Nuclear bombs are very difficult to design. The technical problems faced by someone trying to construct such a device are staggering and are too complicated for me to cover here. But I have read about them in a book called "Critical Assembly : A Technical History of Los Alamos during the Oppenheimer Years, 1943-1945". This book does not provide the kinds of techical details about the solutions to problems faced by those on "Project Y" to tell the reader how to construct a nuclear device; But, it does give enough detail about those problems to impress the reader with just how difficult these devices were to design. After reading this book, I am convinced that it is very unlikely that a terrorist organization cannot successfully design a nuclear bomb without a great deal of money, the protection of a government, and a high risk of being detected by Western nations. But if you're worried that I'm wrong, read the book, yourself. I think you'll find it convincing.