Hybrid functional/embedded cluster study of uranium and actinide (actinide = Np, Pu, Am or Cm) mixed dioxides bulk and {110} surfaces

Understanding actinide (An) mixed oxides (MOX) is important for the development of minor An bearing U-Pu MOX nuclear fuel, and the long-term storage of spent nuclear fuel. In this work, we systematically simulate AnO2 (An = U, Np, Pu, Am, and Cm) and U-An MOX (An = Np, Pu, Am, and Cm) bulk and {110} surfaces using hybrid density functional theory within an embedded cluster model, focusing on the spin density of An and the density of states of AnO2 bulk and surface for the An series from U to Cm; spin density transfer is found between O and An and is discussed in detail. For U-An MOX bulk and surface, geometric structure, U and An spin densities and the densities of states are presented. Clear conclusions can be drawn regarding the oxidation state of An in the U-Np and U-Cm MOX systems, while U-Pu MOX and U-Am MOX are less clear but and sensitive to the simulation method, suggesting they are in a transition region between the U-Np and U-Cm systems.