Plant operator, Tokyo Electric Power Company, said radiation had been found in the Pacific nearby, perhaps not surprising given crews have been dousing the reactors with sea-water ever since the accident.

Radioactive iodine in the sea samples was 126.7 times the allowed limit, while caesium was 24.8 times over, Kyodo news agency said. That still posed no immediate danger, TEPCO said.

"It would have to be drunk for a whole year in order to accumulate to one millisievert," a TEPCO official said, referring to the standard radiation measurement unit. People are generally exposed to about 1 to 10 millisieverts each year from background radiation caused by substances in the air and soil.