Technical Basis for Case N-766 Nickel Alloy Reactor Coolant Inlay and Onlay for Mitigation of PWR Full Penetration Circumferential Nickel Alloy Welds in Class 1 Items

Primary Water Stress Corrosion Cracking (PWSCC) has been observed in pressurized water reactor (PWR) coolant system pressure boundary components. Recently this cracking has been observed in Alloy 82/182 butt welds. Various repair and mitigation schemes have been proposed or employed to address this cracking. Case N-766 was developed as an alternative method for PWSCC mitigation. It is especially useful for applications in PWSCC susceptible regions where accessibility to the outer surfaces for performing other PWSCC mitigation techniques are difficult or impractical. The method in Case N-766 involves isolating the PWSCC susceptible material from its environment, in this case PWR reactor coolant fluid, thereby eliminating one of the three (3) simultaneous conditions that must exist for PWSCC to occur. Inlays provide for mitigation of flaws or repair of existing flaws and also permit maintaining the inside surface contour essentially in its original configuration, without flow path restriction. Onlays, which do not require an excavation into the pipe ID, permit maintaining the inside surface contour essentially in its original configuration, or permitting weld buildup on the inside surface. This paper presents the technical basis for the design, fabrication, and inspection requirements for inlays and onlays.