Japan Restores Power to Rattled Nuclear Plants

TOKYO—Utilities in northern Japan on Friday restored main grid power to three nuclear facilities hit by a strong earthquake the night before, reminding a nervous populace of the vulnerabilities of nuclear plants throughout the earthquake-prone country.

With the restoration of power, plant operators were able to keep their reactors and spent-fuel pools from overheating, avoiding a replay of the disaster that befell the Fukushima Daiichi complex after the devastating March 11 quake.

The nuclear plants in the region hit by last month's quake had already been shut down. But they remain at risk of overheating if the supply of cooling water to their reactor cores and to the pools holding spent fuel is interrupted for an extended period.

Thursday night's 7.1-magnitude temblor, a powerful aftershock from the March 11 quake, left three people dead and injured 141 others, according to local media reports—adding to the 27,000 dead and missing from last month's 9.0 quake. There were isolated reports of damage and fires triggered by the quake, according to authorities.

The region's five conventional power plants—those powered by oil, coal or natural gas—went into automatic shutdown mode when the quake struck. Tohoku Electric Power Co. said that more than 4 million homes were initially without power, although most service was restored by the end of Friday, a spokesman said.

The plant outages and damage to the power grid caused outages across a wide swath of the region, forcing operators to activate emergency diesel power generators to cool spent-fuel rods at the Higashidori nuclear plant, run by Tohoku Electric, and at the Rokkasho fuel recycling plant run by Japan Nuclear Fuel Ltd.

The incident demonstrated the risks that can come in relying on diesel generators. At Higashidori, operators switched to diesel generator after the power was cut. Although the generator worked properly through the power blackout, it was kept running as a backup even after the power grid connection was restored, only to fail at 2:06 p.m. due to an oil leak, according to Japan's Nuclear and Industrial Safety Agency said. While the plant has two other emergency generators, they are currently out of service due to maintenance, the agency said.

Investigators believe the failure of diesel generators at the Fukushima Daiichi plant on March 11 contributed to the crisis there. All but one of the plant's generators was wiped out by the tsunami that followed the quake.