High-temperature Gas-cooled Reactor Pebble-bed Module (HTR-PM), which is designed by Tsinghua university of China, is under construction in Shidao Bay of China. It will be the world’s first pebble-bed type modular HTGR commercial demonstration plant. In HTR-PM project, steam-Rankine cycle has used in the power conversion system because it represents current state-of-the-art technology. Meanwhile, helium turbine for HTGR has been investigated for many years in Tsinghua University. Mock-up machine for HTR-10GT has been built. Helium turbine for 250MW HTGR, which is based on HTR-PM, has completed conceptual designed. However, supercritical carbon dioxide (S-CO2) Brayton cycle has shown to have great potentials for future HTGR technology in recent years because of its critical properties. Helium turbine cycle and S-CO2 Brayton cycle are both candidates for future HTGR. Therefore, comparative study is conducted in this paper. Comparison is focused on achievable efficiencies for each cycle mentioned above and on cycle layout with respect to simplicity and compactness, which primarily determines capital cost. Firstly, the physical model for helium turbine cycle with recuperator and intercooler is built and cycle performance is analyzed based on the parameters of HTR-PM. Then the model for S-CO2 Brayton cycle with recompression is also built and the cycle efficiency is analyzed with the same parameters of HTR-PM. Secondly, comparison between helium turbine cycle and S-CO2 Brayton cycle is made from the view of thermodynamics. Moreover, parameters optimization of both cycles based on HTR-PM is carried out. At last, advantage and drawback of both cycles are discussed from the engineering point. In conclusion, cycle simplicity and technology maturity of helium turbine cycle are better than S-CO2 Brayton cycle. But on the other side, smaller size equipment and less compression work of S-CO2 Brayton Cycle are more competitive than helium turbine cycle. Helium turbine with higher temperature and S-CO2 Brayton Cycle with higher pressure can achieve higher efficiency than steam Rankine cycle.

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