Corium is the material formed during the meltdown of a nuclear reactor. It is composed of nuclear fuel (uranium or plutonium) and material that melts on contact with the fuel.
When the fuel rods containing the nuclear fuel overheat, the temperature rises and the interior of the reactor starts to melt. Such an event occurs following a cooling system malfunction.
The corium progresses towards the bottom of the reactor and can, if it is still not cooled, begin to erode the bottom of the vessel and the concrete shields separating the reactor from the exterior, as was the case in reactor No.1 at the Fukushima power station in 2011.
In Fukushima the corium from reactor 1 in the nuclear power station eroded part of the concrete. © daveeza, Flickr, cc by sa 2.0