239Pu

Plutonium-239, denoted 239Pu, is a radioactive isotope of plutonium with atomic number 94 and a mass number of 239. It has a half-life of about 24,110 years and decays via alpha emission to uranium-235. It is not found naturally in meaningful quantities and is produced in nuclear reactors.

239Pu is created when uranium-238 nuclei capture neutrons in a reactor, forming U-239, which beta decays to neptunium-239 and then to Pu-239. This production pathway makes Pu-239 a key component of reactor fuel cycles as well as a material used in weapons research and production.

As a fissile isotope, 239Pu readily undergoes fission when struck by thermal neutrons. This property underpins its use in nuclear weapons and in certain reactor fuels, including mixed-oxide (MOX) fuel. Reactor-grade plutonium typically contains Pu-239 along with other plutonium isotopes such as Pu-240 and Pu-241, affecting its behavior in reactors and its suitability for weapons use. The bare-sphere critical mass for Pu-239 is commonly cited around 10 kilograms, with less mass required if a surrounding reflector is present.

Safety and handling are important concerns. Pu-239 compounds are highly radiotoxic if inhaled or ingested, because alpha emitters can damage internal tissues. External radiation from Pu-239 is less hazardous due to alpha particles’ limited range, but long-term contamination and chemical toxicity remain serious concerns. Pu-239 has played a notable role in the history of nuclear weapons and is also used in some civilian reactor fuel applications under strict regulatory control.