Nuclear and radiation accidents and incidents involving HEU or plutonium trafficking: A nuclear meltdown is a severe nuclear reactor accident that results in reactor core damage from overheating. It has been defined as the accidental melting of the core of a nuclear reactor, and refers to the core's either complete or partial collapse. 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
Corium (nuclear reactor) nuclear fuel (the uranium oxide fuel has a melting point of 2865 ˚C, thus the physical nature of the damaged fuel has not been fully determined but it is assumed to have become molten) to melt and form corium. The reactor core isolation cooling system (RCIC) was successfully activated for Unit 3; however, the Unit 3 RCIC subsequently failed, and at about 09:00 on March 13, the nuclear fuel had melted into corium. Unit 2 retained RCIC functions slightly longer and corium is not believed to have started to pool on the reactor floor until around 18:00 on March 14.