A Probabilistic Approach to Leak Before Break Demonstration

Abstract

Listen

Translate article

Recently, the Swedish Nuclear Power Inspectorate has developed guidelines on how to demonstrate the existence of Leak Before Break (LBB). The guidelines define the steps that must be fulfilled to get a conservative assessment of LBB acceptability. In this paper, a complementary probabilistic LBB approach is defined and implemented into the software ProLBB. The main conclusions, from the study presented in this paper, are summarized below. • The probabilistic approach developed in this study was applied to different piping systems in both Boiler Water and Pressurised Water Reactors. The present study shows that the conditional probability of fracture is in general small for the larger diameter pipes when evaluated as function of leak flow rate. However, when evaluated as function of fraction of crack length around the circumference, then the larger diameter pipes will belong to the ones with the highest conditional fracture probabilities. • The total failure probability, corresponding to the product between the leak probability and the conditional fracture probability, will be very small for all pipe geometries. • The influence from an off-center crack position on the conditional probability of fracture is not important when assuming a uniform distribution. This is because the result is dominated totally by the center crack position. However, if the crack position is treated as a deterministic parameter, the conditional probability of fracture is strongly dependent on the position of the crack, especially for large off-center cracks. • The weld residual stresses have quite an impact on the resulting fracture probabilities, especially for smaller cracks. The influence from the weld residual stresses on the calculation of leakage flow rate is largest for a thin-walled pipe. The influence from the weld residual stresses on the calculation of fracture probability is largest for one of the thick-walled pipes. • In the formal sensitivity analyses, it is shown that the standard deviation of the yield strength has the strongest influence on the conditional fracture probability. • This study has given an indication of the relation between the deterministic LBB-criteria and the corresponding conditional fracture probability. As expected, it is easier to fulfil the deterministic LBB-margins for a large diameter pipe compared to a small diameter pipe.