From what I understand, in the worst possibility (which seems to be getting less and less remote) the core of one or more damaged reactors would melt through the floor of the reactor compartment and, being composed mostly of uranium and therefore extremely hot and extremely heavy, would go down all the way through the Earth's crust until it hits the mantle. Wouldn't this produce a new and unusually nasty volcano spewing highly radioactive lava? Or am I making it even worse than it is?