Etude ab initio des énergies de solubilisation et de diffusion du césium dans le dioxyde d'uranium

Titre du congrès :E-MRS 2008 Spring Meeting Ville du congrès :Strasbourg Date du congrès :26/05/2008

  In the hypothetical case of a severe accident on a nuclear power plant, UO2 fuel in the nuclear core may undergo high temperature, which enhances the migration of fission products and allow their release out of fuel pellets. In this context, the "Institut de Radioprotection et de Sûreté Nucléaire" has initiated ab initio studies on the location and migration of fission products in nuclear fuel.
The present work focuses on caesium, which has a significant fission rate, is very noxious, and may likely be released in large amount in case of a severe accident. Since its release can be due to intragranular atomic diffusion in UO2, we have studied the solution energies of caesium in several point defects (uranium and oxygen vacancies, Schottky defects, U-O divacancy pairs, interstitial sites), and its activation energy of diffusion in uranium dioxide using an ab initio method. The latter is studied for the first time with ab initio calculations.
Our Density Functional Theory (DFT) calculations have been performed with VASP (Vienna Ab Initio Simulation Package) code using PAW (Projector Augmented Wave) potentials. Both the generalised gradient (GGA) version of the local density approximation (LDA) and the exchange and correlation corrected version GGA+U have been employed. The results obtained in the latter approximation differ somewhat from those obtained in GGA in the stoichiometric and sub-stoichiometric oxide. In our work, supercells of at least 96 atoms have been used to limit defects interactions due to the cell periodicity. We have compared our solution energies with previous calculations based either on the Mott-Littleton methodology or on DFT LDA (Local Density approximation) with small size supercells. Our method is more precise, however our results and physical conclusions are in general agreement with those obtained by these authors.  Finally, it is pointed out how such theoretical approach may help in providing insigth on the diffusion behaviour of fission products in UO2.