The thermionic converter system is composed of 8 subsystems: a reactor core; core heat pipes (Li/TZM); neutron and gamma shields; thermionic converters; radiator armor and fins (Be); reactor controls; a power conditioner; and support structure which is assumed to be 10% of total system mass. A schematic of the thermionic converter system is shown in Fig. 1(b). The reactor design used for the Rankine system was also used here. In this design the thermionic converters are outside of the core in order to avoid short circuits caused by fuel swelling and radiation damage to the electrodes. Sodium radiator heat pipes are not needed in this design since the condenser sections of the core heat pipes also serve as the radiators. Thermionic emitters are attached to both sides of the heat pipes' condenser sections and are heated by conduction. Collectors are welded directly to the armor which radiates waste heat to space. About 200 converter cells are connected in series to provide 100 volts DC output. In both designs payloads are attached to the end of a 25m boom, where radiation fluences are 5.0 E 12 nvt of neutrons and 5.0 E 6 rad of gammas over five years. Nuclear Reactor. An intermediate spectrum nuclear reactor was selected as the several megawatt thermal power source. A schematic of the heat pipe cooled reactor is shown in Fig. 2. The core consists of about 180 fuel elements. These are made up of 93% enriched UO2 covered with concentric layers of low density pyrocarbon, silicon carbide and high density pyrocarbon which contain the fission product gases within the fuel elements. If zirconium carbide coating technology advances to the point where it can be used instead of silicon carbide, maximum fuel temperature could be increased from 1870 K to 2070 K. Fuel burnup is expected to be 20% FIMA [8]. Beryllium reflectors are located at the top, bottom and sides of the core. The heat pipes are bent just outside the top reflector to reduce neutron streaming in the direction of the payload. Details of the core elements are shown schematically in Fig. 3. The reactor is controlled by 12 rotating drums located in the radial reflector. Each
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