Uranium-238 (U-238) is the most common isotope of uranium found in nature. When hit by a neutron, it eventually becomes plutonium-239 (Pu-239). Around 99.284% of natural uranium is uranium-238, which has a half-life of 1.41 × 1017 seconds (4.46 × 109 years, or 4.46 billion years). Depleted uranium consists mainly of the 238 isotope, and enriched uranium has a higher-than-natural quantity of the uranium-235 isotope. Reprocessed uranium is also mainly U-238, but contains significant q… (More on Uranium-238)