L. Messina, D. Bathellier, C. Julien, E. Bourasseau, M. Freyss.
CEA, DEs, IRESNE, DEC/SESC/LM2C, Centre de Cadarache, Batiment 151, F-13108 Saint-Paul-lez-Durance.
Mixed uranium-plutonium oxide (U,Pu)O2 is used as fuel in pressurized water nuclear reactors (PWR) with around 10 wt.% Pu and is the reference fuel for future Generation IV sodium-cooled fast reactors with around 25 wt. % Pu.
The self-diffusion properties are at the origin of several important phenomena taking place in (U,Pu)O2 during irradiation, in particular the redistribution of Pu in the fuel, which strongly affects the fuel behaviour, and the oxygen diffusion, which governs the local oxide/metal ratio.
In order to characterize the self-diffusion properties of mixed uranium-plutonium oxides, we investigate the activation energy of defects using atomic scale calculations. The activation energy, which involves formation and migration energies, depends on the composition and configuration of the local atomic environment (LAE) around the defect, which can fluctuate statistically or due to the presence of heterogeneities. For this reason, it is not possible to define a single activation energy for a given defect, but rather an interval range that depends on the specific configuration of the LAE. Determining this interval, however, is a difficult task due to the chemical disorder observed in the cationic sublattice in (U,Pu)O2. This gives raise to such a large amount of possible configurations that an exhaustive exploration of the configurational space is practically impossible.
In this work, we present an optimized systematic exploration of the configurational space using empirical potentials for various Pu contents. This enables us to provide an accurate evaluation of this interval range for cationic vacancy defects and the impact of the LAE on the point-defect properties.
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Wednesday – 15th September 2021