A nuclear reactor space power system using out-of-pile thermionic diodes, heat pipes, and a dual central absorber rod type of reactivity control has been studied. Emphasis is placed on the neutronic aspects and general feasibility of the concept. Comparison is made between uranium-233 and -235 nitride and plutonium-239 nitride fuels. From a neutronic standpoint, plutonium-239 nitride is found to be a slightly better fuel than uranium-233 nitride for this fast-spectrum reactor. In this concept, heat is transferred from the reactor core to the thermionic diodes by layers of radial heat pipes stacked alternately with slabs of fuel. For this out-of-pile concept, which would supply ∼130 kW(e), the reactor can be considerably smaller than the equivalent reactor with in-pile diodes. It would be particularly adaptable to a shadow-shielded type of application.