Abstract The Sodium-cooled Fast Reactor (SFR) is one of the most promising Generation IV systems with many performance advantages, but has one dominating neutronics drawback – a positive sodium void reactivity. The starting point for the present study is an SFR core design considered in the Collaborative Project on the European Sodium-cooled Fast Reactor (CP-ESFR). The aim is to analyze, for this reference core, four safety and performance parameters from the viewpoint of four different optimization options, and to propose possible optimized core designs. In doing so, the study focuses not only on the beginning-of-life state of the core, but also on the beginning of equilibrium closed fuel cycle. The four studied optimization options are: (a) introducing an upper sodium plenum and boron layer, (b) varying the core height-to-diameter (H/D) ratio, (c) introducing moderator pins into the fuel assembly, and (d) modifying the initial plutonium content. The sensitivity of the void reactivity, Doppler constant, nominal reactivity and breeding gain has been evaluated. In particular, the void reactivity, which is the most crucial safety parameter for the SFR, has been decomposed into its reaction-wise, isotope-wise and energy-group-wise components using a methodology based on the neutron balance equation. Extended voiding in the upper sodium plenum region – in conjunction with the effect of a boron layer introduced above the plenum – is found to be particularly effective in the void effect reduction while, at the same time, having almost no impact on the other considered parameters. A lower H/D ratio can also reduce void reactivity, but normally corresponds to worse neutron economy and, as such, leads to a less positive breeding gain and a smaller nominal reactivity margin. The Doppler constant, which is very sensitive to the neutron spectrum, can be significantly improved by the spectrum softening in the moderator pins case. Initial plutonium content effectively influences the nominal reactivity and breeding gain at the beginning-of-life, without significant changes of the safety related parameters. Finally, two different “synthesis core” concepts are proposed for further study, the common boundary condition set being to have a positive nominal reactivity margin at the end of equilibrium closed fuel cycle. Their considered safety and performance parameters at different fuel cycle states are presented in the end.