Recent In-Service Experience with Degradation of Low Alloy Steel Components Due to Localized Corrosion and Environmentally Assisted Cracking in German PWR Plants

摘要

Recent in-service experience in commercially operating German PWR type plants revealed minor degradation of low alloy steel components by localized corrosion and, in some cases, by environmentally assisted cracking (EAC). All indications were detected by regular inspection techniques such as leak detection systems, visual inspection or other non-destructive testing. The plant safety was not affected in any of the cases. Subsequent failure analyses exhibited the existence of shallow pitting caused by oxygenated water during shut-down periods, where dissolved oxygen can occur in both BWRs and even PWRs under certain conditions. In some cases cracks with typical features of Strain-Induced Corrosion Cracking (SICC), a special case of EAC, were also present. It was concluded that pitting was a necessary precondition for the initiation of cracks due to slow plastic deformation rates during start-up periods. Appropriate remedial actions were identified and defined for application in order to avoid future incidents.