Uranium-238

Uranium-238 (²³⁸
U or U-238) is the most common isotope of uranium found in nature, with a relative abundance of 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. ²³⁸U 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 ²³⁸U's neutron absorption resonances, increasing absorption as fuel temperature increases, is also an essential negative feedback mechanism for reactor control.

Around 99.284% of natural uranium's mass is uranium-238, which has a half-life of 1.41×10¹⁷ seconds (4.468×10⁹ years, or 4.468 billion years). Due to its natural abundance and half-life relative to other radioactive elements, ²³⁸U produces ~40% of the radioactive heat produced within the Earth. The ²³⁸U decay chain contributes six electron anti-neutrinos per ²³⁸U nucleus (one per beta decay), resulting in a large detectable geoneutrino signal when decays occur within the Earth. The decay of ²³⁸U 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 ²³⁸U 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 ²³⁸U. Reprocessed uranium is also mainly ²³⁸U, 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.