No, it's not possible. The types of clean labs required for nuclear weapons maintenance are barely able to be maintained by great powers at enormous expense. Even something as simple as moving a trigger too close to the sides of a weapon while attempting to install it could cause an instant, high-visibility fizzle. This is not work that can be done by amatuers or local welders in a machine shop or warehouse.
Furthermore, our radiation detection is so acute that we are pulling drivers over on the highway who have had radiation treatment for cancer therapy.