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.
While the 2018 Spring outage season is mostly behind us, we all know a key element in being able to provide safe, reliable, clean and economic power to energy consumers is how successfully plant outages are accomplished. 
Nuclear plant workers accrue most of their radiation exposure during refueling outages, when many plant systems are opened for corrective and preventive maintenance. The total refueling outage radiation exposure can be 100-200 person-Rem at a typical Boiling Water Reactor (BWR), and 30-100 person-Rem at a typical Pressurized Water Reactor (PWR). Accrued refueling outage radiation exposure values can be significantly greater than these values depending upon radiation fields, outage work scope, and emergent work. Outage radiation exposure is one metric used by a plant to determine outage success and by industry regulators in assessing the overall performance of a plant. Plants with high personnel radiation exposure tend to be those plants with more equipment problems and more unscheduled shutdowns; consequently, they may be subjected to increased regulatory oversight. 
As all of us who work with nuclear energy know the US nuclear industry is engaged in a multi-year effort to generate power more efficiently, economically and safely. A key goal includes a significant reduction in operating expenses. This initiative is termed “Delivering the Nuclear Promise” (DNP) and is supported by nuclear utilities, vendors such as Structural Integrity, the Nuclear Energy Institute (NEI), Institute 