Innovative thorium-based fuel assemblies for LW-SMR: In-depth assembly-level neutronic analysis and safety considerations in solid and annular configurations

• This study assesses the feasibility of using thorium-based fuels in the AP300™ SMR assembly. • Neutronic performance was analyzed for both solid and annular fuel configurations. • (Th-²³³U-²³⁵U)O₂ fuel offers extended cycle lengths and improved reactivity feedback coefficients. • The fuel enhances safety margins and increases overall fuel utilization. • Power peaking factors are reduced, promoting more uniform power distribution. This study presents a comprehensive neutronic and safety evaluation of different thorium-based fuel compositions (Th- 235 U)O 2 , (Th- 233 U)O 2 , and a new proposed composition, namely, (Th- 233 U- 235 U)O 2 fuel, compared to conventional UO 2 in solid and dual-cooled annular assembly configurations in the context of Small Modular Reactors (SMR) applications. Results demonstrate that (Th- 233 U)O 2 achieves the highest cycle length in both solid configurations, significantly exceeding UO 2 performance. The proposed, however, (Th- 233 U- 235 U)O 2 composition exhibits favorable safety characteristics, including enhanced negative temperature reactivity coefficients and favorable kinetic parameters. Annular fuel configurations also demonstrate superior thermal performance with centerline temperature reductions of approximately 50 °C compared to solid configurations and consistently lower power peaking factors throughout burnup. The findings indicate that thorium-based fuels, particularly (Th- 233 U- 235 U)O 2 in dual-cooled annular configuration, offer compelling advantages for SMR applications, including extended fuel cycles, improved safety characteristics, enhanced proliferation resistance, and superior thermal performance, supporting their viability as sustainable alternatives to conventional uranium-based fuels.