Neutronic Feasibility of a HALEU-Started Breakeven Molten Salt Fast Reactor with a Simple Closed Fuel Cycle

This study investigates whether the breakeven molten salt fast reactor (BeMFR) concept can be started with high-assay low-enriched uranium (HALEU) while maintaining long-term operation based on spent nuclear fuel (SNF)-derived make-up fuel. The HALEU-started configuration employs KCl–UCl 3 –RECl 3 fuel salt, eliminating the need for transuranic (TRU)-based startup fuel while preserving the closed-fuel-cycle framework. Using Serpent 2 with online fuel feeding and fission product removal, three cases are analyzed based on the onset of make-up fuel feeding to characterize the transition to equilibrium. The results show that excess reactivity remains within 0.5–1 $ during most of the transition and that equilibrium is reached after approximately 50-60 years. The timing of make-up fuel feeding significantly affects fuel-salt volume evolution, indicating strong coupling between transition behavior and startup strategy. Both startup and equilibrium cores exhibit control drum worth of ∼3,000 pcm and strongly negative fuel temperature coefficients of about -9 pcm/K. iMC sensitivity calculations show that fuel circulation reduces the effective delayed neutron fraction by about 200 and 100 pcm for the initial (HALEU-started) and equilibrium cores, respectively, under representative flow conditions. These findings demonstrate that HALEU startup can be combined with sustained SNF utilization in a chloride-based BeMFR.