PWSCC Experience of Pressurizer Dissimilar Metal Welds at Tsuruga Unit-2

During the 13th refuel outage in September 2003, the small boric acid deposit was observed at one dissimilar weld joint between Relief Valve Nozzle and Safe End of Pressurizer in Tsuruga unit 2. The other four indications were also observed at the weld joint by non-destructive examination. Similar indications were also observed at Safety Valve Nozzle. Both Nozzles were under the steam condition at 345°C. As the first root cause investigation, the visual observation after etching was conducted at these weld joint on site. The observation revealed the repair weld print. Both weld joints with cracks were taken to conduct some investigations such as metallographic examination of fracture surface at hot laboratory. Hot laboratory examination results revealed that all cracks were extended axially in the Ni base metal weld joint, and there was no evidence of any cracks extending into the low alloy steel Nozzle and stainless steel Safe end. All cracks were interdendritic and branched. Metallographic examination results revealed that the crack growth mainly was caused by interdendritic stress corrosion cracking (IDSCC). These Nozzles including the weld joint with indications were repaired by jointing the spool piece as new Safe ends. The repair process includes the weld buttering on the low alloy steel Nozzle and joint welding between Nozzle and the spool piece. These welding were conducted by GTAW using alloy 52. It took twenty days to complete the repair works.