Impurityconcentrationbehaviorsinaboilingtubesheetcrevice:PartI.OpencreviceOriginalResearchArticle

Inalocallyrestrictedgeometryonthesecondarysideofsteamgenerator(SG)inapressurizedwaterreactor(PWR),impuritiesinbulkwatercanconcentratebyboilingprocessestoextremepHthatmaythenacceleratethecorrosionoftubingandadjacentmaterials.TosimulatearealSGtubesheetcrevice,ahightemperature/highpressure(HT/HP)crevicesimulationsystemwasconstructed.Thesimulatedcreviceareawasmonitoredwiththermocouplesandelectrodesforthemeasurementoftemperatureandelectrochemicalcorrosionpotential(ECP),respectively,inthecreviceaswellasatthefreespan.Asecondarysolutioncomposedof50ppmNaand200ppbhydrogen(H2)wassupplied.Inanopentubesheetcrevicewith0.15mmradialgap,axialdistributionoftemperatureandECPweremeasuredasafunctionoftimeandavailablesuperheat,ΔT.Thesodiumhydroxide(NaOH)concentrationprocessinthecrevicewascharacterizedwithtemperatureandECPdata.ItwasobservedfromtemperatureresultsthattheliquidpenetrationdepthdecreasedandthedryregionexpandedfromthebottomofthecreviceasΔTincreased.MeasuredECPdatashowedabehaviorsimilartothehydrogenelectrodepotentialbehaviorpredictedbytheNernstequation.Frommeasuredvaluesofboilingpointelevation,themaximumcreviceconcentrationfactorwasestimatedtobeabout500atΔT=20°Candabout1000atΔT=25°Cacrossthetubingwall.