Review of recent events involving degradation of reactor coolant system pressure boundary due to primary water stress corrosion cracking at U. S. PWRs

Watanabe, Norio; Takahara, Shogo  

Primary Water Stress Corrosion Cracking (PWSCC), which occurs on nickel-based alloy (Alloy 600) reactor coolant pressure boundary components, has been a safety concern at pressurized water reactors (PWRs). During the years from 2000 to 2002, in particular, circumferential cracks on reactor pressure vessel head penetrations and significant degradation of vessel head have been observed at U. S. PWR plants and thus, awareness of the safety significance of PWSCC was raised because circumferential cracking and leakage of RPV head nozzles and corrosion of the RPV head resulting from PWSCC may lead to pressure boundary failure if the conditions are not detected and repaired. In response to the recent experience, the Nuclear Regulatory Commission (NRC) issued the inspection Order EA-03-009 for PWR licensees to prepare and implement more effective inspections of the reactor vessel heads and associated penetration nozzles. This report provides the description on the historical NRC's responses to PWSCC issues and the review of the forty-five events, reported as licensee event reports, involving PWSCC on reactor coolant pressure boundary components at U. S. PWRs during the years from 1999 to 2005.