exploited the very low temperature regime of fission-power plants (working at ~600 K), high temperature gas-cooled reactors (HTGRs) operate with helium at 1100 K, and high temperature space reactors at a fuel-limited <3000 K for the working fluid. The energy utilized by all these power systems is purely thermal energy. The vapor core reactors are not only capable of operating at significantly higher fluid temperatures of 4000 K and above (non-fuel materials limited) but can also provide a fissiongenerated nonequilibrium electron gas with temperatures in the 8000-15 000 K range. Energy conversion at these levels of fluid and electron gas temperature can produce a requisite system efficiency of ~25%, a radiator operating temperature between 1600-2100 K, and system specific mass on the order of 0.5 Kg/KWe or better. If, as has been suggested by current investigations, a nonequilibrium partially-ionized plasma can be established by the fissioning gas, the power system might no longer be limited by the second law of thermodynamics. The bulk temperature would no longer be a measure for power and power density, and therefore, these systems could be operated at high power but not necessarily at a corresponding high bulk fluid temperature. 2.3 Radiator A compact and light space radiator for plant heat rejection is an enabling component for multimegawatt power generation. The issues of reliability and survivability escalate with radiator size; the radiator's weight is so dominant as to be the optimization parameter for the power cycle. Besides the many possible radiator shapes and physical arrangement variations, the absolute measure of the heat rejection effectiveness is the T4 dependence. Fig. 5 shows the radiator mass-to-power ratio as a function of radiator
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