuelrods,nozzles,andguidetubesarerepresented,thoughthegeometryofthespacergridsisstillmuchsimplified,anddetailssuchasdebris-trappinggridsareignored.Differentmeshesareusedfordifferentfueltypes.Forthemoment,aconstantvelocityupstreamofthelowercoreplateisusedasaninletcondition.Wehavealsobuiltasmalllowerfuelrodassemblymock-up(1/5scale7 image 7tube,3 image 3assemblies)withwhichweplantoobtaindetailedflowinformation,andbetterqualifytheuseofourCFDcodeswithregardstothistypeofapplication.