ASME Sec. III NB-3600-Based Environmental Fatigue Evaluation Considering Code Case N-779-Based Simplified Elastic-Plastic Analysis

Abstract NRC Regulatory Guide (RG) 1.207 requires securing the fatigue integrity of piping systems by incorporating the effects of LWR water environments. Since the application of RG 1.207 can significantly increase the cumulative usage factor (CUF) determined in air environments due to the high environmental fatigue correction factor (Fen) and conservatively changed design fatigue curve for austenitic stainless steels, it poses difficult challenges in satisfying the RG 1.207 requirement for piping systems. As a result, the NB-3200-based approach, which provides a more realistic and less conservative design-by-analysis method than the NB-3600-based approach, has been widely used to satisfy the RG 1.207 requirement for many piping systems. On the other hand, when applying the NB-3200-based approach, significantly increased analytical costs are expected because it requires detailed 3-D finite element (FE) models and numerous evaluations for many locations in each of the piping components. Therefore, the NB-3200-based approach is not an efficient way to satisfy the RG 1.207 requirement for many piping systems of both new plants and operating plants with license renewal for long-term operation. This paper presents an alternative approach to improve the efficiency of meeting the RG 1.207 requirement for piping systems by applying both the ASME Sec. III NB-3600-based analysis approach, which provides a simplified design-by-rule method, and the Code Case N-779, which provides less conservative simplified elastic-plastic factors (i.e., Kn and Kv), compared with the Ke factor defined in NB-3653.6. The Fen is significantly affected by the strain rate and the temperature. To investigate the effects of strain rate, temperature and Fen determined from the alternative approach presented in this paper, NB-3600-based environmental fatigue evaluations considering the simplified elastic-plastic analysis in accordance with N-779 were performed for the direct vessel injection (DVI) nozzle, and their results were compared with NB-3200-based analysis results. When determining NB-3600-based strain rate, the draft Code Case with Record Number 14-1177 was used.