Plutonium239

Plutonium-239 is an isotope of plutonium with 94 protons and 145 neutrons. It is a highly fissile, synthetic material that has been central to both nuclear weapons and nuclear fuel cycles. Pu-239 has a half-life of 24,110 years and decays by alpha emission to uranium-235.

In nuclear reactors, Pu-239 is produced from U-238 by neutron capture and subsequent beta decays, and is chemically separated from irradiated fuel for use. It was first produced and identified during the Manhattan Project; since then it has been used as a principal fissile material for nuclear weapons and as a component of MOX fuel in some civilian reactors.

Pu-239 is a dense, silvery metal that forms protective oxides in air. It is a potent alpha emitter; external exposure is relatively less dangerous than internal exposure, where alpha particles deposited in lungs or other tissues can cause significant radiation damage over time. Its long half-life contributes to long-term contamination concerns; safety requires specialized handling, containment, and regulatory controls.

Because of its dual use, Pu-239 is subject to strict regulatory control and nonproliferation measures.