Westinghouse owners group risk-informed regulation efforts-options 2 and 3

摘要

The U.S. Nuclear Regulatory Commission (NRC) has initiated efforts to incorporate risk-informed methods to redefine the scope of the existing 10 CFR 50 regulations (Option 2) and to change the technical requirements of the regulations (Option 3). The overall objectives of these efforts are to enhance plant safety, provide a framework for risk-informed regulations, add flexibility to plant operations, and reduce regulatory burden The Westinghouse Owners Group (WOG) has a variety of active programs in the risk-informed area, including a program in the Option 2 and Option 3 areas. These two programs will be summarized including the benefits and the technical approach. The purpose of Option 2 is to make changes to the overall scope of structures, systems and components (SSCs) covered by 10 CFR 50 requiring special treatment by formulating new risk-informed safety classification categories that are linked to current definitions of safety-related and important-to-safety. This initiative would permit possible changes to the current special treatment requirements based on risk insights. The Nuclear Energy Institute (NEI) has developed an Option 2 implementation guideline (NEI 00-04 Draft Revision B). The WOG has initiated a program to validate the NEI guideline and to provide an initial cost-benefit assessment of the revised categorization and treatment under Option 2 via trial application to two systems at both Surry Unit 1 and Wolf Creek. The WOG Option 2 program includes consideration of all of the components in the selected systems, regardless of whether or not they are modeled in the respective plant probabilistic risk assessment (PRA) studies. As a result, quantitative risk measures are not available for many of the components being considered. In this case, the WOG program will provide valuable input to the NEI guideline. Additionally, the WOG program extends the use of both of the dominant methodologies for risk-informed SI (RI-ISI) to address repair and replacement activities of pressure-retaining items per Code Cases under development within ASME. Therefore, feedback is provided on the consideration of passive components for extending both the WOG and Electric Power Research Institute (EPRI) RI-IS methodologies for piping to all pressure-retaining items. In the Option 3 area, the WOG Large Break Loss-of-Coolant Accident (LBLOCA) Redefinition program is a risk-informed approach to improve select regulations (10 CFR 50.46, Appendix A, and Appendix K) such that the plant licensing basis is focused on LOCA break sizes up to a new maximum size. The new maximum break size will replace the existing requirement to consider break sizes up to and including double ended breaks of the largest primary system piping. Plants will retain the capability to mitigate a break of the largest primary system piping as evaluated using realistic success criteria and assumptions. The WOG is also providing input to the development of a risk-informed 10 CFR 50.44 based on insights from the WOG Severe Accident Management Guidance and plant-specific PRA studies.