An analysis in the journal *Science* has revealed that high-assay low-enriched uranium (HALEU), which is being produced with federal subsidies for next-generation small nuclear reactors, poses significant nuclear proliferation and terrorism threats. Contrary to common belief, HALEU can be directly used to make nuclear weapons, raising concerns that have not been fully acknowledged by the federal government and industry.

The analysis, conducted by five leading experts in nuclear proliferation, warns that if HALEU becomes a standard reactor fuel without stringent restrictions from an interagency security review, it could enable other countries to acquire and process weapons-usable HALEU without detection. This would blur the line between peaceful and military nuclear programs, putting the international community at risk with little warning.

The paper emphasizes the need for additional measures to mitigate this risk as HALEU-fueled reactors are deployed internationally. It suggests that the US Congress should instruct the DOE’s National Nuclear Security Administration to commission a fresh review of HALEU’s proliferation and security risks by US weapons laboratory experts.

Current commercial reactors use uranium enriched to below 5%, which cannot sustain an explosive chain reaction. However, many new reactor designs propose using HALEU, enriched to between 10% and 20% uranium-235. Although below the 20% threshold that defines highly-enriched uranium (HEU), HALEU above about 12% uranium-235 could still be used to make nuclear weapons with yields similar to those that destroyed Hiroshima and Nagasaki.

The risks are escalating as the federal government encourages and funds HALEU production. The U.S. Energy Department is covering half the cost of deploying two demonstration nuclear plants using multi-ton quantities of HALEU fuel, including TerraPower’s “Natrium” fast reactor in Kemmerer, Wyoming.