Fukushima operator TEPCO on Friday asked Japan's nuclear watchdog for permission to restart a separate atomic power station, as it resumed cleaning polluted water at the crippled plant.

Tokyo Electric Power switched on treatment systems at the tsunami-wrecked site, seen as key to winning public support for the eventual dumping into the ocean of thousands of tonnes of now-contaminated water.

The developments came as the head of the UN's nuclear body said his organisation could help Japan's ocean monitoring efforts to ensure they conform to international norms -- key if Tokyo is going to overcome worldwide suspicion over the effect on the seas.

"We at Tokyo Electric have learnt the lessons from the Fukushima Dai-ichi plant and we think sharing the lessons with the world is our obligation," senior TEPCO official Takafumi Anegawa told reporters after documents were submitted to the Nuclear Regulation Authority.

TEPCO wants permission to re-fire two of the seven reactors at the Kashiwazaki Kariwa plant, and bring some of its nuclear fleet back online to help soften the blow from burgeoning fossil fuel bills.

While other utilities have put in applications for reactor safety assessments by the watchdog, this is TEPCO's first and came the day after local leaders gave their blessing to the move.

The company -- not known for being fleet of foot in public relations -- had previously waltzed into hot water after announcing it would talk to regulators before consulting politicians in the plant's host prefecture.

The NRA assessment is expected to take at least six months. If the reactors are given the green light, TEPCO will then have to seek permission from politicians locally and nationally.

Separately, TEPCO said it had resumed the trial of a water treatment system at Fukushima that had been suspended because of corrosion problems.

The processing system is intended to remove most radioactive materials from contaminated water. The utility hopes that if it is able to clean the water it currently has stored in tanks at the site, it will be allowed to dump it in the sea.

Contaminated water is accumulating at Fukushima and TEPCO currently has no long term solution for it.

On Thursday in Vienna, the director general of the International Atomic Energy Agency offered his organisation's expertise in monitoring radioactive materials in the ocean.

Yukiya Amano told Japanese media a reliable system conforming to global standards was necessary if Japan was to reassure the international community.

"It is not enough if Japan delivers its own data to the world," Amano said, the Mainichi Shimbun reported Friday.

"A highly reliable monitoring system that conforms to international standards is necessary," he said. Amano is to visit Japan to discuss the issue with government officials, the Mainichi said.

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.

TEPCO has poured thousands of tonnes of water onto the reactors and continues to douse them to keep them cool and to prevent them running out of control again.