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The stuff you need to make this sort of thing isn’t off the shelf, they would have to buy or build a shitload of these
https://en.wikipedia.org/wiki/Gas_centrifuge
and its not cheap, these are just maintenance cost (and upgrades)
https://www.statista.com/statistics/752531/nuclear-weapons-spending-worldwide-by-country/
Ukraine’s gdp is around 170 billion, so it isn’t really something they could afford to just crank up.
On a personal note, I hope we as a species never fire another one of these weapons ever again.
Regulations and taxes could have prevented this (sort of).