Enriched uraniumEnriched uranium is uranium whose uranium-235 content has been increased through the process of isotope separation. Natural uranium consists mostly of the U-238 isotope, with about 0.7 percent by weight (varies per mine though) as U-235 which is the only isotope existing in nature to any appreciable extent that is fissionable by thermal neutrons. For use in commercial nuclear reactors natural uranium is enriched to about 3 percent U-235. Highly enriched uranium (HEU), which is used in nuclear submarine reactors and nuclear weapons, contains at least 50 percent U-235, but typically exceeds 90 percent. During the Manhattan Project enriched uranium was given the codename oralloy, a shortened version of Oak Ridge alloy, after the plant where the uranium was enriched. The term oralloy is still occasionally used to refer to enriched uranium. The U-238 with extremely low U-235 content is known as depleted uranium, and is considerabley less radioactive than even natural uranium.
The ability to enrich uranium is of interest to those concerned about nuclear weapons proliferation.
For information on how uranium is enriched see isotope separation