Trapping Properties of Iodine, Cesium, and Tellurium in Uranium Dioxide: A DFT+U Study

We investigate the trapping properties of iodine, cesium, and tellurium in uranium dioxide, using the Hubbard-corrected density functional theory (DFT+U). In order to avoid the metastable states inherent to this method, we use the occupation matrix control (OMC) scheme, which also allows us to monitor the oxidation states of the different species. The most favorable trapping sites, oxidation states, and solubility of I, Cs, and Te are evaluated in stoichiometric UO₂. To that end, vacancy-like defects under various charge states, including uranium and oxygen vacancies, U-O divacancy and bound Schottky defects, as well as the interstitial position, are considered as potential trapping sites in UO₂. Te is found to exhibit a wide range of possible oxidation states, ranging from Te^- to Te⁴⁺, depending on the stable trapping site considered. For I and Cs, one predominant oxidation state for each fission product, namely, I^- and Cs⁺, is found. This behavior is mainly accommodated by the charge of the defects. By providing accurate trapping sites and oxidation states of volatile fission products in UO₂, this study is expected to contribute in the development of larger scale simulation methods, enabling a better prediction and mitigation of corrosion issues in nuclear fuel cladding.