2.1 The OTR Power System The Space Thermionic Advance Reactor-Compact (STAR-C) concept (Ref. 2), which is a specific version of an OTR power system, is depicted in Figure 2.1. This concept consists of a radiatively cooled reactor core that is surrounded by flat plate thermionic devices located in the reflector region. Its two major advantages are that it does not have a coolant loop, which increases reliability and safety, and it has a small radiator. Its major disadvantage is that it becomes increasingly heavy at power levels above about 50 kWe. STAR-C uses a solid core composed of segmented, annular fuel plates that are supported by graphite trays. Each fuel plate is made up of six, pie-shaped, uranium dicarbide segments fully enriched in U-235. The graphite trays are coated with niobium carbide to suppress carbon sublimation, which could result in carbon attack on the emitters of the thermionic devices. The reactor core is built by stacking the graphite trays and fuel plates; the number of fuel-tray assemblies being dictated by the power level. A 10 kW(e) system has a core that is approximately 26 cm in diameter and 48 cm long. The thickness of the trays can be varied to improve the uniformity of the power profile. Heat generated in the reactor core is conducted radially outward to the core surface where it is radiated to thermionic devices surrounding the core. During operation, the maximum core and core surface temperatures are expected to be approximately 2300 K and 2000 K respectively. The thermionics, which are located in the radial reflector, collect heat radiated across a gap from the core. The nominal operating temperature of the emitter is 1860 K. The collector is cooled by an integral heat pipe which conducts heat to the system radiator surrounding the reactor. The radiator is sized so that the collector operating temperature is lOOOK. Under these conditions, the efficiency of the thermionic devices is approximately 14%. When the other electrical losses are
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