SERVICE EXPERIENCE WITH ALLOY 600 AND ASSOCIATED WELDS IN OPERATING PWRS, INCLUDING REPAIR ACTIVITIES AND REGULATORY AND CODE ACTIONS

摘要

This paper provides an update of the cracking history and the experience with repairs and mitigations of Alloy 600 locations in Pressurized Water Reactors (PWR). Emphasis has been placed on the PWR designs which exist worldwide, but where possible, appropriate experience with other light water designs has been included. This paper begins with a discussion of the regulatory, Code, and industry actions that are driving the inspection of the susceptible locations discussed in this paper. This is followed by a discussion of the service experience for each susceptible location. In the reactor vessel, the majority of the cracking observed to date has been in the reactor vessel head penetrations. While some cracking is still being observed in these penetrations, the majority of the more susceptible heads (higher temperatures) have been replaced, as will be discussed. Emphasis in the US regulatory arena has shifted to the RV primary nozzles, where mitigation is expected to be done in most plants by the end of 2014. To this end, a description of a range of mitigations has been provided, along with their service history. The most recent status of the Alloy 600 nozzles in steam generators is provided, and included are the primary nozzle safe ends, as well as the channel head drains and divider plates. For pressurizers, a list of the cracking events which have occurred is provided, as well as a description of the issue which resulted from the inspection findings in the Wolf Creek pressurizer in 2006. This issue, and the resulting advanced fracture analysis work, demonstrated that the margins which actually exist in these regions are very high.