High Temperature Gas-cooled Reactor (HTR) and Very High Temperature Gas-cooled Reactor (VHTR), are the most promising and achievable fourth-generation nuclear reactor for its inherent safety. In this paper, the performance of Closed Brayton Cycle (CBC) and two sub-critical combined cycles were investigated and compared. The CBC is a recuperated and inter-cooling closed Brayton cycle. Two combined cycles include the sub-critical Rankine cycle without steam reheating (Simple Combined Cycle, SCC) and a sub-critical reheated Rankine cycle (Reheated Combined Cycle, RCC). The topping cycles of SCC and RCC are both a simple Brayton cycle, and connect with the bottoming cycles by a sub-critical heat recovery steam generator (HRSG). Physical and mathematical models of three different thermodynamic cycles were established. Within the temperature range of the HTR and VHTR, the effects and mechanism of key parameters, such as reactor outlet temperature, steam temperature and pressure, on features of three different cycles were investigated. The results showed the elevated reactor outlet temperature could obviously enhance efficiency of three cycles. The results showed that RCC had the highest efficiency while SCC had the lowest efficiency, and the efficiency of CBC is slightly lower than that of RCC. The results could be helpful to understand and develop the power conversion system coupled with (V)HTR in the future.

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