Application of the Leak-Before-Break Approach to BWR Piping

摘要

The requirement of General Design Criterion 4 (GDC-4) of 10CFR50, Appendix A, for the postulation of a sudden double-ended guillotine break (DEGB) in the high energy piping of light water reactors (LWRs) has led to the installation of protective devices such as pipe whip restraints and jet impingement barriers. However, in many cases, these devices impede the regular in-service inspection and maintenance, which in turn, leads to increased personnel exposure and adverse effects on plant safety. Recent report (NUREG-1061) of NRC's Pipe Review Committee has recommended the use of fracture mechanics methodology for demonstrating the applicability of the leak-before-break (LBB) approach as an alternate to the DEGB postulation. The NRC has proposed a rule change for GDC-4 to allow the alternate LBB approach. The LBB analysis demonstrates that the detection of flaws either by in-service inspection or by leakage monitoring systems is assured long before the flaws can grow to critical or unstable sizes which can lead to a DEGB.