The Space Power Advanced Core-Length Element Reactor (SPACE-R) System is a moderated core fueled by uranium oxide. The baseline power level is 40 kWe with a system life of 10 years. The system is launched without radioactivity and is designed to meet all U.S. and international public safety and environmental protection objectives with minimum loading of fissionable 235U and minimum proliferation potential. The key feature of the SPACER system is the core-length, single-cell thermionic fuel element (TFE). The open end of the emitter cavity permits an electrical heater to be inserted for full power operational testing of each TFE and the entire reactor assembly before the UO2 fuel is loaded. The current design is based upon technologies, materials, components, and temperatures that have been space flight proven within the U.S. SNAP reactor program, and the Soviet Topaz reactor programs.