Steam Explosion

A steam explosion (also called a fuel-coolant interaction, FCI) is a violent boiling or flashing of water into steam, occurring when water is either superheated, rapidly heated by fine hot debris produced within it, or the interaction of molten metals (e.g., Fuel-Coolant Interaction of molten nuclear-reactor fuel rods with water in a nuclear reactor core following a core-meltdown). Pressure vessels (e.g., pressurized water (nuclear) reactors) that operate above atmospheric pressure can also provide the conditions for a rapid boiling event which can be characterized as a steam explosion. The water changes from a liquid to a gas with extreme speed, increasing dramatically in volume. A steam explosion sprays steam and boiling-hot water and the hot medium that heated it in all directions (if not otherwise confined, e.g. by the walls of a container), creating a danger of scalding and burning.

Steam explosions are not normally chemical explosions, although a number of substances will react chemically with steam (for example, zirconium and superheated graphite react with steam and air respectively to give off hydrogen, which burns violently in air) so that chemical explosions and fires may follow. Some steam explosions appear to be special kinds of boiling liquid expanding vapor explosion (BLEVE), and rely on release of stored superheat. But many large-scale events (e.g., foundry accidents) show evidence of an energy-release front propagating through the material (see description of FCI below), where the forces created fragment and mix the hot phase into the cold volatile one; the rapid heat transfer at the front sustains the propagation.

If a steam explosion occurs in a confined tank of water due to rapid heating of the water, the pressure wave and rapidly expanding steam can cause severe water hammer. This was the mechanism that caused the SL-1 nuclear reactor vessel to jump over 9 feet (2.7 m) in the air when it was destroyed by a criticality accident. In the case of SL-1, the fuel and fuel elements vaporized from instantaneous overheating.

Events of this general type are also possible if the fuel and fuel elements of a liquid-cooled nuclear reactor gradually melt. Such explosions are known as fuel-coolant interactions (FCI). In these events the passage of the pressure wave through the predispersed material creates flow forces which further fragment the melt, resulting in rapid heat transfer, and thus sustaining the wave. Much of the physical destruction in the Chernobyl disaster, a graphite-moderated, light-water-cooled reactor (an RBMK-1000 reactor), is thought to have been due to such a steam explosion.

In a nuclear meltdown the most severe outcomes would be those leading to early containment failure. Two possibilities are the ejection at high pressure of molten fuel into the containment, causing rapid heating; or an in-vessel steam explosion causing ejection of a missile (e.g., the upper head) into, and through, the containment. Less onerous but still significant would be that the molten mass of fuel and reactor core melts through the floor of the reactor building and reaches ground water; a steam explosion might occur, but the debris would probably be contained, and would in fact, being dispersed, probably be more easily coolable. See WASH-1400 for details.

Steam explosions are often encountered where hot lava meets sea water. Such an occurrence is also called a littoral explosion A dangerous steam explosion can be created when liquid water encounters hot, molten metal. As the water explodes into steam, it splashes the burning hot liquid metal along with it, causing an extreme risk of severe burns to anyone located nearby and creating a fire hazard.