Effect of initial flaw and load assumptions on risk estimate changes

This study concerns the effect of initial flaw and load assumptions on nuclear power plant (NPP) piping component failure probability and risk estimate changes. As the main criterion for the acceptance of a risk informed in-service inspection (RI-ISI) program is that the overall risk shall not increase when moving from the earlier ISI program to the risk informed one, means for robust quantification of the change in risk are needed. Worldwide, several assumptions and recommendations have been published both for NPP pipe weld initial flaw distributions and welding process induced residual stresses (WRSs), the latter being often the dominant load component for welds. Failure potential and risk analyses were performed with probabilistic fracture mechanics code VTTBESIT and Markov process application for three representative NPP piping welds, covering a wide range of initial flaw and load assumptions. The considered degradation mechanism is stress corrosion cracking (SCC). In the light of the analysis results, it can be concluded that both the size distributions of the initial cracks and WRSs have a remarkable impact on the risk values. The larger the initial cracks and higher on the tensile side the WRSs, the higher the resulting risk values, and vice versa.