STEAM GENERATOR REPLACEMENT | BALANCE OF PLANT
Left: The Tennessee Valley Authority’s Watts Bar nuclear facility
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. U
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