Shaft seal leakage-affected off-design performance, safety margins and control strategy evaluation of recompression supercritical CO2 direct cooled reactor Brayton cycle

The supercritical carbon dioxide (sCO2) Brayton cycle is a promising power conversion technology for advanced nuclear reactors due to its high efficiency, compactness, and load-following capability. However, shaft seal leakage under high-pressure and high-temperature conditions markedly degrade cycle performance and pose coupled challenges to thermal safety and operational stability, particularly under off-design conditions. In this study, an off-design model of a recompression sCO2 direct cooled reactor Brayton cycle is developed to evaluate leakage effects, control strategies, and safety margins. Labyrinth seal leakage is incorporated, and a multi-layer iterative solution scheme is employed to ensure thermodynamic consistency under off-design conditions. The off-design performances of coaxial-shaft and split-shaft configurations are systematically compared in terms of thermal efficiency, net power output, and operating flexibility. Allowable leakage rates are evaluated against key safety constraints, including coolant, cladding, and fuel temperatures, as well as compressor stability limits. Five representative individual control strategies, rotational speed, inventory, turbine inlet pressure, turbine inlet temperature, and turbine bypass control are assessed over a wide load range, and segmented hybrid strategies are proposed by combining the best-performing control methods in each load region to enhance efficiency and operational robustness. In addition, the impact of leakage gas reinjection location is examined by accounting for the associated compression and heating work. The results indicate that the split-shaft configuration exhibits superior off-design adaptability due to decoupled rotational speeds. Turbine inlet pressure control is more effective at low-to-medium loads, while turbine inlet temperature control performs better at high loads. The proposed hybrid strategies enhance operational flexibility and sustain efficiency over wide range of variable-load conditions compared to individual control methods. Additionally, reinjecting leaked CO2 at low-pressure locations helps minimize performance losses.