TOKYO - Workers at Japan's crippled Fukushima nuclear plant on Thursday scrambled to check 300 tanks storing highly radioactive water after one sprang a leak that is feared to have spread into the Pacific.
Some 300 tonnes of radioactive water was believed to have seeped from one of the tanks that hold water used to cool the broken reactors, while operator Tokyo Electric Power Co (TEPCO) warned some of the water might have flowed into the ocean.
"We are hurriedly checking if some 300 tanks of the same type holding contaminated water have the same leak problem," a TEPCO spokesman said.
"We have finished pumping out water from the troubled tank, while we have continued removing the soil soaked by the water," the spokesman said.
Spokesman Tsuyoshi Numajiri said Wednesday that traces of radioactivity were detected in a drainage stream.
"There is a possibility that contaminated earth and sand contaminated flowed into the drainage. We cannot rule out the possibility that part of the contaminated water flowed into the sea," he said.
On Wednesday, 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 was raised from level one, which indicates an "anomaly".
It is the most serious single incident since the quake and tsunami-sparked meltdowns at the plant in March 2011, which was a level-seven. The Chernobyl disaster in 1986 is the only other incident to have been ranked as serious.
TEPCO has said puddles of water near the tank were so toxic that anyone exposed to them would receive the same amount of radiation in an hour that a nuclear plant worker is allowed to receive in five years.
Thursday's safety checks came after Nuclear Regulation Authority (NRA) chairman Shunichi Tanaka on Wednesday voiced concern about similar leaks from other tanks.
"We must carefully deal with the problem on the assumption that if one tank springs a leak the same thing can happen at other tanks," he said.
The company -- which faces huge clean-up and compensation costs -- has struggled to cope with the disaster.
Its most serious problem is how to handle the massive amount of water accumulating as a result of continuing water injections to cool the reactors, which has been stored in the hastily built tanks.