Startup core design of metallic-fueled lead-bismuth eutectic-cooled rotational fuel-shuffling breed-and-burn fast reactor

This study explores methodology for startup core design integrating enrichment and shuffling optimization across transient phases. Metallic fuel and lead-bismuth eutectic (LBE) coolant were selected for the core materials due to their excellent neutronic properties and chemical inertness to water and air. Four representative cases were analyzed using the Monte Carlo code SERPENT to evaluate core performance, shuffling interval adjustment, and enrichment optimization. The most promising cases were further evaluated using COMSOL Multiphysics to assess thermal-hydraulic performance across different operating steps, confirming that heat removal was feasible under the proposed power and coolant conditions. Control rod worth was also calculated to confirm sufficient shutdown margin during peak reactivity. The results demonstrate that the proposed RFBB startup core achieves criticality, maintains sufficient excess reactivity, and transitions to equilibrium while satisfying safety and thermal design requirements.