March 20 (Bloomberg) -- Tokyo Electric Power Co. says it still doesn’t know what caused a power outage at four pools used to cool spent nuclear fuel at the crippled Fukushima Dai-Ichi plant.
Electricity was restored to all of the pools by 12 minutes after midnight, Tokyo Electric said in a statement. The outage, which disabled pumps at three of the plant’s wrecked reactors’ storage pools and a shared pool, started at 6:57 p.m. on March 18, according to the utility.
“We haven’t found the cause,” general manager Masayuki Ono said at a briefing in Tokyo. Ono said 25 workers are investigating the outage, which was the longest affecting multiple units at the plant since the March 11, 2011, disaster that triggered three meltdowns.
No radiation spikes were recorded near the plant during the failure and the outage didn’t affect cooling systems for the plant’s reactors, according to the government. Without power to pump cooling water, uranium fuel rods stored in the pools could have heated up over days and released radiation.
Three reactors melted down at the Dai-Ichi plant after the earthquake and tsunami on March 11 two years ago, making it the world’s worst nuclear accident after Chernobyl. About 160,000 people were forced to evacuate and a 20-kilometer no-go zone was set up because of radiation fallout.
The loss of electricity may have been caused by the failure of a distribution box exposed to wind and sand, Ono said. All of the spent fuels affected by the outage have been relying on a single power source since March 6, when the utility started repairs on cables in a separate system, he said.
Spent fuel pools are typically 40 feet deep and built in reactor buildings. They’re made of reinforced concrete several feet thick and steel sheeting to contain radiation and hold water to cool rods after they have been used in a reactor, according to the U.S. Nuclear Regulatory Commission.
Tokyo Electric and Japan’s government announced in December 2011 that the Fukushima reactors reached a state known as cold shutdown.
Japan’s previous government approved phasing out nuclear power by the end of the 2030s, a policy favored by the largest percentage of citizens in a government poll in August.
Japanese Prime Minister Shinzo Abe, whose Liberal Democratic Party won a landslide majority in December, has said that policy needs to be reconsidered to provide the energy needed to help revive the world’s third-biggest economy.