A steam explosion (also called a littoral explosion) is a violent boiling or flashing of water into steam, typically occurring when water is either superheated, or rapidly heated by fine hot debris produced within it. 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 a fragmentation and heat release front propagating through the material (see description of fci below)

Steam explosions are most 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 outcome would be that the molten mass of fuel and reactor core melts through the floor of the reactor building and reaches ground water, where a huge steam explosion would occur, dispersing radioactive contamination over a wide area.