What specific capability does gaseous diffusion enrichment rely on to separate uranium isotopes?

Gaseous diffusion enrichment relies on the slightly different diffusion rates of uranium hexafluoride (UF6) gas molecules containing the uranium-235 (U-235) and uranium-238 (U-238) isotopes through a porous membrane. Uranium isotopes have virtually identical chemical properties, but U-235 is slightly lighter than U-238. In gaseous diffusion, UF6 gas is forced through a membrane with millions of tiny pores. The lighter U-235 molecules diffuse through the membrane slightly faster than the heavier U-238 molecules. This results in a slight enrichment of U-235 on the other side of the membrane. Because the enrichment achieved in a single stage is very small, the process must be repeated hundreds or thousands of times in a series of stages to achieve the desired level of enrichment for nuclear fuel or weapons. Therefore, the key capability is the exploitation of the minute mass difference between the isotopes to achieve a gradual separation based on their differing diffusion rates through a porous barrier.