Environmentally-assisted fatigue evaluations are to be conducted for ASME Code Class 1 piping components in a pressurized water reactor. Environmental fatigue correction factor method for incorporating the effects of light water reactor coolant environments into ASME Section III fatigue evaluations was investigated in this paper. Both ASME Code NB-3200 and NB-3600 methods were used to determine the usage factors of the piping components. Considered in these calculations were the loads which are generally applied to the piping design for the nuclear power plants such as seismic, thermal expansions, thermal transients, thermal stratifications and building-filtered dynamic loadings. For the practical applications of NB-3600 method, regarded as the simple and conservative approach, to the piping components, it was presumed that the stress intensity and/or strain time histories for all or some of the external loadings were not known; therefore the time consistency might not be considered in calculating the usage factors as well as environmental correction factors (F en ). In NB-3200 method in contrast to NB-3600, the stress variations with time for all loads except for the dynamic loads were obtained for the fatigue evaluations in LWR environments, and therefore the time consistency was considered. The results showed that the environmental correction factors as well as in-air cumulative usage factors calculated from NB-3200 methods were significantly less than those from NB-3600 rules. In addition, comparing the results of conventional ASME fatigue evaluation applied until 2006 to the ones in accordance with USNRC RG 1.207 issued on 2007, one may identify that the cumulative usage factors in LWR environments were larger than the conventional one due to the change of design fatigue curves as well as F en factors accounting for the environmental effects on fatigue. Although this work was focused on the detailed calculations of the usage factors and F en values, one might identify or suggest a number of areas requiring further clarification or research through the efforts of this study, which were not yet addressed. A few items needed to be clarified, especially for NB-3600-based fatigue evaluations, are also discussed in this paper.

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.