Uranium-238 (238U 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.

Uranium-238, 238U
10 gram sample
Namesuranium-238, U-238
Protons (Z)92
Neutrons (N)146
Nuclide data
Natural abundance99.2745%
Half-life (t1/2)4.468×109 years
Isotope mass238.05078826 Da
Parent isotopes242Pu (α)
238Pa (β)
Decay products234Th
Decay modes
Decay modeDecay energy (MeV)
alpha decay4.267
Isotopes of uranium
Complete table of nuclides

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).[1] Due to its natural abundance and half-life relative to other radioactive elements, 238U produces ~40% of the radioactive heat produced within the Earth.[2] The 238U decay chain contributes 6 electron anti-neutrinos per 238U nucleus (1 per beta decay), resulting in a large detectable geoneutrino signal when decays occur within the Earth.[3] The decay of 238U to daughter isotopes is extensively used in radiometric dating, particularly for material older than ~ 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.[4]

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