What is so difficult about developing nuclear weapons that makes some countries incapable of making them?

Question

What is so difficult about developing nuclear weapons that makes some countries incapable of making them?

In: 1394

Answer 1

If I may I would like to piggyback of this question because the answers to it raise another one. If you are able to make a fission bomb with uranium or plutonium, how challenging is it to upscale it to make a fusion bomb with deuterium/tritium?