I read an article once on how to create a low-tech nuke in a house. The TL;DR is that you mainly need shaped plutonium or a spicy flavor of uranium (a half ball and socket), a pipe, and some explosives. Shoot the ball at the socket using explosives, ?????, city leveled.
Getting the plutonium is the hard part though. And I'm probably trivializing it / missing some steps.
You either need enough of it so it goes prompt critical on it's own (which is very different from being critical, which means your largely running on natural fission, prompt means that your fission rates become exponential), without refining that probably requires half a ton of U or P material.
Nuclear non-proliferation is about controlling access to highly enriched materials, not the knowledge. If you have enough refined material for critical mass, getting it to blow up is trivial. Maximizing yield, not so much.
Getting the plutonium is the hard part though. And I'm probably trivializing it / missing some steps.
I'm probably on a List now.