Plutonium-239 is an isotope of plutonium that is a radioactive, metallic element created when uranium atoms absorb neutrons. Plutonium-239 is the most common isotope used in the production of nuclear weapons and is also one of three primary isotopes used as fuel in nuclear reactors.
Plutonium-239 is one of the most abundant fissionable isotopes of plutonium, meaning that it has an unstable nucleus that splits apart easily if struck by a neutron. It has a half-life of 24,100 years. Plutonium-239 is created inside nuclear reactors out of uranium-238. When a uranium-238 atom captures a neutron, it becomes uranium-239, which then decays twice to become plutonium-239.