Laser Peening Qualification for the U.S. Application

Primary water stress corrosion cracking (PWSCC) is a degradation process that has plagued nickel alloy components and welds in the nuclear industry for decades. Numerous mitigation techniques have been developed over the years that help reduce the potential for cracking in nickel alloy components exposed to the primary water environment. One such method is Laser Peening (LP), which improves the stress properties and helps to reduce the potential for crack initiation. The LP process has been applied in Japan to both boiling water reactors (BWR) and pressurized water reactors (PWR) for stress corrosion mitigation. The first application of LP in the US for the nuclear industry was applied in the fall of 2016 to the bottom mounted instrumentation (BMI) nozzles of a PWR. The bottom mounted nozzles are made from Alloy 600 tubing and attached with Alloy 82/182 welds, which are known to be susceptible to PWSCC. In order to prevent crack initiation, it is important for the peening mitigation process to induce sufficient compressive stress on the surface of the susceptible materials. However, it is not practical to take stress measurements directly on the reactor components in order to verify compression. Thus, the magnitude of compression induced by the LP process was verified prior to the application at the plant using mockups of the BMI nozzles. As a part of the qualification process, test coupons were peened and stress measurements were taken using X-ray diffraction (XRD). The results of the stress measurements demonstrate that sufficient surface compression was achieved by the LP process in order to provide PWSCC mitigation. This paper presents and discusses key stress measurement results taken during the qualification process for the first application of LP at a U.S. nuclear plant. Although not directly applicable in this case, the guidance in ASME Code Case N-729 Mandatory Appendix II and MRP-335 for PWR upper head nozzles was generally followed.