First-Principles Study of Barium and Zirconium Stability in Uranium Mononitride Nuclear Fuels

Barium and zirconium solution behaviors in antiferromagnetic uranium mononitride (UN) have been studied based on first-principles density functional theory. By calculating the incorporation and solution energies in UN, it is found that the most favorable solution sites are U vacancies for both Ba and Zr, and Zr is more soluble than Ba. The volume of the Ba-doped system keeps expanding with increasing Ba doping concentration, whereas that of the Zr-doped system changes from swelling to contraction with increasing Zr doping concentration. This phenomenon may result from the difference of these two elements in atom radius and coordination mechanism. Furthermore, the solution energies of metallic and nitride phases of Ba and Zr indicate that both phases of Ba are insoluble in the UN matrix, whereas the metallic phase of Zr is insoluble, and its nitride ZrN is soluble in the UN matrix.