Probabilistic Risk Assessment (PRA) insights

Four different probabilistic risk assessments (PRAs) have been briefly reviewed with the broad objective of ascertaining what insights might be gained (beyond those already documented in the PRAs) by an independent evaluation. This effort was not intended to verify the specific details and results of each PRA but rather, having accepted the results, to see what they might mean on a plant-specific and/or generic level. The four PRAs evaluated were those for Millstone 3, Seabrook, Shoreham, and Oconee 3. Full detailed reviews of each of these four PRAs have been commissioned by the NRC, but only two have been completed and are available as further input to this study: the review of Millstone 3 by LLNL and the review of Shoreham by BNL. The review reported here focused on identifying the dominant (leading) initiators, failure modes, plant systems, and specific components that affect the overall core melt probability and/or risk to the public. In addition, the various elements of the methodologies employed by the four PRAs are discussed and ranked (per NUREG/CR-3852).