A
steam explosion 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 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 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.