Summary of Finite Element (FE) Sensitivity Studies Conducted in Support of the NRC/EPRI Welding Residual Stress (WRS) Program

The U.S. Nuclear Regulatory Commission (NRC) and the Electric Power Research Institute (EPRI) are working cooperatively under an addendum to the ongoing memorandum of understanding to validate welding residual stress (WRS) predictions in pressurized water reactor (PWR) primary cooling loop components containing dissimilar metal (DM) welds. These stresses are of interest as DM welds in PWRs are susceptible to primary water stress corrosion cracking (PWSCC) and tensile weld residual stresses are the primary driver of this degradation mechanism. The NRC/EPRI weld residual stress (WRS) analysis validation program consists of four phases, with each phase increasing in complexity from laboratory size specimens to component mock-ups and ex-plant material. This paper focuses on Phase 2 of the WRS program that included an international Finite Element (FE) WRS round robin and experimental residuals stress measurements using the Deep Hole Drill (DHD) method on pressurizer surge nozzle mock-up. Characterizing variability in the round robin data set is difficult, as there is significant scatter in the data set and the WRS profile is dependent on the form of the material hardening law assumed. The results of this study show that, on average, analysts can develop WRS predictions that are a reasonable estimate for actual configurations as quantified by measurements. Sensitivity studies assist in determining which input parameters provide significant impact on WRSs, with thermal energy input, post-yield stress-strain behavior, and treatment of strain hardening have the greatest impact on DM WRS distributions.