I was more referring to the idea that once you lose the ability to pump coolant, the reactor starts to heat up. Then as it heats up, it starts to melt itself. As it melts, it re-attains criticality and begins to generate more heat, which leads to more melting, which leads to more heat, ad nauseum.

We don't really even know when this stops or how it stops or whatever because the only time (so far) that it really started to get away from us (Chernobyl) a bunch of brave men gave their lives and dumped boron straight on the exposed reactor core to stop it. By doing so they spared us from gaining the very painful knowledge of how a meltdown ends naturally, at what point an equilibrium is reached.

EDIT: spelling