Research on control strategies for rapid load decrease events in fluoride salt reactor-supercritical CO2 Brayton cycle systems

This study delves into the control strategies for the Fluoride-salt-cooled High-temperature Reactor-Supercritical CO₂ (SCO₂) Brayton cycle power generation system under rapid grid load reduction scenarios. By developing a dynamic simulation model, the system's dynamic response characteristics under rapid load changes were analyzed, and a combined control strategy emphasizing safety, speed, and economic efficiency was proposed. The research first compared the dynamic response characteristics and steady-state performance of different bypass configurations under load rejection conditions. The computational results indicate that all three bypass control systems exhibit excellent load-following capabilities, effectively responding to a 50 % load step change within 10 s. However, significant differences were observed in their steady-state thermodynamic performance: the upper cycle bypass control achieves the highest steady-state efficiency (η = 29.92 %), followed by the turbine bypass control (η = 27.50 %), with the heat source bypass control showing relatively lower efficiency (η = 26.49 %). Although the upper cycle bypass offers superior efficiency, it leads to a substantial increase in the working fluid flow through the compressor (ΔQ = 15 %) during operation, raising the risk of compressor blockage and necessitating additional flow restriction and anti-blocking interlock control systems. Considering system safety, control complexity, and engineering feasibility, the turbine bypass system, with its relative independence and lower operational risk, is deemed more suitable as the primary control strategy for load rejection conditions. Building on this, the study proposes a combined control strategy that leverages the strengths of both bypass control and inventory control. During the initial phase of rapid load reduction, the bypass control system quickly adjusts the turbine bypass valve opening to promptly respond to grid load changes, ensuring system frequency stability around 50 Hz, with a maximum deviation of only 0.0193 Hz. Once the load stabilizes, the inventory control system gradually adjusts system pressure and flow rate, reducing bypass flow and ultimately enhancing the system's thermal efficiency to a new steady-state level. In the four load reduction cases (50 %, 60 %, 70 %, 80 %), the system's thermal efficiency increases from 27.49 %, 31.25 %, 34.61 %, and 37.63 % to 33.88 %, 36.38 %, 38.43 %, and 40.03 %, respectively. The study also found that even in the 50 % load reduction case, the frequency remains stable around 50 Hz, with a maximum deviation of 0.0193 Hz. This research provides theoretical and practical guidance for the control strategy of SCO₂ Brayton cycle systems under rapid load changes, significantly enhancing system safety, efficiency, and reliability, and laying a technical foundation for future wide-load operation in nuclear energy applications.