The shutdown of a key thermal power plant may mean Hokkaido could see its electricity supplies reduced for over a week, a state minister said Thursday, following a powerful earthquake that rattled the island early that morning.

Efforts to restart the Tomato Atsuma power station in the town of Atsuma have been hampered by infrastructure damage and a fire at the plant after the magnitude 6.7 quake struck.

With the coal-fired power plant that provides roughly half of Hokkaido’s electricity offline, around 2.95 million households served by Hokkaido Electric Power Co. experienced blackouts Thursday. The government said later in the day that supplies had been restored to about 340,000 homes.

The number of households affected by the power outage exceeds the country’s previous high of 2.6 million that lost electricity in the aftermath of the Great Hanshin Earthquake that struck Kobe and its surrounding area in 1995.

The operator of the Tomato Atsuma plant had initially planned to restart power generation at one of its three units, but a fire related to an oil leak meant it was forced to keep the unit idle. The remaining two units suffered quake damage, according to the industry ministry. When fully operational, the plant can generate a combined 1.65 million kilowatts of electricity.

As efforts to restore power continue, available capacity will likely increase to around 2.90 million kilowatts through the restart of some hydropower and thermal power generation sources by the end of Friday — roughly 76 percent of Hokkaido’s peak electricity demand registered on Wednesday.

But Economy, Trade, and Industry Minister Hiroshige Seko told reporters that it will likely take over a week for capacity to return to a level that can meet peak demand.

The idled Tomari nuclear power plant temporarily lost its external power supply, forcing operator Hokkaido Electric to cool a spent fuel pool at its Nos. 1 to 3 reactors using an emergency power supply system, regulators said. The power was restored to all the reactor units by 1 p.m.

The plant, which has been in shutdown since the aftermath of the Fukushima disaster, lost power soon after Thursday’s earthquake, the government said earlier in the day. Chief Cabinet Secretary Yoshihide Suga told reporters said that according to the operator, there had been no radiation irregularities at the plant.

The atomic regulator had said the emergency diesel generators had enough fuel to last seven days.

The March 11, 2011, magnitude 9 earthquake that struck off the northern Honshu coast set off a massive tsunami that devastated a wide swath of the Pacific coastline and left nearly 20,000 dead. The quake knocked out power to the Fukushima No. 1 nuclear power plant, and the tsunami swamped diesel generators placed in low floors within reactor buildings, leading to a series of explosions and meltdowns in the world’s worst nuclear disaster for 25 years.

The crisis led to the shutdown of the country’s nuclear industry — once the world’s third biggest. Seven reactors have come back online after a protracted relicensing process. The majority of Japanese remain opposed to nuclear power after Fukushima highlighted failings in regulation and operational procedures in the industry.

KEYWORDS energy, earthquakes, hokkaido, disasters