Uranium-238 (U-238), is the most common isotope of uranium found in nature. When hit by a neutron, it becomes uranium-239 (U-239), an unstable isotope which decays into neptunium-239 (Np-239), which then itself decays, with a half-life of 2.355 days, into 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 ur… (More on Uranium-238)