Abstract: A small natural-circulation sodium-cooled reactor-AMTEC system was presented as a distributed generation system. According to the criticality calculation and thermal-hydraulic analysis of the core when the total fuel mass was constant, the influence of pellet radius, fuel pin length and the number of rings of fuel pins upon the effective multiplication factor k_eff, pressure drop across the core and heat transfer was analyzed. Additionally, the B₄C absorber thickness and beginning-of-life excess reactivity under different numbers of rings of the fuel pins were studied when the assumed additional shutdown margin was different. The results show that decreasing the number of rings and active fuel length would increase k_eff and decrease pressure drop across the core when the core equivalent diameter and total cross-sectional area of the coolant channel are unchanged. In order to increase additional shutdown margin, the absorber thickness should be increased while the beginning-of-life excess reactivity and power plant economy would decrease.