Uranium-238
10-gram sample | |
| General | |
|---|---|
| Symbol | 238U |
| Names | uranium-238 |
| Protons (Z) | 92 |
| Neutrons (N) | 146 |
| Nuclide data | |
| Natural abundance | 99.274% |
| Half-life (t1/2) | 4.463×109 years |
| Isotope mass | 238.050787 Da |
| Spin | 0 |
| Parent isotopes | 242Pu (α) 238Pa (β−) |
| Decay products | 234Th |
| Decay modes | |
| Decay mode | Decay energy (MeV) |
| alpha decay | 4.270 |
| Isotopes of uranium Complete table of nuclides | |
Uranium-238 (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. 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.
The isotope has a half-life of 4.463 billion years (1.408×1017 s). Due to its abundance and half-life relative rate of decay to other radioactive elements, 238U is responsible for about 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.