ReliabilityevaluationofanaturalcirculationsystemOriginalResearchArticle

Thispaperdiscussesareliabilitystudyperformedwithreferencetoapassivethermohydraulicnaturalcirculation(NC)system,namedTTL-1.Amethodologybasedonprobabilistictechniqueshasbeenappliedwiththemainpurposetooptimizethesystemdesign.Theobtainedresultshavebeenadoptedtoestimatethethermal-hydraulicreliability(TH-R)ofthesamesystem. Atotalof29relevantparameters(includingnominalvaluesandplausiblerangesofvariations)affectingthedesignandtheNCperformanceoftheTTL-1loopareidentifiedandaprobabilityofoccurrenceisassignedforeachvaluebasedonexpertjudgment.Followingproceduresestablishedfortheuncertaintyevaluationofthermal-hydraulicsystemcodesresults,137systemconfigurationshavebeenselectedandeachconfigurationhasbeenanalyzedviatheRelap5best-estimatecode.Thereferencesystemconfigurationandthefailurecriteriaderivedfromthe“mission”ofthepassivesystemareadoptedfortheevaluationofthesystemTH-R. Fourdifferentdefinitionsofaless-than-unity“reliability-values”(whereunityrepresentsthemaximumachievablereliability)areproposedfortheperformanceoftheselectedpassivesystem.Thisisnormallyconsideredfullyreliable,i.e.reliability-valueequalone,intypicalProbabilisticSafetyAssessment(PSA)applicationsinnuclearreactorsafety.Thetwo‘point’TH-RvaluesfortheconsideredNCsystemwerefoundequalto0.70and0.85,i.e.valuescomparablewiththereliabilityofapumpinstalledinan“equivalent”forcedcirculation(active)systemhavingthesame“mission.”Thedesignoptimizationstudywascompletedbyaregressionanalysisaddressingtheoutputofthe137calculations:heatlosses,undetectedleakage,looplength,riserdiameter,andequivalentdiameterofthetestsectionhavebeenfoundasthemostimportantparametersbringingtotheoptimalsystemdesignandaffectingtheTH-R. Asaddedvaluesforthiswork,thecomparisonhasbeenmadebetweenresultsfromthisstudyandresultsfromapreviousanalysiswherethesamemethodologywasadoptedfortheevaluationoftheTH-RofadifferentpassivesystemnamedIsolationCondenser(IC).Thecomparisonshowsthatthecurrentsingle-phaseNCsystemis‘morereliable’thanthetwo-phaseICsystem.Thisconstitutesaproofofqualificationandofconsistencyfortheadoptedmethodology.