A first-principles simulation study on solubility of La, Nd, Zr and Mo in UO2 and U3O8

Solubility of fission products (FPs) in spent nuclear fuels is a key issue in understanding their performance. In this study, a detailed density functional theory study on solubility of La, Nd, Zr and Mo in uranium dioxide (UO₂) and triuranium octoxide (U₃O₈) was carried out. The most favorable sites of La, Nd, Zr and Mo were calculated by analyzing the solution energies of these four FP elements in UO₂ and U₃O₈. Thermodynamic properties of FPs doping in UO₂ as well as in U₃O₈ were determined from 0 K to 2000 K. The results reveal that the effect of FPs on thermodynamic properties of U₃O₈ is slightly larger than that of UO₂ due to the presence of an oxygen vacancy in FP-doped U₃O₈. On this basis, the solubility of La, Nd, Zr and Mo in UO₂ and U₃O₈ was predicted, showing a good agreement with available experiments. Subsequently, the difference in solubility behaviors was studied through electronic properties. The simulation results indicate that the solubility of La, Nd, Zr and Mo in U₃O₈ is much lower than that in UO₂. Therefore, an idea to separate FPs from spent nuclear fuel is using mutual transformations between FP-doped UO₂ and FP-doped U₃O₈ under different temperatures, which is a promising way to promote the development of nuclear fuel cycles.