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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.

Molten Salt Reactor (MSR) employs fissile material dissolved in the fluoride salt as fuel which continuously circulates through the primary loop with the flow cycle time being a few tens of seconds. The nuclei evolution law is quite different from that in a solid fuel reactor. In this paper, we analytically deduce the nuclei evolution law of 135 Xe and 135 I which are entrained in the flowing salt, evaluate its concentration changing with the burnup time, and validate the result with the SCALE6. The circulation of fuel salt could decrease the concentration of 135 Xe and 135 I, and the reduction can achieve to around 40% and 50% for 135 Xe and 135 I respectively at a small power level (e.g., 2 MW) when the core has the same fuel salt volume as that of the outer-loop. Furthermore, it can be found that the reduction is inversely proportional to the core to outer-loop volume ratio, but uncorrelated with the mass flow rate under normal operating condition of a MSR. At low core power scale, the flow effect on 135 Xe concentration reduction is apparent, but it is mitigated as the core power scale increases because of the rise of 135 I concentration, which raises its decay to 135 Xe and compensates the loss of 135 Xe due to decay at the outer-loop. The decreased 135 Xe concentration results in a core reactivity increase varying from around 150 pcm to 1000 pcm depending on the core power and core to outer-loop volume ratio.

Reprocessing separation efficiency (RSE) is one important fuel cycle indicator which determines the loss of actinides left in the nuclear waste and the recovered actinides for reuse in the core, affecting the nuclear waste management, fuel utilization and other fuel cycle characteristics. By changing the reprocessing cycle time (RCT) from 10 days to 360 days, the impacts of RSE varying from 99.9% to 99.999% on the core actinides inventory evolution, breeding ratio (BR) and nuclear waste radiotoxicity were investigated for a Heavy Water moderated Molten Salt Reactor (HWMSR), which is a newly proposed molten salt reactor (MSR) that adopts heavy water rather than graphite as the moderator while employs the usual liquid fuel as in a traditional MSR. The obtained results demonstrate that the inventories of transuranic (TRU) and uranium except U-233 at equilibrium and their transition time to equilibrium both decreases as RSE declines due to their increased mass loss in reprocessing, which in turn brings down the parasitic neutron absorption in the core. Consequently, the U-233 inventory required for maintaining critical operation for a lower RSE drops. While the Th-232 inventory rises, since it is online refueled to maintain the total heavy metal (HM) content in the core constant to ensure the electrochemical stability of the fuel salt. As a result, BR is relatively improved as RSE decreases. But the increased U-233 production resulting from the improved BR could not compensate for the U-233 loss during reprocessing and leads to a longer doubling time for a lower RSE. The radiotoxicity of nuclear waste increases as RSE decreases since more actinides are lost to the nuclear waste. The above effects caused by RSE are mitigated as the RCT prolongs since the frequency for reprocessing the fuel salt of primary loop over a given period decreases, leading to a decrease of HM loss. For an RSE level of 99.9%, the shortest doubling time appears at the RCT of 30 days rather than 10 days because the mitigation of U-233 loss overwhelms the efficiency decrease of FPs removal and Pa-233 extraction. By balancing the Th-U breeding and radiotoxicity, an RCT of 60 days is recommended for HWMSR for the RSE of 99.9%.