Japan is moving a step closer to restarting nuclear reactors as utilities file for safety inspections at their idled plants.

It is the clearest sign of Japan's return to nuclear energy nearly two and a half years after the Fukushima disaster.

With all but two of its 50 reactors off line since the disaster, Japan has been without nuclear energy that once supplied about a third of its power.

Four of nine Japanese nuclear plant operators will apply for safety inspections by the Nuclear Regulation Authority for a total of 10 reactors tomorrow, when new safety requirements take effect.

But critics say the rules have loopholes, including grace periods for some safety equipment.

Pro-industry prime minister Shinzo Abe has pushed for restarting the reactors, saying they are vital to Japan's economy.

The new standards are stricter than in the past and for the first time compulsory. Only reactors that pass the inspections will be allowed to reopen - possibly early next year.

Each inspection could take several months, according to the watchdog, plus obtaining local consent may take another few weeks.

Hit by soaring gas and oil costs to run conventional power generation plants to make up for the shortfall, Japanese utility companies have desperately sought to put their reactors back online.

Nearly all the utilities owning nuclear power plants reported huge losses last fiscal year due to higher costs for fuel imports. Hokkaido Electric Power, for example, said it has been hit with additional daily fuel costs of 600 million yen to make up for three idled reactors. Nuclear operators have already requested rate increases or plan to do so.

Mr Abe, who took office in December, scrapped a phase-out plan set by the previous government. Resumption of nuclear power plants is part of his ruling party's campaign platform in parliamentary elections in two weeks.

The new requirements specify for the first time that plants must take steps to guard against radiation leaks in the case of severe accidents, install emergency command centers and enact anti-terrorist measures. Operators are required to upgrade protection for tsunamis and earthquakes, as well as tornadoes and aviation accidents.

Safety was previously left up to the operators, relying on their self-interest in protecting their own investments as an incentive for implementing adequate measures. Tokyo Electric Power came under fire for underestimating the risk of a tsunami and building a seawall that was less than half the height of the wave that hit the Fukushima Dai-ichi plant on March 11, 2011, knocking out power and cooling systems, which led to meltdowns in three of its reactors. About 160,000 evacuees still cannot return to their homes.

"We decided to apply because we're confident about the safety measures we've taken," said Shota Okada, a spokesman at Hokkaido Electric Power, filing for the triple-reactor Tomari plant. "We'll do everything to accommodate a smooth inspection process."

Hokkaido Electric hopes to restart them in time for the long, cold winter on the northern main island, company president Katsuhiko Kawai said recently.

Critics are concerned at the five-year grace period - given to reactors known as PWRs that come with larger containment chambers considered less likely to suffer from pressure build-up than ones like those ravaged at Fukushima - for installing some mandated new equipment and a full-fledged command centre.

This means about half of the 48 reactors that are PWRs, or pressurised water reactors, could operate without the safety features up to five years.

All 10 reactors set for inspections are PWRs, and filtered vents and command centres are reportedly still under way at many of them.

Opponents say the approvals are aimed at resuming reactor operations, although nearby communities lag in enacting needed emergency and evacuation procedures.