ICONE19-43824 EVALUATION AND OPTIMIZATION OF A SUPERCRITICAL CARBON DIOXIDE POWER CONVERSION CYCLE FOR NUCLEAR APPLICATIONS

Abstract

There have been a number of studies involving the use of gases operating in the supercritical mode for power production and process heat applications. Supercritical carbon dioxide (CO_2) is particularly attractive because it is capable of achieving relatively high power conversion cycle efficiencies in the temperature range between 550℃ and 750℃. Therefore, it has the potential for use with any type of high-temperature nuclear reactor concept, assuming reactor core outlet temperatures of at least 550℃. The particular power cycle investigated in this paper is a supercritical CO_2 recompression Brayton Cycle. The CO_2 recompression Brayton Cycle can be used as either a direct or indirect power conversion cycle, depending on the reactor type and reactor outlet temperature. The advantage of this cycle when compared to the helium Brayton Cycle is the lower required operating temperature; 550℃ versus 750℃. However, the supercritical CO_2 recompression Brayton Cycle requires a high end operating pressure in the range of 20 MPa, which is considerably higher than the required helium Brayton cycle high end operating pressure of 7 MPa. This paper presents results of analyses performed using the UniSim process analyses software to evaluate the performance of the supercritical CO_2 recompression Brayton cycle for different reactor coolant outlet temperatures and mass flow rates. The UniSim model assumed a 600 MW_t reactor power source, which provides heat to the power cycle at a maximum temperature of between 550℃ and 850℃. Sensitivity calculations were also performed to determine the affect of reactor coolant mass flow rates for a reference reactor coolant outlet temperature of 750℃. The UniSim model used realistic component parameters and operating conditions to model the complete power conversion system. CO_2 properties were evaluated, and the operating range for the cycle was adjusted to take advantage of the rapidly changing conditions near the critical point. The UniSim model was then optimized to maximize the power cycle thermal efficiency at the different reactor coolant outlet temperatures and flow rates. The results of the analyses showed that power cycle thermal efficiencies in the range of 40 to 50% can be achieved over the range of temperatures and mass flow rates investigated.