It's mentioned somewhat, but admittedly not terribly clear:
> Of the five corium creations, only Cherobyl’s has escaped its containment. With no water to cool the mass, the radioactive sludge moved through the unit over the course a week following the meltdown, taking on molten concrete and sand to go along with the uranium (fuel) and zirconium (cladding) molecules.
> Of the five corium creations, only Cherobyl’s has escaped its containment. With no water to cool the mass, the radioactive sludge moved through the unit over the course a week following the meltdown, taking on molten concrete and sand to go along with the uranium (fuel) and zirconium (cladding) molecules.