The operator of the crippled Fukushima Daiichi nuclear plant said Wednesday that it had found what it believed was the cause of an extended blackout that disabled vital cooling systems this week: the charred body of a rat.
The operator, Tokyo Electric Power Company, said that when its engineers looked inside a faulty switchboard, they found burn marks and the rodent’s scorched body. The company said it appeared that the rat had somehow short-circuited the switchboard, possibly by gnawing on cables.
The company, known as Tepco, has blamed problems with the switchboard for the power failure that began Monday, cutting off the flow of cooling water to four pools used to store more than 8,800 nuclear fuel rods. It took Tepco almost a day to restore cooling to the first of the affected pools, with cooling of the final pool resuming early Wednesday.
Tepco said it would have taken several days for temperatures in the pools to have risen above the safe level of 65 degrees Celsius, or 149 degrees Fahrenheit. Still, the blackout served as an uncomfortable reminder to many Japanese about the continuing vulnerability of the plant, which had a triple meltdown in March 2011 after a huge earthquake and tsunami knocked out cooling systems. It was the worst nuclear disaster since Chernobyl.
Two years later, the Fukushima plant still relies on makeshift cooling systems, some of which were built as stopgap measures in the frantic weeks and months after the accident. The spent fuel pools have been a particular source of concern because they contain far more radioactive material than the three reactor cores that melted down two years ago, forcing the evacuation of 160,000 people.
A Tepco spokesman, Masayuki Ono, said temperatures in the pools were cooling, though it would take several days for them to get back to their pre-blackout levels.
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.)
http://ajw.asahi.com/article/0311disaster/fukushima/AJ201303210068