The cooling systems for the spent fuel pools of the Nos. 1, 3 and 4 reactors at the Fukushima-Daiichi plant resumed operations Tuesday after being suspended for nearly two days due to a power failure.
Plant operator Tokyo Electric Power Co. said it suspects a rat-like animal of shorting the circuit in a switchboard causing the outage, Kyodo News reported.
The company said it found burn marks Wednesday on a makeshift power switchboard connected to the cooling systems and a dead animal nearby.
The entire incident, however, did not affect the injection of water into the Nos. 1, 2 and 3 reactors that had experienced core meltdowns when the nuclear plant was hit by a devastating earthquake and tsunami on March 11, 2011.
Besides the cooling systems, Tepco also reportedly has restored operations at several other facilities affected by the power outage, including an installation to remove radioactive substances from water used to cool reactors and a cooling system for a common pool located inside another building at the site.
There had, however, been no threat of a radiation release as no major radioactivity level changes were observed by monitoring posts.