TOKYO (AP) -- The operator of Japan's tsunami-crippled nuclear power plant said Tuesday that about 300 tons of highly radioactive water have leaked from one of the hundreds of storage tanks there - its worst leak yet from one of the vessels.
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 valve connected to a gutter around the tank.
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.
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.
Four other tanks of the same design had similar leaks since last year. Ono said the latest leak was the worst from a tank in terms of volume.
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. There have been other leaks of contaminated water at the plant, and some of it is entering the sea. Plant workers are trying to reduce the leaks using measures such as building chemical underground walls, but they have made little improvement.