United States Nuclear Regulatory Commission - Protecting People and the Environment

Estimating Loss-of-Coolant Accident (LOCA) Frequencies Through the Elicitation Process: Main Report (NUREG-1829, Volume 1)

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Publication Information

Manuscript Completed: March 2008
Date Published: April 2008

Prepared by:
R. Tregoning (NRC), L. Abramson (NRC)
P. Scott (Battelle-Columbus)

A. Csontos, NRC Project Manager

Office of Nuclear Regulatory Research
U.S. Nuclear Regulatory Commission
Washington, DC 20555-0001

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Abstract

The NRC is establishing a risk-informed revision of the design-basis pipe break size requirements in 10 CFR 50.46, Appendix K to Part 50, and GDC 35 which requires estimates of LOCA frequencies as a function of break size. Separate BWR and PWR piping and non-piping passive system LOCA frequency estimates were developed as a function of effective break size and operating time through the end of the plant license-renewal period. The estimates were based on an expert elicitation process which consolidated operating experience and insights from probabilistic fracture mechanics studies with knowledge of plant design, operation, and material performance. The elicitation required each member of an expert panel to qualitatively and quantitatively assess important LOCA contributing factors and quantify their uncertainty. The quantitative responses were combined to develop BWR and PWR total LOCA frequency estimates for each contributing panelist. The distributions for the six LOCA size categories and three time periods evaluated are represented by four parameters (mean, median, 5th and 95th percentiles). Finally, the individual estimates were aggregated to obtain group estimates, along with measures of panel diversity.

There is general qualitative agreement among the panelists about important technical issues and LOCA contributing factors, but the individual quantitative estimates are much more variable. Sensitivity studies were conducted to examine the effects on the estimated parameters of distribution shape, correlation structure, panelist overconfidence, panel diversity measure, and aggregation method. The group estimates are most sensitive to the method used to aggregate the individual estimates. Geometric-mean aggregation produces frequency estimates that approximate the medians of the panelists’ estimates and also are generally consistent with both operating experience and prior LOCA frequency estimates, except where increases are supported by specific material aging-related concerns. However, arithmetic-mean and mixture-distribution aggregation are alternative methods that lead to significantly higher mean and 95th percentile group estimates. Because the results are sensitive to the aggregation method, a particular set of LOCA frequency estimates is not generically recommended for all risk-informed applications.

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