The Atomic Energy Act put a licensing requirement on all civilian use of nuclear materials, in this case for uranium concentrations above what is found in nature. Enriching is definitely in this category.
To enrich uranium legally in the US you would have to get a license by justifying why you were doing it and there are not many legitimate reasons to do so outside slightly enriched uranium for power plants or highly enriched for a few medical isotopes.
But if you just decide to do it, yes it’s illegal and the NRC will probably find you.
Well, after much searching I didn't find the parts of the Atomic Energy Act that required certification for any and all materials processing.
By default things are legal.
I've also spoken with scientists who do research that, at times, abuts nuclear science, and have heard stories of them just... not doing paperwork because it's annoying.
To enrich uranium legally in the US you would have to get a license by justifying why you were doing it and there are not many legitimate reasons to do so outside slightly enriched uranium for power plants or highly enriched for a few medical isotopes.
But if you just decide to do it, yes it’s illegal and the NRC will probably find you.
https://www.nrc.gov/about-nrc/governing-laws.html#aea-1954