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 clean up of the worst nuclear accident since Chernobyl.

The storage tank breach of about 300 tonnes of water is separate from contaminated water leaks reported in recent weeks, plant operator Tokyo Electric Power Co said yesterday.

The latest leak is so contaminated that a person standing half a metre 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.

“That is a huge amount of radiation. The situation is getting worse,” said Michiaki Furukawa, who is professor emeritus at Nagoya University and a nuclear chemist.

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 yesterday.

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.

“We failed to discover the leak at an early stage and we need to review not only the tanks but also our monitoring system,” he said. Tokyo Electric, also known as Tepco, said it did not believe water from the latest leak had reached the Pacific Ocean, about 500 metres away. Nonetheless, continued leaks have alarmed Japan’s neighbours South Korea and China.

Tepco has been criticised for its failure to prepare for the disaster and been accused of covering up the extent of the problems there.

In recent months, the plant has been beset with power outages and other problems that have led outside experts to question whether Tepco is qualified to handle the clean up, which is unprecedented due to the amount of radioactive material on the site and its coastal location.

The government said this month it will step up its involvement in the cleanup, following Tepco’s admission, after months of denial, that leaked contaminated water had previously reached the ocean.

Fukushima Governor Yuhei Sato told an emergency meeting of prefectural officials yesterday it was a “national emergency”, and that the local government would monitor the situation more strictly and seek additional steps as needed.

Massive amounts of radioactive fluids are accumulating at the plant as Tepco floods reactor cores via an improvised system to keep melted uranium fuel rods cool and stable.

The water in the cooling system then flows into basements and trenches that have been leaking since the disaster.

Highly contaminated excess water is pumped out and stored in steel tanks on elevated ground away from the reactors. About 400 tonnes of radioactive water a day has been stored at Fukushima.

A puddle that formed near the leaking tank is emitting a radiation dose of 100 millisieverts an hour about 50 centimetres above the water surface, reporters were told at a news briefing.

Sign up to our free newsletters

Get the best updates straight to your inbox:
Please select at least one mailing list.

You can unsubscribe at any time by clicking the link in the footer of our emails. We use Mailchimp as our marketing platform. By subscribing, you acknowledge that your information will be transferred to Mailchimp for processing.