30 Mars 2016
March 30, 2016
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.
By HIROMI KUMAI/ Staff Writer
The Nuclear Regulation Authority gave the go-ahead to Tokyo Electric Power Co.’s plan to freeze the soil around the reactors at the crippled Fukushima No. 1 nuclear power plant from the seaside on March 30.
The aim of the frozen soil wall is to block the flow of groundwater into the reactor buildings to prevent it from becoming contaminated with radioactive substances.
The utility has already inserted 1,568 pipes to a depth of 30 meters in the ground around the No. 1 to No. 4 reactor buildings. The plan is to circulate liquid with a temperature of minus 30 degrees through the pipes to freeze the surrounding soil.
TEPCO’s plan is to first freeze the entire wall on the seaside and about half of the wall on the mountain side.
The effects of completing the frozen wall on the seaside are expected to show after about six weeks with water being prevented from flowing through. Then, the frozen portions on the mountain side will be gradually increased. When 95 percent of the wall is frozen, TEPCO will suspend the freeze, leaving cracks in seven places to allow some water through.
The utility predicts that with 95 percent of the entire soil wall frozen, about half of the groundwater will be blocked.
To freeze the entire wall on the mountain side, TEPCO will have to gain further approval from the NRA.
Initially, the electric power company planned to freeze soil only on the mountain side. However, the NRA pointed out that if groundwater is totally blocked from the mountain side, the level of water within the frozen soil near the reactors could become too low and with nothing outside to stop it, highly contaminated water inside the reactor buildings could more rapidly flow out.
Because of that, TEPCO decided in February that it will freeze the soil mainly from the seaside and collect data on the level of groundwater and, after that, it will freeze the entire wall.
“It is important to collect sufficient data in a continuous manner and implement the freezing while keeping watch,” said NRA chairman Shunichi Tanaka.
The plan to create the frozen soil wall was developed by an economy ministry committee in May 2013 as an important part of measures to decrease the volume of contaminated water. The work to insert pipes into the ground was completed in February.