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The thermal inertia results in net heat absorption in natural circulation lead-cooled reactors during power variation processes, increasing the risk of core meltdown. In this study, a dynamic simulation model of a natural circulation lead-cooled fast reactor coupled with a supercritical carbon dioxide Brayton Cycle power generation system has been developed based on Vpower. The influence of thermal inertia on system operating temperature under different power variation rates was discussed. The results indicate that during power reduction, different decline rates only affect the temperature variation range of the system without impacting the final stable state. However, during power increase, different ramp rates not only affect the temperature variation range but also lead to varying degrees of system temperature rise. To address this issue, an optimized turbine throttling scheme is proposed. Compared to the original plan, the new plan successfully stabilizes the temperature at the rated value after increasing the system power. This study provides insights for the operation control strategy of similar reactor types. [ABSTRACT FROM AUTHOR]

• The system analysis program for the 5 MW Small Modular Supercritical CO 2 Brayton-Cycle Reactor System is established and validated. • When the reactor power decreases, the power decrease percentage of the regenerator is the largest and the heat transfer may deteriorate. • At the case of the reactor rod ejection accident, the reactor system is with the good ability of the negative feedback regulation and certain self-stability. • When the temperature of cooling water only increases by 1 °C, the system output power can reduce about 10%. Ensuring the sufficient cooling capacity of the cooler is significant. • The compressor is more sensitive to the input parameter disturbance caused by the reactor and cooler. Small nuclear reactor system (SMR) has good development prospect in energy supply because of its flexible arrangement, wide application, and low cost. Among many small nuclear reactor systems, the Brayton cycle reactor system with supercritical CO 2 (SCO 2) as a working medium stands out. It has the advantages of high conversion efficiency, simple design, compact structure, high inherent safety, and is suitable for almost all nuclear power systems, especially small modular reactors (SMR). However, due to the unique thermophysical properties of SCO 2 , the transient operation characteristics of the system and the coupling effect between the main equipment are not precise. In this study, a dynamic model of the 5 MW modular SCO 2 Brayton cycle reactor system was established, and the system's dynamic operation characteristics and off-design control strategy were evaluated. Firstly, the steady-state simulation of the system is carried out to verify the physical model of the main equipment. Secondly, the typical transient operating conditions such as the reactor power fluctuation, reactor reactivity insertion, cooling water flow rate and temperature fluctuation, and loss of the external load are calculated. The dynamic characteristics of the system and the coupling effect between the main equipment are analyzed. The simulation results show that the control system is with good control effect and regulation ability under the condition of the reactor control rod ejection accident. The system is quite sensitive to the change of the cooling water temperature and flow rate. When the external load is lost, the turbine bypass control method can quickly match the system output power and keep the rotor speed reasonably. The dynamic modeling of the system is beneficial for the analysis of the operation characteristics and control effect of the system under variable conditions. It can provide the basis for the design, evaluation, and control strategy formulation of the system. [ABSTRACT FROM AUTHOR]

The supercritical carbon dioxide (SCO2) Brayton cycle, as a substitute for the steam cycle, can be widely used in a variety of power generation scenarios. However, most of the existing SCO2 cycle studies are restricted to basic thermodynamics research, parameter optimizations, system design in different application fields, and even economic analysis. Considering the load variability and control flexibility of the power generation system, the dynamic performance research of the SCO2 cycle is also crucial, but the work done is still limited. Based on the previous studies, Simulink software is used in this paper to develop a dynamic model of the 20 MW-SCO2 recompression cycle, which specifically includes component models that can independently realize physical functions and an overall closed-loop cycle model. A series of comparative calculation are carried out to verify the models and the results are very positive. The SCO2 recompression power system is built with the developed models and the dynamic model runs stably with a maximum error of 0.56%. Finally, the simulation of the dynamic switching conditions of the 20 MW-SCO2 recompression cycle are performed and the analysis results supply instructive suggestions for the system operation and control. [ABSTRACT FROM AUTHOR]