The most common isotope used for the production of atomic energy is uranium-235 (U-235). It is a fissile isotope, which means that it can undergo nuclear fission when struck by a neutron, releasing a large amount of energy in the process. U-235 is found in natural uranium, but it is only present in very small amounts (about 0.7%). Therefore, it needs to be enriched to higher concentrations for use in nuclear power plants and nuclear weapons.