Development of a Generalized Flaw Distribution as Input to the Re-evaluation of theTechnical Basis for U.S. Pressurized Thermal Shock Regulation of Pressurized WaterReactor Vessels

摘要

The U.S. Nuclear Regulatory Commission (NRC) is re-evaluating the guidance and criteria in the code of federalrnregulations (CFR) as it relates to reactor vessel integrity, specifically, pressurized thermal shock (PTS) which challenges thernintegrity of the reactor vessel's inner wall. The current regulations in 10 CFR 50.61 for PTS were derived from computationalrnmodels and technologies that were developed in the early-to-mid 1980's. Since that time there have been severalrnadvancements and refinements to the various models and technologies. Preliminary studies to date indicate that technicalrnbases can be established to support a relaxation of the current federal regulation for PTS. A potential revision of the PTSrnregulation could have significant implications for plants reaching the end-of-license periods and future plant license-extensionrnconsiderations.rnPressurized thermal shock (PTS) transients can lead to reactor vessel failure. These transients have occurred atrnoperating reactors but to date they have not resulted in vessel failure. To properly determine the probability of vessel failurernfrom a PTS event, an accurate estimate of fabrication flaws is necessary. The characteristics of fabrication flaws are inputs tornfracture mechanics structural calculations that will determine the probability of vessel failure during a PTS event.rnThis paper provides an overview and status of the development of a generalized flaw distribution. It discusses thernbackground of PTS, the fabrication process and the introduction of flaws, the non-destructive (NDE) and destructivernexaminations (DE) of reactor vessel weld and base metal at Pacific Northwest National Laboratory, and an expert judgmentrnprocess and proposed methodology that be used in developing the flaw distribution.