2.5 The SP-100 With Rankine Power Conversion Here, as with the SP-100 Stirling power system, we assumed that six turbo-alternator sets would be used for reliability and that the specific mass of the conversion system was constant with power level. This potassium Rankine system utilizes a primary lithium heat transport loop, as depicted in Figure 2.12, to deliver heat from the nuclear reactor core to the potassium working fluid of the Rankine cycle via the intermediate heat exchanger. A vapor separator in the potassium loop is located at the outlet to the intermediateheat exchangerand provides nearly 100% quality vapor to the turbine inlet. The potassium vapor is then expanded through the axial flow turbine and completely condensed in the condenser heat exchanger. Waste heat is radiated to space with a heat pipe radiator that is directly connected to the condenser heat exchanger. The liquid potassium is finally pumped back through the intermediate heat exchanger to complete the cycle. The lithium heat transport loop and nuclear reactor core are similar in construction to the SP-100 thermoelectric and Stirling power systems. Electrical energy is provided by a turbine driven iron-core generator, with a net system efficiency of about 20% for operating temperatures between 1325 K (boiling) and 950 K (condensing). The materials of construction are expected to be nickel superalloys and refractory based alloys. 2.6 The SP-100 With Brayton Power Conversion The SP-100 nuclear reactor and primary heat transport loop were also mated with a simple Brayton power conversion cycle. Again, six power conversion units (each unit composed of a compressor, turbine, and alternator on the same rotating shaft) were provided for the required redundancy. Thermal energy from the lithium primary heat transport loop was provided to the helium-xenon working fluid of the Brayton cycle by the intermediate heat exchanger. The high pressure He-Xe enters the turbine at temperatures of about 1195 K, expands through the turbine, is cooled in the heat pipe radiator, and is recompressed before entering the intermediate heat exchanger to complete the cycle. Turbine work provided by the expanding gas powers both the compressor and alternator. No recuperator is employed in this simple Brayton cycle. With compressor and turbine efficiencies of 85% and 90%, respectively, the overall cycle efficiency is about 20% to obtain the minimum total power system mass. The Brayton cycle upper temperature limit would allow fabrication from nickel alloys for most components, although the intermediate heat exchanger is assumed to be made of niobium alloy. The turbines are assumed to have blade, center-shaft, and disk cooling to provide less massive rotating parts. The blades are cooled to 1100 K, while the center shaft and disk are cooled to 950 K. The SP-100 Brayton flow diagram is the same as the SP-100/Rankine flow diagram (Figure 2.12) with two exceptions. First, the Brayton system does not have a vapor separator. Second, the Brayton system has a compressor rather than a pump, and the compressor is on the same shaft as the turbine.
RkJQdWJsaXNoZXIy MTU5NjU0Mg==