Incore thermionic space reactor design concepts which operate at a nominal power output range of between 20 and 50 kWe are described. Details of the neutronic, thermionic, thermal, and shielding performance are presented. These moderated reactor concepts use enriched uranium dioxide fuel, zirconium hydride moderator, reinforced tungsten emitters, niobium collectors, alumina insulators, and sodium-potassium coolant in a long, single cell configuration. Due to the strong absorption of thermal neutrons by natural tungsten, and the large amount of that material within the reactor core, two options for the reactor are considered. The first uses enriched tungsten (greater than 70 weight percent W-184) emitters and only thermionic fuel elements (TFEs) in the core to achieve criticality and sufficient lifetime. The second option uses natural tungsten and driver fuel elements in addition to the TFEs in the core. An overall systems design code has been developed to model advanced incore thermionic energy conversion based nuclear reactor systems for space applications. The code modules include neutronics and core criticality, a thermionic fuel element performance module with integral thermal hydraulic calculation capability, a radiation shielding module, and a module for waste heat rejection. Coupled thermal hydraulic and thermionic performance calculations are presented. The model includes the effects of radiation and conductive heat transfer as well as electron cooling of the emitter, and the resistive lead losses on long emitter TFE concepts. Radiation shielding design and overall system heat rejection analyses are also presented.