Burnup-dependent nuclear safeguards and security assessment of HALEU-Fueled sodium-cooled fast reactors

This study presents a burnup- and cooling-time-dependent assessment of safeguards- and security-relevant characteristics of spent fuel from high-assay low-enriched uranium (HALEU)-fueled sodium-cooled fast reactors (SFRs). A physics-based framework is adopted to evaluate spent fuel in terms of three functions: material attractiveness, self-protection, and monitorability. Within this framework, decay heat, photon emission rate, spontaneous fission rate, and external dose rate are treated as key spent-fuel attributes that collectively determine safeguards and security functions. Burnup-dependent isotopic inventories were generated using the Serpent 2 Monte Carlo code and tracked through post-discharge decay. The resulting isotopic compositions were further used in photon and neutron transport calculations to quantify radiation signatures and dose rates over cooling times ranging from 1 day to 100 years. For reference and verification, analyses were also performed for a conventional low-enriched uranium pressurized water reactor (PWR) and a recycled-fuel SFR. Results show that material attractiveness, self-protection, and monitorability exhibit strong dependence on both discharge burnup and cooling time. The fast neutron spectrum of the HALEU-fueled SFR limits the buildup of higher-mass transuranic isotopes such as 244 Cm, resulting in comparatively weak spontaneous-fission neutron signatures and negligible neutron dose contributions. Consequently, radiological self-protection is dominated by photon-emitting isotopes, and passive neutron monitoring may be less effective than photon-based approaches. Higher-burnup fuel maintains greater decay heat, photon emission rates, and dose rates at long cooling times, resulting in stronger intrinsic barriers and enhanced monitorability. These results demonstrate the importance of accounting for the combined effects of discharge burnup and post-discharge cooling time on spent-fuel characteristics when developing safeguards and physical protection strategies for advanced reactor systems.