would be a melt down that would stay in containment, but
this guy says it could be worse:
If workers are unable to get additional cooling water into the reactor vessel, the molten fuel core will collapse into the water in bottom of the vessel. Eventually the heat from the decaying fuel would boil away the water that's left, leaving the core sitting on the vessel's lower head made of steel.
Should that happen, "It'll melt through it like butter," Allen said.
That, in turn, would cause a "high-pressure melt injection" into the water-filled concrete cavity below the reactor. Because the concrete would likely be unheated, the reaction created by the sudden injection of the reactor's ultra-hot content would be immense, he said.
"It'll be like somebody dropped a bomb, and there'll be a big cloud of very, very radioactive material above the ground," Allen said, noting that it would contain uranium and plutonium, as well as the fission products.
I'll say it now: GAH!
That sounds uncomfortably close to what was described in the China