It is vaporized and advantage is taken of the difference in the molar masses of the three isotopes to separate them selectively by passage of UF6 through a porous wall, a barrier. The lightest isotopes, uranium 235 and 234 pass more easily than uranium 238. Because enrichment by means of a single barrier is very slight, it is necessary to repeat the operation a great number of times. The stages are placed in a series. The stages are joined into a whole of ten to twenty units that constitute a group.
Several groups constitute the cascade.
UF6 is introduced into the center of the cascade. The UF6 that has been enriched in uranium 235 is withdrawn at one end and the depleted UF6 at the other. France studied the enrichment procedure in laboratories and pilot units at Saclay, at Grenoble, and at Pierrelatte. Two plants were put into service, both at Pierrelette, the military plant at Pierrelatte and the plant belonging to Eurodif-Production. This procedure uses the effect of centrifugal force on gaseous UF6 contained in a recipient turning very fast around an axis.