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Japan has succeeded in bringing its tsunami-stricken atomic power station to a safe “cold shutdown”, the government said on Friday, concluding the most dangerous phase of its nuclear crisis after a nine-month struggle.

Yoshihiko Noda, prime minister, announced the end of the emergency at a meeting of officials from his administration and Tokyo Electric Power, owner of the Fukushima Daiichi plant 240km north of Tokyo.

“The reactors have achieved cold shutdown, and it is judged that the power plant accident itself is resolved,” he said.

Three reactors at Fukushima Daiichi melted down in March after a powerful tsunami swamped the facility, knocking out its cooling systems. Radiation leaks forced more than 100,000 people to abandon homes nearby and polluted some 3 per cent of Japan’s land mass to levels requiring decontamination.

The accident was the worst caused by the nuclear power industry since the 1986 explosion and meltdown at Chernobyl in the then Soviet Union.

Cold shutdown is a technical term meaning temperatures inside the reactors’ cores are stably below the 100-degree centigrade boiling point. It indicates that a nuclear chain reaction is highly unlikely to recur, though Fukushima Daiichi’s damaged units remain highly radioactive at close range and will remain so for years.

Teams of engineers and emergency workers have been pumping water into the reactors since the first days of the crisis – initially using fire hoses to spray them with seawater, and later employing a jury-rigged system of tanks and pipelines.

A side effect of their early efforts was that tens of thousands of tonnes of contaminated water built up at the site, some of which leaked into the sea.

The next step will be permanently to decommission the facility, a process that is expected to take decades and cost tens of billions of dollars. It will involve sealing or dismantling the three meltdown-hit reactors and at least one more damaged unit on the six-reactor site.

Simply removing the units’ uranium fuel will be difficult and time-consuming: a still unverified amount of fuel has burned through the reactors’ central containment vessels, forming dangerous pools on the floors below. In addition, technicians must deal with four damaged storage tanks containing still-radioactive spent fuel.

Tepco is due to complete a decommissioning plan later this month. Depending on its cost projections, the government could be forced to inject additional capital into the financially struggling utility, in effect nationalising it, according to people familiar with the matter.

A government-appointed panel estimated in October that sealing or dismantling the site’s four ruined reactors could cost Y1,100bn, but some government officials and outside experts argue that the true amount could be several times higher.

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