If you have two pieces of plutonium and put them too close together you have accidentally created a nuclear weapon… so yeah nukes are open source, plutonium breeding isn’t.
I love it when people make this “nuke” argument because it tells you a lot more about them than it does about anything else. There are so many low information people out there, it’s a bit sad the state of education even in developed countries. There’s people trotting around the word “chemical” at things that are scary without understanding what exactly the word means, how it differs from the word mixture or anything like that. I don’t expect most people to understand the difference between a proton and a quark but at least a general understanding of physics and chemistry would save a lot of people from falling into the “world is magic and information is hidden away inside geniuses” mentality.