Plutonium-240 is an isotope of the element plutonium, which is a heavy, radioactive metal. It has a mass number of 240, meaning it has 240 nucleons (protons and neutrons) in its nucleus. This isotope is produced in nuclear reactors and is notable for its relatively long half-life of about 6,560 years, making it less radioactive than some other isotopes of plutonium. Plutonium-240 is primarily used in nuclear research and as a component in nuclear weapons. Its presence in nuclear fuel can affect the efficiency of nuclear reactions, as it can absorb neutrons and reduce the likelihood of a chain reaction.