Excerpt From "Engineering and Design of Nuclear Weapons"
Lack of knowledge has never been an obstacle to any nation in developing nuclear weapons. The problem is in obtaining the necessary tools and materials...
Interestingly enough, the United States government conducted a controlled experiment called the Nth Country Experiment to see how much effort was actually required to develop a viable fission weapon design starting from nothing. In this experiment, which ended on 10 April 1967, three newly graduated physics students were given the task of developing a detailed weapon design using only public domain information. The project reached a successful conclusion, that is, they did develop a viable design (detailed in the classified report UCRL-50248) after expending only three man-years of effort over two and a half calendar years. In the years since, much more information has entered the public domain so that the level of effort required has obviously dropped further.
This experiment established an upper limit on the required level of effort that is so low that the hope that lack of information may provide even a small degree of protection from proliferation is clearly a futile one...
http://nuclearweaponarchive.org/Nwfaq/Nfaq4.html
Great points.