Plant operator TEPCO said the outage to the cooling system was not an immediate threat, and lasted about three hours. This cooling system is used to regulate optimal temperatures at the spent fuel ponds are reactor No. 3.
A previous cooling system outage in March cut off power to four of the plant's spent fuel ponds, which contain highly radioactive fuel rods that generate thermal heat and must be cooled.
Tepco said that throughout the outage, the temperature of the spent fuel ponds remained within safe limits. Cooling for the reactors themselves was never affected.
Plant operators have been working to contain damaged to Fukushima plant since the March 11, 2011 earthquake and tsunami caused a trio of meltdowns at the plant.