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(March 2011)| Nuclear meltdown is an informal term for a severe
nuclear reactor accident that results in core damage from
overheating. The term is not officially defined by the
International Atomic Energy Agency or by the U.S. Nuclear
Regulatory Commission. 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. "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. This differs from a fuel element failure, which is not
caused by high temperatures. 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 w