IdentificationofdampingmechanismofTRR-IIreactorcontrolrodduringfreefallinsertionOriginalResearchArticle

Inlightwaterreactors,controlrodsareingeneralinsertedintoreactorsbygravity.Inordertoachievearapidshutdown,itisrequiredtoinsertcontrolrodsasfastaspossible.Ontheotherhand,acontrolrodwithafastfallingvelocitywouldimposeasubstantialimpacttoreactorstructureaswellastotheroditself.Therefore,adampingforcemustcomeintoeffect,especiallyduringthefinalstageofthefreefallofthecontrolrod.Thepurposeofthisstudyistodevelopamathematicalmodelandanumericalsimulationtodescribeandidentifythedampingmechanism;andapplythismodeltothedesignofthecontrolrodusedinTRR-IIreactoroftheInstituteofNuclearEnergyResearch(INER)ofTaiwan. Thedampingeffectofafallingcontrolrodcomesfromtwofactors:theviscousshearstressoccurredinanarrowgapbetweentherodandanoutertubewhichconfinesthelateralmovementoftherod,andthepressureforceexertedontherodbythecompressedwaterundertherod.Theviscousshearstresscanbeanalyzedbyassumingacouetteflowbetweentherodandtheoutertubesimilartotheviscousforceoccurredinrheology.Indoingthis,theflowrateineachflowpathiscloselyrelatedtothepressuregradientintheflowpathandcanbeevaluatedusinganelectricalcircuitanalogy.TheresultsofthecodepredictionwerecomparedtotheexperimentalresultsascarriedoutbytheINER.Finally,aparametricstudywasappliedtoestimatetheeffectsofthevariousfactorsincludinggapthickness,sizeoftheflowholes,andothergeometricconsiderationsontherodfallingvelocity.Theresultsofthisstudycanservesometechnicalsupportduringthestageofroddesignandmanufacture.