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 burnup 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 233U-to-HM ratio (233U/HM) from 5.0% to 20.0% are carried out. Neutronics characteristics, including the effective multiplication factor (keff), the TCR and the conversion ratio (CR) are analyzed in terms of neutron usage, core safety and burnup. The results show that the fuel pebble with a higher 233U/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 233U/HM ratios. In order to evaluate the 233U saving for thorium based fuel, the burnup per fissile mass (Bu) is introduced. Bu can be improved through reducing C/HM, and it increases firstly and then achieves saturation as 233U/HM increases. A burnup level of 122 GWd/tHM can be achieved for the recommended pebble design with 233U/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.