2007:43 ProLBB - A Probabilistic Approach to LeakBefore Break Demonstration

The SKI regulation SKIFS 2004:2 allows for the use of Leak Before Break (LBB) as one way to provide assurance that adequate protection exists against the local dynamic consequences of a pipe break. The way to demonstrate that LBB prevails relies on a deterministic procedure for which a leakage crack is postulated in certain sections of the pipe based on the leak detection capability of the plant. It shall then be demonstrated that certain margins exist against the critical crack length at which a pipe break can be expected. In certain situations, probabilistic methods can strengthen the conclusion that LBB prevails. Then it is necessary to demonstrate that the likelihood of a pipe failure is sufficiently low and that there is a sufficient margin between a detectable leak and pipe rupture. The research project presented in this report provides information on failure probabilities for both leak and rupture for pipes of different sizes in Swedish BWR- as well as PWR-plants. No active degradation mechanisms are assumed to exist. Defects are assumed only to originate from welding defects from the manufacture.

The purpose of the project is to evaluate leak- and rupture probabilities for pipes of different sizes in Swedish BWR- and PWR-plants. The project will also give information on which failure probability corresponds to a precise fulfilment of the deterministic LBB-criteria.