Any developed nation _with a well developed nuclear energy industry_ would be able to build a bomb with years of effort. There's a lot that goes into a bomb beyond refining (e.g. miniturization of warhead). And to deploy the bomb a ton of work to achieve the gold standard of a nuclear triad, plus build enough bombs to be survivable past an initial strike (to maintain deterrence).

If you learn of any countries with a secret program, let your local intelligence agency know. They monitor this closely, and intervene.