Why Historical Material Degradation Experience Might Not Represent Future Response

摘要

Materials degradation in light water reactors, particularly of primary pressure boundary components, 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 tended to develop on a roughly geometric time scale (e.g., 1.5X), so that new generations of issues surfaced after 1.1, 1.7, 2.5, 3.8, 5.7, 8.5... years. In general, each problem resolution created a sense of optimism that all degradation problems were resolved, when in fact most problems were mitigated but not eliminated, and new problems emerged. In 2002, efforts to anticipate 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 roughly 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 paper highlights a variety of reasons why historical experience does not provide 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. This is especially important as extended lifetimes of 60 -100 years are considered.