Physicist: Nuclear reactors are very 19th century in a way.
The nuclear fuel is basically a bunch of very hot metal, and the more of it you get together in one place, the hotter it gets. That heat is used to turn water into steam, which turns turbines.
Uranium and plutonium (nuclear fuel) undergo radioactive decay on their own, at random. This is the base radioactive rate. But some isotopes can also be made to decay by hitting them with an extra neutron (it’s called being “fissile“). As luck would have it, when uranium (and plutonium) decays it generates a spray of neutrons, and can induce nearby material to decay as a result.
So, you can “throttle” how much heat the fuel rods (bars of nuclear fuel) produce by shielding or not shielding them from each other: the more they’re exposed to each other, the more they make each other react, and the more heat they make. So you can never “turn them off”. Each fuel rod will continue to produce heat, even on it’s own (just less). We’re used to thinking of things cooling off on their own, but that’s exactly what nuclear fuel doesn’t do.
In a reactor, the average number of other atoms set off by each decaying atom is less than one. This means that the rate of radioactive decay levels out at a relatively low level. I mean, you wouldn’t want to be in the room, but it’s not going to blow up either. The base radioactive rate (the random decays) keeps the rate from dropping all the way to zero.
However, if the average is more than one, then the rate of decay will keep increasing exponentially. This is called a “run away nuclear chain reaction” or, in the more common vernacular; “a bomb”. The amount of material you need to bring together to produce this effect is called “critical mass”.
In an emergency, the rods are generally dropped into slots so that they can’t interact (this didn’t happen at Chernobyl, and resulted in a runaway chain reaction). However even without critical mass, if there’s a failure in the cooling system then the rods, and all the supporting material, will eventually melt. Depending on the types of materials used you can also get nasty chemical effects, like the coolant water being broken down into hydrogen and oxygen (this is one of the things that happened at 3 Mile Island).
There isn’t really a maximum temperature that the nuclear fuel can reach. Or at least, it’s really, really high. It’ll just keeps heating up until it can spread out, somehow.
And that’s a meltdown.
Some of the worst case scenarios are: the fuel melting through the floor of the power plant and getting into the water table, melting and pooling together at critical mass, or getting so hot that it vaporizes. There are a lot of safety precautions in place to keep this stuff from happening.
It would be bad if all of the fuel in a nuclear plant vaporized (mostly to nearby people). But, to put it in perspective, it’s much, much worse to leave coal plants running normally. It has been estimated that in 1982 alone more than 10,000 tons of Uranium and Thorium (both nasty) was released into the air by coal powered generators world wide. There are a lot more coal plants open these days, but I can’t find the exact data.
Not to go off on a tangent; but the world would be much better off with a Chernobyl sized disaster every month, than it is with the amount coal pollution we produce today.
For those of you (not presently living in Japan) worried about radiation exposure, just spend an extra minute or two not in direct sunlight. That should more than make up for it.