Validationofatransientthermal-fluidsystemsCFDmodelforapackedbedhightemperaturegas-coolednuclearreactorOriginalResearchArticle

Thispaperprovidesanoverviewofthetheoreticalbasisforanewthermal-fluidsystemsCFDsimulationmodelforhightemperaturegas-cooledreactors,containedintheFlownexsoftwarecode.Flownexprovidesfordetailedsteady-stateandtransientthermal-fluidsimulationsofthecompletepowerplant,fullyintegratedwithcoreneutronicsandcontrolleralgorithms.Thereactormodelisfoundedonafundamentalapproachfortheconservationofmass,momentumandenergyforthecompressiblefluidflowingthroughafixedbed,aswellastheheattransferinthepebblesandcorestructures.Thetime-wiseintegrationoftheresultingdifferentialequationsisbasedonanimplicitpressurecorrectionalgorithm.Thisallowsfortheuseofratherlargetimestepsmakingitverysuitableforsimulatingtheslowtransientsthatcanbeexpectedtofollowincidentslikereactorshutdowns.ThepaperalsocomparestheFlownexresultsforfourtransienttestswiththemeasuredresultsfromtheSANAtestfacilityaswellastotheresultsofsimulationswiththeThermix/DIREKTcodethatweredoneattheResearchCentre,Jülich.TheFlownexresultscomparewellwiththeThermix/DIREKTresultsforallthecasespresentedhere.Goodcomparisonwasalsoobtainedbetweenthesimulatedandmeasuredresults,exceptattwopointswithinthepebblebedneartheinnerwall.Thefactthatquickcomputersimulationtimeswereobtainedindicatesthatthenewmodelindeedachievesafinebalancebetweenaccuracyandsimplicity.However,thediscrepanciesobtainedatthetwopointsneartheinnerwall,togetherwiththefactthatadditionaluncertaintywasintroducedintheoriginalSANAtestset-upbynotbeingabletocontrolthetemperatureoftheouterwall,highlighttheneedforadditionalsystematicteststobeperformedinordertobettervalidatethenewmodel.