Boiling liquid expanding vapor explosion in the context of U.S. Chemical Safety Board

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). Steam explosions are instances of explosive boiling. 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.

There have been many extremely large explosions, accidental and intentional, caused by modern high explosives, boiling liquid expanding vapour explosions (BLEVEs), older explosives such as gunpowder, volatile petroleum-based fuels such as petrol, and other chemical reactions. This list contains the largest known examples, sorted by date. An unambiguous ranking in order of severity is not possible; a 1994 study by historian Jay White of 130 large explosions suggested that they need to be ranked by an overall effect of power, quantity, radius, loss of life and property destruction, but concluded that such rankings are difficult to assess.

The weight of an explosive does not correlate directly with the energy or destructive effect of an explosion, as these can depend upon many other factors such as containment, proximity, purity, preheating, and external oxygenation (in the case of thermobaric weapons, gas leaks and BLEVEs).