Here is depicted a possible scenario.
quote :
Reactor vessel breachingIn absence of adequate cooling, the inside of the reactor overheats, deforms as the portions undergo thermal expansion, then structurally fails once the temperature reaches the melting point of the structural materials. The melt then accumulates on the bottom of the reactor vessel. In case of adequate cooling of the corium melt, it can solidify and the spread of damage is limited to the reactor. However, corium may melt through the reactor vessel and flow out or be ejected as a molten stream by the pressure inside the reactor. The reactor failure may be caused by overheating of its bottom by the corium melt, resulting first in creep failure and then in breach of the vessel. High level of cooling water above the corium layer may allow reaching a thermal equilibrium below the metal creep temperature, without reactor vessel failure.[5]
If the vessel is sufficiently cooled, a crust between the melt and the reactor wall can form. The layer of molten steel on top of the oxide creates a zone of increased heat transfer to the reactor wall;[1] this condition, known as "heat knife", exacerbates probability of formation of a localized weakening of the side of the reactor vessel and subsequent corium leak.
In 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 forms in the center of the hole and gas is discharged together with the melt, resulting in rapid decrease of pressure inside the reactor; the high temperature of the melt also causes rapid erosion and enlargement of the vessel breach. If a 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, retaining its portion inside the reactor.[6] Melt-through of the reactor vessel may take from few tens of minutes to several hours.
After breaching the reactor vessel, the conditions in the reactor cavity below the core govern the 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.[7]
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