Preliminary Study on 950℃ Coolant Outlet Temperature in HTR-PM under the OTTO Scheme

The High-Temperature Reactor Pebble-bed Module (HTR-PM) adopts helium as the coolant, with 250/750 ℃ for core inlet/outlet temperature. Based on the public HTR-PM design parameters, this study adopts the once-through-then-out (OTTO) refueling scheme with a coolant outlet temperature of 950℃. The VSOP-THERMIX code is employed to analyze the distribution of key parameters in the HTR-PM during the equilibrium core phase. The coupled results of the neutronic–thermal hydraulics show that the maximum fuel temperature under steady-state conditions reaches 1157℃, which is below the safety limit of 1200℃, meeting the temperature criterion for retaining radioactive fission products under steady-state operating conditions. To further investigate the safety of HTR-PM with a 950℃ outlet temperature under accident conditions, a depressurized loss-of-forced-cooling (DLOFC) accident is selected to analyze the changing of maximum fuel temperature. The results indicate that 14.4 hours after the accident, the maximum fuel temperature reaches 1931.7℃, exceeding the accident temperature limit of 1620℃, but remains below the melting points of graphite and silicon carbide. Therefore, the core meltdown will not occur in the DLOFC accident. After this time point, the maximum fuel temperature gradually decreases. Moreover, the results reveal that the location of the maximum fuel temperature moved from the bottom to the upper part of the core during the DLOFC accident. To further analyze the influence of fuel enrichment in the DLOFC accident, four different fuel enrichments ranging from 8.0% to 9.5% are compared under the same refueling and operating conditions. The results show that the steady-state power peak of the equilibrium core shifts upward with increasing fuel enrichment under the OTTO scheme. Under DLOFC conditions, the maximum fuel temperatures are 1949.2, 1931.7, 1916.2℃, and 1900.8℃, respectively, showing a decreasing trend with higher enrichment levels.