The most common way to enrich uranium is by spinning uranium gas in a centrifuge, where lighter U-235 separates from the heavier U-238. This is done in stages, using multiple centrifuges that enrich the uranium bit by bit.
The early stages are especially labor-intensive because there is so much more U-238 to strip away, said nuclear security specialist Sébastien Philippe at the University of Wisconsin-Madison. Over time, the enrichment process gets easier.