About 300 tons of highly radioactive water had leaked from a tank at the damaged Fukushima No. 1 nuclear power plant as of Tuesday afternoon, Tokyo Electric Power Co. said.

Tepco claims none of the water, which had been used to cool its stricken reactors and is highly radioactive, has flowed directly into the Pacific, apparently contained by a 30-cm-high waterproof concrete barrier surrounding dozens of tanks, including the leaking No. 5 tank. But some of the water may have been absorbed into the ground, joining with already tainted groundwater.

Where the leak is occurring in the tank, which is bolted together and has sealed seams, remains elusive, however, even after workers finished pumping water from inside the barrier Tuesday afternoon.