Preparation and First-principles Study of New Thorium-based MOX Fuel

Abstract

The development of thorium-based nuclear fuel is of significance for the long-term sustainable development of nuclear power. Among these, Th1−xPuxO2 stands as a highly promising nuclear fuel with proven applications in various reactor types. This study aims to explore the structural and qualitative characteristics of thorium-based mixed oxide (Th-MOX) fuel Th1−xPuxO2. With the substitution of Ce for Pu, the sol-gel method was employed to synthesize Th1−xCexO2 (x=0, 0.25, 0.50, 0.75, 1) at varying sintering temperatures (800, 1 000, 1 200, 1 400, 1 600 ℃). The morphology and structure of Th1−xCexO2 were characterized by X-ray diffractometer (XRD), scanning electron microscope (SEM) and Raman spectrometer. The lattice constant and electronic density of state (DOS) of Th1−xCexO2 were calculated using the first-principles calculation software VASP based on density functional theory. The experimental findings demonstrate that the prepared Th1−xCexO2 material exhibits excellent properties in terms of density and particle uniformity. With the increase in sintering temperature, the results of density testing indicate that the material’s density exhibits a trend of initially increasing and then decreasing, ultimately reaching a maximum of 98.4% of the theoretical density. This phenomenon occurs due to as the sintering temperature increases, the diffusion process induces the migration of grain boundaries, enlarging the contact area between grains and reducing the presence of pores, thereby increasing the material density. However, further elevating the sintering temperature results in the formation of a liquid phase on the material surface, impeding the migration of grain boundaries. This leads to an expansion of gaps between particles, an increase in the number of pores, and ultimately a reduction in density. The XRD analysis reveals that variations in sintering temperature have a negligible effect on the lattice constant of Th1−xCexO2. Nevertheless, the grain size demonstrates a trend of initial enlargement followed by reduction as the sintering temperature increases, indicative of a migration-driven process. SEM images reveal a uniformly dense structure in samples sintered at 1 400 ℃. In comparison to CeO2 and ThO2, the Raman spectrum of Th1−xCexO2 (x=0.25, 0.50, 0.75) reveals an additional, relatively weaker peak at around 582 cm−1, suggesting the occurrence of oxygen vacancies within the lattice during the formation process of the mixed oxide. In addition, as the Th content increases in the mixed oxide, there is a reduction in grain size, an expansion of the bandgap, and a narrowing of the conduction band width, the differences in atomic radii contribute to an increase in the average bond length of the lattice constant, Th—O bond and Ce—O bond. The lattice constant of Th1−xCexO2, as determined through VASP calculations, closely match experimental value and follow Vegard’s law. The local DOS (LDOS) of Th and Ce atoms increases with the increase of the element content. Near 2.2 eV, the LDOS of Th atom is very low but Ce atom is very large due to the contribution of Ce 4f state electrons. This study offers a practical approach for the preparation of Th-MOX fuels.

Keywords

• th-mox fuel
• sol-gel method
• first-principles
• density of state