Japan said Wednesday that decommissioning the tsunami-wrecked reactors at Fukushima could take as long as 40 years, with melted nuclear fuel possibly stuck where it is for a quarter of a century.

A roadmap produced by the government and plant operator Tokyo Electric Power (TEPCO) envisages engineers will use much of the next decade just trying to work out how to get at the fuel, which has partly eaten through its containment vessels.

Underlining the challenges involved in dealing with the disaster, the plan says that as-yet uninvented technology will be key to safely disposing of the waste left behind by the world's worst nuclear accident since Chernobyl.

"Each and every one of the steps toward decommissioning currently poses many technological challenges," said Goshi Hosono, the government minister with responsibility for dealing with the Fukushima disaster.

"We expect extremely difficult work to remove fuel debris (from the reactors). We must accelerate work to develop technologies, particularly remote control robots," he told reporters.

"We will gradually make progress by assessing both the situation on site and the state of research and development work," he said, adding that the government will pay whatever necessary to finish the work.

The roadmap also promises transparency in the decommission work -- its progress and problems -- and says plans will be continually reviewed to ensure the safety of both those working at the site and those living around it.

A government panel has estimated the decommisioning work alone could cost at least 1.15 trillion yen ($14.8 billion).

The unveiling of the plan comes days after Prime Minister Yoshihiko Noda announced the stricken reactors on Japan's tsunami-ravaged northeast coast had reached a state of "cold shutdown".

The announcement, greeted by the press and public as at best an incremental step, means only that the temperature inside the reactors has remained below 100 degrees Celsius (212 Fahrenheit) for a period of time and that emissions of radioactive materials have dropped off.

Under the plan revealed Wednesday, TEPCO will continue to pour water on the reactors as coolant until all debris is removed.

It will also upgrade various makeshift tools at the wrecked power plant, including a system to process radioactive waste water and to cool the damaged reactors, to ensure more stable operation.

Presently some of the equipment being used is temporary, with many measures improvised by workers in the days and weeks after March 11 when a huge earthquake under the Pacific unleashed a devastating tsunami.

Towering waves rolled into the plant, paralysing cooling systems and sending reactors into meltdown as explosions sent radioactive particles into the air and sea.

No one is known to have died as a direct result of the atomic disaster, but the tsunami killed around 20,000 people as it wiped whole communities off the map.

Tens of thousands of people were evacuated from homes in a 20-kilometre (12-mile) radius of the plant and from pockets beyond, with warnings that some places will remain uninhabitable for decades because of radioactivity.