Loading Condition Evaluation for Structural Integrity Assessment of RPV due to PTS Event Based on Three-Dimensional Thermal-Hydraulics and Structural Analyses

For structural integrity assessment on a reactor pressure vessel (RPV) of pressurized water reactor during the pressurized thermal shock (PTS) events, thermal histories of coolant water and heat transfer coefficient between coolant water and RPV are important influence factors. The former is determined on the basis of thermal-hydraulics (TH) analyses simulating PTS events and the latter is derived from Jackson-Fewster correlation using TH analysis results. Using these factors, subsequently, loading conditions for structural integrity of RPVs are evaluated by structural analyses. Nowadays, three-dimensional TH and structural analyses are recognized as precise methods for assessing structural integrity of RPVs. In this study, we performed the TH and structural analyses using a three-dimensional model including cold-leg, downcomer and beltline region of RPV in order to evaluate loading conditions during a PTS event more accurately. Distributions of temperature of coolant water and heat transfer coefficient on the surface of RPV were calculated by TH analysis. Loading condition evaluation was then performed by structural analysis using these values and taking the weld residual stress due to weld-overlay cladding and post-weld heat treatment into consideration. From these analyses, we obtained histories and distributions of loading conditions at the reactor beltline region of RPV. Based on the analysis results of loading conditions, we discussed the conservativeness of current structural integrity assessment method of RPV prescribed in the current codes through the comparison of the loading conditions due to a PTS event.