Evolution of licensing methodology for large break loss of coolant accident analysis and associated PCT margin quantification

Abstract

The evaluation of methodology for large break loss of coolant accident (LBLOCA) licensing analysis involves two kinds of methodologies, namely deterministic methodology and risk-informed methodology. According to the existing 10 CFR50.46, LBLOCA is one of the most essential design-basis accidents and a deterministic methodology shall be applied to perform LBLOCA analysis based on a so-called surrogate sequence. Without considering how low this sequence occurrence probability is, this surrogate sequence satisfies all the required licensing assumptions. However, in the to-be-issued 10 CFR 50.46a, the LBLOCA can be categorized as an accident beyond design basis and the margin of peak cladding temperature (PCT) shall be evaluated in a risk-informed manner. Regarding the deterministic LBLOCA methodologies, the PCT margin quantified by the traditional appendix K methodology with conservative appendix K models and bounding plant parameters can be enlarged by 41.5K with the deterministic-realistic hybrid methodology (DRHM), in which realistic plant parameters and statistical uncertainty analysis are utilized. It was noted that the PCT margin released by realistic plant parameters can be about 1/4–1/5 of the margin generated by a full-scoped BEPU methodology with both best-estimate models and realistic plant parameters. Furthermore, by relaxing the licensing sequence assumptions, an additional LBLOCA PCT margin of 44.3–62.2K can be released from the deterministic approaches by applying the risk-informed safety margin characterization (RISMC) methodology. It was also observed from the risk-informed LBLOCA analysis, to have a cumulated occurrence probability over 99% in the load spectrum, the third sequence by order of ascending PCT was referred and the associated sequence occurrence probability of this sequence is 5.07*10−3. While in the deterministic methodology, the occurrence probability of the traditional surrogate or licensing sequence is only 5.46*10−5.