Assessment of atomistic data for predicting the phase diagram and defect thermodynamics in UO$_2$

Serge Maillard (1) , David Andersson (2) , Fabien Bruneval (3)

1 LM2C - Laboratoire de Modélisation Multi-échelles des Combustibles
2 Materials Science and Technology Division [Los Alamos]
3 SRMP - Service de recherches de métallurgie physique

This paper develops a unified presentation of the thermodynamic parameters of eleven point-defect models in UO$_2$ (defect formation energies and entropies); four of them are fitted to experimental data, while the remaining seven are obtained through atomistic simulations. This allows us to compare all the models on the same basis both among themselves and with a large set of experimental data of various physical quantities. Combining the assessed defect thermodynamics and the TAF-ID functions for U and U$_4$O$_9$ phases makes it possible to compute the U - O phase diagram in the vicinity of stoichiometric UO$_2$. The defect formation energies and entropies are very different from one model to another. Concerning the ability to reproduce experimental data, the fitted models usually correctly reproduce the data sets according to which they were fitted. For one atomistic-based model, the measurements of the oxygen potential as a function of the temperature and the O/M ratio are reproduced in a satisfactory manner while the phase diagram is more approximate. No model, either fitted or atomistic-based, reproduces simultaneously the measured conductivity and the oxygen concentration as functions of the oxygen potential. The difficulties of the atomistic-based models in predicting the O/M ratio as a function of the oxygen potential are thought to partly arise from an erroneous calculation of the oxygen molecule energy derived from ab initio techniques and probably also from a poor evaluation of the electron-hole Gibbs energy of formation; more generally improving the technique for calculating reliable defect entropies of formation appears of great importance. The difficulty of obtaining reliable experimental data close to the stoichiometry might also contribute to the limited agreement between calculations and measurements, which is reason enough to reassess the behavior of the material in this stoichiometry region comprehensively, using all the possible characterization techniques on each material sample. In particular, since several independent studies predict an unexpected crucial role of the uranium vacancies on the evolution of O/M, experimental assessment of this feature should be sought.

Domaines: Science des matériaux [cond-mat.mtrl-sci]

Serge Maillard, David Andersson, Fabien Bruneval. Assessment of atomistic data for predicting the phase diagram and defect thermodynamics in UO$_2$. Journal of Nuclear Materials, 2022, 569, pp.153864. ⟨10.1016/j.jnucmat.2022.153864⟩. ⟨cea-04001325⟩