Experimental investigation of the bed structure in liquid salt cooled pebble bed reactor

Highlights: • The article provides the methods for pebble loading and defueling in liquid salt cooled pebble beds. • The packing factor in liquid salted cooled beds is measured to be less than that in the gaseous environment due to the liquid. • Flow field, loading rate and vibration significantly affect the pebble bed, while the first two mainly affect the bottom geometry of the pebble bed. - Abstract: Based on the advanced high temperature reactor (AHTR), which is an advanced concept combining attractive attributes by adopting liquid salt and coated particle fuel, more and more reactor designs that use fuel pebbles are developed. The bed structure in liquid salt cooled pebble bed reactor is important for reactor design. To investigate the bed structure in the reactor, the Pebble Recirculation Experiment Device (PRED) and the Pebble Bed Dense Experiment facility (PBDE), which were mock-ups for TMSR-SF (Thorium-based Molten Salt Reactor with Solid Fuel) were developed. Packing experiments were performed under different conditions. The results show that, the packing factor of TMSR-SF is supposed to be about 0.574, which is much less than 0.60 in gaseous environment; the loading rate of pebbles and the velocity of inlet flow has great effect on the bottom shape of the pebble bed; the structure of the pebble bed remains stable during normal operation and changes under accident conditions; loss of the coolant in the core may cause rearrangement of pebble bed; increase in packing factor that caused by strong vibration may induce reactivity in reactor core. The pebble bed behavior under different conditions that investigated in this paper provides the basis phenomenological analysis for reactor design. Moreover, the running of PRED demonstrates the feasibility of the methods for pebble loading and defueling in liquid salted cooled pebble bed reactors.