It's more than three years since the magnitude-9.0 earthquake and tsunami that struck Japan, killing more than 15,000 people and wiping out towns and villages along the country's eastern coast. One consequence is that the name Fukushima has come to signify nuclear disaster, joining Chernobyl and Three Mile Island as synonymous with the failed promise of nuclear power as cheap, clean and safe, regardless of whether the meltdown of three of its six cores was preventable if the company that built the plant had built a sea wall high enough.
Repairing the damage requires, in large part, trying to keep the situation from getting any worse. There is a 20km radiation exclusion zone around the Fukushima Daiichi Nuclear Power Plant, which experienced something like ten to 20 per cent of the radioactive fallout that the countryside around Chernobyl experienced, and that radiation has been seeping into the groundwater since the meltdown. That groundwater has been, in turn, seeping into the ocean, irradiating seawater that has travelled as far as North America - though it must be stressed that radiation levels in seawater pose no danger whatsoever once it reaches the other side of the Pacific thanks to the short two-year half-life of Caesium-134. Nevertheless, officials from Japan's Nuclear Regulation Authority (NRA) understandably want to limit the amount of radiation that makes it into the groundwater as much as possible, as part of the Sisyphean effort to contain Fukushima's fallout.
What complicates matters, however, is that the water that was desperately pumped into the three reactors that were undergoing meltdown in an attempt to cool them has begun to leak out from the basement walls. The Tokyo Electric Power Company, or Tepco, has admitted that between 300 and 400 tonnes of groundwater is contaminated every day by leaks of highly-irradiated water from containment tanks, at levels 244 times higher than considered safe for a normal nuclear plant in safe operation. It has started to divert the groundwater by pumping it out of the ground in the hills, before it reaches the nuclear plant, and testing to ensure that it's clean before releasing it into the ocean. That alone isn't enough, though, so a further plan is being trialled: ice walls.
The Japan Times reports that the NRA has given the go-ahead to Tepco to start the first trial of freezing the ground around the building containing reactors one through four at Fukushima. A 1.5km-long series of pipes is being dug between the hills and the plant, in the path of the groundwater flow. Coolant pumped into them will freeze the soil solid, creating an impermeable barrier that groundwater is forced to flow around. At a total cost of $470m, it isn't cheap - and it's the first time such an idea has been tried with nuclear fallout. It's believed that, even if up to 16mm of sinking is expected, it will not threaten the plant's foundation strength.
Freezing the earth is a tried-and-tested method in one field, however - tunnel construction. Both London and Paris have benefited from it in the construction of their subway systems. In Paris, civil engineer Fulgence Bienvenüe was, in 1908, faced with the difficulty of building what was meant to be Line 4 of the Paris Métro along the banks of the Seine, beneath ground that is as much water as it is mud and sand. Bienvenüe's genius idea was to freeze the ground solid using giant cooling pumps, allowing workmen to dig it out like it was solid rock. Workers could then lay tracks and signalling equipment inside the dry tunnel as the mud thawed on the other side. During the construction of the Jubilee line extension in London, in the 1980s, engineers were inspired by Bienvenüe when trying to figure out how to stop Big Ben falling over while tunnels were dug beneath the Houses of Parliament - only this time, they sprayed concrete out through hundreds of pipes beneath the ground, instead of temporarily freezing it.