Trapping Properties\nof Iodine, Cesium, and Tellurium\nin Uranium Dioxide: A DFT+<i>U</i> Study

We investigate the trapping properties of iodine, cesium,\nand tellurium\nin uranium dioxide, using the Hubbard-corrected density functional\ntheory (DFT+<i>U</i>). In order to avoid the metastable\nstates inherent to this method, we use the occupation matrix control\n(OMC) scheme, which also allows us to monitor the oxidation states\nof the different species. The most favorable trapping sites, oxidation\nstates, and solubility of I, Cs, and Te are evaluated in stoichiometric\nUO<sub>2</sub>. To that end, vacancy-like defects under various charge\nstates, including uranium and oxygen vacancies, U–O divacancy\nand bound Schottky defects, as well as the interstitial position,\nare considered as potential trapping sites in UO<sub>2</sub>. Te is\nfound to exhibit a wide range of possible oxidation states, ranging\nfrom Te<sup>–</sup> to Te<sup>4+</sup>, depending on the stable\ntrapping site considered. For I and Cs, one predominant oxidation\nstate for each fission product, namely, I<sup>–</sup> and Cs<sup>+</sup>, is found. This behavior is mainly accommodated by the charge\nof the defects. By providing accurate trapping sites and oxidation\nstates of volatile fission products in UO<sub>2</sub>, this study\nis expected to contribute in the development of larger scale simulation\nmethods, enabling a better prediction and mitigation of corrosion\nissues in nuclear fuel cladding.