TOKYO (Reuters) - Japan declared its tsunami-stricken Fukushima nuclear power plant to be in cold shutdown on Friday, taking a major step to resolving the world's worst nuclear crisis in 25 years but some critics questioned whether the plant was really under control.

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

In making the much-anticipated announcement, Prime Minister Yoshihiko Noda tried to draw a line under the most acute phase of the crisis and highlighted the next challenges: the clean-up and the safe dismantling of the plant, something the government says may take more than 30 years.

"The reactors have reached a state of cold shutdown," Noda told a government nuclear emergency response meeting.

"A stable condition has been achieved," he added, noting radiation levels at the boundary of the plant could now be kept at low levels, even in the event of "unforeseeable incidents."

A cold shutdown is when water used to cool nuclear fuel rods remains below 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 that state by the year-end.

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

Both Noda and his environment and nuclear crisis minister Goshi Hosono said that while the government still faced huge challenges, the situation at the plant was under control.

That provoked an angry response from senior local officials, Greenpeace and some reporters even as the Vienna-based U.N. nuclear agency welcomed "significant progress" at the plant.

"We hope that this will be a fresh step towards going back home but it does not change the fact that the path to bringing the crisis under control is long and tough," Fukushima governor Yuhei Sato said, according to the Asahi newspaper website.

Greenpeace dismissed the announcement as a publicity stunt.

"By triumphantly declaring a cold shutdown, the Japanese authorities are clearly anxious to give the impression that the crisis has come to an end, which is clearly not the case," Greenpeace Japan said in a statement.

Hosono acknowledged that there were some areas where it would be difficult to bring people back and said there could be small difficulties here and there, but he told a briefing: "I believe there will be absolutely no situation in which problems escalate and nearby residents are forced to evacuate."

The water temperature in all three of the affected reactors fell below boiling point by September, but Tepco had said it would declare a state of cold shutdown only once it was satisfied that the temperatures and the amount of radiation emitted from the plant remained stable.

Jonathan Cobb, an expert at the British-based World Nuclear Association, said the authorities had been conservative in choosing the timing of the announcement.

"The government has delayed declaration of cold shutdown conditions, one reason being to ensure that the situation at the plant was stable," Cobb said, adding that the evacuation zone should get progressively smaller as more of it was decontaminated.

Kazuhiko Kudo, professor of nuclear engineering at Kyushu University, said authorities needed to determine exactly the status of melted fuel inside the reactors and stabilize a makeshift cooling system, which handles the tens of thousands of tons of contaminated water accumulated on-site.

The government and Tepco will aim to begin removing the undamaged nuclear rods from the plant's spent fuel pools next year. However, retrieval of fuel that melted down in their reactors may not begin for another decade.

The enormous cost of the cleanup and compensating the victims has drained Tepco financially. The government may inject about $13 billion into the company as early as next summer in a de facto nationalization, sources told Reuters last week.

An official advisory panel estimates Tepco may have to pay about 4.5 trillion yen ($57 billion) in compensation in the first two years after the nuclear crisis, and that it will cost 1.15 trillion yen to decommission the plant, though some experts put it at 4 trillion yen ($51 billion) or even more.

Japan also faces a massive cleanup task outside the east coast plant if residents are to be allowed to go home. The Environment Ministry says about 2,400 square km (930 square miles) of land around the plant may need to be decontaminated, an area roughly the size of Luxembourg.

The crisis shook the public's faith in nuclear energy and Japan is now reviewing an earlier plan to raise the proportion of electricity generated from nuclear power to 50 percent by 2030 from 30 percent in 2010.

Japan may not immediately walk away from nuclear power, but few doubt that nuclear power will play a lesser role in future.

Living in fear of radiation is part of life for residents both near and far from the plant. Cases of excessive radiation in vegetables, tea, milk, seafood and water have stoked anxiety despite assurances from public officials that the levels detected are not dangerous.

Chernobyl's experience shows that anxiety is likely to persist for years, with residents living near the former Soviet plant still regularly checking produce for radiation before consuming it 25 years after the disaster.

(Additional reporting by Yoko Kubota, Fredrik Dahl in VIENNA and Nina Chestney in LONDON; Writing by Tomasz Janowski; Editing by Mark Bendeich and Robert Birsel)