LOCA frequency evaluation using expert elicitation

The double-ended-guillotine break (DEGB) criterion of the largest primary piping system in the plant, which generally provides the limiting condition for the emergency core cooling system requirements, is widely recognized as an extremely unlikely event. As a result, US Nuclear Regulatory Commission (NRC) staff are currently considering a risk-informed revision of the design-basis break size requirements for commercial nuclear power plants. In support of this effort, loss-of-coolant accident (LOCA) frequency estimates have been developed using an expert elicitation process by consolidating service history data and insights from probabilistic fracture mechanics (PFM) studies with knowledge of plant design, operation, and material performance. Baseline LOCA frequency estimates for the 5th percentile, median, mean and 95th percentile were determined from each panelist's elicitation responses. Group estimates were determined by aggregating the individual estimates using the geometric mean of the individual estimates for each frequency parameter. Group variability was estimated by calculating 95% confidence bounds for each of the group frequency parameters (i.e., median, mean, and 5th and 95th percentiles). A number of sensitivity analyses were conducted to examine the effects on the quantitative results from varying the assumptions, structure and techniques of the baseline analysis procedure.