Lanthanum energetics in cubic ZrO2 and UO2 from DFT and DFT + U studies

Lanthanum (La) is a fission product prevalent in spent nuclear fuel. Density funtional theory (DFT) calculations were used to determine the incorporation and solution energies of La in cubic ZrO2. Based on the results of this study, the zirconium vacancy site is predicted to be the most stable incorporation site for La. This can be understood based on Coulombic considerations. Using DFT + U calculations in combination with a point defect model, we also examined La incorporation and solution energies as a function of stoichiometry in UO2±x. Comparing these two sets of results, we find that both La incorporation and solution energies in UO2±x are higher than in cubic ZrO2. Our calculations suggest that, thermodynamically, the fission product La should prefer to reside in the inert matrix phase, i.e., cubic ZrO2, instead of in the fissile phase, UO2.