A former adviser to the Japanese cabinet has revealed the government has known for months that thousands of evacuees from around the Fukushima nuclear plant will not be able to return to their homes.

Nearly seven months after the meltdowns at Fukushima, about 80,000 people are still living in shelters or temporary housing.

Former special adviser to Japan's prime minister and cabinet Kenichi Matsumoto has told the ABC that the government has known for months that many who live close to the Fukushima plant will not be able to return to their homes for 10 to 20 years because of contamination.

The history professor and author has given the ABC an insider's account of what happened in the hours and days after March 11, as three of the Fukushima reactors bubbled towards meltdown after a tsunami knocked out backup power to the plant.

Professor Matsumoto witnessed both the government's and the plant operator's responses to the worst nuclear accident in a quarter of a century.

He says the government is simply too scared to tell Fukushima residents that they cannot return.

"The cabinet knew right after the disaster that some people would not be able to live in their communities for 10 to 20 years, especially those a few kilometres from the plant," he said.

"The government should have conveyed the truth to the evacuees. But it felt scared; it feared telling the truth to the people."

Professor Matsumoto also confirms the prime minister at the time - Naoto Kan - also contemplated evacuating tens of millions of people from in and around Tokyo.

"It's true that the prime minister said we might have to evacuate people from Tokyo," he said.

"There was no clue about the amount of radiation coming from the Fukushima plant or if it was spreading over 100 or 200 kilometres.

"If that was the case, Tokyo would be in danger. And prime minister Kan actually said that eastern Japan might not be able to keep functioning; that it might collapse."

Professor Matsumoto says in the end, talk of tens of millions being evacuated was dismissed, with fears it would cause mass panic and chaos worse than the nuclear crisis itself.

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"I don't think he [Mr Kan] handled it well. Because it was such a terrible accident, information should have been shared with the whole cabinet. But it wasn't," he said.

"The information stopped with Mr Kan, who handled it alone. So the cabinet was isolated and wasn't able to formulate its advice properly."

Request rejected

Professor Matsumoto has also revealed details about the stricken plant's operator, TEPCO.

He says TEPCO wanted to abandon the plant at the height of the crisis, but its request was rejected.

"First TEPCO did not convey accurate information about the accident to the prime minister. It tried to make the disaster look small," he said.

"Then TEPCO's headquarters wanted to evacuate the nuclear plant, but the chief of the facility vowed not to leave. So prime minister Kan was outraged because he wasn't getting proper information or the truth."

Mr Kan faced intense pressure and a drop in popularity over the government's handling of the tsunami and nuclear crisis until he ultimately resigned in August.

Professor Matsumoto has now left his government adviser role and returned the the world of academia, vowing to write down the history of the Fukushima nuclear crisis from his unique perspective from the inside.

The ABC's PM program approached Mr Kan for a response but received no reply.

A spokesman for TEPCO told the ABC the company never tried to downplay information about the nuclear disaster, but acknowledged there were mistakes made and some confusion at the start of the crisis.