Principle of gaseous diffusion
Uranium is injected in the form of hexafluoride of gaseous uranium. Hexafluoride molecules must pass through very fine membranes pierced by billion pores per cm2. The lighter molecules containing the isotope-235 cross a little faster these barriers. At the end of 1400 barriers of the Georges-Besse plant; the enrichment level of about 4%, required for reactor nuclear fuels, is achieved.