The tech to make nuclear weapons is not really that difficult. There have been a few students in western universities over the years who have made them (usually minus the fissionable materials). The tricky part is isolating the fissionable materials and those materials are dangerous to handle. It takes spending a fair bit of money to build dedicated facilities and proper protection for the people working on them to isolate the materials. Any nation with a small cadre of scientists with the right training and the willingness to sink the funds into the project will eventually be able to make enough for a few bombs.
If I remember right India took a short cut and separated the materials for their first bombs from spent nuclear reactor fuel. I believe they were reprocessing a lot of the world's waste fuel at the time. I don't have time to look it up at the moment, I may be misremembering.
It's also rumored that Israel, South Africa, and Japan have had secret nuclear weapons programs. In Japan it's rumored they keep the parts for several weapons disassembled on the "shelf" in case China gets too threatening.
Any country known to be working on nuclear weapons have tremendous pressure put on them to stop, the country risks becoming a political pariah on the world stage, and it can be a massive chunk of a small country's GDP to run the facilities. A lot of countries don't go down that route because it's more trouble than it's worth. A fair number of countries have joined alliances like NATO so they can be under the nuclear umbrella of a country like the US.
For countries with an adversarial relationship with the US or don't have promises the US will protect them with nukes, they have found other members of the nuclear club aren't generally willing to put their country under their umbrella, so some have chosen to go their own way.
www.ifw-kiel.de
mil.in.ua
