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.
Power outages struck Tohoku’s other nuclear plant, Onagawa, where power was lost at three of the four power lines that were feeding electricity to the plant late Thursday. The one surviving line allowed Tohoku to keep the plant operating normally, the nuclear regulator said.
Radioactive water, estimated at about four liters, spilled from the spent fuel pool at Onagawa from the force of the quake. Agency officials said the water was lightly radioactive but didn’t disclose further details.
The temblor also triggered a shutdown of power supply, leaving the cooling systems out of service for about an hour. Power supply was restored early Friday after workers confirmed the safety of the facility. While conditions can vary widely, an absence of cooling for more than 24 hours can lead to overheating of nuclear fuel and a potential radiation build-up.
The earthquake caused no structural damage to the plant, the agency said.
At the Fukushima Daiichi nuclear power plant, where three of its six reactors are still at risk, operations to pump cooling water into Reactors Nos. 1, 2 and 3 and nitrogen into Reactor No. 1 to forestall hydrogen explosions continued unhindered. Hydrogen explosions happen when hydrogen produced by damaged nuclear fuel reacts with oxygen in the atmosphere.
But the temperature of Reactor No. 1 briefly jumped from 223.3 degrees Celsius to 260.7 degrees Celsius before coming down shortly thereafter. Authorities offered no explanation for the short-term rise in temperature that came as they are seeking to cool down the units over time.
Authorities said that the Daiichi plant, and the Daini plant located 13 kilometers south, suffered no outage in electricity from the quake.
As the crisis at the Fukushima Daiichi plant drags on, authorities have said that they would consider changes to the current evacuation zone of 20 kilometers, with an additional 10-kilometer band in which residents are urged to stay outdoors. It is unclear how many people are in the area, though Japanese media estimates suggest about 20,000 people remain within the 10-kilometer band.
Chief government spokesman Yukio Edano said that a decision on the evacuation zone would be made in a few days but offered no clues on what actions the government may make. Residents and officials in some towns northwest of the plant have expressed increasing concerns over the presence of radiation levels that would be over normal safety limits for exposure over long periods of time.
Mr. Edano didn’t disclose Friday whether the government was responding to any specific new data or findings. Tokyo has maintained that it is examining radiation data and discussing it with regulators on an ongoing basis.
He said that with some radiation levels declining in parts of the region, bans on shipments of spinach and milk from parts of Fukushima prefecture and neighboring Gunma prefecture will be dropped.
