Divers Are Why Your Power Stays On in August

By Walt Kesler, Power plant maintenance manager, nuclear-qualified. Reviewed by Dana Whitcombe, technical reviewer.

Cathodic Protection & Anodes | Offshore & Subsea Diving August 12, 2016. The intake bay in Waterford, Connecticut. The air was 98 degrees, the water 78. A patch of hydrilla and plastic bags had half-blocked the traveling screen. The differential pressure was climbing toward the trip setpoint. On that day, the plant was at full load supplying air conditioning to New England. We had two options: trip the pump and derate the unit, or send the divers in. The dive team was on site. We signed the permit, they went over the rail and into the grease and murk. Forty minutes later, the screen was clear. The differential dropped to three inches. Nobody in the control room applauded. They just dialed the output back up. That's the truth about summer reliability: most people never think about the divers, but the divers are why their lights stay on.

The parts of the plant that live underwater

The intake structure is the plant's mouth. It pulls water from a river or sea to condense steam. That water goes through trash racks, then through traveling screens, then into pumps, and then through the condenser water boxes. The condenser tubes are the heat exchanger. If anything clogs that path, the plant loses efficiency or trips. Marine life loves those dark, warm spaces. Zebra mussels, Asiatic clams, even barnacles start with larvae that settle and then grow. A mat of mussels can narrow a tube by half. The only economical way to see and clear that is with a diver. We use remotely operated vehicles for some checks, but they don't have hands. A diver can identify, photograph, clean, and cut away. In nuclear, we have strict requirements for trained divers. It's not just a guy with a wetsuit. They are qualified to work in safety-related systems, which means paperwork, but also means reliability.

What an underwater work day looks like

A typical dive is surface-supplied, not scuba. At a plant you need a tender, a standby diver, and a supervisor who can talk to the control room. The diver uses a full-face mask with communication, and maybe a camera that feeds to a monitor on deck. In the intake bay, visibility is often zero. You work by touch and by hand signals that are vocal, because you hear the comms in your ear. The temperature dictates the suit. In August, the water in the Northeast is maybe 75 degrees, so a wet suit is fine. In January, you're in a dry suit with thermal layers, and we preheat the dive location with space heaters. The job starts with a permit, a lot of paperwork, and a specific game plan. The diver goes down, feels each trash rack slot, clears the debris, and collects samples of what's growing. Samples go to our chem lab. That tells us if a biological wash is due. The diver also inspects for cracks in the concrete and checks the seal around the intake screens. That often saves us from a longer outage later.

Why August is the breaking point

In August, three things hit at once. First, the river or sea is warmest. Warmer water carries less dissolved oxygen and that slows the growth of organisms, but it also speeds up their metabolism, so they filter and grow faster. It's a race. Second, the water is often lower, so the debris is more concentrated. And third, the demand is at its peak: every air conditioner in the region is running. A small thing, a single clogged screen, can force a unit off line. When that happens, the grid operator has to bring on an old peaker plant or ask for voltage reduction. The diversity of the generation fleet is what keeps the grid stable. Our divers are part of that diversity. They are not a luxury; they are a planned tool. We have a standing contract for August, with dive teams on call. We also schedule preventive dives in July, so that August is just monitoring. The dive team knows their role. They show up at 5 a.m. Before the heat spike. They are in the water by 7. That is the season's rhythm.

The day I depended on a dive I didn't schedule

A few years back, we had a condenser vacuum problem. The tube sheet was fouled with mussels and we had to backwash, but backwashing only dislodged the shells. The real issue was on the tube interior, and the tube was too narrow for a brush on a rod. Our options were a chemical cleaning with a nasty stabilant, or a diver with a hydrolance. The diver went in, connected a lance to a high-pressure hose, and shot water through each tube that was plugged. It took two days, but we avoided a mini-outage. The reason I didn't schedule it was that the previous week's inspection used a camera, and it showed a clean tube. The camera missed the few tubes that had a partial blockage. I learned something: a camera is a tool, but a diver is a sensor. He can feel the difference in flow. He can tell you about the texture of the deposit. That's an advantage you can't put a price on. It did cost us an unplanned dive, but we had the diver call. That was cheap compared to a three-day forced outage in August.

What a young plant manager should hear

If you're new to this, schedule your dive inspections for late spring and late summer. In spring, you establish the baseline. In late summer, you see what the growth season did. Don't wait until a screen trips. The cost of a dive team is a fraction of a derate. Make friends with your dive contractor when they are not busy. Then when they're needed, they'll take your call at midnight. And always, without exception, test the comms before the diver enters the water. You don't want to be asking a man to clear a trash rack while you can't hear anything. The other thing is to invite a grid operator or plant manager to watch a dive. They will never think about a screen the same way. They will learn to respect the men and women who keep the plant breathing.

Frequently Asked Questions

Do divers work in the intake while the plant is generating? Yes. We isolate one pump or one screen train, place a permit, and verify that the intake gate is closed. The divers work in a protected space with zero flow. The plant can stay at full output on the other pumps. It's planned and safe. How does a diver know where to go when the water is black? They are in constant voice contact with the surface. They follow handrails and guide lines that are pre-set. They also use a fiber-optic camera and a small light on the helmet, but when visibility is zero, they work by touch, following the structure and counting bolt holes. Is this dangerous? It can be, which is why the safety rules are non-negotiable. In nuclear plants, the divers are qualified to work in a radiation control area, and the water is constantly monitored. The biggest risk is entanglement or crushing, so we have a standby diver in the water at all times, and a crane to lift the screen.