Principles of plutonium separation are well known (https://en.wikipedia.org/wiki/PUREX), but preventing non-nuclear weapon states from having access to nuclear materials usable for nuclear weapons (Plutonium, Highly enriched uranium) is still cornerstone of US foreign policy. See the current events in Iran. Or the discussions with South Korea:

"The U.S. State Department did not give specific responses when asked if the U.S. was open to changing the agreement and what sort of discussions it had agreed to, but a spokesperson said: "America has a longstanding policy to limit the spread of enrichment and reprocessing capabilities around the world and to seek the highest nonproliferation standards achievable in all 123 agreements.""

https://www.reuters.com/business/energy/south-korea-us-agree...

This also the reason for monitoring and inspections by International Atomic Energy Agency in all facilities handling nuclear materials (nuclear reactors, fuel manufacturing, nuclear waste storage) or capable of producing nuclear materials - in non-nuclear weapon states.

https://www.iaea.org/topics/additional-protocol