STRESS CORROSION CRACKING IN WELDS OF REACTOR VESSEL NOZZLE AT OHI-3 AND OF OTHER VESSEL'S NOZZLE AT JAPAN'S PWR PLANTS

During the 13th periodic inspection, which started in February 2008, KEPCO's Ohi unit 3 (1,180MWe PWR) implemented voluntary ECT in addition to the visual inspection of the RV hot and cold leg nozzle welds to confirm the integrity of the concerned section. As a result of inspection, in March 2008, a flaw extending in the depth direction along dendritic grain boundaries of the weld metal was found in the RV A-loop hot leg nozzle. With traces of machining, which could cause residual tensile stresses, it was suspected that SCC initiated and grew at the concerned section. After grinding the section to remove the entire flaw, WJP was applied as the corrective action. Ohi-3 restarted operation on November 2008. It is planned to apply repair welding to the ground out section with alloy 690 during the next periodic inspection [1]. Several Japan's PWR plants have experienced similar incidents in the nozzle welds. This paper presents the details of repair technologies, which have been developed to address PWSCC found in Ohi-3 RV hot leg nozzle and previous similar incidents in the RV and other vessel's nozzle welds at Japan's PWR plants.