A nuclear meltdown describes a malfunction of a nuclear reactor. The term "nuclear meltdown" is commonly used by the public and by news media, but nuclear engineers usually refer to it as a core melt accident. A nuclear meltdown occurs when the middle portion of the nuclear reactor containing the fuel rods (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. Due to decay heat, a nuclear meltdown can occur even in a reactor that is shut down. The uranium and plutonium liquified in a nuclear meltdown, mixed with fission products, melted zirconium from the fuel rod cladding, and other materials, is called corium. Corium is highly radioactive and remains hazardous for many centuries after a meltdown. The zirconium is a hazard because at high temperatures, it can react with the cooling water and make flammable hydrogen gas.
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. The Chernobyl meltdown created a mass of corium which has been nicknamed "the Elephant's Foot" and is one of the most radioactive objects in the world.
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.
A nuclear meltdown is sometimes called the "China syndrome", which refers to a scenario, not meant to be taken literally, where a reactor core could melt through the Earth "all the way to China". The movie The China Syndrome is named after this scenario. There is no way such an event could happen in the real world. A reactor core could not melt through the Earth's crust, and even if it did melt to the center of the Earth, it would not go back up to the surface against gravity.