Seismic Loads for Crack Stability Assessment in a Review of Lead-Before-Break (LBB) Applicability

This study was performed to define the seismic loading conditions for use in the crack stability assessment of a BWR for the applicability of the leak-before-break (LBB) criterion. The LBB has been applied to the design of Class-1 piping in Japanese light-water reactors. Crack propagation in the LBB applicability review is classified into the Level C service condition. Here an S₁-earthquake (maximum design earthquake) is currently assumed, rather than an S₂-earthquake (extreme design earthquake). In order to justify this assumption, the frequency of an S₁-earthquake occurring during coolant leakage due to crack propagation was determined. The frequency of coolant leakage from Class-1 piping must be less than that of the Level C service condition (2.5×10⁻² to 1×10⁻⁴ per year) in order to assume that an S₁-earthquake is appropriate. Accordingly, the frequency of coolant leakage from Class-1 piping was calculated using a probabilistic fracture mechanics (PFM). The results of this analysis indicate that the frequency of coolant leakage from Class-1 piping is less than the expected occurrence of the Level C service condition. As the results, it is concluded that the assumption of the seismic loading employed in an LBB applicability review should be appropriate.