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The crippled nuclear reactors at Japan's Fukushima power plant have finally been stabilised, Prime Minister Yoshihiko Noda has announced.

An earthquake and tsunami in March knocked out vital cooling systems, triggering radiation leaks and forcing the evacuation of thousands of people.

Mr Noda's declaration of a "cold shutdown" condition marked the stabilisation of the plant.

The government says it will take decades to dismantle it completely.

The six-reactor Fukushima Daiichi nuclear plant was badly damaged by the 11 March earthquake and tsunami. Blasts occurred at four of the reactors after the cooling systems went offline.

Workers at the plant, which is operated by Tokyo Electric Power Company (Tepco), have been using sea water to cool the reactors. Waste water has built up and some contaminated liquid has been released into the sea.

A 20km (12m) exclusion zone remains in place around the plant.

"The nuclear reactors have reached a state of cold shutdown and therefore we can now confirm that we have come to the end of the accident phase of the actual reactors," Mr Noda told a news conference.

"We are now moving from trying to stabilise the nuclear reactors to decommissioning them.

"The Japanese government promises to clarify the roadmap from here and do our utmost, while ensuring we operate the nuclear reactors as safely as possible, to decommission them."

The "battle is not over", he said, adding that the next phase would focus on the clean-up operation, including decontaminating the ground around the plant.

With the reactors stable, Mr Noda said the government would review the evacuation zones established in the immediate aftermath of the incident.

Earlier this year, the government said it was aiming for a cold shutdown by the end of the year.

This is where water that cools nuclear fuel rods remains below boiling point, meaning that the fuel cannot reheat.

Tepco has also defined it as bringing the release of radioactive materials under control and reducing public radiation exposure to a level that does not exceed 1mSv/year at the site boundary.

Speaking to cabinet ministers of his nuclear task force earlier on Friday, Mr Noda said: "We can now maintain radiation exposure at the periphery of the plant at sufficiently low levels even in the event of another accident."

But some nuclear experts have said that the repairs made to the plant after the accident are makeshift and could break down without warning.

Forty years

More than 80,000 people had to leave the area, but radiation levels in some places remain too high for them to return home.

Earlier this week, the government said it could take up to 40 years to fully decommission the plant and clean up surrounding areas.

Spent fuel rods and melted fuel inside the reactors must be removed. Waste water must also be safely stored.

Contamination has been found in foodstuffs from the region including rice, beef and fish, while radioactive soil has also been found in some areas.

Some experts have also warned that the plant could be further damaged if a powerful aftershock were to strike.

Engineers are also continuing to encounter new problems - last week Tepco officials confirmed that 45 cu m (1,590 cu ft) of water had leaked into the sea from a crack in the foundation of a water treatment facility.