Impact of input uncertainties on the failure frequency of Korean nuclear piping systems based on probabilistic fracture mechanics

Double-ended guillotine break (DEGB) of primary piping is widely recognized as an extremely unlikely event in nuclear power plants. Demonstrating that pipe rupture occurs with extremely low probability therefore requires evaluation of rupture frequency. Deterministic leak-before-break and statistical approaches based on operating experience have been used for this purpose, but they do not explicitly model degradation mechanisms or allow the influence of individual parameters on rupture frequency to be examined. Probabilistic fracture mechanics provides a framework for estimating rupture frequencies while accounting for the stochastic nature of material behavior, degradation processes, loading conditions, and inspection performance. Within this framework, the influence of modeling parameters on rupture behavior can be systematically examined, allowing the variables governing pipe rupture to be identified. In this study, sensitivity analyses were performed using the xLPR (eXtremely Low Probability of Rupture) code to investigate rupture behavior of representative Korean nuclear power plant piping systems. Shutdown cooling piping and surge line nozzle welds were selected as evaluation targets. Stress corrosion cracking was considered as the degradation mechanism. The influence of weld residual stress, crack growth rate, weld overlay repair, and probability of detection on rupture frequency and leak-before-break behavior was evaluated. The base case predicted a rupture frequency of 4.82E-07 per year for the shutdown cooling piping, whereas no rupture was observed for the surge nozzle due to compressive residual stresses near the inner surface. The results show that variations in modeling parameters significantly influence predicted rupture frequencies and clarify the factors governing failure behavior of nuclear piping systems.