Corium (nuclear reactor)

Corium (also called fuel containing material (FCM) or lava-like fuel containing material (LFCM)) is the name for the lava-like mixture of fissile material created in a nuclear reactor's core during a nuclear meltdown.

It consists of 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.

The heat causing the melting of a reactor may originate from the nuclear chain reaction, but more commonly decay heat of the fission products contained in the fuel rods is the primary heat source. The heat production from radioactive decay drops quickly, as the short half-life isotopes provide most of the heat and radioactive decay, with the curve of decay heat being a sum of the decay curves of numerous isotopes of elements decaying at different exponential half-life rates. A significant additional heat source can be the chemical reaction of hot metals with oxygen or steam.

The temperature of corium depends on its internal heat generation dynamics: the quantities and types of isotopes producing decay heat, dilution by other molten materials, heat losses modified by the corium physical configuration, and heat losses to the environment. A compact corium mass will lose less heat than a thinly spread layer. Corium of sufficient temperature can melt concrete. A solidified mass of corium can remelt if its heat losses drop, by being covered with heat insulating debris, or if water that is cooling the corium evaporates.

Crust can form on the corium mass, acting as a thermal insulator and hindering thermal losses. Heat distribution throughout the corium mass is influenced by different thermal conductivity between the molten oxides and metals. Convection in the liquid phase significantly increases heat transfer.

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