Neutronic and kinetic performance of thorium–U-233 and thorium–Plutonium fuel cycles in the SMART integral PWR

Abstract

Thorium-based fuel cycles have attracted increasing interest as a potential alternative to conventional uranium fuels, particularly in small modular reactors (SMRs), due to their favorable neutronic characteristics and resource sustainability. In this work, a comparative neutronic analysis of thorium–U-233 and thorium–plutonium fuel cycles is performed for the SMART integral pressurized water reactor. A total of eleven fuel cycles are investigated, including a reference SMART cycle and ten thorium-based configurations with varying fissile content. Neutronic burnup calculations are carried out using the MCNP6 Monte Carlo code over a 1000-day cycle. The evolution of the effective multiplication factor is analyzed to identify fuel configurations capable of sustaining critical operation, from which one representative Th–U-233 and one Th Pu case are selected for detailed analysis. The results show that both thorium-based fuel cycles achieve average burnup levels of approximately 24–25 GWd/MTU, comparable to the 23 GWd/MTU of the reference SMART UO2 core. The Th–U-233 configuration preserves smooth radial flux and power distributions, with moderate power peaking, whereas the thorium–plutonium configuration exhibits higher power peaking and greater spatial heterogeneity due to the radially homogeneous fissile loading. Kinetic analyses indicate a reduction of the effective delayed neutron fraction to approximately 50% of the reference UO2 value, while maintaining delayed-neutron-driven kinetic behavior and characteristic generation times of the same order of magnitude as those of the reference core. Overall, the study demonstrates that thorium–U-233 fuel is more naturally compatible with the original SMART core design, while thorium–plutonium fuel cycles are capable of sustaining critical operation under the adopted assumptions, but exhibit higher power peaking and would require dedicated optimization of radial loading and reactivity control strategies before practical implementation.