Traditionally, leak-before-break (LBB) analysis has been applied to large diameter main loop piping in pressurized water reactors (PWRs) to address the dynamic effects of pipe break required by the General Design Criteria 4, Appendix A of 10CFR50.
Recently, however, LBB is being sought for Class 1 pressure boundary piping systems such as the safety injection and residual heat removal systems which have small diameter piping (12-inch NPS and less) to mitigate the effect of dynamic loads on reactor internals and other concerns. In NUREG-1061, Volume 3, it is stated that LBB approach means the application of fracture mechanics to demonstrate that high energy fluid piping is unlikely to experience double-ended ruptures or their equivalent as longitudinal or diagonal splits. In application of traditional LBB to large diameter piping, only circumferential flaws are considered since it is generally believed that leaks through axial flaws for such piping are relatively large in comparison to circumferential flaws. However, this may not be the case for small diameter piping.
This paper examines the application of LBB to small diameter stainless steel piping in PWRs by determining the leak rate requirements to meet the criteria specified in SRP 3.6.3 and NUREG-1061, Volume 3 for both circumferential and axial flaws.