The natural-circulation decay heat removal capability of a 550-kW(electric) SP-100 reactor power system for a lunar outpost is investigated. A transient thermal-hydraulic model of the decay heat removal loop (DHRL) is developed to investigate the effects of the radiator surface area, the dimensions and elevation of the decay heat exchanger (DHE), and the diameter of the rise and down pipes on the passive decay heat removal of the system. The effect of gravity is also investigated in order to examine the applicability of earth-based test results to the actual system on the lunar surface. Results show that natural circulation of lithium coolant in the DHRL would keep the SP-100 reactor safely coolable after shutdown. However, the lithium coolant in the adiabatic rise pipe, directly downstream from the reactor core, could overheat by as much as 175 K above its nominal operation value of 1355 K at ∼200 s after shutdown. This coolant temperature increase can be reduced by as much as 50 K by increasing the height of the DHE duct to 15 cm; a further increase in the duct height would have little effect on the decay heat removal. Increasing the elevation of the DHE slightly improves the decay heat removal. Results also show that the maximum coolant temperature in the DHRL and the maximum fuel temperature in the reactor core at 1 g could be as much as 140 and 50 to 100 K lower than their values on the lunar surface, respectively. Conversely, the coolant flow rate could be more than twice that occurring on the lunar surface after reactor shutdown.