Plutonium-239 ($\text{Pu-239}$) is a synthetic, radioactive, and fissile isotope of the element plutonium (atomic number 94). It is a concept central to nuclear technology, possessing a long half-life of approximately 24,110 years.
The isotope was first discovered in February 1941 by a team led by Glenn Seaborg at the University of California, Berkeley, as the decay product of neptunium-239. It was created to solve the problem of finding a highly potent fissile material for nuclear weapons, and it was the core material for the "Fat Man" bomb dropped on Nagasaki in 1945. The first large-scale production reactor, the B Reactor at Hanford, was completed in September 1944 to produce it.
Plutonium-239 works by undergoing nuclear fission when struck by a neutron, splitting its nucleus to release a large amount of energy and more neutrons, thereby sustaining a nuclear chain reaction. In nuclear reactors, it is primarily produced from Uranium-238 (a fertile material) by capturing a neutron, which then undergoes two beta decays to become Plutonium-239. In commercial nuclear power reactors, the fission of Plutonium-239 provides about one-third of the total energy produced.
This isotope connects directly to Uranium-235, the other primary fissile material, and to the concept of nuclear proliferation because it can be recycled from spent nuclear fuel. It is used in MOX fuel (Mixed Oxide fuel) for power generation. A critical distinction is the presence of the contaminant Plutonium-240; weapons-grade plutonium must contain no more than 7% Plutonium-240 to prevent pre-detonation in an implosion-type weapon. The fundamental properties and applications of Plutonium-239 have not been replaced or amended, but the ongoing challenge of Plutonium-240 contamination, first noted in 1944, continues to define its use.