A heavy, man-made, radioactive metallic element. The most important isotope is pu-239, which has a half-life of 24,000 years. Pu-239 can be used in reactor fuel and is the primary isotope in weapons. One kilogramme is equivalent to about 22 million kilowatt-hours of heat energy. The complete detonation of a kilogramme of plutonium produces an explosion equal to about 20,000 tonnes of chemical explosive. All isotopes of plutonium are readily absorbed by the bones and can be lethal, depending on the dose and exposure time.