corium isn't an element- it's a term for the material created during a nuclear meltdown. In the case of Chernobyl:
The largest known amounts of corium were formed during the
Chernobyl disaster.
[15] The molten mass of reactor core dripped under the reactor vessel and now is solidified in forms of
stalactites,
stalagmites, and lava flows; the best known formation is the "Elephant's Foot," located under the bottom of the reactor in a Steam Distribution Corridor.
[16][17]
The Chernobyl corium is composed of the reactor uranium dioxide fuel, its zircaloy cladding, molten concrete, and decomposed and molten
serpentinite packed around the reactor as its thermal insulation.