Small modular reactor core design for civil marine propulsion using micro-heterogeneous duplex fuel. Part I: Assembly-level analysis

In an effort 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% ²³⁵ U enrichment) civil nuclear marine propulsion small modular reactor (SMR) core that provides at least 15 effective full-power-years (EFPY) life at 333 MWth using 18% ²³⁵ U enriched micro-heterogeneous ThO ₂ -UO ₂ duplex fuel and 15% ²³⁵ U enriched homogeneously mixed all-UO ₂ 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 (Gd ₂ O ₃ ), erbia (Er ₂ O ₃ ) and ZrB ₂ 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 effects, this design maintains low and stable assembly reactivity with relatively little burnup penalty. Thorium-based duplex fuel offers better performance than all-UO ₂ fuel with all BP options considered. Duplex fuel has ∼20% lower reactivity swing and, in consequence, lower initial reactivity than all-UO ₂ 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 (B ₄ C), hafnium, and Ag-In-Cd alloy. All the candidate materials exhibit greater rod worth for the duplex design. For both fuels, B ₄ C has the highest rod worth. In particular, one of the major objectives of this study is to offer/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 UO ₂ fuel and ‘open the option’ of designing the functional cores with both the duplex and UO ₂ fuel cores. A companion paper will examine key physics and core safety analysis parameters in the whole-core environment.