The SL-1 nuclear reactor is destroyed by a steam explosion in the only reactor incident in the United States to cause immediate fatalities.
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 fuelcoolant 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 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 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 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 fuelcoolant interactions (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 physical destruction in the Chernobyl disaster, a graphite-moderated, light-water-cooled RBMK-1000 reactor, is thought to have been due to such a steam explosion.
In a nuclear meltdown, the most severe outcome of a steam explosion is early containment building 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 (such as the upper head) into, and through, the containment. Less dramatic but still significant is 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 cooled. See WASH-1400 for details.
Steam explosions are often encountered where hot lava meets sea water or ice. Such an occurrence is also called a littoral explosion. A dangerous steam explosion can also be created when liquid water or ice 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.
Stationary Low-Power Reactor Number One, also known as SL-1 or the Argonne Low Power Reactor (ALPR), was a United States Army experimental nuclear reactor located at the National Reactor Testing Station (NRTS), basis of what is now the Idaho National Laboratory, west of Idaho Falls, Idaho, United States. It is best known for the meltdown and steam explosion which occurred at 9:01 pm, on the night of January 3, 1961, killing all three of its young military operators, one of whom was pinned to the ceiling of the facility by a reactor vessel plug. The direct cause was the improper withdrawal of the central control rod, responsible for absorbing neutrons in the reactor's core. The event is the only reactor accident in U.S. history to have resulted in immediate fatalities. The accident released about 80 curies (3.0 TBq) of iodine-131, which was not considered significant due to its location in the remote high desert of eastern Idaho. About 1,100 curies (41 TBq) of fission products were released into the atmosphere.The facility housing SL-1, located approximately 40 miles (65 km) west of Idaho Falls, was part of the Army Nuclear Power Program. The reactor was intended to provide electrical power and heat for small, remote military facilities, such as radar sites near the Arctic Circle, and those in the DEW Line. The design power was 3 MW (thermal), but some 4.7 MW tests were performed in the months prior to the accident. Operating power was 200 kW electrical and 400 kW thermal for space heating. During the accident, the core power level reached nearly 20 GW in just four milliseconds, precipitating the steam explosion.