Predictionofmoisturemigrationandporepressurebuild-upinconcreteathightemperaturesOriginalResearchArticle

Predictionofmoisturemigrationandporepressurebuild-upinnon-uniformlyheatedconcreteisimportantforsafeoperationofconcretecontainmentvesselsinnuclearpowerreactorsandforassessingthebehaviouroffire-exposedconcretestructures.(1)Changesinmoisturecontentdistributioninaconcretecontainmentvesselduringlong-termoperationshouldbeinvestigated,sincethedurabilityandradiationshieldingabilityofconcretearestronglyinfluencedbyitsmoisturecontent.(2)Thepressurebuild-upinaconcretecontainmentvesselinapostulatedaccidentshouldbeevaluatedinordertodeterminewhetheraventingsystemisnecessarybetweenlinerandconcretetorelievetheporepressure.(3)Whenconcreteissubjectedtorapidheatingduringafire,theconcretecansufferfromspallingduetopressurebuild-upintheconcretepores. Thispaperpresentsamathematicalandcomputationalmodelforpredictingchangesintemperature,moisturecontentandporepressureinconcreteatelevatedtemperatures.Apairofdifferentialequationsforone-dimensionalheatandmoisturetransferinconcretearederivedfromtheconservationofenergyandmass,andtakeintoaccountthetemperature-dependentreleaseofgelwaterandchemicallyboundwaterduetodehydration.Theseequationsarenumericallysolvedbythefinitedifferencemethod.Inthenumericalanalysis,thepressure,densityanddynamicviscosityofwaterintheconcreteporesarecalculatedexplicitlyfromasetofformulatedequations. Thenumericalanalysisresultsarecomparedwithtwodifferentsetsofexperimentaldata:(a)long-term(531days)moisturemigrationtestunderasteady-statetemperatureof200°C,and(b)short-term(114min)pressurebuild-uptestundertransientheating.Theseexperimentswereperformedtoinvestigatethemoisturemigrationandpressurebuild-upintheconcretewallofareactorcontainmentvesselathightemperatures.Theformerexperimentsimulatedtheeffectoflong-termsteady-statelinertemperatureduringnormaloperation,andthelattersimulatedasituationwhereanaccidentresultedinsudden,short-termheatingtoapproximately400°C. Finally,concretespallingissimulatedbythenumericalanalysis;andtheresultsshowhowthemoisturecontentandporepressuredistributionsinconcreteexposedtofirechangewithtimeandtemperature.Thenumericalanalysiscanpredictthetime,positionandtemperatureatwhichspallingoccurs.