A Loss of Coolant Accident (LOCA) is a mode of failure for a nuclear reactor; in a nuclear reactor, the results of a LOCA could be catastrophic to the reactor, the facility that houses it, and the immediate vicinity around the reactor. Each US plant's Emergency Core Cooling System (ECCS) exists specifically to deal with a LOCA. Nuclear power station at Leibstadt, Switzerland. ... Nuclear power station at Leibstadt, Switzerland. ... US,Us or us may stand for the United States of America us, the oblique case form of the English language pronoun we. ...

Nuclear reactors generate heat internally; to convert this heat into useful power, a coolant system is used. If this coolant is lost, the nuclear reactor may continue to generate the same heat while its temperature rises to the point of damaging the reactor. Particularly dangerous is the possibility that the high temperatures may prevent the control systems from slowing the reaction; if this happens, the temperature will continue to rise until something drastic happens.

• If water is present, it may boil, bursting out of its pipes. (For this reason, nuclear power plants are equipped with Pressure-Operated Relief Valves.)
• If graphite and air are present, the graphite may catch fire, spreading radioactive contamination. (This situation exists only in RBMKs and weapons-production reactors.)
• The fuel and reactor internals may melt; if the melted configuration remains critical, the molten mass will continue to generate heat, possiblly melting its way down through the bottom of the reactor. Such an event is called a nuclear meltdown and can have severe consequences. The so-called "China syndrome" would be this process taken to an extreme: the molten mass working its way down through the soil to the water table (and below) - however, current understanding and experience of nuclear fission reactions suggests that the molten mass would become too disrupted to carry on heat generation before descending very far; for example, in the Chernobyl accident the reactor core melted and core material was found in the basement, too widely dispersed to carry on a chain reaction (but still dangerously radioactive).

A reactor may passively (that is, in the absence of any control systems) increase or decrease its power output in the event of a LOCA or of voids appearing in its coolant system (by water boiling, for example). This is measured by the void coefficient. All modern nuclear power plants (except, under some conditions, RBMKs) have a negative void coefficient, indicating that as water turns to steam, power instantly decreases. Boiling water reactors are designed to have steam voids inside the reactor vessel. A nuclear power plant (NPP) is a thermal power station in which the heat source is one or more nuclear reactors. ... Graphite (named by Abraham Gottlob Werner in 1789, from the Greek γραφειν: to draw/write, for its use in pencils) is one of the allotropes of carbon. ... The radiation warning symbol (trefoil). ... RBMK is an acronym for the Russian reaktor bolshoi moshchnosty kanalny which means reactor (of) large power (with) channels, and describes a now-obsolete class of nuclear power reactor which was built only in the Soviet Union. ... A nuclear meltdown occurs when the core of a nuclear reactor melts. ... The nuclear power plant at Chernobyl. ... In nuclear engineering, the void coefficient (more properly called void coefficient of reactivity) is a number that can be used to estimate how much the thermal output of a nuclear reactor increases (or decreases, if negative) as voids (steam bubbles) form in the reactor moderator or coolant. ... A nuclear power plant (NPP) is a thermal power station in which the heat source is one or more nuclear reactors. ... RBMK is an acronym for the Russian reaktor bolshoi moshchnosty kanalny which means reactor (of) large power (with) channels, and describes a now-obsolete class of nuclear power reactor which was built only in the Soviet Union. ... A boiling water reactor (BWR) is a light water reactor design used in some nuclear power stations. ...

Modern reactors are designed to prevent and withstand loss of coolant using various techniques. Some, such as the pebble bed reactor, passively shut down the chain reaction when coolant is lost; others have extensive safety systems to shut down the chain reaction. The Pebble Bed Reactor is an advanced nuclear reactor design. ...

See Also

• containment building
• nuclear power
• pressurized water reactor