The atomic behavior of helium and krypton in uranium dioxide has been studied using an ab initio simulation technique. The electronic structure calculations are based on the local density approximation (LDA) applied to the density functional theory (DFT). They have been performed in the framework of the linear muffin-tin orbital (LMTO) method in the atomic sphere approximation (ASA). Incorporation energies and solution energies of the two rare gases have been calculated. Krypton atoms are found to be insoluble in this standard nuclear fuel whatever the trapping site considered and their presence in the lattice induces swelling when they are located in interstitial sites or in oxygen vacancies. Due to its smaller atomic size, the predicted helium behavior is very different. Indeed, helium is found to be soluble in stoichiometric and hyperstoichiometric uranium dioxide in the presence of uranium vacancies or divacancies constituted by one uranium and one oxygen vacancy. Moreover helium atoms induce a lattice parameter contraction except in interstitial sites for which a slight expansion is calculated. Calculations on iodine and cesium are in progress and preliminary results for iodine are given. The experimental validation of the calculations is in progress. X-ray absorption spectroscopy (XAS) measurements using synchrotron radiation are planned on polycrystalline stoichiometric uranium dioxide samples doped with iodine and/or cesium by ion implantation. The spectra collected at the K and L{sub}(III) edges of the two elements will allow us to determine their local environment and their chemical state.
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