News & Views, Volume 44 | Weld Overlay Repair Mitigates Thermal Fatigue Flaw Growth
By: David Segletes
A circumferential flaw in a 14-inch diameter suction pipe-to-elbow stainless steel weld was identified in both units of a nuclear power plant as depicted in Figure 1. The two units are Westinghouse designed four-loop pressurized water reactor (PWR) plants and are mirror images of each other. The pipe-to-elbow weld is the first junction remote from the hot leg piping. The circumferential flaw at this location was first discovered on Unit 2 during the spring of 2016 and subsequently on Unit 1 in the spring of 2017. The flaws are located at comparable circumferential positions, given the two pipes are mirror images of each other and at the same distance from the RHR nozzle. Structural Integrity (SI) performed the flaw evaluation for each unit at the time of discovery. The flaws are ID connected and located at the weld heat affected zone (HAZ) on the pipe side. Although stress corrosion cracking has not be observed in the HAZ of austenitic stainless steel in PWR systems, the flaws were evaluated for both fatigue crack growth and stress corrosion crack growth. The flaw evaluations indicated there was life remaining for a short period of operation, with the appropriate safety margin, but not sufficient to allow the client to operate the plant until the end of the operating license for the given unit. Subsequently, a repair plan was developed to allow the units to operate to the end of the operating license.