Abstract: Aiming at improving the economy and inherent safety of lead-based fast reactor, the SPALLER-100 core concept design of 100 MWt ultra-long life small natural circulation lead-based fast reactor was proposed. Based on the selection of PuN-ThN fuel and ²⁰⁸Pb-Bi coolant, a fuel assembly design scheme with the addition of solid moderator BeO was carried out. The core layout and control rod system design were presented, and the core neutronics characteristics and steady state natural circulation features were analyzed. The results show that under the conditions of low fuel loading and small core volume, the refueling cycle of SPALLER-100 core is up to 32 years, the average discharge burnup is up to 210.38 MW·d/kg(HM), and the reactivity coefficients are negative throughout the lifetime. Under steady-state operating conditions, the maximum temperatures of fuel cladding and pellet are lower than the safety limit, and the core has primary circuit natural circulation capability and certain automatic flow distribution capability.