FHR, HTGR, and MSR Pebble-Bed Reactors with Multiple Pebble Sizes for Fuel Management and Coolant Cleanup

Abstract

Three reactor types can be designed with pebbles (carbon spheres) as the reactor core: the pebblebed high-temperature gas-cooled reactor (PB-HTGR), the pebble-bed fluoride-salt-cooled high-temperature reactor (PB-FHR), and the thermal-spectrum molten salt reactor (MSR) with fuel dissolved in coolant. In the HTGR and FHR, the pebbles are fuel (coated-particle fuel) and moderator (graphite). In a MSR the pebbles would be the moderator (no fuel). Recent advances enable prediction and modeling of pebble beds with two or more sizes of pebbles. This may enable the use of pebble beds with multiple size pebbles that create new options. A second smaller size of HTGR/FHR fuel pebble that fills some of the space between the regular pebbles can increase the power output for the same size reactor. For the FHR the second pebble size would reduce inventory of expensive coolant and may widen choices of salt coolants. In an HTGR or FHR, smaller pebbles with high actinide loadings and high heat transfer rates could be used to burn actinides while the larger pebbles are the driver fuel. Multiple pebble sizes in MSRs may enable varying the carbon-to-fuel ratio to optimize the neutron spectrum over time to more efficiently utilize the fuel and allow easy replacement of moderator. The smaller pebbles with no fuel and a high surface-to-volume ratio could be designed to remove (1) HTGR/FHR/MSR tritium from the coolant and (2) noble metal fission products and potentially other impurities in MSRs. We examine the potential incentives for pebble beds with multiple size pebbles. With the tools now available to quantify pebble-bed behavior with multiple size pebbles, the next step is to begin to quantify benefits and limitations for different applications of pebble-bed reactors with multiple sizes of pebbles. Keywords-Fluoride-salt-cooled high-temperature reactor, molten salt reactor, high-temperature gas-cooled reactor, pebble-bed reactor. Note-Some figures may be in color only in the electronic version.

Investigation on the mechanism of enhancing heat transfer in a pebble bed by adding in the non-fixed number of smaller-sized spheres

“…The possible reasons for the above findings will be discussed in details in the following sections. As the ratio of the surface area to the volume decreased from case 1 to case 3, however, it was found that the heat transfer coefficient did not show a similar decreasing trend as presented by Forsberg and Peterson [6]. The reason might be that the small spheres were not generating heat and only worked as the expanded surfaces to adjacent pebbles, which were able to enlarge the convective surface area and therefore result in a better convective heat transfer.…”

“…To reduce the possibility of hotspots appearance, researchers have studied the heat transfer characteristics of different pebble beds [3][4][5]. Moreover, Forsberg and Peterson [6] proposed that packing a bed with multi-sized pebbles rather than mono-sized pebbles could help the reactor reach a higher power density and also enhance the heat transfer of the bed. However, thus far, the research on the heat transfer characteristics of beds packed with multi-sized spheres has barely been conducted.…”

Fusion Blankets and Fluoride-Salt-Cooled High-Temperature Reactors with Flibe Salt Coolant: Common Challenges, Tritium Control, and Opportunities for Synergistic Development Strategies Between Fission, Fusion, and Solar Salt Technologies

“…The specific characteristics of this reactor core design may allow the fuel and graphite pebbles in the core to be used as a tritium removal system by heating the pebbles after exiting the core to remove the weakly bound tritium. There is the option 47 to add smaller pebbles of graphite that fit in the spaces between the fuel pebbles that are designed specifically for tritium removal. Such nonfuel pebbles are at lower temperatures than the fuel pebbles and thus have a greater capacity for tritium adsorption.…”