System Description Basic Concepts. The principles on which a space nuclear power reactor operates are the same as those which govern terrestrial installations. The system consists of a reactor, heat transport system, electrical generating system, and a heat rejection system. Electrical generation and heat rejection are carried out somewhat differently than they are on Earth. In space, heat can only be rejected by radiation - and the fourth power dependence of rejected heat on rejection temperature encourages the use of a substantially higher temperature for the low temperature end of the generating cycle than would be considered on Earth. The generally high systems temperature due to this opens the possibility of using any of a variety of conversion cycles-such as the Brayton, Rankine or Stirling cycle or of using thermoelectric or thermionic converters. The Rankine cycle and thermionic conversion were selected as reference schemes after a comparison of the options above. The Rankine cycle system is composed of 9 subsystems: a reactor; core heat pipes (Li/TZM); neutron and gamma shields; 4 potassium Rankine loops (each containing a boiler, condenser, turbine and electromagnetic pump); radiator heat pipes (Na/SUS); radiator armor and fins; reactor controls; power conditioner; and support structure. A schematic of the Rankine cycle system is shown in Fig. 1(a). The reactor core is composed of hundreds of fuel elements and a beryllium reflector. The core is cooled by lithium heat pipes. The condenser sections of the core heat pipes boil the potassium and superheat the vapors to 1500 K. Shell and tube heat exchangers were used in this design and a two stage impulse turbine was selected due to its simplicity. Four power conversion loops each generating 250 kWe were used to prevent single point failure. The boiler and condenser are common to the four energy conversion loops. Sodium heat pipes are used in the radiator, which is armored with enough beryllium to be micrometeoroid resistant. The high temperature loop must resist liquid lithium and potassium at the operating temperature. Tantalum, molybdenum, columbium and tungsten have excellent corrosion resistance under these conditions, so their alloys are the materials of choice for the Rankine loop.
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