reasons for the steep slope. The main reason is the low efficiency of the thermoelectric conversion system; the multicouple efficiency is 4.7% and the total system efficiency 4.2%. As a result, the masses of the reactor and heat rejection radiator both increase faster with increasing power requirement than for the other liquid metal reactor options. The second reason for the steep slope is that the specific mass (in kg/kWe) of the thermoelectric conversion units is high. 4.1.3 The SP-l(M) With Brayton Power Conversion Aside from the potential reductions in mass of the STAR-C concept, probably the most interesting result obtained in our analyses is the excellent scalability of the SP-100 reactor used in conjunction with a Brayton power conversion system. It is the least massive of all of the near-term power systems at power levels above 25 kWe. In fact, it even scales better than the SP-100 with a refractory Stirling power conversion system, which is not a near-term system. The only power system that scales better is the SP-100/Rankine system. It is 20% lighter than the SP-100/Bray- ton system at 100 kWe and 43% lighter at 1000 kWe. The above results obtained for the SP-100/Brayton system are different than those obtained by Rockwell in work they have done recently for 10 to 40 kWe SP-100/Bray- ton systems (Refs. 20 and 21). The difference between our results and those of Rockwell were identified and can be attributed in large part to the design assumptions that were made (Ref. 22). For the case of the 10 kWe system, the Rockwell mass is 28% higher than ours. We believe differences in basic design assumptions and requirements, especially in the areas of boom and power conditioning, account for the majority of this difference (Ref. 22). Our design assumptions have been made to be consistent with the other systems discussed in this report. As was mentioned earlier, the gas-cooled reactor/Brayton power system was not analyzed in detail because of financial constraints; we did not have the time to develop the criticality model for the reactor. However, at 1000 kWe, where the reactor will not be criticality limited, our mass estimate for a gas-cooled reactor/Brayton system is within 1% of the mass of the SP-100/Brayton system. Illis leads us to believe that the masses of these two types of systems will be comparable at other power levels as well. 4.1.4 TFE Based Power Systems The TFE based power systems do not scale very well at the very low power levels. They are the most massive power system below about 22 kWe. However, as the power level increases, they become much more attractive. Above 85 kWe, they are lighter than all other near-term power systems except the SP-100/Brayton system. The reason for the poor system performance at low power levels is that the reactor and shield are much more massive than for the other systems. 'Hie reactor is more massive, because it contains moderator and more structure and has larger dimensions. The shield is more massive, because it has a larger diameter, which is caused by a larger reactor. TFE based systems perform better at higher power levels
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