The following constraints were imposed in the minimization process: • neutron fluence at the payload below 5.0 E 12; • gamma fluence at the payload below 5.0 E 6 rad; • core heat pipe radial power limitation of less than 120 W/cm2; • radiator heat pipe radial power limitation of less than 200 W/cm2; • maximum fuel temperature limited to 2073 K; • maximum turbine inlet temperature limited to 1500 K; • shuttle cargo bay compatible dimensions—less than 4.5m dia. X 18.6m. The minimization was performed using MPLSUMT in the HIT AC numerical analysis library. After optimization, system mass was found to be most strongly dependent on turbine inlet temperature for the Rankine cycle system and on collector temperature for the thermionic conversion system. The values of the important variables within their operating limits are shown in Fig. 12. Fig. 13(a, b) shows system mass as a function of these variables for the two different conversion technologies. For the Rankine system, mass decreased monotonically as temperature increased over the range of 1320 to 1560 K. Minimum achievable systems mass was 12930 kg at a turbine inlet temperature of 1500 K which was chosen as an upper limit. The collector temperature for the thermionic system was varied from 800 to 1300 K. The minimum systems mass of 7480 kg was obtained at a collector
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