Uranium contains a number of isotopes in different concentrations. The
two primary isotopes of interest are U235 and U238 which have natural
abundances of .72% and 99.275% by mass, respectively. For most reactors
and nuclear applications, U235 is the primary fissile nuclide.
Generally, the percentage of this isotope needs to be increased. This
increase in the percentage of U235 is called enrichment. Typical
methods for enrichment include: Gaseous
diffusion, Centrifuge Separation, Electromagnetic separation,
Aerodynamic processes, and Laser enrichment.