Reassessment of the BWR scram failure probability

As part of the Severe Accident Policy Statement implementation, the probabilistic quantification of accident sequence frequencies that may lead to core damage is a key element in demonstrating a plant's safety status relative to US Nuclear Regulatory Commission (NRC) staff goals. One of the key quantitative inputs in a boiling water reactor (BWR) probabilistic risk assessment is the probability of a failure to scram. The assessment of this failure probability has been the subject of a long and continuing debate over the adequacy of available data and analytic modeling. This report provides a summary of the status of this debate, including the latest data, and provides a revision to the characterization of the failure probability originally published in NUREG 0460 and the Utility Group on Anticipated Transient Without Scram (ATWS) Petition.