RESEARCH ACTIVITIES ASSOCIATED WITH THE TREATMENT OF POTENTIAL COMMON-CAUSE FAILURES IN EVENT AND CONDITION ASSESSMENT

The U.S. Nuclear Regulatory Commission (NRC) and the nuclear power industry have evaluated and discussed various assumptions and methods for treatment of potential common-cause failures (CCFs) in event and condition assessments (ECA), specifically risk assessments performed as part of the NRC’s Significance Determination Process (SDP) in recent years. The basis for how potential CCF is treated in SDP risk assessments is provided in NUREG-2225, “Basis for the Treatment of Potential Common-Cause Failure in the Significance Determination Process.” In light of new information and advancement in the probabilistic risk assessment (PRA) technology, the NRC and Idaho National Laboratory (INL) have continued development of the causal alpha factor method (CAFM) for potential use in ECA. The NRC and INL evaluated the suitability of using CAFM in SDP evaluations and reviewed the methodology to identify any potential data gaps. Furthermore, a focused review of the existing method that estimates CCF parameters was performed to determine if the assumptions used in the existing process result in CCF parameters that are representative of current industry performance. It was also desired to gather a better understanding of the aspects of the alpha factor method and data calculation process that either have significant effect on the CCF parameters and/or increase the uncertainties associated with these parameters. Finally, an investigation was performed on the practice of common-cause component groups (CCCGs) to determine if changes are needed to ensure that CCF is appropriately accounted for in the Standardized Plant Analysis Risk (SPAR) models. This paper provides a summary of recently completed work, including insights, conclusions, and recommendations from this effort.