Performance assessment of neutron coincidence collars for safeguards measurements of fresh high-assay low-enriched uranium advanced reactor fuels

Advanced reactor (AR) fuel elements differ markedly from light water reactor (LWR) fuels in size, composition, and form, and many employ high-assay low-enriched uranium (HALEU, 5–20 wt % 235U), creating potential challenges in applying existing nondestructive assay (NDA) international safeguards verification instruments to fresh AR fuel elements. We evaluated the applicability of existing neutron collar technologies to AR fuel elements using Monte Carlo N-Particle models of Uranium Neutron Coincidence Collar-II and Fast Neutron Collar interrogations for elements representative of 7 of the 10 designs supported by the US DOE Advanced Reactor Demonstration Program, which provides a common screening comparison across three different AR fuel forms. Partial defect scenarios were simulated by substituting HALEU in the fuel with depleted uranium. Relative to LWR assemblies, several AR geometries were incompatible with detector designs and produced lower responses, yielding weak sensitivity to enrichment/fissile content and degraded detectability of partial defects for some designs. Sensitivities varied strongly with fuel form, moderation, and element dimensions. In several cases the collars’ responses were insufficient for reliable enrichment verification or partial defect detection without design-specific fixtures or modified operating modes. These findings indicate that collar-based verification is not universally transferable to AR fuels, and that alternative or significantly adapted NDA approaches will be required for some fuel types. Early development and qualifications are therefore needed to meet safeguards timelines.