Thermal decomposition of concrete produces water vapor and carbon dioxide, which may further react with the metals in the melt, oxidizing the metals, and reducing the gases to hydrogen and carbon monoxide. The decomposition of the concrete and volatilization of its alkali components is an endothermic process. Aerosols released during this phase are primarily based on concrete-originating silicon compounds; otherwise volatile elements, for example, caesium, can be bound in nonvolatile insoluble silicates.
Several reactions occur between the concrete and the corium melt. Free and chemically bound water is released from the concrete as steam. Calcium carbonate is decomposed, producing carbon dioxide and calcium oxide. Water and carbon dioxide penetrate the corium mass, exothermically oxidizing the non-oxidized metals present in the corium and producing gaseous hydrogen and carbon monoxide; large amounts of hydrogen can be produced. The calcium oxide, silica, and silicates melt and are mixed into the corium. The oxide phase, in which the nonvolatile fission products are concentrated, can stabilize at temperatures of  for a considerable period of time. An eventually present layer of more dense molten metal, containing fewer radioisotopes (Ru, Tc, Pd, etc., initially composed of molten zircaloy, iron, chromium, nickel, manganese, silver, and other construction materials and metallic fission products and tellurium bound as zirconium telluride) than the oxide layer (which concentrates Sr, Ba, La, Sb, Sn, Nb, Mo, etc. and is initially composed primarily of zirconium dioxide and uranium dioxide, possibly with iron oxide and boron oxides), can form an interface between the oxides and the concrete farther below, slowing down the corium penetration and solidifying within a few hours. The oxide layer produces heat primarily by decay heat, while the principal heat source in the metal layer is exothermic reaction with the water released from the concrete. Decomposition of concrete and volatilization of the alkali metal compounds consumes a substantial amount of heat.
The fast erosion phase of the concrete basemat lasts for about an hour and progresses to about one meter in depth, then slows to several centimeters per hour, and stops completely when the melt cools below the decomposition temperature of concrete (about ). Complete melt-through can occur in several days even through several meters of concrete; the corium then penetrates several meters into the underlying soil, spreads around, cools and solidifies.
During the interaction between corium and concrete, very high temperatures can be achieved. Less volatile aerosols of Ba, Ce, La, Sr, and other fission products are formed during this phase and introduced into the containment building at a time when most of the early aerosols are already deposited. Tellurium is released with the progress of zirconium telluride decomposition. Bubbles of gas flowing through the melt promote aerosol formation.
The thermal hydraulics of corium-concrete interactions (CCI, or also MCCI, "molten core-concrete interactions") is sufficiently understood.
The dynamics of the movement of corium in and outside the reactor vessel is highly complex, however, and the number of possible scenarios is wide; slow drip of melt into an underlying water pool can result in complete quenching, while the fast contact of a large mass of corium with water may result in a destructive steam explosion. Corium may be completely retained by the reactor vessel, or the reactor floor or some of the instrument penetration holes can be melted through.
The thermal load of corium on the floor below the reactor vessel can be assessed by a grid of fiber optic sensors embedded in the concrete. Pure silica fibers are needed as they are more resistant to high radiation levels.
Some reactor building designs, for example, the EPR, incorporate dedicated corium spread areas (core catchers), where the melt can deposit without coming in contact with water and without excessive reaction with concrete.
Only later, when a crust is formed on the melt, limited amounts of water can be introduced to cool the mass.
Materials based on titanium dioxide and neodymium(III) oxide seem to be more resistant to corium than concrete.
Deposition of corium on the containment vessel inner surface, e.g. by high-pressure ejection from the reactor pressure vessel, can cause containment failure by direct containment heating (DCH).