[en] Designs for two candidate reject heat systems (RHS) have been developed for a Brayton closed-cycle space nuclear power system. The most important constraints imposed on the design are the size and mass limits and the rigid survival standard of 99% probability that the RHS be functional at full power at the end of a seven-year mission. The two basic design concepts developed are (1) a secondary loop RHS with a redundant heat exchanger and (2) a direct-pumped gas RHS. Both concepts make extensive use of heat pipes to provide redundancy. The secondary loop design includes a pumped-NaK radiator and a heat-pipe heat exchanger. The radiator is of conventional design with armored flow tube passages and fins arranged in rectangular panels placed at right angles to each other. The heat exchanger is a cylindrical pressure vessel with four radiator loops and two converter loops. The direct-pumped gas RHS is designed with many thin-walled, finned heat pipes placed side by side to form rectangular radiator panels that are arranged at right angles to each other. The gas heat exchanger is an armored manifold composed of multiple parallel flow tubes placed at the evaporator end of the radiator heat pipes. Results show that this system is the lightest configuration investigated