Application of probabilistic fracture mechanics to seismic fragility analysis of piping systems

Azuma, Kisaburo*; Yamaguchi, Yoshihito ; Katsuyama, Jinya  ; Li, Y. 

Lessons learned from the TEPCO Fukushima Daiichi accident indicate the importance of risk assessment for extreme external events. Seismic probabilistic risk assessments (SPRAs) are the standard methods to quantify the seismic risk of operating nuclear power plants. One of the major factors that affect the results of SPRAs is the quality of the seismic fragility analysis. Seismic fragilities represent the failure probabilities of components under earthquakes and the associated uncertainties. However, seismic fragility data of aging pipes are not readily available due to the complex behavior under seismic loading. For example, stress corrosion cracking (SCC) is one of the main material degradation phenomena of aging pipes. There is a possibility that extreme seismic loading on aging pipes may cause the propagation of cracks originally initiated by SCC. It is unclear how these crack propagations can affect the seismic fragility of the piping system. In this study, failure probabilities of carbon steel pipes and austenitic stainless steel pipes were evaluated using a probabilistic fracture mechanics code, PASCAL-SP. SCC and fatigue crack growth were considered as age-related degradations of the pipes. Comparing the failure probabilities for brand new pipes and those for aging pipes, we discussed the effects of crack growth on seismic fragilities.