Determination of the Continued Adequacy of LOCA Frequencies From NUREG-1829

Abstract In 2008, the NRC published NUREG-1829, “Estimating Loss-of-Coolant Accident (LOCA) Frequencies Through the Elicitation Process.” This report was part of the technical basis for a proposed voluntary risk-informed revision of the emergency core cooling system requirements, 10 CFR 50.46a, “Risk-Informed Changes to Loss-of-Coolant Accident Technical Requirements.” However, this rulemaking was never promulgated. As part of that technical report, a formal elicitation process was used to estimate several LOCA frequencies as a function of break diameter. The original technical basis supporting the LOCA frequencies was based on evaluating the impacts of degradation on piping and non-piping passive components using operating experience (OE) data up to approximately 2004 coupled with additional analysis. Recently, the NRC staff conducted an effort to determine whether the LOCA frequencies estimated in the early 2000’s reasonably represent the LOCA frequencies of today. The staff conducted this effort by • considering recent OE and its impacts on the LOCA frequencies, • conducting a detailed probabilistic fracture mechanics study to verify the NUREG-1829 LOCA frequencies, • conducting an internal and external elicitation to identify possible scenarios not considered, or underestimated, in NUREG-1829, and • conducting a database study to estimate whether U.S. OE would modify the LOCA frequencies. The results of the analyses suggest that the LOCA frequencies in NUREG-1829 remain a reasonable, and likely conservative, representation of the expected LOCA frequencies of today.