On assessment of initial cracks for RI-ISI analysis purposes

The assessment of initial cracks for Risk Informed In-Service Inspection (RI-ISI) analyses of Nuclear Power Plant (NPP) piping systems is considered here. In RI-ISI analyses piping component specific risks are assessed based on estimated failure and consequence potentials, and typical analysed locations are welds. In general there exist two main approaches for assessment of piping failure potential in current relevant RI-ISI procedures: qualitative and quantitative. Here discussion concerning assessment of initial cracks connects primarily to the latter approach. Estimates of initial crack sizes are among the most influential input parameters in terms of RI-ISI analysis response. Of the quantitative analysis approaches structural reliability procedures can best capture the relative piping component failure potential/probability differences caused by component specific differences in physical characteristics, loading conditions and inspection histories. The foremost of these procedures for piping components is probabilistic fracture mechanics (PFM). The initial crack sizes are part of the needed input data in the PFM analyses. Two developed approaches to recursively calculate estimates for initial cracks are covered here. According to application results by decreasing the average size of initial cracks from 1.0 mm to 0.3 mm leads to approximately 100 times smaller yearly pipe component failure probabilities.