Steam explosion

Littoral explosion at Waikupanaha ocean entry at the big island of Hawaii was caused by the lava entering the ocean

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).^{[citation needed]} 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.

Flash boiling in cooking

There is also a cooking technique called flash boiling, in which a smaller amount of water is used so as to quicken the process of boiling. An example of this technique is used to melt a slice of cheese onto a hamburger patty, whereby the cheese slice is placed on top of the meat on a high-heat surface (e.g., a hot frying pan), and a small quantity of cold water is thrown onto the surface near the patty. A vessel (such as a volume-rich small pot or frying-pan cover) is then used to quickly seal the steam-flash reaction, which disperses much of the steamed-water on the cheese/patty, which results in a large release of heat in a transfer resulting from the vaporized water condensing back into a liquid, resulting in an energy release (a principle also utilized in refrigerator and freezer production).

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 passing hot gases through tubes in a body of water, 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 fail due to poor build quality or lack of maintenance (such as corrosion of the fire tubes, or fatigue of the boiler shell due to constant expansion and contraction). A failure of fire tubes forces large volumes of high pressure, high temperature steam back down the fire tubes in a fraction of a second and often blows the burners off the front of the boiler, whereas a failure of the pressure vessel surrounding the water would lead to a full and entire evacuation of the boiler's contents in a large steam explosion. On a marine boiler, this would certainly destroy the ship's propulsion plant and possibly 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 may be 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 either use a damp cloth or a tight lid on the pan; both help deprive the fire of oxygen, and the cloth also serves to cool it down. Alternatively, a non-volatile purpose designed fire retardant agent or simply a fire blanket can be used instead.

See also

• BLEVE
• Boiler explosion
• multiphase flow
• 2007 New York City steam explosion

Bibliography

• Triggered Steam Explosions by Lloyd S. Nelson, Paul W. Brooks, Riccardo Bonazza and Michael L. Corradini ... Kjetil Hildal