Lanthanum energetics in cubic ZrO_2 and UO_2 from DFT and DFT + U studies

摘要

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 ZrO_2. 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 UO_2±x. Comparing these two sets of results, we find that both La incorporation and solution energies in UO _2±x are higher than in cubic ZrO_2. Our calculations suggest that, thermodynamically, the fission product La should prefer to reside in the inert matrix phase, i.e., cubic ZrO_2, instead of in the fissile phase, UO_2.