Pu239

Plutonium-239 (Pu-239) is a radioactive isotope of plutonium with a mass number of 239. It is fissile, meaning it can sustain a nuclear chain reaction with thermal neutrons. Pu-239 decays by alpha emission to uranium-235 and has a half-life of about 24,110 years. Its radiological hazard is greatest when ingested or inhaled, due to its long residence in the body and its alpha radiation.

Pu-239 is not produced naturally in meaningful quantities. It is manufactured in nuclear reactors from uranium-238, via neutron capture to U-239, which beta decays to neptunium-239 and subsequently to Pu-239. In civilian and military contexts, Pu-239 is separated from spent nuclear fuel in reprocessing facilities and fabricated into various forms for use as fuel or weapons material.

Chemically and physically, Pu-239 behaves like other actinides. It is a dense metal that readily oxidizes, and its most common oxide is PuO2. The metal and its compounds are highly radiotoxic when managed improperly. Pu-239 has a relatively high specific activity; a bare sphere of material has a critical mass typically on the order of around 10 kilograms, with smaller masses possible when reflected or tamped.

Applications and regulation: Pu-239 is a principal fissile component in some nuclear weapons and has been used in mixed oxide (MOX) fuel for nuclear reactors. Its production, transport, and disposal are subject to strict safeguards and national and international regulation to prevent illicit use and limit civil and military risks.

History: Pu-239 was identified during the Manhattan Project in the 1940s, with Glenn T. Seaborg and colleagues contributing to its isolation. It played a key role in the development of plutonium-based nuclear weapons, notably in the Fat Man device, and remains a central material in nuclear policy and nonproliferation discussions.