"Nuclear meltdown" is an informal term for a severe nuclear reactor accident that results in core damage from overheating.
A nuclear and radiation accident is defined by the International Atomic Energy Agency as "an event that has led to significant consequences to people, the environment or the facility."
A nuclear reactor core is the portion of a nuclear reactor containing the nuclear fuel components where the nuclear reactions take place and the heat is generated.
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The term is not officially defined by the International Atomic Energy Agency or by the Nuclear Regulatory Commission.
The International Atomic Energy Agency is an international organization that seeks to promote the peaceful use of nuclear energy, and to inhibit its use for any military purpose, including nuclear weapons.
The Nuclear Regulatory Commission is an independent agency of the United States government tasked with protecting public health and safety related to nuclear energy.
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However, it has been defined to mean the accidental melting of the core of a nuclear reactor, and is in common usage a reference to the core's either complete or partial collapse.
Melting, or fusion, is a physical process that results in the phase transition of a substance from a solid to a liquid.
"Core melt accident" and "partial core melt" are the analogous technical terms for a meltdown.
A core melt accident occurs when the heat generated by a nuclear reactor exceeds the heat removed by the cooling systems to the point where at least one nuclear fuel element exceeds its melting point.
The melting point of a solid is the temperature at which it changes state from solid to liquid at atmospheric pressure.
This differs from a fuel element failure, which is not caused by high temperatures.
A fuel element failure is a rupture in a nuclear reactor's fuel cladding that allows the nuclear fuel or fission products, either in the form of dissolved radioisotopes or hot particles, to enter the reactor coolant or storage water.
A meltdown may be caused by a loss of coolant, loss of coolant pressure, or low coolant flow rate or be the result of a criticality excursion in which the reactor is operated at a power level that exceeds its design limits.
A coolant is a fluid which flows through or around a device to prevent the device from overheating, transferring the heat produced by the device to other devices that either use or dissipate it.
A loss-of-coolant accident is a mode of failure for a nuclear reactor; if not managed effectively, the results of a LOCA could result in reactor core damage.
Alternately, in a reactor plant such as the RBMK-1000, an external fire may endanger the core, leading to a meltdown.
The RBMK is a class of graphite-moderated nuclear power reactor designed and built by the Soviet Union.
Once the fuel elements of a reactor begin to melt, the fuel cladding has been breached, and the nuclear fuel and fission products within the fuel elements can leach out into the coolant.
Nuclear fission products are the atomic fragments left after a large atomic nucleus undergoes nuclear fission.
Subsequent failures can permit these radioisotopes to breach further layers of containment.
A containment building, in its most common usage, is a reinforced steel or lead structure enclosing a nuclear reactor.
Superheated steam and hot metal inside the core can lead to fuel-coolant interactions, hydrogen explosions, or water hammer, any of which could destroy parts of the containment.
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.
Water is a transparent fluid which forms the world's streams, lakes, oceans and rain and is the major constituent of the fluids of organisms.
Superheated steam is a steam at a temperature higher than its vaporization point at the absolute pressure where the temperature is measured.
A meltdown is considered very serious because of the potential for radioactive materials to breach all containment and escape into the environment, resulting in radioactive contamination and fallout, and potentially leading to radiation poisoning of people and animals nearby.
Nuclear fallout, or simply fallout, is the residual radioactive material propelled into the upper atmosphere following a nuclear blast or a nuclear reaction conducted in an unshielded facility, so called because it "falls out" of the sky after the explosion and the shock wave have passed.
Radioactive decay, also known as nuclear decay or radioactivity, is the process by which the nucleus of an unstable atom loses energy by emitting radiation, including alpha particles, beta particles, gamma rays, and conversion electrons.
Radioactive contamination, also called radiological contamination, is the deposition of, or presence of radioactive substances on surfaces or within solids, liquids or gases, where their presence is unintended or undesirable.