uranium238

Uranium-238 (238U or U-238) is the most common isotope of uranium found in nature. It is not fissile, but is a fertile material: it can capture a slow neutron and after two beta decays become fissile plutonium-239. 238U is fissionable by fast neutrons, but cannot support a chain reaction because inelastic scattering reduces neutron energy below the range where fast fission is probable.