Unit 2 of the Tennessee Valley Authority’s Watts Bar nuclear power plant returned to service last summer after replacing the reactor’s four steam generators. Although Watts Bar Unit 2 is America’s newest nuclear plant, construction actually began in 1972. It was suspended in 1985 having been about 55% complete when construction stopped. Work was resumed in 2007 and commercial operations at the 1165MWe Westinghouse PWR started in October 2016. A year earlier the NRC had approved a forty- year operating license for Unit 2, which is located on a 1700 acre (688 Ha) site on the northern end of Chickamauga Reservoir, near Spring City in Eastern Tennessee. Operated by TVA, Unit 2 was built at a construction cost of some $4.7 billion.

Although unit 2 has only been operating for around six years, the existing steam generators had been installed during the earlier construction phase. “The steam generators were the original steam generators that were installed and not in use since the 1970s or early 1980s. They were laid up for that time so they were older steam generators,” explains Carol Barajas, vice president of the Watts Bar steam generator replacement project.

“We expected them to last approximately 10 years and we knew we would have to replace them so when we started unit two back in 2016. We had ordered from Westinghouse the four new steam generators. They didn’t arrive on site until 2018,” Barajas adds. Westinghouse had supplied the original units also.

The decision to replace the steam generators was based on the fact that they were an older design and used inferior alloys that are known to be susceptible to stress corrosion cracking and can therefore prematurely develop leaks and other problems.

Indeed, unit 1 at Watts Bar had already had its steam generators replaced in the early 2000s with an upgraded version of the Westinghouse design using the new alloy 690 which is more robust and resistant to stress corrosion cracking.

“Most of the industry in the United States have, for pressurized water reactors, replaced their steam generators with the new Westinghouse alloy 690 version and TVA had already upgraded all of its existing reactor fleet. We did upgrade them on Watts Barr unit one and both Sequoia unit one and two. The steam generators were replaced with the new Westinghouse generators in the 2000 to 2012 time frame and unit two at Watts Bar was the last pressurized water reactor where we had to replace the steam generators,” says Barajas.

TVA owns and operates several additional nuclear installations and seven reactors in the state, including Browns Ferry and Sequoia.

A long-term steam solution

Although the old units hadn’t failed after only six years of use and they were still operational, by replacing them early the plant owner aimed to remove some risk of the plant shutting down as a result of failures within the steam generators. The plan was to avoid an unexpected outage. TVA monitors crack propagation within the steam generator and chose to execute the replacement programme before actual degradation occurred to the point where any cracking had been observed in the steam tubes in the old units. “We do condition monitoring on the steam generators during every refuel outage. We’re monitoring this stress corrosion cracking and trend over time the condition. We determined that we needed to replace them,” notes Barajas.

In fact, having determined that replacements were required early on during the operational life of the plant, TVA accelerated its replacement programme at unit 2 by around 18 months. Barajas explains: “We ended up replacing the steam generators on unit 2 one refuelling cycle before we had originally planned to be proactive and ensure that we weren’t going to have any issues.”

As part of this industry-wide replacement programme TVA is also anticipating a potential life extension of this plant and others within its reactor fleet.

“The purpose of investing in these new steam generators is so that we can operate the plant safely and reliably over many years to come. We expect the new steam generators to last the remainder of the plant life – that could be anywhere from 40 to 60 years,” she says, adding: “The plants have potentially a 40-odd year life remaining and of course unit two at Watts Bar just came online and it has a 40-year life expectancy but we do expect down the road to potentially request license renewal.”

If the life extension operating licence is requested and approved a full 60-year life would see unit 2 operating for another 54 years. However, the life expectancy of the replacement steam generators offers scope for an even longer life. As Barajas says: “There’s no actual expiration of these steam generators, they could last a lot longer than that. With this new design we expect it to last the lifetime of the plant and that means anywhere from a minimum of 40 years potentially up to 60 or 80 years based on how they progress.”

The Steam Generating Team (SGT), a joint venture between Framatome and United Engineers & Constructors, Inc., was contracted by TVA to execute the steam generator replacement programme.

“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 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.