The process of raising the amount of a substance in another. In nuclear reprocessing, enrichment involves raising the amount of one radioactive isotope in a mixture of isotopes. The enrichment of uranium-235 from a mixture of uranium-238 involves converting the uranium isotopes into uranium hexafluoride and separating them in a gas centrifuge process. This involves using an array of centrifuges in which the heavy atoms are separated from the lighter atoms by centrifugal forces. The separation is dependent on the difference in mass between the isotopes being separated.