30 Juin 2016
June 30, 2016
In March this year, Tokyo Electric Power Co. (TEPCO) began work on a subterranean wall of frozen soil mainly on the seaward side of the disaster-stricken Fukushima No. 1 nuclear plant, with most of another wall on the landward side begun in June. The purpose of the barriers is to stop the flow of groundwater into the plant buildings -- a problem that has resulted in enormous volumes of contaminated water. However, three months since the freezing process began, TEPCO is ominously silent on the ice wall's effectiveness, and the plan is quickly approaching its do-or-die moment.
The problem itself is simply put. Every day, some 850 metric tons of groundwater flows down from the mountains and under the Fukushima No. 1 plant property. Some of the water collects in the shattered reactor buildings, coming into contact with melted nuclear fuel and other radioactive substances and becoming heavily contaminated. TEPCO needs to stop the groundwater from getting into these buildings.
In September 2015, the utility started digging a chain of wells called subdrains to catch and drain the groundwater. This is just one of many countermeasures tried so far, including the ice wall. Work on the latter began in June 2014, and eventually 1,568 pipes were sunk along a 1.5-kilometer perimeter around the No. 1-4 reactors and turbine buildings. The plan calls for coolant chilled to minus 30 degrees Celsius to be pumped into the pipes, freezing the soil around them to a depth of about 30 meters and creating a solid barrier.
"Ice walls are often used in public works projects, but the one at the Fukushima No. 1 nuclear plant is by far the largest ever tried," says Mie University associate professor Kunio Watanabe. When building a tunnel, for example, ice walls are used to prevent groundwater from flowing into the construction area after the bedrock has been fractured. In Japan, the method has been used on some 600 such projects since 1962. The largest ice wall ever created was about 37,700 cubic meters, during construction of a subway line in Tokyo. The Fukushima plant ice wall is nearly double that, at about 70,000 cubic meters.
TEPCO tested the method in April 2015, freezing one section of the subterranean wall. To stop contaminated groundwater from flowing into the ocean, the utility started injecting coolant in the pipes on the seaward side and part of the landward wall in late March in an attempt to create about an 820-meter-long subterranean barrier -- or 55 percent of the eventual total length. Saying that the temperatures were dropping according to plan, the utility started freezing operations on most of the remaining landward section at the beginning of June, and now only seven sections totaling 45 meters on the landward side are left.
TEPCO has stated that "the ice wall is going according to plan." However, the Nuclear Regulation Authority (NRA) has pointed out that the volume of groundwater collecting in waterfront wells has not decreased, casting doubt on TEPCO's claim.
At a meeting this month, NRA committee member Toyoshi Fuketa stated, "This is not a wall in a true sense. Perhaps it's more akin to a bamboo screen, with groundwater trickling through the gaps." TEPCO has responded that the quick flow of the groundwater likely makes it hard to freeze the soil in some places, and it is proceeding with work to create cement barriers to slow the water down.
There are also worries that the large volumes of highly contaminated water already collecting in the reactor and turbine buildings could leak into the environment if only the landward ice wall proves effective and the seaward wall has gaps. While TEPCO is looking to expand the ice wall, the NRA has not altered its stance that it must first confirm the effectiveness of the freezing operations already undertaken. The ice wall has already cost 34.5 billion yen in government funds.