Three-dimensional Numerical Investigation on the Heat Transfer and Bypass Flow Characteristics in a Pebble-Bed High Temperature Gas-Cooled Reactor

The pebble-bed high temperature gas-cooled reactor (HTGR) is a type of the Generation IV reactor. For the necessary thermal and radiation protection, the walls around the pebble bed are stacked with the graphite reflector layer and the carbon bricks. The bypass flow fails to effectively cool the fuel pebbles in the pebble bed and thus influences the efficiency and safety of HTGR. In this paper, a comprehensive three-dimensional model for HN-750 self-designed by HUANENG Group of China is established. A physically realistic mathematical model is proposed to investigate the convective heat transfer and the bypass flow characteristics in HN-750. The results indicate that the maximum temperature of the pebble bed with bypass flow is about 200 K higher than that without bypass flow, while the pressure drop in pebble bed is about 13 kPa lower than that without bypass flow. The bypass flow accounts for 29.25 % of the total helium flow, 24.43 % of which is the vertical gap bypass flow. The synergistic combination of square keys and sealing strips can effectively block nearly half of the vertical gap bypass flow. The present large-scale three-dimensional numerical calculations can support the thermal-hydraulic optimization and safety analysis of pebble-bed HTGRs.