In the absence of adequate cooling, the materials inside of the reactor vessel overheat and deform as they undergo thermal expansion, and the reactor structure fails once the temperature reaches the melting point of its structural materials. The corium melt then accumulates at the bottom of the reactor vessel. In the case of adequate cooling of the corium, it can solidify and the damage is limited to the reactor itself. Corium may also melt through the reactor vessel and flow out or be ejected as a molten stream by the pressure inside the reactor vessel. The reactor vessel failure may be caused by heating of its vessel bottom by the corium, resulting first in creep failure and then in breach of the vessel. Cooling water from above the corium layer, in sufficient quantity, may obtain a thermal equilibrium below the metal creep temperature, without reactor vessel failure.
If the vessel is sufficiently cooled, a crust between the corium melt and the reactor wall can form. The layer of molten steel at the top of the oxide may create a zone of increased heat transfer to the reactor wall; this condition, known as "heat knife", increases the probability of formation of a localized weakening of the side of the reactor vessel and subsequent corium leak.
In the case of high pressure inside the reactor vessel, breaching of its bottom may result in high-pressure blowout of the corium mass. In the first phase, only the melt itself is ejected; later a depression may form in the center of the hole and gas is discharged together with the melt with a rapid decrease of pressure inside the reactor vessel; the high temperature of the melt also causes rapid erosion and enlargement of the vessel breach. If the hole is in the center of the bottom, nearly all corium can be ejected. A hole in the side of the vessel may lead to only partial ejection of corium, with a retained portion left inside the reactor vessel.
Melt-through of the reactor vessel may take from a few tens of minutes to several hours.
After breaching the reactor vessel, the conditions in the reactor cavity below the core govern the subsequent production of gases. If water is present, steam and hydrogen are generated; dry concrete results in production of carbon dioxide and smaller amount of steam.