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.