In an e↵ort to de-carbonise commercial freight shipping, there is growing interest in the possibility of using nuclear propulsion systems. In this reactor physics study, we seek to design a soluble-boron-free (SBF) and low-enriched uranium (LEU) (<20% 235U enrichment) civil nuclear marine propulsion small modular reactor (SMR) core that provides at least 15 e↵ective full-power-years (EFPY) life at 333 MWth using 18% 235U enriched micro-heterogeneous ThO2-UO2 duplex fuel and 15% 235U enriched homogeneously mixed all-UO2 fuel. We use WIMS to develop subassembly designs and PANTHER to examine whole-core arrangements. The assembly-level behaviours of candidate burnable poison (BP) materials and control rods are investigated. We examine gadolinia (Gd2O3), erbia (Er2O3) and ZrB2 integral fuel burnable absorber (IFBA) as BPs. We arrive at a design with the candidate fuels loaded into 13⇥13 assemblies using IFBA pins for reactivity control. Taking advantage of self-shielding e↵ects, this design maintains low and stable assembly reactivity with relatively little burnup penalty. Thorium-based duplex fuel o↵ers better performance than all-UO2 fuel with all BP options considered. Duplex fuel has ⇠20% lower reactivity swing and, in consequence, lower initial reactivity than all-UO2 fuel. The lower initial reactivity and smaller reactivity swing make the task of reactivity control through BP design easier in the thorium-rich duplex core. For control rod design, we examine boron carbide (B4C), hafnium, and Ag-In-Cd alloy. All the candidate materials exhibit greater rod worth for the duplex design. For both fuels, B4C has the highest rod worth. In particular, one of the major objectives of this study is to o↵er/explore a thorium-based candidate alternative fuel platform for the proposed marine core. It is proven by literature reviews that the ability of the duplex fuel was never explored in the context of a single-batch, LEU, SBF, long-life SMR core. In this regard, the motivation of this paper is to observe the neutronic performance of the proposed duplex fuel with respect to the UO2 fuel and ‘open the option’ of designing the functional cores with both the duplex and UO2 fuel cores. A companion paper will examine key physics and core safety analysis parameters in the whole-core environment.