Acquiring nuclear-explosive material remains the most challenging step in the weapons acquisition process. Given such material, a sophisticated terrorist organization or a state could plausibly construct a rudimentary nuclear bomb.

As the now-defunct U.S. Congressional Office of Technology Assessment concluded: “A small group of people, none of whom have ever had access to the classified literature, could possibly design and build a crude nuclear explosive device… Only modest machine-shop facilities that could be contracted for without arousing suspicion would be required… The group would have to include, at a minimum, a person capable of searching and understanding the technical literature in several fields and a jack-of-all-trades technician.”⁹⁸ This assessment is particularly relevant for HEU, because its usefulness in the most rudimentary gun-type design makes it the material of choice for terrorists as well as states seeking a more rapid or less challenging route to a crude nuclear bomb. A few tens of kilograms are sufficient for such a primitive design, similar to that used in 1945 to destroy Hiroshima. As enrichment levels decline, more material is required, yielding both a larger and less powerful device, but the former might not be problematic for terrorists contemplating delivery via truck or ship and even a relatively weak device could have a yield equivalent to thousands of tons of conventional explosive.

Luckily, HEU is extremely difficult to manufacture. Unfortunately, thousands of tons were manufactured over the past half-century for diverse uses, from nuclear weapons to targets used in civil production of medical isotopes. State weapons establishments are more likely to provide high degrees of security for their nuclear-explosive materials, although even here there is room for concern. Conversely, non-military or civil stockpiles of HEU are often less well secured, although their usefulness for constructing nuclear explosives is not necessarily commensurately lower.

Although comprehensive, detailed data on civil sites possessing nuclear explosive materials has not been compiled, estimates suggest that there are approximately 100 metric tons of civil HEU worldwide.⁹⁹ Located in civil research and test reactors, critical and subcritical assemblies, and medical isotope production facilities worldwide, some of this material is in facilities with high levels of security, some in facilities secured with little more than a padlock and a guard.¹⁰⁰ Today there are more than 140 research reactors in more than 40 countries fueled