A study, funded by the National Aeronautics and Space Administration’s (NASA’s) Prometheus Nuclear Systems and Technology Program, has been conducted to examine the performance of a potassium Rankine cycle power conversion system (PCS) for use in space. The potassium Rankine cycle, coupled to a fast spectrum nuclear reactor, offers the potential for very high power output, low-mass system designs. Higher operating temperatures, higher system efficiencies, and higher heat rejection temperatures keep both system mass and required radiator areas comparatively low, potentially allowing the Rankine cycle to be applied over a wide range of powers, from tens of kilowatts all the way to the multimegawatt level. The conceptual design study presented in this paper examined the design and performance characteristics of major components included in the potassium Rankine cycle PCS: boiler, turbine, condenser, radiator, turbine-driven boiler feed pump, and a rotary fluid management device used to provide positive phase separation. Although not the focus of this effort, the reactor, shield, and primary coolant system were also included to perform system studies. The performance characteristics and mass dependencies for each component were incorporated in a system model, and trade studies were performed to optimize the system design. The trade studies included evaluations of structural materials, thermodynamic cycle features such as feed heat and reheat, multiple heat rejection system designs, and operating state points. The paper includes results of these studies for two system power levels, 100 kW(e), and 250 kW(e), and presents results for minimized mass system designs.