If you only care about fission weapons, a practically viable weapon is easy, and does not require testing. This was demonstrated by South Africa, who developed

If you only care about fission weapons, a practically viable weapon is easy, and does not require testing. This was demonstrated by South Africa, who developed a fission weapon, using an indigenous and entirely novel isotope separation technique and no outside help. They produced a stockpile of around five weapons, with absolutely no testing. The Vela incident often referred to is outwith the relevant timescale, being in 1979 and the weapons program ran from 1984 to 1994. The devices themselves were gun-type bombs using uranium, similar to the 'little boy' (Hiroshima) design, with improvements in safety primarily. This was as a hedge against Soviet agression in the Angolan civil war, and they were intending to detonate one of the bombs as proof of their capability if the situation ever became desperate. As a pariah state due to Apartheid, the idea was that the US and allies would be forced to abandon their arms embargos and step in to help with conventional assistance in order to prevent World War III. This never happened, and South Africa is still the only state ever to have developed a viable nuclear weapons capability and then voluntarily given it up. The details are available in an excellent book by David Albright of ISIS (not that one, the Institute foir Science and International Security...) at https://isis-online.org/uploads/isis-reports/documents/RevisitingSouthAfricasNuclearWeaponsProgram.pdf https://isis-online.org/uploads/isis-reports/documents/Revis...