Fukushima plant begins to discharge 11,500 tonnes of water into Pacific to make space for more highly contaminated liquid

The operator of Japan's Fukushima nuclear plant has started breaking its own regulations by discharging 11,500 tonnes of contaminated water into the Pacific to make space for more highly radioactive liquid.

The release of water that is 100 times the legal limit is an unprecedented breach of operating standards, but it is considered necessary so workers can concentrate on containing more severe leaks.

The government justified the action as the lesser of two evils. Recent samples of contaminated seawater from the leak show radiation levels at 4,000 times the legal standard.

"We didn't have any other alternatives," the chief cabinet secretary, Yukio Edano, told reporters. "This is a measure we had to take to secure safety."

The plant's operator, Tokyo Electric, said it would discharge 10,000 tonnes of water from its waste treatment facility and a further 1,500 tonnes that have collected in pits outside reactors No 5 and No 6.

The International Atomic Energy Agency has been notified of the discharge. On a website update, it said the Japanese authorities explained the move was necessary "to have sufficient capacity to store highly contaminated water found in the basement of the Unit 2 turbine building".

Tokyo Electric estimates the potential additional annual dose to a member of the public would be approximately 0.6 millisieverts if they ate seaweed and seafood caught near the plant every day for a year. The annual permissible level for the general public in Japan is one millisievert.

Workers have been battling to control radiation leaks since the magnitude 9 earthquake and tsunami crippled the plant's cooling system on 11 March, leading to a partial meltdown of the reactor.

Earlier attempts to cool the reactor by hosing water from fire engines and helicopters have left pools of contaminated water and flooded basements, hampering the containment operation and efforts to restart the cooling pumps.

Highly radioactive water is seeping from at least one point at reactor No 2, where a 20cm crack has been found in a concrete pit. It is thought to be leaking into an inflow conduit for seawater, but there may be other paths of contamination. Plant workers have started to dye the water a milky white colour so they can trace its route.

At the weekend workers tried and failed to plug the crack by using 80kg of highly absorbent polymer (more commonly used in nappies) mixed with shredded newspaper and sawdust. A previous attempt to use concrete had a similar outcome.

Edano said the situation must be stabilised as soon as possible because a long-term leak "will have a huge impact on the ocean".Critics of Japan's nuclear industry said the authorities were confronted with a dilemma that was unique in the history of nuclear power: whether to keep cooling the reactors and spent fuel or reduce the water being pumped into the plant, which is overflowing the capacity of the trenches.

"As a result of Tokyo Electric's desperate but failed efforts to cool the reactors, they are about to release perhaps an unprecedented amount of radioactivity into the environment," said Shaun Burnie, a nuclear consultant to Greenpeace Germany. "If the Japanese government were to take a cupful of this water and take it outside their territorial waters it would be illegal under the law of sea dumping convention."

As a temporary measure to ease the leak, Japan's nuclear and industrial safety agency has recommended the construction of an undersea silt barrier."A silt fence ensures that mud down deep doesn't seep through," said Hidehiko Nishiyama, Japan's spokesman on nuclear safety.

Officials said the situation is unlikely to be under control for several months. Independent analysts warn it might take years.

Giant concrete pumps are being sent to the area from overseas. The government has also asked Tokyo Electric to look into the possibility of covering the plant with sheets pinned to a steel frame.

But the more radioactivity that enters the air and water, the harder such countermeasures become. Tokyo Electric has said the plant will never recover and some areas are so contaminated that workers cannot get near them.

"I don't know if we can ever enter the No 3 reactor building again," Hikaru Kuroda, the company's chief of nuclear facility management, said on Sunday.

The situation dominated a meeting in Vienna of signatories to the convention on nuclear safety, which was supposed to prevent a repeat of the disasters at Three Mile Island and Chernobyl."I know you will agree with me that the crisis at Fukushima Daiichi has enormous implications for nuclear power and confronts all of us with a major challenge," Yukiya Amano, head of the International Atomic Energy Agency, told participants. "We cannot take a 'business as usual' approach."

Although the nuclear threat has yet to claim a life, it has overshadowed the severe humanitarian crisis faced by survivors of the tsunami, which killed 12,157 people and has left 15,496 missing. Hundreds of thousands of people have lost their homes and millions are still affected by shortages of electricity.