Cascade of gas centrifuges used to produce enriched uranium. This photograph is of a the U.S. gas centrifuge plant in Piketon, Ohio from 1984

(Source: DOE photo)

"the ratio of the combined weight of the isotopes uranium-233 and uranium-235 to that of the total uranium in question," usually stated as a percentage. Although this definition deals with the combined weight of the two fissile uranium isotopes, in practice they are rarely mixed and are normally accounted for separately. The term 'enrichment' is also used in relation to an isotope separation process by which the abundance of a specified isotope in an element is increased, such as the production of enriched uranium or heavy water, or of plutonium with an increase in the fissile isotope.