URANIUM ENRICHMENT

Enriched uranium is a sample 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. However, 235U is the only isotope existing in nature (in any appreciable amount) that is fissionable by thermal neutrons.
See more at Wikipedia.org...