Alloy 82/182 Dissimilar Metal (DM) butt welds have been the subject of significant work due to their susceptibility to primary water stress corrosion cracking (PWSCC). The current rules for inspection of DM butt welds are contained in ASME Code Case N-770 (latest revision). These rules are based on crack growth rates that were originally published in MRP-115 in 2004. Since that time significant work has been performed to better understand the crack growth behavior of Alloy 82/182 materials in pressurized water reactors (PWRs) and updated crack growth rates have recently been published in MRP-420 (Rev. 1). These updated crack growth rates show a significant difference at lower temperatures which line up with the cold leg temperatures for PWRs (approx. 540–560°F). Additionally, the US Nuclear Regulatory Commission (NRC) working along with the Electric Power Research Institute (EPRI) recently released the xLPR computer code which is a probabilistic fracture mechanics code for evaluating DM butt welds. This paper evaluated various cold leg DM butt welds for PWRs using the xLPR probabilistic computer code to determine if a change in the frequency between inspections for unmitigated DM butt welds (Item B-1) from their current requirements in ASME Code Case N-770 is warranted. Failure probabilities were determined for the current technical basis using MRP-115 crack growth rates (with the current inspection duration) and compared to the probabilities using the new crack growth rates in MRP-420, Rev. 1 (with increased inspection durations). The evaluation results demonstrate that the duration between volumetric examinations for small and medium diameter DM butt welds from Code Case N-770, Inspection Item B-1, can be safely increased to once per 10-year ISI interval from the current requirement of once every 7 years when the specific requirements described in this paper are met.

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