First-principles DFT + U modeling of actinide-based alloys: Application to paramagnetic phases of UO 2 and (U,Pu) mixed oxides

We report first-principles DFT$\phantom{\rule{0.16em}{0ex}}+\phantom{\rule{0.16em}{0ex}}U$ calculations of the paramagnetic phases of uranium dioxide (UO${}_{2}$) and (U,Pu) mixed oxides (MOX). We use a combination of two techniques, the first of which enables us to treat the presence of metastable states and the other the random distribution of spins (for paramagnetism) and of plutonium atoms (for MOX). The resulting method is first used to determine the ground-state properties of paramagnetic UO${}_{2}$ and is then applied to MOX with $12.5%$ and $25%$ Pu, for which both experimental and theoretical studies are lacking. We find the paramagnetic phase of fluorite UO${}_{2}$ to be more stable than the antiferromagnetic phase as expected from experimentation, with ground-state properties in excellent agreement with experimental data. Results on MOX show that most of the ground-state properties of UO${}_{2}$ are affected by addition of $12.5%$ Pu and do not seem to be much different when the plutonium content changes, at least for Pu contents below $30%$. For both UO${}_{2}$ and MOX, it is found that the true paramagnetic order has a significant effect on the calculated elastic constants, compared to the antiferromagnetic and ferromagnetic orders. By contrast, we find that regarding energetics, the antiferromagnetic order may be used as a very good approximation of the paramagnetic state.