A fifth of Japan's electricity supply should come from nuclear power generation, the country's industry ministry said Tuesday, despite widespread opposition in the aftermath of the Fukushima disaster.

With none of the nation's viable nuclear reactors in operation, the target indicates an intention to bring most, if not all of them, back online.

Environmentalists Greenpeace said the proposition showed the government was "deluding itself".

Japan's intended energy mix -- what proportion of power comes from which sources -- has been a subject of hot debate for months, not least because without it, Tokyo has been unable to make international commitments to reduce greenhouse gas emissions.

In its proposal, which will be debated by the government by the end of next month, the industry ministry said about 20-22 percent of the country's electricity should come from nuclear power by 2030.

Supply from renewables such as solar and wind power should also be doubled to 22-24 percent.

The industry ministry favours nuclear power as a way to cut emissions of carbon dioxide and other greenhouse gases, despite a groundswell of public opposition since the nuclear crisis in Fukushima.

Reactors at the Fukushima Dai-ichi plant went into meltdown in March 2011 after a tsunami swamped their cooling systems -- setting off the worst nuclear disaster since Chernobyl in 1986.

Japan's entire nuclear stable was gradually switched off following the crisis, while tens of thousands of people were evacuated due to concerns about radiation exposure.

Many local residents are still unable to return to their homes and scientists have warned that some areas around the plant may remain uninhabitable for decades or more.

- 'Deluding itself' -

Pro-nuclear premier Shinzo Abe and the country's business sector have since pushed to restart plants that once supplied more than one quarter of Japan's electricity, as a plunging yen has sent energy import bills through the roof.

Abe's government has pledged to lower the country's dependence on nuclear power and promote so-called "green energy", but insists it is unrealistic to rely too much on renewables because of cost and stability issues.

Japan's pro-nuclear lobby said last week that 2015 would be the year reactors are restarted, despite public wariness.

Four reactors were decommissioned on Monday after failing to meet new, stricter safety standards, and another will be permanently taken out of operation on Thursday, Jiji Press reported.

But the 20-22 percent supply figure indicates that most, if not all, of the remaining 43 reactors will have to be restarted, though most have yet to receive the green light from regulators.

Shaun Burnie, nuclear specialist at Greenpeace Germany, said many of these reactors would never come back online.

"The Japanese government is deluding itself if it thinks that nuclear power will play a significant role in its future energy supply," he said.

"The scale of the challenges facing the nuclear industry are such that generation from reactors is likely to collapse during the coming decade. Many reactors will never restart, and most reactors over the coming years will be too old to operate."

The industry ministry said if the proposed energy mix is implemented, Japan will be able to reduce greenhouse gas emissions by 21.9 percent in 2030 from 2013 levels.