Uranium-238

Uranium-238 (
U or U-238) is the most common isotope of uranium found in nature, with a relative abundance above 99%. Unlike uranium-235, it is non-fissile, which means it cannot sustain a chain reaction in a thermal-neutron reactor. However, it is fissionable by fast neutrons, and is fertile, meaning it can be transmuted to fissile plutonium-239. U cannot support a chain reaction because inelastic scattering reduces neutron energy below the range where fast fission of one or more next-generation nuclei is probable. Doppler broadening of U's neutron absorptionresonances, increasing absorption as fuel temperature increases, is also an essential negative feedback mechanism for reactor control.

The isotope has a half-life of 4.463 billion years (1.408×10 s). Due to its abundance and half-life relative rate of decay to other radioactive elements, U is responsible for about 40% of the radioactive heat produced within the Earth. The U decay chain contributes six electron anti-neutrinos per U nucleus (one per beta decay), resulting in a large detectable geoneutrino signal when decays occur within the Earth. The decay of U to daughter isotopes is extensively used in radiometric dating, particularly for material older than approximately 1 million years.

Depleted uranium has an even higher concentration of the U isotope, and even low-enriched uranium (LEU), while having a higher proportion of the uranium-235 isotope (in comparison to depleted uranium), is still mostly U. Reprocessed uranium is also mainly U, with about as much uranium-235 as natural uranium, a comparable proportion of uranium-236, and much smaller amounts of other isotopes of uranium such as uranium-234, uranium-233, and uranium-232.