Loss of coolant

Loss of Coolant is a mode of failure for a nuclear reactor; in a nuclear fission reactor, the results of loss of coolant can be catastrophic to the reactor, the facility that houses it, and the immediate vicinity around the reactor.

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.
• If graphite and air are present, it may catch fire, spreading radioactive contamination in the form of small particles.
• The fuel and reactor internals may melt; if the melted configuration remains critical, the molten mass will continue to generate heat, conceivably melting its way down through the bottom of the reactor. Such an event is called a nuclear meltdown and would have severe consequences. The so-called "China syndrome" would be this process taken to its logical extreme: the molten mass working its way down through the soil to the water table (and below). Current understanding 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 loss of coolant or of voids appearing in its coolant system (by water boiling, for example). This is measured by the void coefficient.

Modern reactors are designed to prevent and withstand loss of coolant using various techniques. Some, such as the pebble bed reactor or the pressurized water reactor, passively shut down the chain reaction when coolant is lost; others have careful safety systems to shut down the chain reaction.