Some of the hundreds of tonnes of radioactive water which leaked from a tank at Japan's crippled Fukushima nuclear plant may have flown into the sea, the plant's operator said Wednesday.
Tokyo Electric Power Co (Tepco) is desperately trying to seal a tank at the plant which has leaked about 300 tonnes of radioactive water.
Spokesman Tsuyoshi Numajiri said traces of radioactivity were detected in a drainage stream.
"There is a possibility that earth and sand contaminated with the leaked water flowed into the drainage. We cannot rule out the possibility that part of the contaminated water flowed into the sea," he said.
"We intend to make detailed examinations of the matter."
Japan also issued its strongest warning about the plant since the crisis began in 2011, as it struggled to seal the tank.
Plant operator Tokyo Electric Power Co (Tepco) said some of the water might have flown into the Pacific Ocean.
Nuclear regulators said the leak represented a level-three "serious incident" on the UN's seven-point International Nuclear Event Scale (INES), which measures radiation accidents.
The alert, raised from level one which indicates an "anomaly," is the most serious declared at the ruined plant since March 2011, when a quake-generated tsunami knocked out reactor cooling systems and sparked meltdowns.
At its height the Fukushima disaster was classified as level seven – one of only two events ever rated in that category along with the Chernobyl disaster in 1986.
The NRA said in a statement the amount of radiation leakage and the "fact that there is no safety protective layer remaining at the facility" meant the level-three warning needed to be declared.
A Tepco official said earlier Wednesday the leak was thought to be continuing from the tank but its source had not yet been pinpointed.
The comnpany was also "hurriedly checking" if any of 350 similar tanks at the plant were also leaking.
Numajiri said workers are removing soil contaminated by the leaked water, and pumping the remaining water from the leaky tank.
He said there were no significant changes in radiation levels outside the plant.
An earthquake-generated tsunami knocked out reactor cooling systems and sparked meltdowns at the plant beside the Pacific in March 2011, in the worst nuclear accident since Chernobyl in 1986.
