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It's the same process either way, and the first few percent are the hardest (as it's super dilute so you need to handle a lot of material). It's much easier to go from 20% to 90%, than it is to get to 20%.

https://en.wikipedia.org/wiki/Separative_work_units

The 90% level is not a strict number. You can have nuclear explosion with lower levels of enrichment, but the total mass of the uranium is very large, therefor not a practical weapon.

For example you need 100kg of 60% enriched Uranium-235

https://www.researchgate.net/figure/Critical-mass-of-a-bare-...

5% enrichment (regular reactor grade) is 2/3 the way to weapons grade already.

https://world-nuclear.org/information-library/nuclear-fuel-c...

Hence there technically being no enrichment cap for any country that signed the NPT and stuff (not to be confused with protection from US bombs or rogue states that never signed the NPT).