Review of Studies Regarding Risk-Informing Regulations for Normal Operating Transients

摘要

The current regulations, as set forth by the United States Nuclear Regulatory Commission (USNRC), to insure that light-water nuclear reactor pressure vessels (RPVs) maintain their structural integrity when subjected to planned startup (heat-up) and shutdown (cool-down) transients are specified in Appendix G to 10 CFR Part 50, which incorporates by reference Appendix G to Section XI of the ASME Code. The technical basis for these regulations contains many aspects that are now broadly recognized by the technical community as being unnecessarily conservative. During the past decade, the NRC conducted the interdisciplinary Pressurized Thermal Shock (PTS) Re-evaluation Project that established a technical basis to support a risk-informed revision to current PTS regulations (10CFR Part 50.61). Once the results of the PTS re-evaluation are incorporated into a revision of the 10 CFR 50.61 guidance on PTS, the technical basis for the fracture toughness that is required to withstand a PTS event (an accidental loading) will differ from the technical basis for the fracture toughness that is required by Appendix G to 10 CFR Part 50 for normal operating transients. The new PTS guidelines will be based on more realistic risk-informed models and inputs whereas the existing Appendix G requirements contain many known conservatisms that place unnecessary burdens on plant operation. Consequently, a goal of this project is to develop technical information supporting a risk-informed revision to the current requirements of Appendix G to 10 CFR 50 in a manner that is consistent with that used to develop a risk-informed revision to the PTS regulations. This research has consisted of the application of the FAVOR computer code for cool-down transients associated with reactor shutdown and the development and application of the FAVOR computer code for heat-up transients associated with reactor startup. This paper provides a brief overview of the current results of this research project.