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

Abstract 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 233 U-to-HM ratio ( 233 U/HM) from 5.0% to 20.0% are carried out. Neutronics characteristics, including the effective multiplication factor ( k eff ), 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 233 U/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 233 U/HM ratios. In order to evaluate the 233 U 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 233 U/HM increases. A burnup level of 122 GWd/tHM can be achieved for the recommended pebble design with 233 U/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.