Enrichment

Natural uranium essentially contains two isotopes, 235U (0.7 %) and 238U (99.3 %).
Enriching the uranium consists in increasing its content in 235U.
This operation is performed by ultracentrifugation in a cascade arrangement: under the effect of centrifugal acceleration, the heaviest molecules are pushed out to the edge, inducing isotopic separation. For current pressurized water reactors, the content in the isotope 235 of uranium is raised to between 3 % and 5 %.