The Fluoride-salt-cooled High temperature Reactor (FHR) is a new reactor concept, which combines low-pressure liquid salt coolant and high-temperature TRISO particle fuel. The refractory TRISO particle coating system as well as the dispersion in graphite matrix enhances the nuclear proliferation resistance. Compared to the conventional high temperature reactor cooled by helium gas, the liquid salt system features significantly lower pressure, larger volumetric heat capacity, and higher thermal conductivity. The FHR is, therefore, considered as an ideal candidate for the transportable reactor concept. In this context, a 20 MWth compact core aiming at an 18-month once through fuel cycle is currently under design at MIT. One of the key challenges of the core design is to minimize the reactivity swing induced by fuel depletion, since too much excess reactivity will increase the complexity in control rod design and also result in criticality risk during the transportation process. In this paper, burnable poison particles (BPPs), made of B_4C with natural boron (i.e. 20% enriched B-10), are adopted as the key measure for the fuel cycle optimization. It has been found that the overall inventory and the individual size of the BPPs are considered as the two most important parameters that determine the evolution path of the multiplication factor along the fuel cycle. The packing fraction in the fuel compact and the height of active zone are of secondary importance. The neutronic influence Li-6 depletion has also been quantified. Eventually, the 18-month once-through fuel cycle is optimized and the depletion reactivity swing is reduced to one beta. Two core configurations have been proposed, intended to provide a quantitative reference, which could require more detailed neutronic and thermal-hydraulic analysis.
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