In this reactor physics study, we attempt to design a high power density (HPD) core that fulfills the objective of providing 15 effective futl-power-years (EFPY) life at 333 MWth using 19% 235U enriched micro-heterogeneous ThO2-UO2duplex fuel and 16% 235U enriched homogeneously mixed all-UO2fuel. We use WIMS to develop subassembly designs and PANTHER to examine whole-core arrangements. In order to design cores with power densities between 82 and 111 MW/m3i, three HPD cases have been chosen by optimizing the fuel pin diameter (D), pin pitch (P) and pitch-to-diameter ratio (P/D). Taking advantage of self-shielding effects, the duplex option shows greater promise in the burnable poison design for all the HPD cases. For the poison design with ZrB2, duplex fuel contributes ∼5% more initial reactivity suppression and ∼20% lower reactivity swing. Higher power density cases (e.g. 111 MW/m3) require less burnable absorber than lower power density cases (e.g. 82 MW/m3) for both candidate fuels. For control rod design with boron carbide (B4C), rod cluster control assembly (RCCA) worth increases with increasing power density. RCCA worth is ∼2% higher for the duplex fuel than UO2. In this assembly-level analysis, optimised assemblies for all the HPD cases are loaded into a 3D reactor model in PANTHER in order to determine whether the proposed HPD assemblies can obtain the designated core life. PANTHER results confirm that at the end of the 15-year cycle, the candidate cores are on the border of criticality for both fuels, so the assembly-level analysis fissile loading is well-designed for the desired lifetime. A companion paper will examine key physics and core safety analysis parameters in the whole-core environment.