Application of Leak-Before-Break to Small Diameter Piping Nozzles With Dissimilar Metal Butt Welds Susceptible to PWSCC Using xLPR

To address the dynamic effects of pipe break required by the General Design Criteria 4 (GDC-4), Appendix A of 10CFR50, deterministic leak-before-break (LBB) analysis (based on margins on leak rate and critical flaw size) as described in Standard Review Plan (SRP) 3.6.3 has been utilized by the nuclear industry. The LBB deterministic margins are generally met for large diameter piping (NPS 10 and greater) using a traditional LRD of 1.0 gpm. For smaller diameter piping, it is typically difficult to meet the required margins using LRD of 1.0 gpm necessitating a lower LRD. An alternate approach is to employ a probabilistic method to determine the probability of rupture and compare it to an appropriate probabilistic acceptance criterion. Recent release of the Extremely Low Probability of Rupture (xLPR) software has made the use of such a probabilistic approach possible. In this paper, a feasibility study was performed using xLPR to investigate if low probability of rupture can be demonstrated for a typical small diameter piping nozzle with a dissimilar metal (DM) butt weld that is susceptible to primary water stress corrosion cracking (PWSCC). This study assumes a small surface crack and grows this crack to determine if rupture occurs by a through-wall crack or a surface crack. Several sensitivity studies were performed to investigate the effects of key input variables on the rupture probabilities. The study has shown that it is feasible to justify low probability of rupture for small diameter DM welds susceptible to PWSCC. However, the WRS profile used in the evaluation has strong influence on the probability of rupture. The study has also shown the limitations in applying the deterministic LBB approach on a broader basis to small diameter DM welds in the presence of PWSCC, since rupture by surface cracks instead of through-wall cacks cannot be summarily dismissed and is a fundamental assumption in deterministic LBB with SRP 3.6.3.