A steam explosion is an explosion caused by violent boiling or flashing of water or ice into steam, occurring when water or ice is either superheated, rapidly heated by fine hot debris produced within it, or heated by the interaction of molten metals (as in a fuel–coolant interaction, or FCI, of molten nuclear-reactor fuel rods with water in a nuclear reactor core following a core-meltdown). Pressure vessels, such as pressurized water (nuclear) reactors, that operate above atmospheric pressure can also provide the conditions for a steam explosion. The water changes from a solid or 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 react chemically with steam (for example, zirconium and superheated graphite (inpure carbon, C) react with steam and air respectively to give off hydrogen (H2), which may explode violently in air (O2) to form water or H2O) 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 the release of stored superheat. But many large-scale events, including foundry accidents, show evidence of an energy-release front propagating through the material (see description of FCI below), where the forces create fragments and mix the hot phase into the cold volatile one; and 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, in Idaho, USA, in 1961, caused the SL-1 nuclear reactor vessel to jump over 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 water-cooled nuclear reactor gradually melt. The mixture of molten core structures and fuel is often referred to as "Corium". If such corium comes into contact with water, vapour explosions may occur from the violent interaction between molten fuel (corium) and water as