The only step that takes time and space is creating enough highly enriched uranium for the bomb. You need few hundred or thousand centrifuges running a some tim

The only step that takes time and space is creating enough highly enriched uranium for the bomb. You need few hundred or thousand centrifuges running a some time. I have a friend who is nuclear physicist working for the nuclear proliferation issues in international organization. According to him, every country would do modern boosted fission weapon as their first nuke, and it can take less than year when you have easy access to experts and modern machines. Few subcritical tests (easy to hide) are needed to verify that everything works, numerical simulations do the rest. Boosted fission weapon is still considered a fission weapon, but it uses fusion to boost the yield. You need a breeder reactor or small particle accelerator to make the necessary tritium but basically anyone can do it. Including North Korea.