The experience and analysis of vent pipe PWSCC (primary water stress corrosion cracking) in PWR vessel head penetration

One of the Korea pressurized water reactors (PWRs), Yong-Gwang Unit 3 (YGU3), has experienced a boron leakage near the vent line pipe (3/4 in.) nozzle of pressure vessel head penetration on February 25, 2010, during its 12th annual overhaul. The amount of boric acid leakage was evaluated and estimated to be 31.8 l which had leaked to the inside containment. Using the non-destructive testing such as MRPC ECT and UT, two cracks were confirmed near the nozzle welding part. The vent pipe was Alloy 600 and installed in carbon steel head by J groove welding (Alloy 182 buttering and Alloy 182/82 welding). Alloy 600 was known as a sensitive material for PWSCC (primary water stress corrosion cracking). To find the root cause of the cracks, we did additional non-destructive test for crack sizing and destructive test for crack morphology. And stress analysis and operating environmental analysis were also performed. Through the non-destructive test, destructive test, residual stress analysis and operating environment analysis, it was confirmed that the cracks of YGU3 were related with the boric acid leakage by the axial PWSCC. Especially, the high residual stress seemed to have one of the biggest effects on PWSCC, and the random Cr carbide precipitates could be an important factor for inter-granular crack growth. On the other hand, the YGU3 was re-started after performing 5 action items (including the integrity assessment of nozzle repair).