Violator said:
What would be the absolute worst case nuclear reactor meltdown?
The absolute worst case was Chernobyl. I don't know why another poster said "times ten" because I don't see what could be worse: a serious part of the core material (and other stuff, as the Chernobyl reactor was also a military irradiation facility) thrown high into the atmosphere by a huge fire enhanced by a still working reactor for more than 36 hours, at the moment of shutdown (that means, with the highest possible amount of fission products).
But beware: that's not a *meltdown*. It was a blowing up of the reactor by sudden overheating. Also, a nuclear reactor can *never* undergo a nuclear explosion (as in an atomic bomb). It is physically impossible. You can at most have strong heat production which will make the confinement give up under heat and pressure. At Chernobyl, it was the water that got heated too much in the tubes that blew up the reactor structure.
A *meltdown* is a much less severe accident, and it doesn't imply necessarily any release of radioactivity to the environment. A meltdown happens when the fuel elements are not cooled enough, so that they reach their melting point - inside the reactor. For instance, a meltdown can happen when there is a loss of coolant, or in a PWR, a loss of pressure, so that the cooling water starts boiling off. This is what happened (by wrong manipulations by the control crew) in the 3-miles island accident: the rods containing the fuel can get so hot as to melt. This can happen even with the reactor shut off (in fact, with a loss of coolant, normally the reactor shuts itself off immediately), but because of the heat of radioactive decay. We are talking here about much lower power productions than the 30 GW that blew up the Chernobyl reactor, but nevertheless they can melt the fuel. The thing to do is to restore the cooling, and the incident is closed (but the reactor is damaged). If the cooling cannot be restored, then things can get worse: the heat can end up damaging the reactor vessel, and as such, the fuel can escape the reactor vessel and arrive in the confinement building. If one still doesn't cool this, the confinement building can end up damaged by the hot fuel, and some fuel can escape to the outside (but not in a plume in the atmosphere!). When spread out enough in the neighbourhood, the fuel will cool down and that's it. This is very heavy, but local, contamination.
Again, serious as this is, this is nothing comparable to Chernobyl. In Chernobyl, everything was really put together to make the accident as bad as possible. That's why I cannot imagine anything worse.
In fact, in modern reactor design (such as the EPR), a meltdown is considered as a possible accident that should be handled. Hence, there is a cooled "core catcher" underneath the reactor vessel which stops the accident right there, without any release to the outside.
What sorts of effects could one expect and at what types of distances? And bear in mind, I really am asking about the worst it could possibly get, as in, no one there to SCRAM the control rods, no fire crews responding, that sort of disaster scenario.
You have to know that western reactors are all so designed (contrary to the Chernobyl type reactors, which are therefor considered unsafe - and this was well-known, even before the accident) that they *naturally* shut down by physical mechanisms when things go wrong seriously. There's no action required nor from the control crew, nor from any automatic system: the physics of the reactor is such, that the reactor stops when it gets too hot or when it looses a serious part of its coolant.
This is why even a worst-case accident with a western power plant can never be as severe as the Chernobyl accident (where the reactor was unstoppable because of bad design - they stopped it finally by dumping boron on the carcass with helicopters).
So the only driving force for an accident that remains is decay heat, which is far less than the reactor power.