Core meltdown

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If the reactor core cooling fails, e.g. due to a major leakage in the reactor cooling circuit, and the emergency core cooling system fails simultaneously, the residual heat in the fuel created by the radioactive decay of the fission products heats up the reactor core - possibly until the fuel melts. During the meltdown, the core support structures also fail so that the whole molten mass drops into the lower hemispherical area of the reactor pressure vessel. It can be assumed that the heat released by the molten mass melts through the bottom of the reactor pressure vessel. The density of the containment is important for the extent of radioactive substances released to the environment in the case of such a core meltdown accident.

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