ASME Sec. III NB-3200-Based Environmental Fatigue Analysis of Safety Injection Piping for Determining Postulated Rupture Locations

Branch Technical Position (BTP) 3-4 provides a guideline to determine postulated rupture locations for ASME Class 1 piping. This guideline contains criteria related to the maximum cyclic stress ranges and cumulative usage factor (CUF) by using only NB-3600-based procedure which may have conservative analysis results for determining postulated rupture locations. Recently issued BTP 3-4 Rev.3 provides two different CUF limits of 0.1 for air environments and 0.4 for Light Water Reactor (LWR) environments, respectively, for determining postulated rupture locations. To calculate CUFen considering the effects of the LWR environments, the fatigue usage factor determined in the air environments based on NB-3200 or NB-3600 of ASME B&PV Sec. III is multiplied by the environmental fatigue correction factor (Fen) based on Regulatory Guide 1.207 (RG 1.207). The Fen values may vary depending on the LWR environment conditions and the maximum Fen can be determined as a factor of approximately 14 for stainless steels. Also, RG 1.207 requires to use the new design fatigue curves (DFC), which have been developed recently by Argonne National Laboratory, to perform the environmental fatigue analysis. Since the new DFC predicts much shorter fatigue lives than the current DFC given in ASME B&PV Sec. III for stainless steels, the CUFen in the LWR environments could be significantly increased. For these reasons, many points in piping systems could be determined to be postulated rupture locations due to exceeding the CUFen limit of 0.4 in the LWR environments. In this paper, NB-3200- and NB-3600-based stress analyses and fatigue analyses considering both the air environments and the LWR environments for the safety injection (SI) piping have been performed to evaluate the conservatism of NB-3600-based stress analysis results and to review the effects of the LWR environments for determining postulated rupture locations.