A Critical evaluation of methods for computing small failure probabilities for ductile piping components

This paper focuses on the application of the failure assessment diagram (FAD) approach to accomplish robust and reliable calculations of small failure probabilities, as required for safety relevant nuclear components. Specifically, the firstand second-order reliability methods (FORM, SORM) as well as different sampling algorithms are investigated as to their performance and numerical stability. In particular, computational problems are noticed in the assessment range corresponding to the transition from ductile fracture to plastic collapse which is a typical operating regime for ferritic piping. Numerical errors are found to result due to the occurrence of multiple design points in the FORM procedure, thus leading to an underestimation of failure probabilities. Subsequent benchmark calculations reveal a high efficiency of the improved numerical algorithms within a very broad range of failure probabilities. Another aspect considered in the paper is the application of FOR M/SORM to the assessment of crack instability taking into account the scatter in crack resistance curves. Finally, the methods and computer tools developed within the framework of this research are applied for evaluating some pipe burst tests described in the literature.