Japan's Fukushima Daiichi nuclear power plant lost the ability to cool radioactive fuel rods for about three hours on Friday.
It was the second failure of the system to circulate seawater to cool spent fuel rods at the plant in the past three weeks.
Tokyo Electric Power Co (Tepco) said it was alerted by an alarm that the cooling system for the No. 3 reactor had shut down at around 2:27 p.m. (0527 GMT) after a power board failed.
Technicians fixed the problem by about 5:20 p.m., the utility said.
Tepco said workers appeared to have had inadvertently caused the power outage when they were trying to install a net to keep small animals from crawling into the reactor building.
A rat caused a power outage at Fukushima last month when it triggered a circuit breaker by bumping into exposed wires. In that case, the cooling system was shut down for almost a day.
The utility has faced a range of problems with controlling ground water and maintaining the massive cooling system built to keep the reactors stable.
The plant was the site of the worst nuclear disaster since Chernobyl in March 2011 when a magnitude 9.0 earthquake triggered a tsunami that destroyed back-up generators at Fukushima and disabled the plant's cooling system. Three of the reactors melted down.
Work to decommission the plant is projected to take decades to complete.