PIPING BASE CASE RESULTS OF BENGT LYDELL

An Application of the Parametric Attribute-
Influence Methodology to Determine Loss of
Coolant Accident (LOCA) Frequency Distributions

Report No. 2 to the NRC Expert Panel on
LOCA Frequency Distributions

Prepared for

U. S. Nuclear Regulatory Commission
Washington (DC)

June 2004

D-1

ACKNOWLEDGEMENTS

The work documented in this report was performed for the U. S. Nuclear Regulatory Commission under Subcontract No. 177115 (Battelle Memorial Institute).

Mr. Karl N. Fleming (Technology Insights, Inc., San Diego, CA) provided constructive review comments on a draft of this report. Mr. Fleming also provided the Markov model solutions supporting the calculation of time-dependent LOCA frequencies.

D. 1 Background

Limited to consideration of Code Class 1 piping failures, Base Case Report Number 2 documents an assessment of BWR and PWR loss of coolant accident (LOCA) frequency distributions. The assessment is a demonstration of the role of statistical analysis of service experience data and Markov modeling in a “bottom — up” approach to piping system reliability analysis.

D. 1.1 Objectives

Using primary coolant piping design information for three reference plants (one BWR plant and two PWR plants), the overall objective is to determine LOCA frequency distributions that are representative of currently operating U. S. nuclear power plants, including current in-service inspection (ISI) practices and degradation mitigation strategies. This determination is done analytically using a parametric model of piping reliability. The LOCA frequency distributions are determined for three time periods. To address today’s piping reliability state-of-knowledge the LOCA frequency is determined at T = 25 years. Next the LOCA frequency is extrapolated to T = 40 years to represent the primary system piping reliability status at the end of a 40-year operating license. Finally an extrapolation is made to T = 60 years to account for a possible license renewal. Analytically, this extrapolation is concerned with the potential impact on the structural integrity of the piping by material aging as well as by reliability improvement efforts.

As implied by the report title, the objective is to develop LOCA frequency distributions. The report addresses two aspects of LOCA frequency distributions. It develops LOCA frequencies associated with a distribution of flow rate threshold values ranging from 380 lpm (100 gpm) at the low end to beyond 380,000 lpm (100,000 gpm) at the high end. Additionally the study develops statistical uncertainty distributions for each set of LOCA frequencies to account for the uncertainty in the input parameters to this piping reliability analysis.