Browns Ferry and the fight against eelgrass

Opportunities to improve maintenance efficiency can arise unexpectedly at nuclear power plants.

At Browns Ferry in Athens, Ala., one opportunity arose when an unexpected surge of nonnative eelgrass in the Tennessee River clogged the cooling water intake, prompting the shutdown of two reactors and the derating of a third to about 78 percent power. Power plant engineers said the single surge reportedly cost the Tennessee Valley Authority about $15 million-$20 million in revenue. A 2026 report from the International Atomic Energy Agency said the same incident resulted in a production loss of roughly 0.5 terawatt-hours.

Prior to this event, Browns Ferry relied on a bucket excavator to run over the intake system's trash racks to pick up debris. But as system engineer Antonio Diaz-Lucier pointed out, this sometimes would tear up collected eelgrass, push it past the trash racks, and throw it onto the traveling water screens, clogging the system.

To combat this, Browns Ferry moved forward with an upgraded trash rack system featuring automated raking. As described by the IAEA, the new trash racks were "designed with tighter bar spacing to capture more biotic fouling, increasing protection for the systems downstream." The custom-made rake allows for better capture and removal of material from the intake. What's more, the rack now applies force to push the debris away from the traveling water screen system rather than toward it.

Browns Ferry learned the hard way how vegetation and wildlife can become unexpected nuisances for water intake systems. And the plant is far from alone-just this summer, a swarm of jellyfish overwhelmed the Gravelines nuclear power plant in France to the point where three of its six reactors were shut down.

Diaz-Lucier and two others from the Browns Ferry Operations Support Engineering team, senior manager Ryan Grimme and shift manager John Ridinger, spoke with Nuclear News Associate Editor Rafael Guerrero about the upgraded system, eelgrass, and the impacts of both on TVA's largest nuclear power plant.

Why is eelgrass particularly threatening for operations? What makes it stand out from other invasive species?

Ridinger: We have procedures in place for biofouling. Most of the debris that we've seen come in at Browns Ferry is what they call shad runs, where small fish will come in and school up at certain times of the year and in certain weather patterns.

We have traveling water screens, which basically are vertical conveyors with buckets, and they pick up any kind of debris or fish and then dump them over into a sluice trench. Right on the other side of the screens are the trash racks.

In the past, we've seen Japanese milfoil [an invasive Eurasian plant], which kind of floats on top. We were usually able to screen that off.

But one of the properties of eelgrass that made it so challenging is it has a neutral buoyancy. It really wouldn't float on top, nor would it really sink to the bottom. It made it difficult to catch before it got to the screens.

What other invasive species have you dealt with?

Diaz-Lucier: It's just eelgrass, to my knowledge. We still get a little bit of milfoil and other aquatic plants, but mainly it's eelgrass.

I don't know why, but the life cycle [of the eelgrass] ramped up in 2019, 2020. And then in 2021, it seemed like it was already less. In the last year or two, it hasn't been bad. There were times where once or twice a shift, we would be taking the trailer and dumping it and filling it back up. Now we can go sometimes three or four days with only raking once or twice. So it's kind of low right now. But with the new system, we're in a good position, and I don't foresee any severe impacts.

What can the new system do, and how is it different from the process used before?

Diaz-Lucier: Our system is actually first of a kind in a couple of ways. The trash racks are in the vertical position. That gave [contractor WTR Engineering] a little bit of a headache; we've had to deal with some minor tweaks since installing it. Since our talon [rake] potentially swings out beyond the security-protected area as it picks up the trash, there could be a diver who tries to sneak in, so we can only run the system when security is watching it.

It used to take a team of three mechanics to get a crane and two guys on the bucket in order to rake the trash racks. It would take them four to six hours, and it didn't even do that great of a job of removing debris. With this system, it takes one operator about an hour and a half to do the entire screen. There's been an efficiency gain in the time to actually do the raking and cleaning of the screens, and it's more effective, which means we don't do it as often.

How does the rake work?

Diaz-Lucier: The rake itself lines up perfectly with the trash racks such that when you drop that rake, it falls in between big funnels. The wheels and guide sliders fall into that funnel and then follow guide rails all the way down to the bottom. And as it's doing that, it pitches the rake tines back behind the trash rack bars themselves.

