Here's the crazy thing about the plan to build an almost mile long, 90-foot deep, subterranean ice wall around the Fukushima nuclear plant: It's not really very crazy at all. Building cryogenic barriers sounds like the specialty of an obscure supervillain, but it's a well-established technique in civil engineering, used regularly for tunnel boring and mining. Ground freezing was even tested as a way of containing radioactive waste in the 1990s at Oak Ridge National Laboratory and performed admirably.
Joe Sopko, the civil engineering firm Moretrench's director of ground freezing, has spoken with several consultants about the details of the project, and he's convinced it's certainly possible. "This is not a complicated freeze job. It really isn't," he told me. "However, the installation, because of the radiation, is."
Ed Yarmak of Arctic Foundations, which installed the system at Oak Ridge, agreed. "It's a large system, but I don't think it's out there, where people can't do it and can't do it efficiently."
Here's the problem this technology could solve. The Fukushima nuclear plant, which was devastated by an earthquake and tsunami in March of 2011, is located on a slope. This fact of topology means that groundwater running down from the Abukuma plateau to the east pass right into the site.
Japanese officials estimate that 400 tons of water reach the plant every day and mix with water used to cool the reactors. That's roughly 96,000 gallons of radioactive water. About 300 tons (or 72,000 gallons) of contaminated water flow out to sea daily, according to Japan's National Resources and Energy Agency.
TEPCO, the Japanese utility, has been trying to deal with these problems, but the volume of water poses a formidable logistical challenge, and some of it continues to leak out to the ocean.
So, what to do? Engineers have been pumping and treating the water, but the scale has made that too difficult and prevented other cleanup work from happening. They've contemplated diverting groundwater approaching the site into the sea or building clay walls, too. But it appears that officials have settled on the ice-wall containment strategy, as suggested by Japanese contractor Kajima.
In news stories, this operation was presented as difficult on account of the scale. "The technology has been used before in the construction of tunnels, but never on the massive scale that the Fukushima plant would require," CNN wrote. Even Cabinet Secretary Yoshihide Suga told reporters, "There is no precedent in the world to create a water-shielding wall with frozen soil on such a large scale."
But the more I dug into ground freezing, the more I realized it was one of those corners of engineering that's been quietly helping the world's infrastructure get built for decades. There are journal articles about it and books, too. There's J.S. Harris' Ground Freezing in Practice and the definitive textbook, Frozen Ground Engineering by Orlando Andersland and Branko Ladanyi. There have been hundreds of ground-freezing projects, an evaluation by the Department of Energy, and dozens of international conferences.
Here's how it works. Freeze pipes, made from normal steel, are sunk into the ground at regular intervals. The spacing is normally about one meter. Then, some type of coolant is fed into the pipes. Sopko uses a brine -- salty liquid which can be cooled far below the freezing point of fresh water without turning into a solid. On the surface, a big refrigerator chills the liquid, which is pumped into the pipes. The liquid extracts heat from the ground, and returns to the chiller, where it is recooled and sent back down. It's not a fast process and can take many months. (Sometimes, for speed's sake an expendable refrigerant like liquid nitrogen is used, but it requires trucking in tanks full of the stuff.)
First, ice forms in columns around the freeze pipes. Then, as time goes on, the ice spreads out, linking the columns. Finally, an impermeable wall forms. For containment, it's important that the ice extend all the way down to the bedrock, so that the walls of ice form a box with the bedrock at the bottom. If an earthquake cracks the ice or the power goes out for a period of times, refrigerating the ground again re-seals the wall.
"You have all this cold frozen soil that water wants to leak through," Yarmak said. "But as the water leaks its way through, it freezes, and the wall heals itself back up."