Incorporation and migration of xenon in uranium-plutonium mixed nitride; a Density Functional Theory Study

Actinide nitride materials are promising candidates for advanced nuclear fuels. In this work, we investigate the bulk properties of the mixed nitrides U0.75Pu0.25N and U0.5Pu0.5N, and study the incorporation and migration behaviour of the fission gas Xe. The disordered U0.75Pu0.25N and U0.5Pu0.5N structures are constructed using the special quasi-random structure method. Their lattice parameters are closer to the experimentally determined values than the corresponding ordered structures. The density of states show that Pu f states are located at lower energy than U f, consistent with the trend of increasing f orbital stability across the actinide series. The actinide vacancy formation energy (E_f) in disordered and ordered U0.5Pu0.5N is highly dependent on the chemical environment around the vacancy: it increases as the number of U atoms in the first nearest-neighbour shell (NU(1NN)) increases, but decreases as the number of U atoms in the second nearest-neighbour shell (NU(2NN)) increases. The Xe incorporation energy (E_i) is found to be independent of vacancy species, depending only on the chemical environment of the vacancy. As does E_f, E_i increases with increasing NU(1NN), while decreases with increasing NU(2NN), because the smaller the NU(1NN) and the larger the NU(2NN), the larger the vacancy steric space. The E_i of Xe and Kr are calculated to be within the range 4.47 – 6.01 eV and 3.30 – 4.64 eV, respectively. The Xe migration energy barrier in ordered U0.5Pu0.5N allows us to set the energy range for Xe diffusion in disordered U0.5Pu0.5N as 0.50 – 1.75 eV. A lower range of 0.30 – 1.25 eV is found for Kr diffusion.