BEIJING -- The operator of the
Still, it was a serious setback in the efforts to contain and clean up the damage that crippled the power plant in March 2011, the world's worst nuclear disaster since Chernobyl.
About 1,000 tanks have been built on the ground around the plant complex to contain radioactive water that had been used to cool its damaged nuclear reactors. Three nuclear reactors melted down after the tsunami knocked out cooling systems.
On at least four previous occasions since the initial disaster, workers have discovered leaks in the tanks, though none as serious as the leak reported Tuesday.
"We are extremely concerned," Hideka Morimoto, a spokesman for Japan's Nuclear Regulation Authority, was quoted as telling reporters Tuesday in Tokyo.
A spokesman for Tokyo Electric said workers had put concrete barriers and sandbags around the damaged tank to contain radioactive water.
Japanese Prime Minister