Uranium 238

10 gram sample

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. 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 U-238'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 1.41×10¹⁷seconds (4.468×10⁹ years, or 4.468 billion years). Due to its natural abundance and half-life relative to other radioactive elements, ${\displaystyle ^{238}U}$ produces ~40% of the radioactive heat produced within the Earth.${\displaystyle ^{238}U}$ decay contributes 6 electron anti-neutrinos per decay (1 per beta decay), resulting in a large detectable geoneutrino signal when decays occur within the Earth. The decay of ${\displaystyle ^{238}U}$ to daughter isotopes is extensively used in radiometric dating, particularly for material older than ~ 1 million years.

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