BALANCE OF PLANT | STEAM GENERATOR REPLACEMENT V “We worked through planning and preparations for


approximately a year and selecting SGT was really because of their record, their expertise and their knowledge of industry operating experience. SGT had performed one of our steam generator replacements at Sequoia previously so we were familiar with them. They were able to take all of the different lessons learned from all of the steam generator replacements they’ve done and make sure that we put the right contingencies in place to do it efficiently and effectively,” says Barajas. She adds: “Because completing the task within a relatively narrow time window was an important part of the process, when we brought them on site we went through the process methodically making sure that the key people that were brought in had the experience that we needed.” Barajas emphasises the importance of experience


in executing such projects: “Obviously, it’s a very large construction project on an operating nuclear site so the reliance on an expert that has done many steam generator replacements before was beneficial.”


Heading through the top The steam generators are large components. Each of the original units is 67-feet (20m) long and weighs some 360-tonnes. To remove and replace them through the reactor building, temporary openings had to be cut in the reactor building dome, containment and steam generator enclosures. “Because of the design of Watts Bar we had to cut two


large holes, one on each side, on top of our containment approximately 45 feet by 22 feet (13.7 x 6.7m) to access the two steam generator compartments. We have two steam generators per compartment, so we had to cut the top of the concrete shield building dome which is about two feet (60cm) thick,” explains Barajas. “We did use a machine cutting robot that sat on top of containment to cut and process the concrete to remove it,” she says, adding: “Once we were done cutting the shield building, we had to cut the steel reactor vessel liner to about the same size and then in the steam generator compartment we had to cut two holes


in the concrete to access each of the two steam generators. It was pretty much a deconstruction of the shield building and the reactor vessel in order access and remove the four steam generators through the top of the building.” This method was chosen because it wasn’t possible


to disassemble the steam generators from within the containment building itself due to their level of contamination. “We wanted to keep them intact and once we removed the old steam generators they were stored on site in the secure dry storage mausoleum. They’ll remain on site until final decommissioning, however long that is,” notes Barajas. For the removal of the old steam generators a


combination of both machine and manual cutting using different methods were deployed to sever the pipe connections. Barajas says: “Across the four steam generators you’ve got small bore pipes, large bore pipes, main steam pipes, a lot of instrument lines that had to be cut. We have to use tools for cutting that are used by our pipe fitters and pipe fitter welders but it really is a combination of these approaches.” Precision measurements and meticulous metrology


practices were used to support the necessary lifting operations, which were performed using a Sarens DMAG 8800 crawler crane. Some 22 significantly heavy lifts were performed during the project. “SGT hired Sarens, which constructed the world’s second largest crane on site in order to be able to remove the four steam generators and install the new steam generators in addition to removing concrete shield building dome pieces and the steel containment vessel liner on both sides,” says Barajas. Once the original steam generators had been removed


the replacements were lifted into place with 3D fit-up solutions and specialised machining enabling precise placement back into the existing plant configuration. More than 4000 welds were required to install the new steam generators. Due to the potential radiological hazard associated with working close to the reactor pressure vessel several of the installation welds were performed by robotic welding systems developed by Framatome. “One


Right: More than 4000 welds had to be completed to connect the four new steam generators to the reactor


24 | March 2023 | www.neimagazine.com


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