Estimation of rupture frequencies of Korean nuclear piping for the determination of transition break size

In nuclear power plant design, double-ended guillotine breaks (DEGB) of the largest primary system pipes have traditionally been included in the design basis accidents, despite being widely regarded as extremely unlikely. To justify their exclusion, the concept of transition break size (TBS) has been proposed, which requires demonstrating the improbability of ruptures exceeding a certain threshold. One approach is the leak-before-break (LBB) approach, which uses deterministic analysis to ensure leakage detection occurs well before rupture, assuming no active degradation mechanisms. Alternatively, probabilistic approaches, based on operating data, expert judgment, or probabilistic fracture mechanics (PFM), can directly estimate rupture frequencies while accounting for degradation and mitigation strategies.This study applied a PFM approach using xLPR (eXtremely Low Probability of Rupture) to estimate rupture frequencies for six LBB-approved pipes in a Korean nuclear power plant. Relevant degradation mechanisms such as fatigue and stress corrosion cracking were considered. In addition to rupture frequencies, key LBB-related quantities such as the time interval from detectable leakage of the coolant to pipe rupture and the margin between leakage flaw size and critical flaw size were also evaluated to provide additional insights into piping integrity.