Trade-off between repository footprint and radiotoxicity in geological disposal of vitrified high-level waste from spent MOX fuels: quantitative assessment of simplified minor actinide separation

Abstract

Radioactive waste management remains a critical challenge for the sustainability of nuclear energy systems, particularly for closed fuel cycles utilizing mixed oxide (MOX) fuel. MOX fuel contains significantly higher concentrations of minor actinides (MAs) and heat-generating nuclides compared to uranium dioxide (UO 2 ) fuel, necessitating distinct disposal strategies. This study evaluates the effect of a proposed “simplified MA separation” process (recovery ratios of 70% and 90%) on the environmental load of geological disposal for vitrified high-level waste from spent MOX fuel (MOX-HLW). The term environmental load is defined as the physical/spatial footprint. We quantitatively assessed the repository footprint and radiological impact based on nuclide migration and potential human intrusion scenarios. The results demonstrate that, without separation, MOX-HLW of no-countermeasure baseline case requires a repository footprint approximately 6.7 times that of UO 2 -HLW due to the high heat generation from Am-241. Simplified MA separation with 90% efficiency effectively reduces the footprint to levels comparable to those of UO 2 -HLW by enabling higher waste loading. However, a fundamental trade-off was identified: increasing waste loading to minimize footprint maintains a constant Am-241 inventory per canister due to thermal constraints, resulting in no reduction in the potential exposure dose from Am-241 radiotoxicity in inadvertent human intrusion scenarios. Furthermore, the exposure dose from nuclide migration is dominated by long-lived fission products (specifically Cs-135), which are unaffected by MA separation. These findings reveal that the effects of upstream nuclear fuel cycle conditions, such as MA separation during reprocessing and the waste loading of vitrified waste, are deeply embedded in the repository thermal design and the evaluation of future human-induced events for MOX-HLW disposal. Therefore, it is necessary to derive a comprehensive optimal solution from various combinations of conditions by focusing on these trade-off relationships.