NRA approves TEPCO's plan to freeze underground walls of soil at Fukushima plant

The Nuclear Regulation Authority (NRA) decided on March 30 to approve Tokyo Electric Power Co. (TEPCO)'s plan to gradually freeze underground walls of soil around the crippled Fukushima No. 1 nuclear plant, starting with shields on the ocean side.

With the NRA's approval, TEPCO, the operator of the crippled Fukushima nuclear complex, is to begin work as early as March 31 to freeze the walls built around the buildings of reactors Nos. 1 through 4 at the plant. The walls are designed to prevent underground water from flowing into the reactor buildings. But such a large-scale "wall of ice" has not been introduced anywhere in the world and it is unclear how much underground water the frozen shields will be able to prevent from flowing into the crippled nuclear complex.

Under the project to build the frozen soil walls, coolant chilled to a temperature of minus 30 degrees Celsius is to circulate through 1,568 pipes that are driven into the ground to a depth of around 30 meters, to create a "wall of ice." The project is aimed at preventing underground water from entering the reactor buildings and reducing the amount of contaminated water being generated. If the project goes as planned, work to freeze the walls is expected to be completed in about eight months. TEPCO estimates that the walls will help the utility reduce the inflow of underground water to several dozen tons per day from the current 150 to 200 tons.

TEPCO is to gradually freeze the walls, starting with the one (about 690 meters) on the ocean side first, while leaving seven sections (a total of about 45 meters) on the mountain side unfrozen. TEPCO had initially planned to freeze all of the walls at once. But if the levels of underground water around the reactor buildings drop drastically, contaminated water remaining in the reactor buildings could flow out. So the NRA called for the gradual freezing of the walls. TEPCO then accepted the NRA's suggestion.

The frozen-soil wall project is considered to be a key measure to deal with contaminated water along with the so-called "subdrain" project designed to reduce the amount of water being contaminated by removing underground water from wells around the reactor buildings. TEPCO started inserting pipes into the ground in June 2014 and completed its preparations to begin freezing the walls in February this year.