Modelling studies on the diffusion of gaseous fission products in UN

Sammanfattning

Uranium Nitride (UN) has attracted considerable interest as a candidate for advanced nuclear fuels due to its high fissile density, elevated thermal conductivity, and improved performance over Uranium Dioxide (UO2). Despite these advantages, its behavior under irradiation is not yet fully understood, particularly concerning defect energetics and the retention and release of volatile fission products such as Xenon (Xe) in the presence of Oxygen (O), resulting from the oxidation of UN.Using Density Functional Theory (DFT), later improved with DFT+U parameters, within the VASP framework, a UN supercell was built in order to evaluate defect formation energies, solute-vacancy binding, and migration barriers via the Climbing Image Nudged Elastic Band (CINEB) method.The results show that DFT+U provides a more accurate description of the magnetic state of UN supercells and yields formation energies of uranium and nitrogen vacancies (5–6 eV and 4–5 eV, respectively) in closer agreement with literature values than standard DFT.Incorporation energies show that Xe is energetically costly to insert into the lattice, while O incorporates spontaneously with a strong driving force.Binding energies reveal the largest discrepancies between DFT and DFT+U, reflecting their different ground-state energetics. Migration energy barriers, obtained with CINEB, demonstrate that oxygen in the vicinity of the migration path increases the barrier for both Vacancy (V) and xenon migration. In particular, vacancy–solute exchange energies for Xe–V were found to be significantly lower than those for U–V, suggesting that Xe–V exchanges can occur frequently, whereas U–V jumps, though less frequent, are the key contributors to long-range diffusion.