Historical vs. Future Material Degradation in LWRs

Peter L. Andresen
U.S. National Academy of Engineering

EXTENDED ABSTRACT: Materials degradation of primary pressure boundary components in light water reactors has had a significant effect on capacity factor. Early degradation issues developed within months of operation – such as bulk cold worked stainless steel in boiling water reactors (BWRs) – and were resolved. Subsequent problems developed on a roughly geometric time scale (e.g., 1.5X), so that new generations of issues surfaced after 1, 1.5, 2.5, 4, 6, 9, 14, 20… years. Each problem resolution created optimism that all degradation problems were resolved, when most problems were reduced but not eliminated, and new problems emerged. In 2002, efforts to anticipate materials degradation were initiated at the Nuclear Regulatory Commission (NRC) and at the Electric Power Research Institute (EPRI) in the form of proactive materials degradation management, which relied on experience and expert judgment. The systematization of the vulnerabilities unfortunately led to steadily diminished research to address the many gaps. Additionally, the geometric timescale between the appearance of new issues has led some to believe that if we haven’t seen it yet, it probably won’t happen – the same thinking that existed since the inception of light water reactor (LWR) operation. This presentation highlights the reasons why historical experience is not an adequate basis for anticipating future degradation, and encourages sustained expertise, laboratory capability and plant inspection so that future degradation can be identified and managed.