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Why would a shut down reactor be running at a higher temperature than when it’s running normally?

What have we got so far?

Japanese nuclear reactors shut down automatically when the earthquake hit the country but the electrical system failed after being swamped by a 10m sea surge which lead to a loss of coolant/coolant scenario. The residual decay heat generated in the fuel rods raised the pressure in the uncooled reactors and the forced  release of a steam/hydrogen mixture ignited resulting in the demolition of several buildings.

Residual decay heat generation falls away rapidly when a reactor is shut down and it would seem that the dry storage ponds should be the most pressing concern rather than the reactors themselves.

Having said that, the following from Reuters/ZeroHedge is something of a puzzle :

The news this time comes straight from TEPCO which finally admits that the temperature of Reactor 1 is 380-390 Celsius (715-735 Fahrenheit), which apparently is a “worry” as the reactor was meant to run at a temperature of 302 C (575 F). That is when the reactor is fully operational, not when it is supposed to be in a cold shut down mode.From Reuters:

Hidehiko Nishiyama, the deputy-director general of Japan’s nuclear safety agency, later said the smoke at reactor No.3 had stopped and there was only a small amount at No.2.

He gave no more details, but a TEPCO executive vice president, Sakae Muto, said the core of reactor No.1 was now a worry with its temperature at 380-390 Celsius (715-735 Fahrenheit).

“We need to strive to bring that down a bit,” Muto told a news conference, adding that the reactor was built to run at a temperature of 302 C (575 F).

Asked if the situation at the problem reactors was getting worse, he said: “We need more time. It’s too early to say that they are sufficiently stable.”

It could be that bugger all water is in the reactor so the structure is just sitting there hot and slowly cooling – given the mass of material involved it would be plausible.

It could also be that some fuel rods have melted and are sustaining their own reaction in the bottom of the reactor. As the current temperature is higher than when the reactor is running normally it does suggest that some additional heat is being generated to raise the temperature.

Either way, they still haven’t really got a grip on this but the news cycle has moved on to Libya so no one but those living nearby really cares anymore it seems.

I just hope this incident isn’t used by the likes of Chris Huhne to press for the installation of even more bird mincers instead of a safe, base load generation system which we so desperately need here in the UK.

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