Development of a Category I Quantity for High-Assay Low-Enriched Uranium

Most of the non-light-water-reactor projects under development around the world, and some marine-based light-water reactors already deployed, require the use of high-assay low-enriched uranium (HALEU) fuel (enriched from 10 percent to under 20 percent uranium-235) in quantities ranging from hundreds of kilograms to tens of tonnes. However, the nuclear terrorism and proliferation impacts of a transition from the use of uranium enriched to much lower levels in current-generation light-water reactors to the use of HALEU in new types of power reactors has not been adequately considered. Publicly available U.S. national laboratory reports state that the direct use of HALEU in nuclear weapons is impractical but not impossible. Although HALEU-based nuclear weapons are likely less efficient than highly enriched uranium (HEU)-based weapons of similar design, their larger critical masses may still allow for significant yields, and more advanced designs may allow for efficiency improvements. However, in both domestic and international material categorization schemes HALEU is never classified as Category I (high strategic significance) material, no matter how large a quantity is present. This paper provides an analysis based on publicly available information and codes that suggests quantities of HALEU containing about 20 kilograms or more of 235U should be considered of comparable attractiveness and protected in a similar manner to Category I quantities of HEU, uranium-233, and plutonium.