TOKYO - Some 300 tonnes of radioactive water is believed to have leaked from a tank at Japan's crippled nuclear plant, the worst such leak since the crisis began, the operator said Tuesday.
Tokyo Electric Power Company (TEPCO) said the leak was believed to be continuing Tuesday at Fukushima and it had not yet pinpointed the source of it.
TEPCO said puddles with extremely high radiation levels - about 100 millisieverts per hour - have been found near the water tanks at the ruined plant.
"This means you are exposed to the level of radiation in an hour that a nuclear plant worker is allowed to be exposed to in five years," a TEPCO spokesman told a press conference.
The company later said it had identified which tank was faulty but had yet to find the spot from where it was leaking.
"We have instructed TEPCO to find the source of contaminated water...and to seal the leakage point," an official from the Nuclear Regulation Authority told AFP.
"We have also instructed them to retrieve contaminated soil to avoid a further expansion of toxic water, and to strengthen monitoring of the surrounding environment."
There were no significant changes in radiation levels outside the plant, he added.
Since a quake-generated tsunami struck Fukushima in March 2011, knocking out reactor cooling systems and sparking meltdowns, there have been four similar leaks from tanks of the same design.