TOKYO (AFP) - Japan's industry minister will Saturday meet workers battling to cool overheating reactor cores and plug radioactive leaks in the first government visit to the country's tsunami-crippled nuclear plant.

With Tokyo warning the crisis is far from over, Banri Kaieda will don full protective gear for his brief trip to the damaged Fukushima Daiichi plant, his ministry said.

Kaieda, who has overall responsibility for all of Japan's 50-plus nuclear reactors, will be the first government figure to step inside the compound since the giant tsunami of March 11 knocked out cooling systems.

The visit comes 24 hours after it emerged small amounts of radioactive water spilled from spent fuel cooling pools at another nuclear plant as a powerful aftershock rocked northeast Japan.

The cooling systems at three plants were forced onto back-up power when the 7.1 magnitude tremor late Thursday shut down electricity generation across a swathe of the country.

On Friday, Tohoku-Electric Power, the operator of the Onagawa nuclear plant, said around four litres (a gallon) of mildly radioactive water had spilled from the spent fuel pool of one reactor.

The entire plant had been shut down since the March 11 disaster and the leak did not present any danger, the company said.

A spokesman for the Ministry of Economy, Trade and Industry said Kaieda wanted to see for himself the work being done at Fukushima.

"He will express his appreciation to the workers at Fukushima Daiichi and will see the situation first hand," the spokesman said.

Kaieda will also visit the "J-Village" sports complex, which is being used as a base for workers at the plant.

The complex lies inside a 20-kilometre (12-mile) exclusion zone around the plant, from which thousands of people were evacuated when levels of radiation in the area soared following the emergency.

The crippled plant has leaked radiation that has made its way into tap water and farm produce, sparking food export bans covering a large area.

Some highly radioactive water has leaked into the Pacific Ocean and this week Tokyo Electric Power Co. (TEPCO) began dumping 11,500 tonnes of low-level radioactive water from the plant into the sea to free up urgently needed storage space, a process they hoped would finish on Saturday night.

Workers at the plant on Thursday began injecting inert nitrogen gas into reactor No. 1 in a bid to head off a possible explosion from a build-up of hydrogen reacting with oxygen from the air.

On Friday, chief government spokesman Yukio Edano said the situation remained "unstable" as it was reported reactor-maker Toshiba said it could decommission crippled reactors in 10 years, four years less than it took to dismantle Three Mile Island in the United States after a 1979 incident.

"The Japanese government has always hoped to draft a detailed roadmap (towards decommissioning of the reactors)," he said.

"But the very fact that the reactors are unstable puts us in a situation where we have to continue to debate whether we can issue a responsible outlook," he said.

Edano has repeatedly said he cannot "prejudge" the outcome of the accident and warned that the situation could still take a turn for the worse.

As of Saturday morning, nearly 300,000 households in Iwate, Miyagi and Fukushima prefectures remained without power after Thursday's aftershock.

The tremor set nerves jangling in an area where tens of thousands of people remain in evacuation centres following the monster 9.0 magnitude quake of March 11 and the towering tsunami it spawned.

Nearly 13,000 people are known to have died in the disaster, with around 15,000 still officially listed as missing.