The objective of the trade study presented in this report is to identify configurations for a water-cooled reactor system that satisfies a set of constraints. In this report we have illustrated the neutronics results for a trade study aiming at sorting among different choices in terms of geometries and components given a certain number of constraints and having as the objective, minimizing the core volume. The investigation was articulated in two steps. The first one, in a form of a parametric study, was applied to both UO2 fuel (reference) and the aLEU U10Mo. That study has indicated that the moderation ratio of a p/d=1.15 allows a more compact configuration. Regarding the reflector choice among the different candidates, beryllium provides better results than others, but some concern could exist in terms of its use (environmental impact), so that the water reflector was kept as backup solution. Concerning coolant, heavy water looks preferable to light water, but future studies on its use are postponed until reactors alternative to LWRs will be investigated. The subsequent study involved use of control rods, burnable poisons, and reactivity coefficients and power distribution constraints. A preliminary optimization aiming at minimizing the core volume was carried out for different configurations involving combinations of the two fuels, UO2 (reference) and U10Mo, the two reflectors (water and beryllium), and the two fuel forms, rods and plates. The rod form used pins arranged in a hexagonal assembly, while plates are arranged in a square assembly. The two fuel forms provide quite similar results with a slight preference for the rod form in a hexagonal assembly. The use of beryllium as a more beneficial reflector has been confirmed. In the end, at this stage, there is not a clear advantage of using the U10Mo fuel instead of the UO2 from a neutronics perspective on core volume. The most compact core, volume of 2.5 m3, was obtained using pins and UO2 fuel. All eight preliminary, optimized configurations have been transmitted to a project team member who will perform the related thermal-hydraulics and safety analyses. Moreover, the same configurations will be kept for future investigations for with a double use: comparisons of results, and as starting configurations.
