Two Means of Calculating Very Low Failure Probability

Computation of large break probabilities in pipes when initiating cracks are the dominant degradation mode is difficult, because the problem is dominated by the probability of initiating multiple cracks around the pipe circumference and having them coalesce and grow to become long prior to penetrating the wall to become a leak. The purpose of this paper is to describe two techniques for evaluating very low large break probabilities in pipes with multiple initiating cracks: (i) combining initiation and growth probabilities by a convolution integral, and (ii) sorting through sets of sampled random variables and performing detailed (lifetime) calculations only for particularly “severe” sets. These techniques are demonstrated in an example problem involving primary water stress corrosion crack (PWSCC) initiation and subsequent growth in a piping weldment with high residual stresses by use of a probabilistic fracture mechanics code, PRAISE-CANDU, which is under development to address specific degradation issues in CANDU® reactors.Copyright © 2011 by ASME and Her Majesty The Queen in Right of Canada