Uranium enrichment is the process of increasing the concentration of U235 isotope in natural uranium and decreasing that of U238 isotope.
Uranium is a naturally occurring element containing U235 and U238 isotopes. Only the U235 isotope is fissionable. Enrichment is a critical step in transforming natural uranium into nuclear fuel to produce electricity.
Natural uranium contains about 0.7% U235. But the fuel assemblies that power a commercial nuclear reactor at an electric utility generally need uranium with a 4% – 5% concentration of U235. To produce this fuel, companies enrich the concentration of U235 in natural uranium hexafluoride (UF6) to the appropriate level and sell the fuel to utility customers.
Uranium enrichment is sold as separative work units (SWU), which represent the level of effort required to increase the concentration of U235 in natural uranium. Higher levels of U235 require more SWU.
For the past half century, commercial uranium enrichment has employed one of two technologies: gaseous diffusion or gas centrifuge. Both use UF6 as the chemical form of uranium for processing, in part because UF6 readily becomes a gas when heated. Both rely on the mass differences between U235 and U238 to achieve separation, either through a semi-porous membrane (diffusion) or by spinning at high speed (centrifuge). Gaseous diffusion technology is no longer in commercial use, having been displaced by more economical gas centrifuge technology in recent decades.