Study on Cold End Characteristic of SCO2 Brayton Cycle in Small Fluoride-salt-cooled High-temperature Reactor

Abstract

Fluoride-salt-cooled high-temperature reactor coupled with SCO2 Brayton cycle power generation system has the advantages of high inherent safety, high efficiency, compact structure, and waterless cooling. However, the cold end of the SCO2 Brayton cycle system operates near the critical point and is very sensitive to the cold end parameters, which seriously affects the performance of the cycle system. In addition, when the cold end parameters cross the critical point, the liquefied mass can seriously threaten the safety of the compressor and even the cycle system, therefore, the investigation of the characteristics of the cold end parameters during the dynamic process becomes the key to the safe and stable operation of the SCO2 Brayton cycle system. This paper took the SCO2 Brayton cycle system of a small fluoride-salt-cooled high-temperature reactor as the research object. Matlab/Simulink software was used as the simulation platform, and the Refprop interface was embedded to calculate physical properties. The one-dimensional dynamic simulation model was established for compressors, turbines, heat exchangers, reactors, and other thermal equipment through energy conservation and mass conservation. On this basis, the dynamic characteristics of the system under the disturbance of the cooling water flow of the SCO2 pre-cooler were studied, and the influence of the cold end parameters on the system performance during the variable load was explored. The results show that the disturbance of the cooling water flow of the SCO2 pre-cooler directly affects the inlet temperature of the main compressor, which in turn seriously affects the stable operation of the whole system, so it is necessary to impose certain control means to ensure the stability of the pre-cooler outlet temperature during the operation of the system. Due to the role of the thermal inertia of the reactor and the interaction between the reactor and the heat exchanger, the hot end of the system fluctuates obviously and then affects the cold end, so the reactor is the main reason for the longterm fluctuations of the cold end. In the process of load reduction, the cold end parameters of the system gradually approach the critical point, resulting in increasingly drastic changes in carbon dioxide density at the inlet of the main compressor, which leads to the sensitivity of the main compressor to the inlet temperature and inlet pressure gradually increases. At the low load state, the main compressor inlet temperature is disturbed by 1 ℃, the highest change of main compressor inlet density is 262 kg/m3, the highest change of main compressor outlet pressure is 7 MPa, and the highest change of main compressor efficiency is 6.82%. The main compressor inlet pressure is disturbed by 0.1 MPa, the highest change of main compressor inlet density is 228 kg/m3, the highest change of main compressor outlet pressure is 6.33 MPa, and the highest change of main compressor efficiency is 6.23%. However, the inlet temperature and inlet pressure of the main compressor have a small and almost negligible effect on the outlet temperature.

Keywords

• fluoride-salt-cooled high-temperature reactor
• sco2 brayton cycle
• dynamic characteristic
• variable load