The grapple closes at the bottom of the pit and pulls up all the trash versus just kind of scraping it and letting it wash around down at the bottom or get pushed through to the traveling water screens. With this new system, we're able to effectively remove debris from the trash racks and not just scoop up what we can and hope for the best.

Was the new system designed in-house?

Diaz-Lucier: WTR Engineering was the firm we picked. I wasn't actually part of the project team that vetted the various vendors, but we went with WTR because they were the most widely known and they were willing to work with us.

Our trash racks are essentially vertical, and WTR and every other raking company does not recommend having a vertical trash rack. They like one that is at an angle of some 75-80 degrees out into the river or into your forebay.

Even though our forebay is off the river, it's still considered part of the river, such that you would have to do a permit request change, and that's like a three-year process at the quickest. We knew we couldn't change our trash rack physically where it extends into the river, so we went with WTR because they were pretty confident that their system would work, even though it was in a vertical position. We had some challenges when we installed the system, but we worked with the vendor, and we were able to get it knocked out pretty quickly and efficiently.

When was installation and how long did it take?

Diaz-Lucier: We first started seeing eelgrass in the winter of 2019. In the summer of 2020, we realized it might be a big issue. In July 2020, we had two units shut down, and the third unit had to be rated to about 78 percent power-that's when we knew we were in trouble.

We had the new trash racks by April 2021. That spring, we had installed them and pretty quickly mobilized to get up and running.

Toward the end of July 2021 is when the new system was fully installed and we were running through the site acceptance test. And then we had WTR on-site for a week or two to go through everything. I think by August we were up and rolling.

Why is it important to properly maintain Browns Ferry's water intake?

Grimme: In our process, when we're running these plants, once you put steam through the turbine, you have to condense that steam in the condensers. We use river water coming through tubes to condense the steam to cool it off and condense it, and it stays in a vacuum.

Basically, we have set points that trip our turbines and scram our reactors that are at a certain vacuum level. If you can't get enough water in from the river to maintain that cooling effect and condense that steam, then you automatically lose your reactor.

We had that happen on two units and were in the process of lowering power to recover the vacuum on a third unit. We were able to recover it at about 70 percent, where we had enough water coming in that we could maintain that condenser vacuum. So that's kind of what this is all about. There are other systems that rely on river water for cooling, too, but the major immediate impact was that we couldn't cool our condenser and keep the units on line. This is a really big deal for generation, being able to keep our trash racks clean and keep the water flowing through the system.

Diaz-Lucier: Not to get too into the weeds, but the first unit we shut down, we had a known issue at the time, so we were already hurting on that aspect. On Unit 1, we derated to about 45 percent and were actually keeping the unit on line and doing alright until we were out there raking with the old system. As we raked, we were washing a bunch of grass onto the screens, and we had two of the traveling water screens fail on us. When they fail-that doesn't happen often-you have to shut down the pump. When we had to shut down the pump, that led to losing condenser vacuum and having to scram the reactor. It was one of those events that they talk about where "all the holes in the cheese line up."

Is the new system automated?

Diaz-Lucier: It's automated in a way where once [the operator] gets security on-site, he can go and basically tell it to run the clean cycle and it just goes. Or he can manually use a remote control and just go down to this screen and push "cleaning." He'll go and clean that one screen, or he can go and do all 18 by himself, manually telling it to clean each one. It also has an automatic feature where he essentially can push "go" and then either sit there and watch it the entire time or go and drink coffee for an hour and come back.

What results have you seen at this point?

Diaz-Lucier: We haven't had any real issues with eelgrass challenging the units recently. We have had shad runs, like John [Ridinger] was talking about, but eelgrass-wise, we haven't had anything specific to call out.

Guntersville is a dam upstream of Browns Ferry, and they would sometimes "spill,"where they wouldn't send their water to their turbines and would just spill off a bunch of eelgrass. Prior to the 2020 event, there was no communication or very limited communication between Browns Ferry and Guntersville. But then after everything happened, we've had better communication back and forth, also with Wheeler Dam, downstream of Browns Ferry. All three were keeping an eye on the eelgrass situation and talking with natural resources. We were learning as we went, unfortunately, but we did open those lines of communication. To this day, we still have eelgrass notifications that go out weekly, I believe.