Japan was set to announce on Friday it has finally tamed leaking reactors at Fukushima, in what authorities say is a vital step on the long road to recovery, nine months after its nuclear crisis began.

Prime Minister Yoshihiko Noda was expected to tell a disaster-weary public that all reactors at the plant, struck by a titanic tsunami in March, were in a state of cold shutdown and were no longer at any risk of spontaneous fission.

Stabilisation of the reactors, whose molten cores spewed radioactive particles into the air and sea, will mark the end of what the government has dubbed 'Step Two' of the clean-up.

The initial success of Step One -- the stable cooling of reactors and used fuel pools -- was announced in July, after the quake-triggered tsunami pummelled the plant on March 11 and laid waste to much of the northeast coast.

"This step (Two) means that the reactors have continued to be in a stable condition for some time, so we can consider that they are now under control," said Takashi Sawada, vice-chairman of the Atomic Energy Society of Japan, a pro-nuclear group of academics and industry specialists.

Sawada stressed that the use of the term 'cold shutdown' by the government and plant operator Tokyo Electric Power (TEPCO) did not indicate that all four disaster-hit reactors were now safe.

"But I think it's okay to say that the reactors have basically reached a stable condition of cooling," he said, adding the amount of radiation leaking from the plant is now a tiny fraction of what it was in March.

After the March disaster, an exclusion zone around the plant was established with tens of thousands of people evacuated to avoid their being exposed to dangerously high levels of radiation.

Swathes of this zone remain polluted, with the clean-up plodding along amid warnings that some towns could be uninhabitable for three decades.

While the natural disaster claimed 20,000 lives, the nuclear emergency has recorded no direct casualties, but it has badly dented the reputation of a technology on which Japan previously depended for a third of its electricity.

Chief Cabinet Secretary Osamu Fujimura said the government's nuclear task force would meet Friday afternoon to make the decision on declaring cold shutdown.

"Today from 3:30 pm (0630 GMT), the government will hold the 22nd meeting for the nuclear disaster taskforce to discuss and decide on the issue," he said.

Noda was scheduled to make an announcement at 0900 GMT, Fujimura said, while adding: "Even after completing Step Two, it does not mean an end (to the disaster). The taskforce will certainly continue coping with it."

On March 11, waves up to 14 metres high swamped the reactors' cooling systems, sparking meltdowns, explosions and the release of radioactive material in the world's worst nuclear accident since Chernobyl in 1986.

TEPCO was caught short by the disaster, with its tsunami defence systems overwhelmed and back-up power generators knocked offline, leaving a small band of men -- dubbed the Fukushima 50 -- to try a series of jury-rigged solutions, including the use of seawater to cool the melted fuel rods.

Water used in the cooling process subsequently became a major headache for TEPCO, which had to release tonnes of the contaminated liquid into the Pacific, provoking the ire of fishermen both locally and in South Korea and China.

Farmers in the area also suffered, with produce shunned by consumers or banned by the government because of radioactive contamination.

Sawada at the Atomic Energy Society said another big quake or tsunami could undo the hard work at Fukushima, and stressed that decommissioning the reactors and cleaning up the surrounding area would last decades.

"The process of decommissioning and people finally feeling safe to go back there are two different things," he said. "We still need to do a lot of work to decontaminate the area."