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Abstract: Design concepts of the Sectored Compact Space Reactor for Small power (SCoRe-S) have been developed for the avoidance of single-point failures in reactor cooling and energy conversion and a wide range of thermal powers. These modular, fast neutron spectrum, lithium cooled reactors with 16.0cm thick BeO radial reflector are designed for at least +$2.00 hot-clean excess reactivity, and with a sufficient reactivity shutdown margin. They employ 157GdN additives in the UN fuel and a 0.10mm thick coating of 157Gd2O3 on the outer surface of the reactor vessel to ensure that the bare reactors, when submerged in wet sand and flooded with seawater following a launch abort accident, remain at least −$1.00 subcritical. In addition to identifying the smallest SCoRe-S concept that satisfies the design reactivity requirements, the benefit of using a lunar regolith as a supplementary reflector to decrease the thickness of the BeO radial reflector and hence, the launch mass of the SCoRe-S concepts for a lunar outpost is investigated. Calculations performed using MCNP5 confirmed that SCoRe-S7 with a 16cm thick BeO reflector is the smallest to satisfy the stated reactivity requirements. Results also show that a lunar regolith reflector alone is inadequate for this reactor to achieve a critical state at the beginning of life. However, when the regolith is used in conjunction with a BeO reflector of a reduced thickness, this reactor not only becomes critical, but also satisfies the reactivity design requirements at a significantly reduced launch mass. Using a supplementary reflector of regolith decreases the thickness of the BeO reflector for the SCoRe-S7 from 16cm to 8.0cm, and to 5.7cm for the SCoRe-S11 of the largest core. The resulting decreases in the launch mass of the SCoRe-S concepts are ∼34% or 150–200kg. [Copyright &y& Elsevier]