It?s been estimated (by the ISIS, http://www.isis-online.org/...) that, in the last sixty years, some 4,000 tons of weapons grade nuclear material (enriched uranium, or plutonium) has been manufactured. This is enough for nearly 400,000 nuclear weapons. That estimate includes the belief that at least fifty nations possess at least eleven pounds of that material, which is enough to build a small nuclear weapon.

Why aren?t their more nuclear weapons, given the widespread availability of the plans for older nuclear weapons, and the presence of so much nuclear weapons material in so many nations? Several reasons. Many nations that could quickly design and manufacture nuclear weapons have not done so because they promised not to. Japan, Germany, Taiwan and South Korea all fall into this category. But the most important reason more nations have not built nuclear weapons is because a nuclear weapon is not an easy thing to build. Even sixty years after the first one was built and successfully tested, it is still a tricky bit of engineering. Despite the avalanche of new technology developed in the last six decades, little of it was ?dual use? in the nuclear weapons sense. The difficulty one has building nuclear weapons is demonstrated daily, as we continue following the trials and tribulations of countries like Iran and North Korea, who have been openly working on nukes for over a decade. If it were easy, they would have their bombs by now. But it isn?t, and they don?t.