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Fukushima: Japan plans to corral radioactive water behind frozen dam

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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.

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