The possible application of 6Li as a burnable poison and U-Np nitride as a fuel for space nuclear reactors has been studied. The analysis was performed for an infinite lattice with a leakage in the form of buckling and (U-Np)N fuel with 20% uranium enrichment. The combination of 7Li as a coolant and 6Li as a burnable poison results in a favorable criticality behavior during burnup. The parameters taken into consideration include the different fuel and coolant compositions, the form of absorber material, and the various absorber mass and concentrations. It was found that absorption properties of 6Li allow reaching the burnup value up to 67 GWd/tHM while reactivity swing is comparable with eff. The corresponding reactor lifetime is ~10 to 30 yr.