Another leak at the Fukushima No. 1 power plant has been pouring radioactive water filled with high amounts of cesium into the Pacific Ocean for an unknown period of time, but Tokyo Electric Power Co. was able to plug the leak.
Radioactive materials are also apparently penetrating the silt fence installed in the sea near the crippled plant, raising concern that a wider area of the Pacific may be contaminated.
The leak was found Wednesday at a pit close to the seawater intake for the No. 3 reactor. Seawater sampled in the area found concentrations of cesium-134 at a 32,000 times the permissible level and concentrations of cesium-137 to be 22,000 times the limit, Tepco said Wednesday.
The water in the pit contained cesium-134 at 620,000 times the legal level and cesium-137 at 430,000 times the limit, it said.
The radiation on the water's surface was giving off scorching readings of 1.5 millisieverts per hour.
The leak was stopped Wednesday night after filling the pit with concrete and other materials. A worker there heard the sound of water flowing nearby at around 10:30 a.m., but it is not known when the leak began. No data were provided on the leak's size or rate.
"This is an extremely serious problem," Goshi Hosono, special adviser to Prime Minister Naoto Kan, told a news conference Wednesday attended by Tepco and government officials to explain their ongoing efforts to contain the nuclear crisis.
In April, the utility found that highly contaminated water was leaking into the sea near a seawater intake for the No. 2 reactor at the plant, which was crippled by the quake and tsunami.