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Estimating Loss-of-Coolant Accident (LOCA) Frequencies Through the Elicitation Process (NUREG-1829)
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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
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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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