Underwater Welding In Nuclear Power Plants
Diving welders keep cool heads in hot water
BY ROSS HANCOCK
On top of the everyday dangers of welding, imagine welding in a deep, narrow pool of water, where your accustomed realities of gravity, light, and sound are all distorted. Imagine the water is hotter than your body temperature, subjecting your brain to heat stress while you concentrate on performing a precision weld. Now add time pressure: every second costs the utility company thousands of dollars. Every second brings you closer to heat exhaustion. Every second exposes you to accumulated alpha, beta, gamma, and neutron radiation.
It takes a special breed of welder to plunge deep into a nuclear reactor pool filled with 100°F-plus, radioactive water to make a weld repair - while the reactor is running at peak power. But the economic reality of the nuclear power industry, where an hour or so of outage can cost half a million dollars, demands drastic measures. It's all part of the job of the underwater nuclear welder to make this extraordinary situation as routine and professional as possible.
This is no job for the faint of heart, but neither is it a job for the reckless daredevil. To be a nuclear plant diver in the United States, one must be cleared by the FBI, undergo drug and alcohol testing, and pass a psychological screening. These criteria are above and beyond welding certification, diving certification, and special training required of all nuclear plant personnel.
Nuclear diving was pioneered in the 1970s, when the first nuclear power plants were reaching middle age. Plant engineers found they could perform minor underwater maintenance activities during scheduled refueling outages. Those involved nonwelding operations such as replacement of steam plugs, as well as quick welding repairs to the transfer carts used to move fuel rods. Soon, more elaborate operations were being performed, such as reinforcing fuel pool liners with large stainless steel plates and modifying spent fuel storage racks.
In a sense from a UCC training video, a diver works on a spent fuel rod assembly.
A typical reactor produces about 20 metric tons of spent, but highly radioactive, uranium every year. This uranium, encased in spent fuel rods, is often stored in steel-lined concrete vaults filled with water. As the plants run out of spent fuel storage space, they employ underwater welders to "re-rack" the old rod storage assemblies, making them more space-efficient.
In some ways, it is safer to work on fuel rod racks and other plant projects under water than it would be to drain the containment pool. Radiation is well shielded by demineralized water, so a commercial diver working in a nuclear environment can often perform a given task with less exposure than a worker in the same environment if it were "de-watered."
"Nuclear reactor diving is the best diving, the safest diving," said Kyle Wilkins, a nuclear diver and project manager for Underwater Construction Corporation (UCC) in Essex, Conn. He pointed out that there is unlimited visibility and no marine life in the highly purified water.
The nuclear diver can never be complacent, however. Special procedures and equipment for underwater dosimetry have been developed. Electronic radiation monitors are worn on several places on a diver's body, carefully monitored by the "Rad Man," a technician above water who has the authority to terminate the dive. Measurement of exposure in a diving project conducted by American Inland Divers in the 1990s, involving repair to a plant's fuel transfer cart, found that a diver received only 150 millirems of exposure from materials that would have involved an exposure of 10 rems in the open air.
A hyperbaric nuclear mockup tank at UCC can simulate a depth of 95 feet.
Exposure is a function of time, distance, and shielding. The shielding offered by the water environment must be complemented by the diver working almost as fast as a conventional welder would. Proper planning helps reduce exposure time and can help a diver control the amount of time spent near radiation sources that are incidental to the specific mission.
Significant radiation exposure is a fact of life, but the decision to use a commercial diver instead of plant welders is not seen as sacrificing the diver since the diver does not receive the daily exposure that a plant worker might, and thus would not normally exceed mandated exposure limits.
Another form of exposure for nuclear divers is heat. The water in some nuclear sites exceeds 100°F. Safe immersion at that temperature is limited to less than 45 minutes. It can be very costly for a plant to cool down its containment pool water, so divers must limit their time below surface. Working in conjunction with space suit engineers, UCC has developed a diving suit system cooled by a line carrying 40°F water that extends diving in 120°F reactor pools to more than an hour.
"That makes the dive more comfortable and less fatiguing," said Wilkins. "But it's still a demanding job." Even though the diver's body is cooled and his or her vital systems are being monitored topside, the spun brass diving helmet still gets hot and so does the breathing air.
Plants often require divers to perform mock-up training for a particular job in a simulated underwater work site. Underwater Construction Corporation has its own training tanks where it builds elaborate simulated work sites to practice specific projects to be conducted during planned outages. These rehearsals are performed with actual welding in the required positions at the depth of the upcoming job.
A UCC diver wearing a nuclear diving helmet inspects a weld in the torus vessel of a nuclear plant.
"You have a scope of work to do in a short timeframe," said Wilkins. "We'll spend three to four months preparing for a 20-day job." Most of his company's work involves shielded metal arc welding (SMAW) of stainless steel, though some old plants require repair and modification of high-carbon steel. Mechanical cutting is performed with pneumatic band saws and cutting wheels. Handheld plasma cutting torches have recently come into vogue.
Underwater repairs and modifications are extremely difficult, Wilkins said. He was a welder by trade who went on to become a commercial diver.
"Others are divers who have been exposed to welding in the field," he said. These welding novices have to become welding pros, first by mastering topside SMAW, then by undergoing extensive training offered by nuclear contractors.
Rob Murray, training director for Divers Academy International, Camden, N.J., said commercial diving schools provide general preparation for a nuclear diving career, including training for contaminated water, but nuclear plant diving is so specialized that the contractors provide the ultimate training expertise.
"The diving training is the vehicle that gets you there," said Murray. "And the more of a welding background you bring with you, the easier the transition will be."