The heaviest naturally occurring element (atomic number 92). It is metallic and slightly radioactive. Its only substantial use today is as a raw material for generating electricity via nuclear fission. Ranging in atomic mass from 227 to 240, uranium has 14 isotopes, of which only three occur naturally: uranium-238, uranium-235, and uranium-234.

Uranium-238 – comprises 99. 283% by weight of natural uranium (ie uranium whose isotopic composition, as it occurs in nature, has not been altered)and is not fissile, but is fertile. It usually absorbs any neutrons which strike it and as a result is transformed into plutonium-239, a fissile isotope of plutonium.

Uranium-235 – the only naturally occurring isotope of uranium which is capable of fission. Natural uranium contains approximately 0. 711% by weight of uranium-235.

Uranium-234 – exists in very small quantities in natural uranium (0. 005% by weight) and is not fissile in thermal reactors.

The remaining isotopes are created synthetically. Uranium-233 is fissile and is obtained by the irradiation of thorium-232 with neutrons. Uranium-232, -236, and –237 are found in irradiated nuclear fuel and are not fissile.