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 investi-gate 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 (Ef) 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 num-ber of U atoms in the second nearest-neighbour shell (NU(2NN)) increases. The Xe incorporation energy (Ei) is found to be independent of vacancy species, depending only on the chemical environment of the vacancy. As does Ef, Ei increases with increasing NU(1NN), while decreases with increasing NU(2NN), be-cause the smaller the NU(1NN) and the larger the NU(2NN), the larger the vacancy steric space. The Ei of Xe and Kr are calculated to be within the ranges 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.(c) 2023 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY license ( http://creativecommons.org/licenses/by/4.0/ )

Automated summary

This work investigates the properties of mixed nitrides U0.75Pu0.25N and U0.5Pu0.5N, as well as the incorporation and migration behavior of fission gas Xe. The results show that the disordered structures have closer lattice parameters to experimental values and lower energy states for Pu f orbitals than U f orbitals. Furthermore, the vacancy formation energy and Xe incorporation energy depend on the chemical environment.