Pretending that the Fukushima reactors achieved a state of “cold shutdown” was a political – rather than scientific - decision.
Tepco itself said the state of cold shutdown could only continue so long as the temperature within the nuclear reactors stayed below 100 degrees Celsius. (Because the thermometers within the reactors have a 20 degree margin of error, Tepco says that any reading over 80 degrees violates the conditions for a cold shutdown.)
As Bloomberg notes today:
Tokyo Electric Power Co. said the temperature in one of the damaged reactors at its Fukushima nuclear station rose to levels above safety limits even as it injected increased amounts of cooling water.
One of three thermometers indicated the temperature at the bottom of the No. 2 reactor pressure vessel rose to 93.7 degrees Celsius (200.7 Fahrenheit) today, higher than the 80 degrees limit, Ai Tanaka, a spokeswoman for the utility known as Tepco, said by phone today.
The thermometers have a margin of error of as much as 20 degrees.
But major Japanese news sources Yomiuri and Jiji note that the thermometer in reactor 2 has since climbed to 272.8 degrees Celsius, and then hit the upper limit of the thermometer at 400 degrees Celsius (752 degrees Fahrenheit).
In other words, the thermometer is showing temperatures more than 4 times higher than the 100 degree Celsius limit for cold shutdown.
Tepco claims that such a high reading means that the thermometer must be broken, and is maintaining its declaration of cold shutdown based upon the reading of other thermometers. Of course, the fuel is moving around, so there could be hot spots and cooler spots within each reactor.
Of course, Tepco could be right: the thermometer could be broken. But I am not yet convinced, given that – ever since the earthquake last year – Tepco has repeatedly claimed that an instrument is broken whenever there is a new reading of things gone haywire. (Indeed, one Japanese writer said that Tepco’s spokesman sounded “testy” when asked how Tepco knew that the thermometer was broken.)
In good news, a second, nearby 4-reactor nuclear complex which almost melted down last March – the Fukushima Daini complex (referred to as the Fukushima “2? complex), which is 7 miles away from the infamous, leaking 6-reactor Fukushima Daiichi complex – apparently is in a true state of cold shutdown.