Fuel pebble optimization for the thorium-fueled Pebble Bed Fluoride salt-cooled high-temperature reactor (PB-TFHR)

Thorium-fueled Pebble Bed Fluoride salt-cooled High-temperature Reactor (PB-TFHR) is a newly developed reactor concept featuring the technology of High Temperature gas-cooled Reactor (HTR) and the molten salt cooling for thorium-based fuels. To attain a deep bumup with enough negative temperature coefficient of reactivity (TCR), a systematic study for the graphite-to-heavy metal ratio (C/HM) from 66 to 800 and the U-233-to-HM ratio (U-233/HM) from 5.0% to 20.0% are carried out. Neutronics characteristics, including the effective multiplication factor (ken), the TCR and the conversion ratio (CR) are analyzed in terms of neutron usage, core safety and bumup. The results show that the fuel pebble with a higher U-233/HM owns a wider range of under moderated area, which is favorable to improve TCR. Moreover, a strongly negative TCR is obtained by optimizing the C/HM and U-233/HM ratios. In order to evaluate the U-233 saving for thorium based fuel, the bumup per fissile mass (Bu) is introduced. Bu can be improved through reducing C/HM, and it increases firstly and then achieves saturation as U-233/HM increases. A bumup level of 122 GWd/tHM can be achieved for the recommended pebble design with U-233/HM of about 12.5% and C/HM of about 124. With the optimized design, an improved TCR can be obtained with a value of about -2.77 pcm/K, which is much more superior to our previous work.