DOE OSTI2017
Fluoride-salt-cooled high-temperature reactors (FHRs) are thermal-spectrum nuclear reactors defined by their use of liquid-fluoride-salt coolants, together with tri-isotropic (TRISO)-coated particle fuels and graphite moderator materials (Forsberg, 2004). FHRs are also characterized by their improved thermodynamic efficiency relative to gas-cooled reactors. The liquid salt coolant is designed to operate with high core outlet temperature and nearly atmosphere pressure. These features allow the FHR to possess higher core power density compared to gas-cooled TRISO-based systems. However, industry fabrication limitations result in a low volumetric fraction of fuel kernel in TRISO particles. In order to obtain reasonable operation periods, the enrichment needs to be in the high LEU (low enriched uranium) range as was proposed in the original design (Greene, 2010). Recent studies (Ilas, 2014) presented newly repurposed SmAHTR-CTC reactor concept that relies on prior experience obtained from the 2010 initial SmAHTR design (Greene, 2010). This design consists of 19 hexagonal fuel assemblies arranged in a 3-ring hexagonal geometry with 18 fuel plates each. The active length of fuel plates in each fuel assembly is 4 meters. The proposed SmAHTR-CTC achieves substantial improvements from the neutronic performance standpoint over the original concept. The results indicate that a fuel cycle length of over two years with discharged fuel burnup of 40 gigawatt-day per metric ton of initial heavy metal (HM) is possible with an 8% fuel enrichment in a once-through fuel cycle. However, the lower uranium enrichment loading requires a more frequent refueling strategy and hence considerably reduces the cycle length. It is necessary to achieve reasonable cycle length values but at the same time to extend the residence time of the fuel, especially since the costs associated with TRISO-fuel are substantially higher (Shropshire et al., 2008) than traditional UO{sub 2} fuel ($10,000/kg for TRISO vs $200/kg for UO{sub 2}). This paper presents an approach to extend the operational time and improve fuel utilization of FHR by means of spectral shift control. This is achieved by incorporating movable graphite structures into the design, instead of the static graphite blocks presently envisaged. At beginning of life (BOL), the graphite structures are withdrawn. The reactor is under-moderated, with excess neutrons being primarily captured in {sup 238}U, breeding {sup 239}Pu. Towards the end of life (EOL), the graphite structures are inserted, thermalizing the neutron spectrum and increasing reactivity. The extra {sup 239}Pu bred during the cycle is then burned, allowing the cycle and discharge burnup to be extended. Such movable structures regime should be feasible from an engineering point of view, given that it is broadly equivalent to inserting and withdrawing control rods. The preliminary results indicate that such a design may considerably improve the fuel utilization.
11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