The Japanese government unveiled a plan Tuesday to keep radioactive water at the stricken Fukushima Daiichi nuclear plant from contaminating ground water that flows into the sea, in large part by surrounding the plant with an underground wall of frozen soil.
By some accounts, the $470 million plan’s reliance on frozen soil represents a use of the technology on an unprecedented scale.
At the same time, some nuclear-energy analysts suggest, the ad-hoc effort to deal with contaminated water highlights a shortcoming in the industry more broadly.
Much attention had been focused on making sure plenty of water reaches a reactor to keep it cool during an accident, they say, but relatively little thought has been given to how to manage that contaminated water.
The plan for Fukushima was unveiled following the Tokyo Electric Power Company's admission on Aug. 20 that some 300 tons of radioactive water had leaked from storage tanks built to hold the water, which has been used to cool the plant's reactors and highly radioactive spent fuel since March 2011. This followed an admission a month earlier that contaminated water from the plant was leaking into the sea.
At the same time, some 400 tons of groundwater are said to be flowing into the plant's environs each day, where it seeps into the damaged buildings and mixes with contaminated water before making its way to the sea.
The sea-side power plant, located at Okuma on the east coast of Japan's largest island, Honshu, was devastated by a series of safety-system failures following a magnitude 9 earthquake offshore that sent a 45-foot tsunami sweeping across the nuclear plant as it moved inland. Follow-up investigations showed that the utility underestimated the tsunami hazard the plant faced, leading to design flaws that left it more vulnerable to tsunamis than it might otherwise have been.
The plant was built above sea-level, but the tsunami still covered the site with up to 16 feet of water as it moved inland. All of the main and back-up systems designed to provide power to the plant's cooling systems failed. Hydrogen gas built up in three of the plant's six reactor buildings, triggering explosions that also damaged a fourth reactor building.
The explosions and subsequent fires released a radioactive plume that within three weeks of the accident was detected over southern Spain. The plant's owner, Tepco, has been pouring water over the damaged reactors and the spent-fuel pools ever since to keep heat from doing further damage to the reactors and spent-fuel assemblies.
The plan the government announced Tuesday would spend $150 million on equipment to remove virtually all of the contaminants from the water. But the bulk of the $470 million would pay for the construction of a frozen, subterranean wall around the plant stretching for some 1.4 miles. Japanese officials say they expect the wall to be finished by March 2015.
For more than 100 years, engineers have frozen soil as a way to prevent cave-ins at mines or excavation sites, or to prevent water from seeping into excavations. In the Arctic, artificially frozen soil have been used to shore up foundations affected by melting permafrost. More recently it has been used to curb the flow of pollutants.
The technique has been used at least once on a much smaller scale to isolate water in a contaminated pond at the Oak Ridge National Laboratory, starting in the late 1990s. The pond cooling water for a prototype reactor built in the early 1950s as the Atomic Energy Commission was sorting among the designs it would license for civilian use.
The pond was leaking water contaminated with tritium, contaminating area streams, recalls Dirk Van Hoesen, a former environmental clean-up manager at the lab who retired last year. The lab hired a company out of Alaska to set up a frozen barrier around the pond.
"As I recall, it was pretty effective at hanging on to the tritium, which is pretty tough stuff to hang on to," he says. "It worked."
The wall remained in place until the mid '00s, when the lab cleaned up the pond, Mr. Van Hoesen says.
The approach involves sinking pipes into the ground at closely space intervals, then pumping coolant into them, explains Larry Applegate, president of SoilFreeze, a Seattle-based company that sets up and maintains frozen underground walls of soil. Typically, the pipes are driven into rock or clay layers impervious to flowing water. At Fukushima, that means driving the pipes roughly 100 feet deep.
Once coolant begins circulating, the soil freezes in an expanding circle around a pipe until it joins with the expanding frozen circle generated by it neighbors. The result is a continuous wall of artificial permafrost. While the initial freeze-up is energy intensive, requiring a large number of chillers, once the wall is established, only about half of the original number of chillers is needed to keep the wall intact, Mr. Applegate says.
The walls are fairly immune from power failures at the surface because is takes a long time to thaw frozen soil deep underground. If an earthquake strikes, any crack in the wall can be patched fairly quickly, sometimes automatically by liquid trying to flow through the crack, which can freeze as it encounters the frozen soil.
Applegate notes that if the Fukushima project sounds large, the technique also is being considered for corralling effluent from tar-sands oil extraction in Canada.
At Fukushima, the wall would hold any water leaking from the plant or storage tanks on site for remediation. It also would force flowing groundwater around the plant area, rather than through it.
While the technique has potential, however, it's unclear if the Japanese government or Tepco have a sufficiently accurate picture of the groundwater flows at the plant to put the wall in the right place, cautions Edwin Lyman, a senior scientist who focuses on nuclear issues at the Union of Concerned Scientists in Washington.
"Fukushima itself is an unprecedented event," he says. "There's never been a situation where you've had this great a volume of radioactively contaminated water that doesn't seem to be controlled. They are in uncharted territory," he says.
The on-going crisis at Fukushima underscores what he sees as an insufficiently addressed element in dealing with a nuclear accident: keeping contaminated water from reaching a wider environment.
Local hydrology comes up when a potential site for a nuclear plant is considered, Dr. Lyman says. But he adds that despite the nuclear industry's assertions that it's adequately addressed the safety issues Fukushima raised, no one is thinking about preparations for long-term water management in case an accident occurs.
With a severe accident, "the strategy is still: get as much water as you can into the reactors for a long period of time," he says. And reactor designers have been striving to develop designs that deliver the water more reliably without much need for human intervention.
"They're strengthening their ability to get water in, but they're not thinking about what they're going to do with the overflow and the effluent," he notes. "There needs to be more-serious attention given to post-accident water management, not just focusing on getting water into the reactors."