Problem with nukes is that every reactor in the world has a signature, similar by type of design, but utterly unique in specifics. These are pretty much all known by all the major powers thanks to fallout patterns, air sampling after even below ground testings, etc.
You use a weapon and the world knows, down to the reactor location, exactly where it came from. That point of origin will have some 'spainin to do. If it is from a stolen device every country on Earth will start turning over rocks to find the doers, they would far too dangerous for even the most radical state to let them run loose (if for no other reason than the hosting state becomes a target for retaliation). If it ISN'T from a stolen device, then God have mercy on whoever supplied it.