Steam explosion
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A steam explosion (also called a littoral explosion, or fuel-coolant interaction, FCI) is a violent boiling or flashing of water into steam, occurring when water is either superheated, or rapidly heated by fine hot debris produced within it. Pressure vessels that operate at above atmospheric pressure can also provide the proper conditions for 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 reacts with steam to give off hydrogen, which burns violently in air) so that chemical explosions and fires often follow. Some steam explosions appear to be special kinds of Boiling Liquid Expanding Vapor Explosion, and rely on release of stored superheat. But many large-scale events (eg '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.
Steam explosions are often encountered where hot lava meets sea water. 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.
Events of this general type are also possible if, under extreme circumstances, the fuel of a liquid-cooled nuclear reactor becomes molten. Such explosions are known as fuel-coolant interactions or 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 destruction in the Chernobyl accident is thought to have been due to such a steam explosion.
In a full-fledged 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 (eg. 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.
[edit] Other rapid boiling phenomena
High steam generation rates are possible under other circumstances, such as boiler-drum failure, or at a quench front (for example when water re-enters a hot dry boiler). Though potentially damaging, they are usually less energetic than events in which the hot ('fuel') phase is molten and so can be finely fragmented within the volatile ('coolant') phase. Some examples follow:-
Steam explosions are naturally produced by certain volcanos especially a stratovolcano and are a major cause of human fatalities in volcanic eruptions.
When a pressurized container such as the waterside of a steam boiler ruptures, it is always followed by some degree of steam explosion. A common operating temperature and pressure for a marine boiler is around 950 P.S.I. (6.55 MPa) and 850 °F (454 °C) at the outlet of the superheater. A steam boiler has an interface of steam and water in the steam drum, which is where the water is finally evaporating due to the heat input, usually oil-fired burners. When a water tube fails due to any of a variety of reasons, it will cause the water in the boiler to expand out of the opening into the furnace area that is only a few P.S.I. above atmospheric pressure. This will likely extinguish all fires and expands over the large surface area on the sides of the boiler. To decrease the likelihood of a devastating explosion, boilers have gone from the "fire-tube" designs, where the heat was added by burning inside of the tubes as the pressure vessel of water surrounded the tubes, to "water-tube" boilers that have the water inside of the tubes and the furnace area is around the tubes. Old "fire-tube" boilers were known to blow up inside the fire tubes, which was enough to damage the pressure vessel of water surrounding it, which led to a steam explosion. This destroyed at least the ship's propulsion plant and normally the corresponding end of the ship.
In a more domestic setting, steam explosions can be a result of incorrectly handled chip pan fires. When oil in a pan is on fire, the natural impulse is to extinguish it with water. However, doing so will cause the water to become superheated by the hot oil. Upon turning to steam, it will disperse upwards and outwards rapidly and violently in a spray also containing the ignited oil. It is for this reason that the correct course of action for dealing with such fires is to use a damp cloth to help deprive the fire of oxygen as well as serve to cool it down. Alternatively, a non-volatile purpose designed fire retardant agent or simply a fire blanket can be used instead.
