Contaminated water with dangerously high levels of radiation is leaking from a storage tank at Japan's crippled Fukushima nuclear plant, the most serious setback to the cleanup of the worst nuclear accident since Chernobyl.
The storage tank breach of about 300 metric tons of water is separate from contaminated water leaks reported in recent weeks, plant operator Tokyo Electric Power Co said on Tuesday.
The latest leak is so contaminated that a person standing half a meter (1 ft 8 inches) away would, within an hour, receive a radiation dose five times the average annual global limit for nuclear workers.
After 10 hours, a worker in that proximity to the leak would develop radiation sickness with symptoms including nausea and a drop in white blood cells.
The embattled utility Tokyo Electric has struggled to keep the Fukushima site under control since an earthquake and tsunami caused three reactor meltdowns in March 2011.
Japan's Nuclear Regulation Authority has classified the latest leak as a level 1 incident, the second lowest on an international scale for radiological releases, a spokesman told Reuters on Tuesday.
It is the first time Japan has issued a so-called INES rating for Fukushima since the meltdowns. Following the quake and tsunami, Fukushima was assigned the highest rating of 7, when it was hit by explosions after a loss of power and cooling.
A Tokyo Electric official said workers who were monitoring storage tanks appeared to have failed to detect the leak of water, which pooled up around the tank.
Tokyo Electric, also known as Tepco, said it did not believe water from the latest leak had reached the Pacific Ocean, about 500 meters (550 yards) away. Nonetheless, continued leaks have alarmed Japan's neighbors South Korea and China.