Results of analyses performed using the UniSim process analyses software to evaluate the performance of both a direct and indirect supercritical CO_2 Brayton power plant cycle with recompression at different reactor outlet temperatures are presented. The direct supercritical CO_2 power plant cycle transferred heat directly from a 600 MW, reactor to the supercritical CO_2 working fluid supplied to the turbine generator at approximately 20 MPa. The indirect supercritical CO_2 cycle assumed a helium-cooled Very High Temperature Reactor (VHTR), operating at a primary system pressure of approximately 7.0 MPa, delivered heat through an intermediate heat exchanger to the secondary indirect supercritical CO_2 recompression Brayton cycle, again operating at a pressure of about 20 MPa. For both the direct and indirect power plant cycles, sensitivity calculations were performed for reactor outlet temperature between 55O°C and 850°C. The UniSim models used realistic component parameters and operating conditions to model the complete reactor and power conversion systems. CO_2 properties were evaluated, and the operating ranges of the cycles were adjusted to take advantage of the rapidly changing properties of CO_2 near the critical point. The results of the analyses showed that, for the direct supercritical CO_2 power plant cycle, thermal efficiencies in the range of approximately 40 to 50% can be achieved over the reactor coolant outlet temperature range of 550°C to 850°C. For the indirect supercritical CO_2 power plant cycle, thermal efficiencies were approximately 11 - 13% lower than those obtained for the direct cycle over the same reactor outlet temperature range.
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