As the power level increases, the power conversion system and radiator become more important. At 1000 kWe, the radiator is 57% of the total system mass. 4.23 Radiation Hardened Electronics The radiation hardness of the electrical subsystem and payload electronics can have a significant impact on system mass. This is particularly true at lower power levels. The Weapons Laboratory currently has a program in place that should result in space qualified integrated circuits and memory chips that will have hardness values of 100 Mrad gamma and 1015 nvt neutrons by the year 1995 (Ref. 23, 24, 25, and 26). These values compare to 0.5 Mrad and 1013 nvt, which are used in this report. If a similar program were in place to develop power electronics that could withstand the higher radiation levels as well as single event burn-out, which can be induced by cosmic radiation, shield thicknesses could be significantly reduced. It should be noted, however, that the technical issues associated with hardening high voltage power electronics will be more difficult to solve than those associated with lower voltage integrated circuits and memory chips (Ref. 26). This is an area where a research program could have a high pay-off. Figure 4.6 shows how the mass of an optimized OTR is reduced with these advanced electronics. At 10 kWe the mass decreases by 26%, and at 30 kWe it decreases by 21%. At the same time, the length of the separation boom for the 30 kWe system decreases by more than a factor of two. 4.3 Power System Area Results A comparison of the specific areas of the heat rejection radiators for each of the power systems as well as the power conditioning radiators is given in Figure 4.7. The specific areas are for the power system beginning of life (BOL), i.e., they include an additional 10% area to account for heat pipe redundancy. It should be noted that the power systems were optimized based on total power system mass. Therefore, the radiator areas shown here could be reduced at the expense of mass, i.e., by using higher heat rejection temperatures and less efficient energy conversion. The largest radiator belongs to the SP-100 thermoelectric power systems. This is due in large part to the fact that the system efficiency is 4.2%, which is very low compared to the other systems, and so almost 96% of the power generated in the reactor must be rejected. The next largest radiator belongs to the SP-100/Brayton power system. Its large size is due to the fact that it has a relatively low temperature. The 19% change in the specific mass between 30 and 50 kWe is due to an increase in the effective radiator temperature. The radiator temperature changes from an inlet of 758 K and outlet of 379 K to an inlet of 758 K and outlet of 428 K. This increase in outlet temperature reduces the system efficiency from 23.1% to 20.5%, but reduces overall system mass by reducing the size of the radiator. The third largest radiator is for the SP-100 with a near-term Stirling power conversion system. Its large size is due to the fact that it operates at relatively low temperatures: between 490 and 540 K. The radiator specific area for the SP-100 with
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