A nuclear industry insider has told the ABC that the situation at the stricken Fukushima reactor is still not under control, three years after the disaster there.

Japan's prime minister Shinzo Abe has announced he wants 30,000 residents to return to their homes and the reactors to be switched back on within two years.

But a Fukushima insider and two former prime ministers have told the ABC's 7.30 program that such a move would be irresponsible.

At the risk of losing his job if his identity is revealed, a man who worked at TEPCO's Fukushima plant for more than 20 years says the situation at the reactor is not under control and no-one knows how to fix the problem.

"There are too many systems and they all have problems," he said.

"For example, too many water tanks with too many lines - it's very difficult to operate. It's made worse because all the experienced workers have reached their radiation limits, so TEPCO has to rely on staff that don't know the site and who aren't trained."

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Listen Duration: 3 minutes 45 seconds 3 m 45 s Listen to Mathew Carney's report for PM Download 1.7 MB

The whistleblower says mistakes are made weekly, and contaminated water leaks into the Pacific Ocean every day.

"The other day when contaminated water overflowed from a tank, an alarm was ringing but they didn't go and check. I couldn't believe it. It was ringing for nine hours and they thought the alarm was out of order."

The insider says the damaged reactors can never be decontaminated and that people should not be moved back into the no-go zone, a 20-kilometre exclusion area around Fukushima.

"I feel it is impossible to fix before my death," he said.

"We just don't have the technology to fix it. It currently doesn't exist. We just can't deal with the melted fuel."

TEPCO declined 7.30's invitation to respond to these allegations.

Next month, the government will start moving 30,000 of Fukushima's evacuees back into the no-go zone.

It is all part of the Abe government's plan to turn back on the country's 48 nuclear reactors by the middle of this year. They have been sitting idle since the 2011 disaster.

A government spokesman said there was no such thing as zero risk with a nuclear plant, but the government believed the risks could be managed with new safety standards.

Turning reactors back on 'a risk not worth taking'

The Abe government has said it is in the best interest of the economy to once again make nuclear power the core source of Japan's energy.

It is worried that the reliance on imported coal and gas is threatening the country's fragile economic recovery.

But a former prime minister, Morihiro Hosokawa, says it is a risk not worth taking.

"The causes of the accident haven't been investigated properly. Contaminated water is still leaking, and compensation for victims hasn't been sorted out," he said.

Residents are reluctant to return to their homes. ( ABC News )

"I think in these circumstances it is very irresponsible to turn the reactors back on."

He is backed by another former prime minister, Naoto Kan, who was in power at the time of the nuclear disaster in 2011.

Mr Kan says the current government does not understand the risk.

"They are trying to restart the nuclear reactors without learning the lessons of the March 11 accident," he said.

"If the accident had spread just a little further, then 50 million people around Tokyo would have been evacuated for a long time and that would have put Japan in chaos for 20 to 30 years."

Mr Kan says no national evacuation plan has been developed, and in the rush to turn the reactors back on the government is ignoring the safety of the general public.

"I submitted written questions to Prime Minister Abe and his response from the Nuclear Regulation Authority says it only decides on limited technical issues and won't judge local disaster prevention plans; that is, whether residents can escape safely or whether the residents can ever return.

"It's becoming clear they are trying to restart the reactors with no regard for people's safety."