The tsunami-devastated Fukushima Dai-ichi nuclear power plant has reached a stable state of "cold shutdown" and is no longer leaking substantial amounts of radiation, Japan's prime minister announced Friday.

Prime Minister Yoshihiko Noda's announcement marks a milestone nine months after the March 11 tsunami sent three reactors at the plant into meltdowns in the worst nuclear crisis since Chornobyl.

Experts noted, however, that the plant remains vulnerable to more problems and it will take decades to decommission.

"The reactors at the Fukushima Dai-ichi nuclear power plant have reached a state of cold shutdown," Noda told a Cabinet meeting.

The government's official endorsement of the claim by Tokyo Electric Power Co. that the reactors have reached cold shutdown status is a necessary step toward revising evacuation zones around the plant and focusing efforts from simply stabilizing the facility to actually starting the arduous process of shutting it down.

But its assessment has some important caveats.

The government says Fukushima Dai-ichi has reached cold shutdown "conditions" — a cautious phrasing reflecting the fact that TEPCO cannot measure temperatures of melted fuel in the damaged reactors in the same way as with normally functioning ones.

Even so, the announcement marks the end of the second phase of the government's lengthy roadmap to completely decommission the plant, which is expected to take 30 years or more.

Officials can now start discussing whether to allow some evacuated residents who lived in areas with lesser damage from the plant to return home — although a 20-kilometre zone around the plant is expected to remain off limits for years to come. The crisis displaced some 100,000 people.

A cold shutdown normally means a nuclear reactor's coolant system is at atmospheric pressure and the its reactor core is at a temperature below 100 C, making it impossible for a chain reaction to take place.

According to TEPCO, temperature gauges inside the Fukushima reactors show the pressure vessel is at around 70 C. The government also says the amount of radiation now being released around the plant is at or below 1 millisievert per year — equivalent to the annual legal exposure limit for ordinary citizens before the crisis began.