Tokyo Electric Power Co. suspects two leaks of radioactive water at the Fukushima No. 1 nuclear plant were caused by shoddy workmanship to install devices to detect such spillage.
The latest problem at the stricken plant suggests that the defect could cause leaks at the five other underground water storage tanks because they all have the same structure.
TEPCO, operator of the plant, said April 7 that radioactive water leaked from the No. 3 storage tank. It earlier confirmed that at least 120 tons of contaminated water leaked from the adjacent No. 2 storage tank.
The utility has yet to confirm how the leaks occurred, but it said it suspects a breach where water-shielding sheets had been connected or damage to the sheets.
It noted that when a leak detector is installed, an opening is made in the sheets. If the sealing is inadequate, the opening could widen when the sheets come under the weight of the water.
Storing contaminated water has become a serious challenge for TEPCO at the Fukushima No. 1 nuclear plant as it continues to cool the melted fuel and spent fuel rods in preparation for decommissioning the reactors.
TEPCO has set up facilities that can together store 325,000 tons of contaminated water at the plant, including 58,000 tons at the seven underground tanks.
The storage facilities already hold more than 270,000 tons, but the amount of radioactive water is increasing by 400 tons daily.
“It is extremely difficult (not to use the underground tanks),” Masayuki Ono, acting general manager of TEPCO’s Nuclear Power and Plant Siting Division, said April 7. “We have been able to hold (the radioactive water) only by continuing to build tanks.”
Shunichi Tanaka, chairman of the Nuclear Regulation Authority, ordered TEPCO on April 7 to monitor the leaked water and prevent it from flowing out of the premises.
The Fukushima prefectural government asked the utility to review its radioactive water storage plan, including shifting water from the underground tanks to those above ground.
TEPCO has been moving radioactive water from the No. 2 tank, which stores 13,000 tons, to the No. 1 and No. 6 tanks since April 6. The work is expected to be completed as early as April 11.
But the company is only draining some of the water from the No. 3 tank, which holds 11,000 tons, and is monitoring its conditions.
“We are giving priority to the No. 2 tank, whose conditions are worse,” a TEPCO official said. “We are not leaving the No. 3 tank unattended.”
The No. 3 tank is 56 meters long, 45 meters wide and 6 meters deep. It stores water used to cool melted fuel inside nuclear reactors that is then treated by a cesium adsorption system.
The water contained about 290,000 becquerels of radioactivity per cubic centimeter, roughly half the level of the untreated water that has accumulated in the reactor buildings.
TEPCO detected a small amount of radioactive materials outside the No. 3 tank on April 6. It examined water contained between water-shielding sheets and found 2,200 becquerels of radioactivity per cubic centimeter.
The company also said April 7 it believes that radioactive water began leaking from the No. 2 tank around March 20.
The company has been monitoring water levels and radioactivity concentrations outside the tank. But only after the leak was detected did TEPCO find that water levels had started falling around March 20.
There were also signs of changes in radioactivity concentrations on March 20, but the company did not notice them until April 3.
The tanks are lined with three layers of water-shielding sheets: two sheets of polyethylene and the outer sheet of clay.
TEPCO says the leaked water will not spread extensively because cement has been mixed into soil around the tanks. But contamination could affect the entire area if leaked water mixes with groundwater.
Groundwater has been flowing into the reactor buildings, creating 400 tons of radioactive water daily.
TEPCO planned to pump up groundwater and release it into the sea before it enters the reactor buildings. That plan would be meaningless if the groundwater is already contaminated.
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