* Mag 6.7 quake hit northern Hokkaido island early on Thursday
* Fuel rods cooled using back-up diesel generators
* Fukushima disaster came after tsunami knocked out back-up power
* Restoring full power to island may take a week - minister (Recasts throughout, adds comment)
By Osamu Tsukimori and Aaron Sheldrick
TOKYO, Sept 6 (Reuters) - Power was restored to a nuclear energy plant in Hokkaido, northern Japan on Thursday after a strong earthquake left it relying on emergency generators for 10 nervous hours, but it may be a week before lights are back on all over the major island.
Triggering a blackout just after 3 a.m. local time, the magnitude 6.7 quake left at least seven people dead, more than 100 injured and dozens missing on Hokkaido, an island of about 5.3 million people whose capital is Sapporo. A major coal-fired power station was also damaged in the temblor that shut down the grid.
The situation at utility Hokkaido Electric Power's three-reactor Tomari nuclear plant provided an uncomfortable, if comparatively brief, echo of the Fukushima Daiichi nuclear disaster in 2011. Reactors there melted down after a massive tsunami knocked out back-up generators, designed to maintain power to cool reactors in emergencies.
Though Tomari was shut down after the Fukushima disaster in 2011, it needs electricity to keep fuel rods cool, and had to rely on back-up diesel generators that kicked in after the quake until power was restored to all three reactors by 1 p.m. local time.
Hokkaido Electric is now scrambling to get power restored to households, factories and other customers, even as rescuers bring in heavy machinery and digging equipment to search for survivors that may be trapped in stricken buildings.
In another echo of the 2011 disaster, Hokkaido's situation was compounded by difficulties in receiving supplies from neighbouring regions that remain dominated by separate utilities with their own grids.
A Hokkaido Electric spokesman said the utility was not receiving any supplies from the island of Honshu to the south - home to Tokyo, Osaka and Nagoya - despite there being a 600 megawatt connection for transferring power from the coast of Japan's main island.
"Nationwide, they have been trying to bolster power systems by linking these fairly Balkanized domains, but it's not as easy as it sounds," said Andrew DeWit, professor of energy policy at Rikkyo University in Tokyo. "It requires equipment like transformers that is quite expensive and tailor-made."
Damage to transmission lines, transformers and other equipment from the quake is likely also delaying the restoration of power, DeWit said.
By early afternoon, Hokkaido Electric had restarted one 125 megawatt coal-fired unit at its Sunagawa station.
Its Tomato-Atsuma coal station, which normally supplies about half the island's electricity, remained shut after sustaining damage in the quake.
The abrupt halt in supplies from Tomato-Atsuma caused such a huge imbalance in supply and demand that other power plants on Hokkaido had to be shut own, industry minister Hiroshige Seko told reporters in Tokyo. it may take a week to restore power fully to Hokkaido, he said.
Thursday's blackout prompted the shutdown of Hokkaido's New Chitose Airport, a popular gateway to the most northern of Japan's main islands, the second major airport to be knocked out in the country in two days after a typhoon swamped the nation's third-biggest.
A number of industrial plants and factories were also affected.
A fire that broke out at a steel production facility at Mitsubishi Steel Manufacturing's joint venture with Nippon Steel & Sumitomo Metal Corp in Muroran, Hokkaido, was extinguished with little damage to the facilities, a company official said.
The plant will work towards resuming special steel production after power supply from Hokkaido Electric is restored, the official said. It usually takes about eight hours to resume production after the power supply has been secured, he said.
Meanwhile Idemitsu Kosan, Japan's second-biggest refiner by sales, stopped all refining and product shipments at its 150,000 barrels per day Hokkaido refinery, the only one on the island, an official told Reuters.
Kirin Beer, part of giant drinks maker Kirin Holdings Co , said it closed its Chitose plant, which accounts for about 5 percent of its beer output, because of the power outage. There was no structural damage to the factory, it said.
Sapporo Holdings Ltd said it closed down its Hokkaido brewery because of the lack of power.
Reporting by Osamu Tsukimori and Aaron Sheldrick Additional reporting by Linda Sieg Editing by Kenneth Maxwell