Developing generic prior distributions for common cause failure alpha factors

Common cause failures (CCFs) have been recognized as significant risk contributors since the early launching of probabilistic risk assessments (PRAs). A series of United States Nuclear Regulatory Commission (U.S. NRC) regulation (NUREG) reports have been published since the 1980s to provide guidelines for CCF modeling in PRA. A CCF database system has been developed and maintained by the NRC and Idaho National Laboratory (INL) for the nuclear industry. However, although the CCF database system has been routinely maintained and the CCF parameter estimations updated on a yearly basis, the process for developing CCF parameter prior distributions has not been published and the prior distributions themselves have not been updated since the early 2000s. In this paper, an overview of the history of the NUREG CCF reports is provided. Existing CCF prior distributions are listed. The process for developing prior distributions for CCF parameter estimations is described. Prior distributions for CCF alpha factor are updated with data from 1997 through 2015. The issues identified from the study as well as the suggested future work are discussed.