Closing the Nuclear Loop: Environmental Performance of MOX and REMIX Fuel Cycles in Fast Reactors

This study evaluates the environmental performance of fast reactor fuel cycles operating with Mixed Oxide (MOX) and Regenerated Mixture (REMIX) fuels through a comparative life cycle assessment (LCA) following ISO 14040/44 standards. Three scenarios were analyzed: an open uranium oxide (UO2) cycle, a fast reactor with MOX single recycling, and a fast reactor with REMIX multi-recycling. The assessment considered global warming potential (GWP100), total material requirement (TMR), high-level waste (HLW) volume, and radiotoxicity at 1,000 years. Results show that both MOX and REMIX cycles significantly reduce uranium demand (−50%), radiotoxicity (−70%), and HLW generation (−40%) compared to the open cycle. Lifecycle GHG emissions remain among the lowest of all power sources (8–15 g CO2- eq/kWh) and may decrease further if reprocessing uses low-carbon electricity. The REMIX multi-recycling option offers the highest sustainability gains, doubling the overall performance index relative to UO2. When interpreted through the frameworks of planetary boundaries and Doughnut Economics, fast reactor fuel cycles emerge as technologies capable of providing firm, low-carbon energy within ecological limits. The results highlight the role of closed nuclear fuel cycles as instruments of circularity, intergenerational responsibility, and planetary restoration within future sustainable energy systems.