Reply #16: Corium..... [View All]

Corium (nuclear reactor)
From Wikipedia, the free encyclopedia

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, control rods, structural materials from the affected parts of the reactor, products of their chemical reaction with air, water and steam, and, in case the reactor vessel is breached, molten concrete from the floor of the reactor room.

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Corium-concrete interactions

Thermal decomposition of concrete yields water vapor and carbon dioxide, which may further react with the metals in the melt, oxidizing them and being reduced to hydrogen and carbon monoxide. Decomposition of the concrete and volatilization of its alkali components are endothermic processes. Aerosols released during this phase are primarily based on concrete-originating silicon compounds. Otherwise volatile elements, e.g. caesium, can be bound in nonvolatile insoluble silicates.<2>

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 nonoxidized metals present in it and yielding 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 1300–1500°C for a considerable time. An eventually present layer of more dense molten metal, containing fewer radioisotopes (Ru, Tc, Pd..., 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 below, slowing down the corium penetration and solidifying within a couple of hours. The oxide layer produces heat primarily by decay heat, while the principal heat source in the metal layer is exothermic reaction with water released from the concrete. Decomposition of concrete and volatilization of the alkali metal compounds consumes substantial amount of heat.<2> The fast erosion phase of the concrete basemat lasts for about an hour and progresses into about one meter depth, then slows to several centimeters per hour, and stops completely when the melt cools below the decomposition temperature of concrete (about 1100°C). 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.<3> 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 time when most of early aerosols is already deposited. Tellurium is released with progress of zirconium telluride decomposition. Bubbles of gas flowing through the melt promote aerosol formation.<2>

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