Enriched uranium is a type of uranium in which the percent composition of uranium-235 has been increased through the process of isotope separation. Natural uranium is 99.284% ²³⁸U isotope, with ²³⁵U only constituting about 0.711% of its weight. ²³⁵U is the only nuclide existing in nature (in any appreciable amount) that is fissile with thermal neutrons.