building a weapon that goes boom from hitting critical mass of uranium/other radioactive element is pretty straight forward. Getting enough uranium is one of the hard parts, but it's not the only hard part. getting a nuke somewhere without being stopped is pretty difficult too. sure you can put it in the back of a pickup and blow it up somewhere, but hiding the radiation of that bomb while you travel/assemble isn't easy. Building a rocket is difficult for nations.

Yes, I believe you're right about that, but I believe it's still the case that no one just distributes the plans. You have to learn engineering and nuclear physics then design your weapon yourself.