TECHNICAL BASIS FOR CASE N-766-1 NICKEL ALLOY REACTOR COOLANT INLAY AND ONLAY FOR MITIGATION OF PWR FULL PENETRATION CIRCUMFERENTIAL NICKEL ALLOY WELDS IN CLASS 1 ITEMS

摘要

Primary Water Stress Corrosion Cracking (PWSCC) has been observed in pressurized water reactor (PWR) coolant system pressure boundary components. This type of cracking has been observed in Alloy 82/182 butt welds. Various repair and mitigation methods have been proposed and employed to address PWSCC. 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 required for the other PWSCC mitigation processes is difficult or impractical. The method in Case N-766 involves isolating the PWSCC susceptible material from the primary water environment, thereby eliminating one of the three conditions that must exist for PWSCC to occur. Inlays provide a means for PWSCC mitigation or repair of existing PWSCC flaws while 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 allow for weld buildup on the inside surface with a negligible change to the inner pipe diameter. The technical basis for the design, fabrication, and inspection requirements for inlays and onlays was documented in ASME Pressure Vessel and Piping Division Conference paper PVP2010-26164. Subsequently, the Nuclear Regulatory Commission (NRC) has submitted comments on Code Case N-766. Changes were proposed to the Code Case to address the NRC's comments and it was suggested that a bounding PWSCC crack growth evaluation be added to the technical basis to demonstrate that a flaw which might have been missed by surface examination would not grow through the inlay or onlay before the next inspection. The purpose of this paper is to augment the existing technical basis for Code Case N-766 to support the changes proposed.