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 Tokyo Electric Power Co. said the contaminated water leaked from a steel storage tank at the wrecked Fukushima Dai-ichi plant. TEPCO hasn't figured out how or where the water leaked, but suspects it did so through a seam on the tank or a valve connected to a gutter around the tank.
TEPCO said that because the tank is about 100 meters (330 feet) from the coastline, the leak does not pose an immediate threat to the sea. But Hideka Morimoto, a spokesman for Japan's nuclear watchdog, the Nuclear Regulation Authority, said water could reach the sea via a drain gutter.
Four other tanks of the same design have had similar leaks since last year. The incidents have shaken confidence in the reliability of hundreds of tanks that are crucial for storing what has been a never-ending flow of contaminated water.
"We are extremely concerned," Morimoto told reporters. He urged TEPCO to quickly determine the cause of the leak and its possible effect on water management plans.
TEPCO spokesman Masayuki Ono said the leaked water seeped into the ground after largely escaping piles of sandbags added to a concrete barrier around the tank.
Workers were pumping out the puddle and the remaining water in the tank and will transfer it to other containers, in a desperate effort to prevent it from escaping into the sea ahead of heavy rain predicted later in the day around Fukushima. By Tuesday afternoon they had captured only about 4 tons of the 300, Ono said.
The water's radiation level, measured about 50 centimeters (2 feet) above the puddle, is about 100 millisieverts per hour -- five times the annual exposure limit for plant workers, Ono said.
The plant suffered multiple meltdowns following Japan's massive earthquake and tsunami in March 2011. Hundreds of tanks were built around the plant to store massive amounts of contaminated water coming from the three melted reactors, as well as underground water running into reactor and turbine basements.
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 Aside from the tanks, contaminated water that TEPCO has been unable to contain continues to enter the Pacific Ocean at a rate of hundreds of tons per day. Much of that is ground water that has mixed with untreated radioactive water at the plant.
The water that leaked from the tank had been partially treated, with cesium and salt removed, before being stored.
Ono said the latest leak was by far the worst from a steel tank in terms of volume. The previous four cases involved leakage of only up to 10 tons.
TEPCO says the tanks that have leaked used rubber seams and were intended to last about five years. Ono said TEPCO plans to build additional tanks with welded seams that are more watertight.
The massive amount of radioactive water is among the most pressing issues affecting the cleanup process, which is expected to take decades.
The contaminated water is recycled as reactor cooling water, but its volume grows by 400 tons a day because of underground water inflow. TEPCO plans to secure storage facilities capable of holding 800,000 tons more water by 2015.
To reduce leaks unrelated to the tanks, plant workers are using measures such as building chemical underground walls along the coastline, but they have made little improvement so far.
[Associated
Press; By MARI YAMAGUCHI]
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