U238U235

Uranium has two long-lived isotopes that dominate natural uranium: uranium-238 and uranium-235. Natural uranium is mainly U-238 (about 99.3%), with roughly 0.7% U-235 and trace amounts of U-234. These isotopes share chemical properties but differ greatly in their nuclear behavior.

U-238 decays by alpha emission to thorium-234 and proceeds through a long decay chain to lead-206. Its half-life is about 4.47 billion years. U-235 decays by alpha emission to protactinium-231 and eventually to lead-207, with a half-life of about 0.704 billion years. Both isotopes are part of Earth's crust and are formed in stellar processes.

Nuclear properties distinguish the two: U-235 is fissile, meaning it can sustain a nuclear chain reaction with thermal (slow) neutrons, a property central to most nuclear reactors and weapons. U-238 is fertile: it can capture a neutron and, after a series of decays, produce plutonium-239, which is also fissile. This makes U-238 important for breeding reactors and for enhancing reactor fuel use.

Industrial significance and usage: Civilian nuclear fuel often uses natural uranium that is enriched to increase the U-235 fraction; typical reactor fuel contains about 3–5% U-235 (low-enriched uranium). Weapons-grade material is highly enriched, usually exceeding 85–90% U-235. U-238 also plays a key role in breeding cycles and in shaping reactor behavior through neutron economy and fuel management.

Environmental and safety considerations: Both isotopes contribute radiological activity, and their long half-lives mean persistence in the environment. Handling and use are subject to regulatory controls, shielding requirements, and careful monitoring.