09.04.13 8:45 AM ET
Fukushima N-Plant Will Be Surrounded by a Wall of Ice
At first glance Japan’s plan to surround the crippled Fukushima nuclear plant with a mile-long subterranean wall of ice seems like a crazy, last-ditch gambit by a Tepco employee turning to Game of Thrones for inspiration. But the technique isn’t as farfetched as it sounds. Engineers have been building underground ice walls for over a century, and using one to contain radioactive waste makes a lot of sense, though building it won’t be easy.
For a precedent for Fukushima’s ice wall you don’t need to look to fantasy, but to the 19th-century mining industry. Digging a mine shaft is a never-ending battle against groundwater, which is constantly flowing into the mine and threatening to collapse the walls and flood the project. You can try to pump it out faster than it comes in, or you can dig another shaft to divert it—or you can freeze everything in place.
In 1863 German scientist F.H. Poetsch patented a method of driving metal pipes full of super-cold brine (saltwater can go below 32 degrees without turning to ice) into the soil, freezing the surrounding ground solid as concrete. This allowed miners to dig in peace.
The 1905 guide An Elementary Class-Book of Practical Coal-Mining touts its effectiveness in digging mine shafts through quicksand. “The principle of the Poetsch system is to freeze the ground around the shaft into a solid block, the effect of this being to consolidate the sand and hold the water back whilst the shaft is being sunk and lined through the wet ground.” The mining industry uses essentially the same technique today.
The reason an ice wall is appealing in Fukushima is because the plant’s most pressing problem right now is groundwater. When the tsunami hit two years ago and the reactors melted down, Tepco cooled the overheating fuel rods by flooding them with millions of gallons of water. That water then became contaminated and had to be stored in tanks, tanks that then began to leak into the soil, and ultimately into the sea. Compounding the problem is the 400 tons of groundwater that flows down the neighboring hillside each day and into the damaged reactor buildings. That water also has to be pumped out and put in tanks, and storage space is running out. There are now about 1,000 tanks at the plant, but water continues to flow in—and, alarmingly, out to sea.
The ice wall proposed by the contractor Kajima Corp. would block incoming groundwater while sealing in the radioactive waste that’s already in the soil. It would run about a mile in length, encasing four reactor buildings, and extend down to the clay 100 feet below the surface. Coolant-filled pipes would be driven into the soil at regular intervals—probably every three feet or so—and hooked up to refrigeration units on the surface. Once the system is powered on, coolant flows to the bottom, extracts heat from the soil, and flows back up to be cooled again. As more heat is extracted, the water in the soil freezes in a column surrounding each pipe, eventually converging to form a solid barrier. Once frozen, an ice wall can be as strong as concrete, but its real strength is that it’s self-healing: whenever the soil shifts and a crack forms in the wall, any water that flows in or out will freeze, plugging the leak.
Only in recent years have ice walls been used to contain hazardous wastes. In 1995, after a leak was discovered in the salt dome encasing the National Petroleum Reserve at Weeks Island, Louisiana, a freeze wall was constructed to seal off the site while the oil was pumped out and relocated. At Oak Ridge National Laboratory (PDF) in Tennessee, the Department of Energy made a 300-foot-long freeze wall to contain a pool of radioactive material. Completed in 1998, that wall blocked waste for six years, until regulators ordered DoE to clean up the site. Several years ago, Canada began testing an ice barrier at the arsenic-tainted Giant Mine near Yellowknife.
Ed Yarmak, president of Arctic Foundations, which built the Oak Ridge wall, says the engineering isn’t difficult, especially compared to other nuclear remediation techniques. Instead of digging a trench and filling it with grout, or racing to set up enough pumps and filters to catch shifting groundwater, you can drive pipes into the soil, set up a refrigeration unit, and flip a switch to freeze the contaminants in place. When you’re done, you can turn it off and the soil goes back to being more or less like it was before.
Once Oak Ridge’s ice wall froze, energy costs were low, about 100,000 kWh a year, or about as much energy as 10 U.S. homes consume annually. They used a 30 horsepower refrigeration unit to keep the wall frozen and spent about $15 a day to power it. It takes a lot of time and energy to push water below the freezing point, but once it’s frozen, it doesn’t take much to keep it cold. Even when the refrigeration unit was switched off for a week, to test how it would fare in a power outage, surface temperatures never got above 32 degrees.
As for scaling up from 300 feet to more than a mile, Yarmak doesn’t think that will pose much of a problem. Far larger ice walls have been designed, though for mining and construction, not for waste management. Equipment for a three-mile wall was built for a gold mine in Ontario, but the price of gold dropped and the project was mothballed before the wall was activated. The engineering company Moretrench is currently testing a 500-feet-deep barrier for tar sands excavation in Alberta. “The size of the wall in Fukushima isn’t unprecedented,” says Yarmak.
Not that construction at Fukushima will be easy. Yarmak says the hardest part of the Oak Ridge project was keeping workers safe and preventing the spread of nuclear contamination. They had to stay on a patch of asphalt that had been put down to keep rainwater from bleeding into the contaminated soil. “It looked like a parking lot, like any place you’d go work, but you couldn’t walk in the woods,” says Yarnak. The trees, he said, had taken up radioactive water, so every morning he had to take a leaf blower to his machines to keep them from being contaminated. Whenever holes were drilled for pipes, the extracted soil had to go directly into a sealed box, which then had to be put in a special containment area. Even the air from the drill had to go through a filter. “Engineering-wise, Fukushima isn’t a big deal,” says Yarmak. “It’s getting the thing done safely that’s going to be a big deal.”