Actually, it really can't happen to commercial-grade fuel. The classic movie scenario is three stages:
Fuel starts to get hot and melts the steel fuel holders (fuel bundle sheathing in Canada). This is probably what has happened here and everybody calls it a meltdown for dramatic effect.
Molten metal puddling in the bottom of the reactor vessel (very thick steel), and the ceramic fuel pellets (4% enriched uranium) also melt.
Melted fuel starts globbing together, and starts a sustained reaction, like a nuclear bomb, and burns all the way to China!
So, really, the whole thing only works if you have bomb-grade uranium, like the seismic death trap at Chalk River. For a commercial reactor, we just get slag at the bottom of the reactor. There is nothing to explode. Right now, it looks like the seawater is doing the trick even though they are just pouring it on the outside of the steel vessel. They probably can't get inside to the fuel because all the metal inside has melted. Now, the US made all its bomb plutonium from graphite reactors with bomb-grade uranium. Those things can have a meltdown!