uranium238

Uranium-238, or U-238, is the most abundant isotope of natural uranium. It has a mass number of 238 and comprises about 99.3% of natural uranium. It is a long-lived radioactive isotope with a half-life of roughly 4.468 billion years, and it decays by alpha emission to thorium-234.

U-238 is not fissile with thermal neutrons; it is considered fertile. In nuclear reactors, it can capture a neutron to form uranium-239, which beta decays to neptunium-239 and then to plutonium-239. Pu-239 is fissile and can sustain a nuclear chain reaction, making U-238 central to breeding programs that produce reactor fuel and, historically, weapons-grade material.

The uranium-238 decay chain continues from thorium-234 through several intermediate isotopes until it reaches lead-206. This series, known as the uranium series or uranic decay chain, involves eight alpha decays and six beta decays in its progression to lead.

In nature, most uranium is U-238; depleted uranium used in industry is largely U-238 with reduced U-235 content. U-238 has important applications in geology and archaeology through uranium-lead dating, which uses the long half-life to date ancient rocks and minerals. It also informs radiological safety considerations; as an alpha emitter, its primary hazards arise from inhalation or ingestion of particles rather than external exposure.