I'd read that the problem was residual heat, as the control rods should have dropped as soon as coolant flow halted. This means that no new heat is being added to the system, but that the reactor will experience a gradual decline from its 3000 MW operating capacity. All of that energy is stored as irradiated steam, which can be forced into pools of seawater beneath the plant to cool it, but only so much.
I don't think it's true to say no new heat is being added. The uranium fission process has stopped, but there are secondary radioactive elements created by the decay of uranium. These elements will continue to decay, as such the core is still producing the heat. Though eventually this heat will decline to a safe level, provided there isn't a meltdown beforehand.
The design oversight seems to be that they depend on external AC power or diesel generators for backup. Surely there should be an emergency generator that can run off the heat that the core still produces in order to at least partially operate the cooling system. Much like aeroplanes have a small dynamo that runs off a tiny wind emergency wind turbine so they can still operate the hydraulics with no engines running. A saving grace in the case of Gimli Glider (http://en.wikipedia.org/wiki/Gimli_Glider).
Ah, thanks for the explanation. As I said, that was just what I had read. I appreciate the further info. What you said makes sense, and explains why steam pressure is still building.
No problem. Further to my comments about the design, I read in another story linked to on HN that generation III reactors now use convection based cooling. I gather this means the water is taken elsewhere as steam to cool then returns via gravity to the core. Therefore you wouldn't have this same kind of issue with a modern reactor.
My understanding is that new heat continually gets added because the nuclear byproducts decay. It's exponential decay with a long half-life.
I hadn't thought of this, but it makes sense, considering that normally the cooling rods are kept in ponds for years after being removed from the reactor.
