lava-like material created in the core of a nuclear reactor during a meltdown accident

Corium (also called fuel containing material (FCM) or lava-like fuel containing material (LFCM)) is a lava-like fuel containing nuclear fuel, fission products, control rods, structural materials from the affected parts of the reactor, products of their chemical reaction with air, water and steam, and, in the event that the reactor vessel is breached, molten concrete from the floor of the reactor room. It is created in a nuclear reactor's core while a nuclear meltdown occurs.

Nuclear fuel, fission products, and control rods are what consists of Corium.