I'd say the author's writing is entirely consistent with him knowing full well what corium is. This passage in particular shows he doesn't think it's an element

I'd say the author's writing is entirely consistent with him knowing full well what corium is. This passage in particular shows he doesn't think it's an element (emphasis mine): 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.