In this reactor physics study, we attempt to design a civil marine reactor core that can operate over a 10 effective-full-power-years life at 333 MWth using ThUO2 and all-UO2 fuel. We use WIMS to develop subassembly designs and PANTHER to examine whole-core arrangements, optimizing: subassembly and core geometry; fuel enrichment; burnable and moveable poison design; and whole-core loading patterns. We compare designs with a 14% fissile loading for ThUO2 and all-UO2 fuel in 13X13 assemblies with ZrB2 integral fuel burnable absorber pins for reactivity control. Taking advantage of self-shielding effects, the ThUO2 option shows greater promise in the final burnable poison design while maintaining low, stable reactivity with minimal burnup penalty. For the final poisoning design with ZrB2, ThUO2 contributes 2.5% more initial reactivity suppression, although the all-UO2 design exhibits lower reactivity swing. All the candidate materials show greater rod worth for the ThUO2 design. For both fuels, B4C has the highest reactivity worth, providing 10% higher control rod worth for ThUO2 fuel than all-UO2. Finally, optimized assemblies were loaded into a 3D reactor model in PANTHER. The PANTHER results show that after 10 years, the core is on the border of criticality, confirming the fissile loading is well-designed.