No efforts have been documented which reflect any attempt to demonstrate that an orbiting nuclear reactor including the fuel cycle component can meet this requirement. Since the reactor would operate at synchronous orbit there is no re-entry problem. Nuclear power transfer from low earth orbit and transfer of nuclear materials through the atmosphere would cause some concern however. b . Materials The light weight required of orbiting reactors (to keep their transportation costs feasible) and the high temperature required of either the thermionic* or Brayton conversion systems place a very stringent requirement on the materials to be used in the reactor. For example, the highest practical tubing temperature for the 1985 technology molten salt breeder reactor was judged to be 1350°F (1030 K) for 30 years of continuous utilization. This relatively low temperature severly limits the temperature drop which can be achieved across the Brayton engine or the emitter to collector portions of the thermionic diodes. This, of course, greatly reduces the efficiencies of these two devices. There is also a problem with metal creep over the thirty-year period hypothesized for the satellite, a problem that is exacerbated by high mechanical stresses and operating temperatures. c . Efficiency Neither the Brayton cycle nor the thermionic converter is an inherently high efficiency device. The Brayton system has a current overall efficiency of about 45 percent (Ref. A4 indicates current efficiencies of 35-40 percent with optimum space efficiency on the order of 40 percent), and even at temperatures about twice that available from the reactor, the 1995 projection of thermionic conversion efficiency is only 36 percent. It is currently on the order of 10 to 15 percent. 4. Microwave Transmission All SPS concepts utilize microwave transmission for the transfer of *As indicated in Section III-3, the nuclear thermionic concept has been found to not be feasible. It is included in the discussions in this section for the sake of completely reporting efforts to date.
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