STEAM GENERATOR REPLACEMENT | BALANCE OF PLANT


of the reasons we were successful in our steam generator replacement was good preparation and being ready for any issues with the amount of mock-ups and pre-planning key,” says Barajas, adding: “An example would be the use of machine welding on the reactor coolant system piping that attaches to the steam generators. We worked with Framatome closely and they did some innovative machine welding. By replicating the environment that they would have to do the welding at Watts Bar we were able to work out any issues that they had and were also able to prove that the equipment was going to work. The whole process of welding large bore pipe was very successful and was done on time because of that additional mock-up preparation and testing of the machine welding operation.” Having reconnected all the pipework and instrumentation the openings in the steam generators compartments and the containment vessel had to be repaired. “We had to reinstall the concrete on top of the steam generator enclosure blocks, along with re-welding the steel containment vessel liners on each side. Then we had to pour new concrete on top of the shield building dome to make the shield building dome intact,” says Barajas. During the steam generator programme a range of other


repair and maintenance activities were also undertaken, including a refueling in which 88 of Unit 2’s 193 fuel assemblies were replaced. In addition, more reliable low pressure turbine blades were installed, main turbine controls and feedwater heaters were upgraded and other plant equipment was either replaced or serviced and inspected.


Planning for project success The outage for the steam generator replacement began in March 2022 and was concluded at the end of June. As Barajas says: “For such a big project that seems like a very short amount of time. That’s why I wanted to emphasize the amount of preparation and planning that we did, incorporating operating experience and lessons learned from other steam generator placements to make it as efficient as possible.”


There were additional challenges associated with


executing a large construction project within 50 or 60 feet (15-18 m) of another reactor operating at 100%. “There are some systems that are common between Watts Bar unit 1 and unit 2. A key part was the safety of operating unit one and ensuring unit 1 operated reliably throughout the process of deconstruction and reconstruction of unit 2,” she adds, noting: “A lot of preparation and planning also had to take into account that we had an operating unit and making sure that we were not going to put that unit at risk throughout the process, for example through lifting heavy objects nearby.” Prior to Watts Bar, the last steam generator replacement


carried out by TVA had been over 10 years ago. Enough time so that some of the proficiencies that were gained then could be dissipated. Nonetheless, the latest, and last, replacement programme was deemed a success. “There’s obviously challenges with a large construction project that you have to work through. We focused more on the safety of our people, the environment, nuclear safety and radiological safety so when we say the steam generator placement was successful it was based on those four factors. We had no related recordable injuries so it was very safe and error-free and that’s what we focus on,” says Barajas. Overall, replacement of the steam generators delivered an opportunity to increase operating life of the reactor and to improve the plant reliability by addressing a well- known degrading component. But the steam generator upgrade yielded other benefits too. Although the core steam characteristics haven’t


changed the generators are now more efficient and TVA is actually getting more power out of the plant now the steam generators have been replaced. “One of the features is increased efficiency. There’s also been some internal modifications done but coming out of the Unit 2 outage we saw about a 20 MWe increase based on the replacement of the four steam generators. It definitely benefits the plant and now we can generate more electricity to provide to our customers,” concludes Barajas. ■


Left: Construction on Unit 2 of Watts Bar began in the 1970s but grid synchronisation actually occurred in 2016


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