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Corium, also called fuel containing material (FCM) or lava-like fuel containing material (LFCM), is a lava-like molten mixture of portions of nuclear reactor core, formed during a nuclear meltdown, the most severe class of a nuclear reactor accident. 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. ==Composition and formation== 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. Radioactive chain reaction and corresponding increased heat production may progress in parts of the corium if a critical mass occurs locally. This condition can be detected by presence of short life fission products long after the meltdown, in amounts that are too high to be from the controlled reaction inside the pre-meltdown reactor. Because nuclear chain reactions generate high amounts of heat and highly radioactive fission products, this condition is highly undesirable from a reactor vessel structure and biological safety perspective.〔 The temperature of corium depends on its internal heat generation dynamics: the amount 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.〔 The molten reactor core releases volatile elements and compounds. These may be gas phase, such as molecular iodine or noble gases, or condensed aerosol particles after leaving the high temperature region. A high proportion of aerosol particles originate from the reactor control rod materials. The gaseous compounds may be adsorbed on the surface of the aerosol particles. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Corium (nuclear reactor)」の詳細全文を読む スポンサード リンク
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