INSIGHT: Frozen soil wall at Fukushima plant riddled with uncertainties

September 27, 2013

By TAKASHI KAMIGURI/ Staff Writer

An ambitious plan to create a frozen underground soil wall at the crippled Fukushima No. 1 nuclear power plant to contain contaminated groundwater is facing hurdles, most notably its durability and cost.

An estimated 400 tons of groundwater is flowing into the four reactor buildings daily, where it is mixing with water already contaminated with radioactive materials from melted nuclear fuel.

The Nuclear Energy Response Headquarters on Sept. 3 decided to surround the reactor and turbine buildings with the underground wall of frozen soil to prevent groundwater from flowing into and out of the structures.

The government plans to spend 32 billion yen ($320 million) on construction of the wall, which it hopes to complete by March 2015.

The proposal calls for installing rows of underground ducts at 1-meter intervals where coolants will circulate, freezing the soil. The wall will reach down to an impermeable layer dozens of meters below the surface and run 500 meters north to south and 200 meters east to west.

A major challenge is whether the frozen wall can be maintained over an extended period.

The same technology has been used in tunneling and other civil construction projects in locations where groundwater is abundant.

But at a meeting of the industry ministry’s committee working on the disposal of contaminated water, members said previous walls were only in place for a maximum of 18 months or so.

Pipes, pumps and other equipment may eventually require replacement due to the corrosion caused by the coolants. The 32-billion-yen budget does not include operating and maintenance costs for the cooling system.

Some members noted the possibility that groundwater may still end up seeping even deeper when the flow of water is blocked.

The wall may also experience breaches in locations where groundwater currents are strong.

Areas near concrete water conduits and pipes are considered vulnerable because soil around some objects does not freeze as easily.

At a meeting of the industry ministry panel in April, general contractors proposed a number of methods to prevent groundwater from flowing into the buildings, such as erecting underground concrete barriers and using chemical agents.

The frozen soil wall technique was proposed by general contractor Kajima Corp. A Kajima subsidiary is one of only two Japanese companies that possess the expertise to build the frozen wall.

In terms of cost, the technology was the least competitive because the wall must be maintained by the use of a cooling system. One advantage cited by members was that the wall can be restored quickly if a major earthquake causes a breach.

Kajima and the industry ministry plan to build a working model of the wall to identify potential problems and eventually solutions before primary construction begins.

Masafumi Yokemoto, a professor of environmental policy at Osaka City University, said the government taking the lead in dealing with contaminated water at the nuclear plant is one thing, but public funds being used to implement countermeasures is another.

“If taxpayers money is kept pouring into something when no one knows how much it will cost, it becomes ambiguous who should bear responsibility,” he said.

By TAKASHI KAMIGURI/ Staff Writer
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