Add corium to your Scrabble arsenal. It's basically a lava-life fluid comprised of very hot fuel rods, control rods, and other parts of melted reactor vessel. It's a potential end game for the Japanese reactors. In the case of Three Mile Island the corium did not ultimately breach the reactor vessel. In Chernobyl it did. Or more correctly it got "outside" the reactor core because it, well, blew up.
Here's a picture of it as it oozed out of a pressure release valve in Chernobyl and then solidified.