Full structural weld overlays have been installed on three steam generator hot leg nozzles at unit 1 of Dominion Virginia Power’s North Anna Power Station. The work was carried out by Aquilex WSI, LLC (WSI) and Structural Integrity Associates (SI), known as W(SI)2.
North Anna 1 is a Westinghouse-designed three-loop pressurized water reactor. It is the only PWR in the United States with Alloy 82/182 (600) dissimilar welds joining the low alloy steel steam generator nozzles to the stainless steel primary piping.
During a recent outage, Dominion Virginia Power prepared the area for work, including insulation removal and scaffolding erection, and conducted pre-weld overlay examinations of the Alloy 600 welds. The W(SI)2 scope of work consisted of design and analysis, repair licensing, mockup testing and qualification, on-site implementation and post-mitigation NDE UT of the weld overlays.
Although W(SI)2 has performed hundreds of weld overlays at both US and foreign nuclear sites in its history, the North Anna project involved the largest Alloy 600 nozzle weld yet (41” OD) to be mitigated with weld overlay. Due to the thickness of the dissimilar metal weld, approximately 20% of base material thickness was removed prior to installing the overlay for UT inspection purposes. Through this preparatory machining, the project uncovered and repaired two through-wall cracks in one of the nozzle welds. In spite of this complication, all safety, schedule, quality and dose expectations for the project were satisfactorily completed.
“This first-of-a-kind achievement was the result of more than a year-long team effort between WSI, Structural Integrity, and our Dominion Virginia Power client,” said Doug Jacobs, senior vice president of Aquilex WSI Nuclear.