Control Strategies and Transient Characteristics of a 5MWth Small Modular Supercritical CO2 Brayton-Cycle Reactor System

The combination of supercritical CO2 (SCO2) Brayton cycle and small modular reactors (SMR) is considered as one of the most promising energy conversion system. However, the nuclear safety and thermo-hydraulic constraints will impose more stringent control strategy requirements, and it is unclear whether the existing control strategies can ensure the safe operation of the reactor. Therefore, this paper establishes a dynamic model of the small modular SCO2 Brayton reactor system, which includes bypass, inventory, and combined control strategy. The transient characteristics of the reactor, heat exchangers and rotating machine are analyzed, and the control performance of different control strategies is evaluated and compared. The results show that the maximum load following range of the bypass control strategy (30 % - 100 %) is larger than that of the inventory control strategy (50.6 % - 100 %). When the external load decreases, bypass control strategy will increase the temperature of reactor fuel elements and the system's maximum pressure, and bring a certain degree of thermal shock to the heat exchangers. Although the inventory control strategy has slower control speed, it has better reactor safety and higher system efficiency. The combination control strategy has the advantages of both bypass control and inventory control, which can meet the system loads from 0 % to 100 %. This work can provide guidance for the design and operation of the SCO2 Brayton reactor system.