Japanese power companies on Monday asked for permission to restart 10 nuclear reactors, a move that could presage a widespread return to atomic energy more than two years after the Fukushima disaster.

The firms submitted applications to regulators for safety assessments on units at five separate plants on the day that new beefed-up rules came into force.

The requests are the first step on a journey that could take many months, but which commentators say is likely to result in the resumption of nuclear power generation in Japan.

All but two of the country's 50 nuclear reactors are offline, shut down for safety checks after the Fukushima disaster, the worst the world has seen since Chernobyl in 1986.

If the regulator gives the green light, the companies must then get the nod from national and regional politicians.

Three reactors are at one site on Japan's northernmost main island of Hokkaido, while the remaining seven are in four plants in the west of the country, the utilities said separately.

However, Fukushima operator Tokyo Electric Power (TEPCO), said it has yet to submit an application for a safety assessment of two of the seven units at the Kashiwazaki Kariwa plant, the world's biggest.

"We are considering submitting the application but we have a policy of seeking local agreement on it," a TEPCO spokesman told AFP.

TEPCO's chastened approach came after the company's boss received a public tongue-lashing on Friday from local politicians for announcing it would talk to regulators before having consulted locally.

The crisis at Fukushima, caused when a huge tsunami smashed into the plant and sent reactors into meltdown, fuelled widespread public distrust of nuclear power.

A vocal anti-atomic campaign, whose leading lights say the industry had an overly cosy relationship with its regulators in the decades leading up to Fukushima, nudged the government into establishing a new industry watchdog.

Eager to prove it has teeth, the Nuclear Regulation Authority (NRA) has set strict new standards that operators must show they can meet before they will be granted permission to re-start mothballed reactors.

Even so, anti-nuclear activists gathered in Tokyo, warning against risks posed by active tectonic plates near or underneath nuclear reactors in quake-prone Japan.

They also criticised the nuclear watchdog for what they say is a half-hearted risk evaluation.

"The plant is located in a vary dangerous place with three active faults in the ground underneath it," said Taka Yamaguchi, who came from a community near the Tomari plant in Hokkaido.

"I'm full of anger over the application for the re-start" of three reactors there, she said.

While the NRA determines if a plant meets safety requirements, the decision to allow reactors to come back online rests with politicians.

"It is important that assessment will be done in a strict manner by the Nuclear Regulation Authority based on the new standards," Katsunobu Kato, deputy chief cabinet secretary, told reporters.

"It is a precondition that host communities agree on the re-firing, so we hope utilities give detailed explanations to local residents," he added.

The new safety standards require utilities to prepare measures against severe accidents or terrorist attacks and to better protect their plants from tsunamis and earthquakes.

The power companies are eager to get their reactors back up and running, having been badly hit by the surging cost of generating electricity from fossil fuel alternatives.

Resource-poor Japan has to import the coal, gas and oil it is using to replace nuclear generating capacity, and the falling value of the yen has pushed up the relative cost of the dollar-priced commodities.

TEPCO is also struggling with the vast expense of the clean-up at Fukushima and with the mounting compensation bills for people whose lives or livelihoods were wrecked by the disaster.

Although the natural disaster of March 2011 claimed around 18,000 lives, no one is officially recorded as having died as a direct result of the radiation released by meltdowns at Fukushima.

However, large areas around the plant had to be evacuated, with tens of thousands of people still unable to return to their homes.