A statement from operator Tokyo Electric Power Company (Tepco) said the cause of the power failure, which began at 18:57 on Monday, was still being assessed. But it said there is no immediate danger.
The outage had hit ponds at reactors 1, 3 and 4, it said, but cooling to the reactors themselves was not affected.
It would take four days for the hottest pool to hit safety limits, Tepco said.
"We are trying to restore power by then," spokesman Kenichi Tanabe said.
In an announcement Tepco said, "Upon investigation, it was found that part of the power supply facilities at Fukushima Daiichi Nuclear Power Station have stopped due to the incident and the following facilities are currently being suspended."
According to the AP new agency, Tepco said it was trying to repair a broken switchboard that could have caused the problem.
"In the worst case scenario, we have a system in place to put water into the pools, so we have a double strategy: we have time to find a solution before the temperature gets out of hand and then also the ability to inject water into the pools if needed," Tepco official Masayuki Ono said.
Image: © Tepco
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