Sensitivity Study of the Reactivity-Equivalent Physical Transformation Method for Fluoride Salt Coolant Pebble Bed Reactor Lattice

Conventional dispersed-particle fuel systems, such as TRISO fuel, exhibit typical double heterogeneity. The Reactivity-equivalent Physical Transformation (RPT) method can convert a double-heterogeneity system into a single-heterogeneity system by simplified geometric modelling. In a Pebble-Bed Fluoride-Salt-Cooled Reactor (PB-FHR), the random packing pebbles and the fluoride-salt coolant further impacted the spatial self-shielding effect, fuel loading and moderating ratio, causing the variation in the RPT radius. In this study, a neutronics model of the TMSR-SF1 pebble lattice was established with OpenMC. The VHM, RPT and random TRISO models were compared and validated from the perspective of neutronics. The effects of pebble lattice structure, pebble packing fraction, FLiBe coolant temperature, TRISO loading fraction, various burnable neutron poisons and boundary albedo on RPT radius were systematically evaluated. Results indicated that RPT radius is unaffected by the pebble packing structure, while it was positively correlated with pebble packing fraction or coolant temperature and was negatively correlated with the moderating ratio. Poison with strongly neutron absorption may reduce RPT radius, while poison with weakly neutron absorption has little effect on RPT radius. Reducing neutron leakage will lead to an increase in a RPT radius. This work would provide an effective view for the improvement of RPT method in PB-FHR, and as the basis for the subsequent full core analysis.