The use of graphite-matrix tri-structural-isotropic (TRISO) fuels in high-temperature reactors with high-assay low-enriched uranium (HALEU) can significantly reduce nuclear weapons proliferation risks relative to other fuels and reactor types. The HALEU fuel, with fuels containing 15% to 20% 235U enable used nuclear fuels (UNFs) with thermal neutron–spectrum burnups between 150 000 and 200 000 MWd per ton. At these high burnups, the plutonium isotopics make the direct use for nuclear weapons unattractive and the uranium isotopics unattractive as a feed to a uranium-enrichment plant. On the front end, it would require the theft of ~150 000 pebbles with uranium just under 20% 235U to create the theoretical potential to produce sufficient material for one weapon (1000 kg), which is about a 2-year supply of fuel for these reactors.

The chemical and mechanical processing requirements to convert fresh TRISO fuel to uranium metal for use in a nuclear weapon are beyond nonstate actors. Over 10 sequential chemical process steps would be required, plus uranium recovery from waste streams, to avoid large uranium losses in the conversion processes. If a nation-state wanted to make a nuclear weapon starting with HALEU fuel, they would enrich the HALEU from 19.95% to over 90% 235U, which presumes they already possess enrichment capabilities and can use any uranium feedstock. If enriched to weapons-grade 235U, 1 ton of HALEU has sufficient 235U for multiple weapons.

Separately, it is not clear if a weapon can actually be built with HALEU fuel. The fuel characteristics also reduce risks from sabotage. Consequently, we conclude that reactor safeguards for fresh HALEU TRISO fuel can be similar to those for low-enriched uranium light water reactor fuel; that is, no requirements for added security or other measures. TRISO UNF safeguards and security can be significantly relaxed relative to the requirements for other types of UNF at the reactor site.