When high-pressure UF6 is forced through a barrier having
millions of submicroscopic holes per square centimeter,
molecules containing the isotope 235U pass through the barrier
slightly faster than those containing 238U.
Therefore UF6 contains a slightly higher ratio of 235U to 238U
than it did previously. The gas has become enriched in 235U.
Carrying this process through several thousand passes yields a
product highly enriched in 235U.