A dead rat was found lying near equipment that likely short-circuited this week and plunged critical cooling systems at the Fukushima No. 1 nuclear plant into crisis again.
Officials of Tokyo Electric Power Co., the plant operator, said on March 20 they found the remains of a rodent inside the housing of a scorched switchboard.
They now believe that is where the blackout began, possibly when the rat climbed across live terminals and shorted them together.
Power was lost to a cooling system that keeps the water of fuel rod storage pools from heating up.
The outage lasted a little over a day. All cooling equipment was back up early on March 20.
Company officials subsequently began an investigation. Shortly after noon that day, they discovered charring on a temporary switchboard delivered to the site aboard a truck after the March 2011 accident and which has been in use there ever since. It is the only temporary switchboard now in use at the plant.
Inside the unit's housing, workers found the remains of a small animal, likely a rat, about 15 centimeters from nose to tail, lying near the charred terminal. Officials plan to examine the matter further.
Such a power loss should ordinarily have affected only one electrical network that supplies power to cooling equipment for spent fuel pools at reactors No. 3 and No. 4 and a common pool for spent fuel.
But in this instance, a blackout occurred in a second network, too, which led to the failure of power in equipment that keeps spent fuel cool in the storage pool at the No. 1 reactor.
Ordinarily, the two electrical networks are independent from one another. However, the two were temporarily connected together when maintenance work took place to strengthen the site's systems against the impact of another tsunami. That may be why a blackout in one triggered another simultaneously.
TEPCO officials said the cooling equipment for the nuclear fuel storage pools in the No. 1, No. 3 and No. 4 reactors as well as that for the common storage pool were all functioning normally.
(This article was written by Naoya Kon and Yu Kotsubo.)
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