Tokyo Electric Power Co. concluded March 25 that a charred rat found electrocuted beneath a temporary switchboard was behind a recent extended blackout at the utility's crippled Fukushima No. 1 nuclear power plant.
The blackout, which affected critical cooling systems at the nuclear plant's spent fuel storage pools, began on March 18 and lasted 29 hours.
The temporary switchboard arrived at the plant on the back of a truck shortly after the March 2011 nuclear disaster and has been in use there ever since.
Following the blackout, TEPCO investigators found a terminal with scorch marks inside the switchboard's housing and a dead rat, clearly electrocuted, lying on the floor nearby.
Analysis of the operation records of the cooling systems and other equipment led them to conclude that a short-circuit caused by the rat climbing across live terminals caused the blackout, TEPCO officials said.
The investigators spotted no anomalies in the other switchboards.
Investigators believe the rat entered through an opening in the housing. To prevent a recurrence, workers have disconnected key devices from the temporary switchboard, which remains outdoors, and reconnected them to rat-proof, indoor switchboards, the utility said.
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