The gas diffusion method is a method for uranium enrichment. It is because the gas molecules of a substance whose mass varies by different isotopes diffuse at different rates.
To ( gaseous ) uranium hexafluoride (UF6 ) is introduced into a chamber which is divided by a porous membrane. The pressure on the side on which the uranium hexafluoride is introduced, is higher than on the other. Light molecules (which the lighter isotope 235U included) have a higher diffusion rate than heavier, so the light isotopes accumulate in the chamber with low pressure. Since the separation effect is very small, about 1,000 such plates must be connected in series to a so-called cascade for the effective enrichment. Using uranium hexafluoride, because it already sublimed at 56 ° C, and because fluorine is only a single isotope. The molecular mass thus depends only on the uranium atom.
One of the biggest gas diffusion plants for uranium enrichment is located in Paducah, United States.