U239

Uranium-239 (U-239) is a radioactive isotope of uranium with a mass number of 239. It is not found in meaningful quantities in nature and is typically produced transiently when uranium-238 captures a neutron in nuclear reactors or laboratory experiments. As a uranium isotope, its chemical behavior is that of uranium, but its radioactivity is governed by its rapid decay.

In its decay chain, U-239 beta decays to neptunium-239 with a half-life of about 23.5 minutes. Neptunium-239 then beta decays to plutonium-239 with a half-life of about 2.36 days. Consequently, U-239 is an intermediate in the reactor-based production route to Pu-239, a fissile material used in certain nuclear fuels and historical weapons programs. The short half-life of U-239 means it exists only briefly and is encountered primarily in controlled, research, or fuel-cycle contexts.

Safety and handling are governed by standard radiological practices for short-lived actinides. U-239 emits beta particles and gamma radiation during its decay, with radiological considerations focused on its decay products and their subsequent transformations. As a transient intermediate in the uranium-plutonium cycle, U-239 is mainly discussed in the contexts of nuclear physics, reactor chemistry, and radiochemical research.