Core characteristics of a modified pebble-bed high-temperature gas-cooled reactor design

As a promising candidate of Small Modular Reactors (SMRs), high-temperature gas-cooled reactor (HTGR) proves its technical superiority and maturity with the successful commercial operation of Chinese High Temperature gas-cooled Reactor Pebble-bed Module (HTR-PM) in Shidao Bay, Shandong. HTR-PM employs an online-refueling pebble-bed reactor design. Spherical fuel elements enter the core through the top fueling tube, gradually move downward through the pebble bed, and are discharged via the bottom defueling tube. Simultaneously, the coolant enters the pressure vessel and flows around its periphery before reaching the cold helium plenum at the top of the core. It then cools the pebble core in a top-down flow, merges in the hot helium plenum at the bottom, and finally exits the pressure vessel. Based on the design of the HTR-PM, a modified core design (HTR-UD) is proposed, featuring a top-down movement of spherical fuel elements and a bottom-up flow of coolant. This design leverages the strong negative temperature feedback in HTGR to flatten the axial power distribution within the core. A comparative analysis of the core characteristics between the two designs is conducted, and two core upgrading schemes are investigated for the HTR-UD. The analysis results indicate that the HTR-UD can significantly reduce the maximum fuel temperature by approximately 175 °C under depressurized loss of forced cooling (DLOFC) accident. Furthermore, it demonstrates the potential to increase the thermal power by approximately 40% or increase the reactor outlet coolant temperature to 900 °C compared to the original HTR-PM design.