A nuclear meltdown is a term for a severe nuclear reactor accident. This can occur when a nuclear power plant system or component failure causes the reactor core to cease being properly controlled and cooled to the extent that the sealed nuclear fuel assemblies – which contain the uranium or plutonium and highly radioactive fission products – begin to overheat and melt. A meltdown is considered very serious because of the possibility that the reactor containment will be defeated, thus releasing the core's highly radioactive and toxic elements into the atmosphere and environment. From an engineering perspective, a meltdown is likely to cause serious damage to the reactor, and possibly total destruction.

Several nuclear meltdowns of differing severity have occurred, from localized core damage to complete destruction of the reactor core. In some cases this has required extensive repairs or decommissioning of a nuclear reactor. In the most extreme cases, such as the Chernobyl disaster, deaths have resulted and the near-permanent civilian evacuation of a large area was required.

A nuclear explosion does not result from a nuclear meltdown because, by design, the geometry and composition of the reactor core do not permit the special conditions necessary for a nuclear explosion. However, the conditions that cause a meltdown may cause a non-nuclear explosion. For example, several power excursion accidents have caused coolant to rapidly over pressurize, resulting in a steam explosion.

In some reactor types, the fuel assemblies in the core can melt as a result of a loss of pressure control accident, a loss of coolant accident (LOCA), an uncontrolled power excursion, or any other event that might start a fire around the fuel assemblies.

* In a loss of pressure control accident, the pressure of the confined coolant falls below specification without the means to restore it. In some cases this may reduce the heat transfer efficiency and in others may form an insulating 'bubble' of steam surrounding the fuel assemblies . In the latter case, due to localized heating of the steam 'bubble' due to decay heat, the pressure required to collapse the steam 'bubble' may exceed reactor design specifications until the reactor has had time to cool down.

* In a loss of coolant accident, either the physical loss of coolant (which is typically deionized water, an inert gas, or liquid sodium) or the loss of a method to ensure a sufficient flow rate of the coolant occurs. A loss of coolant accident and a loss of pressure control accident are closely related in some reactors. In a pressurized water reactor, a loss of coolant accident can also cause a steam 'bubble' to form in the core due to excessive heating of stalled coolant or by the subsequent loss of pressure control accident caused by a rapid loss of coolant.
                    
* In an uncontrolled power excursion accident, a sudden power spike in the reactor exceeds reactor design specifications due to a sudden increase in reactor reactivity. An uncontrolled power excursion occurs due to significantly altering a parameter that affects the exponential rate of a nuclear chain reaction (examples include ejecting a control rod or significantly altering the nuclear characteristics of the moderator, such as by rapid cooling). In extreme cases the reactor may proceed to a condition known as prompt critical.

* Structural and core-based fires may also severely endanger the core and potentially cause the fuel assemblies to melt. A structural fire may directly heat the fuel assemblies (such as during a fire on lagging of piping near the core) or in other cases it may damage control electronics or wiring preventing operators from quickly responding to other failures . In certain reactor designs it is possible for hydrogen or graphite to ignite inside the reactor core. A fire inside the reactor may be caused by failure to carefully control the amount of hydrogen in the coolant, an air addition to certain types of nuclear reactors, the uncontrolled heating of the coolant or moderator of the reactor by the types of reactor accidents listed above, or by an external source. Fires can be a much more severe casualty for nuclear reactors that are moderated with graphite because without taking proper precautions Wigner energy may accumulate which will greatly increase the severity of the fire (for example, during the Windscale fire).                                                                                                                                                 top

A nuclear reactor does not have to remain critical for a nuclear meltdown to occur because fires or decay heat can continue to heat the reactor fuel assemblies long after the reactor has shut down.

If the reactor core becomes too hot, it might melt through the reactor vessel (although this has not happened to date) and the floor of the reactor chamber and descend until it becomes diluted by surrounding material and cooled enough to no longer melt through the material underneath, or until it hits groundwater. This type of nuclear meltdown is known as a China Syndrome. Note that a nuclear explosion does not happen in a nuclear meltdown due to the low fissility of the radioactive components. However, a steam explosion may occur if it hits water.

The geometry and presence of the coolant has a twin role, and both cools the reactor as well as slowing down emitted neutrons. The latter role is crucial to maintaining the chain-reaction, and so even without coolant the molten core is designed to be unable to form an uncontrolled critical mass (a recriticality). However, the molten reactor core will continue generating enough heat through unmoderated radioactive decay ('decay heat') to maintain or even increase its temperature.

If the reactor core becomes too hot, it might melt through the reactor vessel (although this has not happened to date) and the floor of the reactor chamber and descend until it becomes diluted by surrounding material and cooled enough to no longer melt through the material underneath, or until it hits groundwater. This type of nuclear meltdown is known as a China Syndrome. Note that a nuclear explosion does not happen in a nuclear meltdown due to the low fissility of the radioactive components. However, a steam explosion may occur if it hits water.

The geometry and presence of the coolant has a twin role, and both cools the reactor as well as slowing down emitted neutrons. The latter role is crucial to maintaining the chain-reaction, and so even without coolant the molten core is designed to be unable to form an uncontrolled critical mass (a recriticality). However, the molten reactor core will continue generating enough heat through unmoderated radioactive decay ('decay heat') to maintain or even increase its temperature.                                                                                                                                                 top

A number of Russian nuclear submarines have experienced nuclear meltdowns. The only known large scale nuclear meltdowns at civilian nuclear power plants were in the Chernobyl disaster at Chernobyl Nuclear Power Plant, Ukraine, in 1986, and the Three Mile Island accident at Three Mile Island, Pennsylvania, USA, in 1979, although there have been partial core meltdowns at:                     

* NRX, Ontario, Canada, in 1952

* EBR-I, Idaho, USA, in 1955

* Windscale, Sellafield, England, in 1957 (see Windscale fire)

* Santa Susana Field Laboratory, Simi Hills, California, in 1959

* SL-1, Idaho, USA in 1961. (US military)

* Enrico Fermi Nuclear Generating Station, Michigan, USA, in 1966

* Chapelcross, Dumfries and Galloway, Scotland, in 1967

* A1 plant at Jaslovské Bohunice, Czechoslovakia in 1977. 25% of the fuel elements in a heavy water moderated carbon dioxide cooled 100 MW(e) power reactor were damaged due to operator error. The operators failed to remove silica gel packs from a new fuel element. The silica gel was used to keep the unused fuel dry during storage and transport. The silica gel packs blocked the flow of the coolant resulting in overheating of the fuel and the pressure channel holding it. As a result of overheating the heavy water leaked into the part of the reactor where the fuel elements are accommodated, the cladding was subject to corrosion and a considerable amount of radioactivity leaked into the primary cooling circuit. Through leaks in the steam boilers (similar basic design to a MAGNOX or AGR plant) some parts of the secondary circuit became contaminated. Not all of these were caused by a loss of coolant and in several cases (the Chernobyl disaster and the Windscale fire, for example) the meltdown was not the most severe problem.
Chernobyl - Russia