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.