Tokyo Electric Power Co lost power used for cooling spent uranium fuel rods at its Fukushima Daiichi nuclear plant.
The company, also known as Tepco, said there was no immediate threat of overheating or radiation releases after the loss of electricity earlier this week.
It has now partially restored power, the company said.
But the power loss shows that the plant remains in a precarious state after an earthquake and tsunami in March 2011 damaged the plant, causing the world's worst nuclear accident in 25 years.
"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," Masayuki Ono, a Tepco general manager, told reporters at the company's Tokyo headquarters.
While the utility says it does not know what caused the loss of power for cooling equipment for four spent fuel pools, Ono said a temporary switchboard linked to the affected areas may provide clues to the power loss.
Tepco has got the cooling system working at the No.1 reactor.
The utility said it expected cooling operations to restart at the No.3 and No.4 reactors later today, and the shared pool on Wednesday morning.
According to the utility's estimates, it would take a minimum of four days for the spent fuel pool at the No.4 reactor to exceed the legal limit of 65 degrees Celsius (149 degrees Fahrenheit) if the power stays off and no additional water is added.
The power loss is just one hurdle Tepco faces as it works to decommission the Fukushima Daiichi complex, in an unprecedented clean-up effort expected to take decades.
Earlier this month, a Tepco official said the company is struggling to stop groundwater flooding into damaged reactors and it may take four years to fix the problem.