Investigation of the bulk and point defect properties in uranium–plutonium mixed oxides (U,Pu)O2 using DFT+U: Effect of a low americium content

Abstract

We report an investigation of the effect of a low Am impurity content on bulk and point defect properties of mixed actinide oxide (U,Pu)O2. Using the generalized gradient approximation, (GGA + U) we study U0.75−zPu0.25AmzO2 with z = 3 and 6%. Several of its bulk properties have never been reported in the literature or are poorly known. The Hubbard term U is used to take into account the strong correlation effects related to the actinide 5f electrons. We find an electronic charge transfer between uranium and americium cations in the bulk crystal. Am(+IV) cations tend to easily reduce to Am(+III), and this reduction of Am(+IV) is compensated by the oxidation of U(+IV) to U(+V) in stoichiometric compounds. In turn, oxygen hypostoichiometry is accommodated by the reduction of U(+V) cations [which result from the presence of Am (+III) cations] before any reduction of the Pu cations. Furthermore, we show that Am induces a significant increase in the mixing enthalpy as well as a decrease in the lattice parameter of the (U,Pu)O2 solid solution, which is in agreement with early experimental studies. Finally, we show that americium facilitates the formation of oxygen vacancies in (U,Am)O2 compared to UO2, whereas it induces an increase in the formation energy of oxygen vacancies in (U,Pu,Am)O2 compared to (U,Pu)O2.