The sodium flow must transfer heat to each diode emitter; the NaK flow of the radiator loop removes heat from the diode collectors. Figure 5-9 shows a possible method of interfacing a heat transfer loop to a thermionic diode. Good thermal contact is required, yet electrical isolation must be provided. Pumping power for the radiator is parasitic upon the diode output. Therein lies the fundamental problem with the nuclear thermionic concept. 1. Maximum temperature is set by the reactor capability. 2. Minimum temperature is set by the size of the radiator system. 3. Larger and cooler radiators require the most pumping power. 4. A high temperature radiator reduces diode efficiency, thus increasing the waste heat to be dissipated. If the diode efficiency is E, and the electrical power required for the MTPS is PmTPS ^ien waste power (power to be rejected) is: R = PMTPS -1) For a radiator pumping power of PpUmp, the heat rejection increases: R = (pMTPS + ppumpX -U But Ppump a Unction of R, i.e., more heat rejection requires a larger radiator and more pumping power. Evaluation of this concept over a wide range of collector temperatures indicated that the system could not generate the pumping power necessary to pump the radiator system required for waste heat generation. As an example, with a 1020 K (1394°F) emitter and a 400 K (260°F) emitter, the diode efficiency is only 23%. Thus 77% of the solar power absorbed must be rejected as a waste heat. The resultant radiator pumping power exceeds the diode output. 5.9 NUCLEAR CLOSED BRAYTON CYCLE (CONCEPT 8) The nuclear Brayton cycle SPS contains sixteen 1 GW busbar output molten salt breeder reactor modules. The main structure of the satellite consists of a spine with sixteen ribs to which are attached the reactor modules and their primary radiators. Each reactor module has secondary radiators for cooling the generators and nuclear fuel processing systems. The ground output of the nuclear Brayton cycle power satellite is 10 GW. The concept is shown in Figure 5-27. Fig. 5-27. Nuclear Brayton Cycle SPS In the baseline concept, Figure 5-28, sixteen of these modules are used to provide 10 GWe ground output. The molten salt breeder reactor (MSBR) is spherical. The shield to reduce the radiation level at the transmitter is located only along lines-of- sight to the transmitter. Molten salt flows to six Fig. 5-28. Reactor Module
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