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A nuclear meltdown describes a malfunction of a nuclear reactor. A nuclear meltdown occurs when the middle portion of the nuclear reactor (its "core") is not properly cooled. This can occur when the cooling system fails or is otherwise defective. If this happens, uranium or plutonium or similar materials inside the nuclear reactor become hot and may start melting or dissolving. It is this melting that is a nuclear meltdown.
Around the world, some nuclear meltdowns have occurred. Some of them were mild, but few of them were very serious. Nuclear meltdowns can kill people from radiation poisoning.
The very last accident was the Fukushima nuclear disaster in March 2011. Four reactors at the Fukushima Daiichi Nuclear Power Plant had cooling problems after back-up diesel generators were destroyed by the tsunami.
In 1986, a nuclear meltdown occurred in a place named Chernobyl (Ukraine). In this case, all the people living in the towns and the villages (near the defective nuclear reactor) had to move to far away places.
Sometimes, the nuclear meltdown may happen immediately. For example, the nuclear reactor at Chernobyl. Sometimes, the nuclear meltdown may take many hours to happen. For example, the nuclear meltdown at Three Mile Island, (Pennsylvania, United States) took many hours to happen.