Japan declared its tsunami-stricken Fukushima nuclear power plant to be in cold shutdown on Friday, in a major step toward resolving the world's worst nuclear crisis in 25 years.

The Fukushima Daiichi plant, 240 km (150 miles) northeast of Tokyo, was wrecked on March 11 by a huge earthquake and a tsunami which knocked out its cooling systems, triggering meltdowns, radiation leaks and mass evacuations.

In making the much-anticipated announcement, Prime Minister Yoshihiko Noda sought to draw a line under the crisis phase of the emergency at the plant itself, though experts have said it could be 40 years before the site is finally cleaned up.

Even if unforeseeable incidents happen, the situation is such that radiation levels on the boundary of the plant can now be maintained at a low level, Noda said at the government's nuclear emergency response meeting.

A cold shutdown is when water used to cool nuclear fuel rods remains below its boiling point, preventing the fuel from reheating. One of the chief aims of the plant's operator, Tokyo Electric Power (Tepco), had been to bring the reactors to cold shutdown by the year-end.

After months of efforts, the water temperature in all three of the affected reactors fell below boiling point by September, but Tepco has been cautious of declaring a cold shutdown, saying it had to see if temperatures and the amount of radiation emitted from the plant remained stable.

The declaration of a cold shutdown could have repercussions well beyond the plant: it is a government pre-condition before it allows about 80,000 residents evacuated from within a 20 km (12 mile) radius of the plant to return home.

(Editing by Tomasz Janowski, Mark Bendeich and Matt Driskill)