Neutronic Feasibility Assessment of Liquid Salt-Cooled Pebble Bed Reactors

This study investigates the neutronic characteristics of the Pebble Bed-Advanced High Temperature Reactor (PB-AHTR), which combines TRISO fuel technology and liquid salt [flibe (2LiF-Be2F)] cooling. Compared to equivalent helium-cooled cores, the flibe-cooled cores feature a significantly larger fraction of neutron loss to coolant absorption but also a reduced neutron loss to leakage. The flibe also significantly contributes to neutron slowing-down and allows an increase of the pebbles' heavy metal-to-carbon volume ratio as compared to helium-cooled cores. In order to guarantee all negative reactivity coefficients, and in particular coolant void and temperature feedbacks, the carbon-to-heavy metal atom ratio must not exceed 300 to 400, depending on the fuel kernel diameter. The maximum burnup attainable from a PB-AHTR that is fueled with 10% enriched uranium and operated in continuous refueling is ˜130 GWd/t HM; this is comparable to the maximum burnup achieved in other high-temperature reactors, either liquid salt or gas cooled. Compared to helium-cooled pebble bed reactors, the PB-AHTR pebbles can be loaded with 2.5 times more fuel, resulting in a smaller number of pebbles to fabricate and a smaller spent-fuel volume to handle per energy generated. Relative to a light water reactor, the PB-AHTR offers improved natural uranium ore utilization and reduced enrichment capacity.