Atomic-scale simulation of soluble fission products in UO2

The continuously increasing amount of energy produced within the nuclear fuel in the reactor is accompanied by an incessant increase of the number of fission products (FPs), which affects the thermo-mechanical behaviour of the fuel rods and eventually limits its lifetime. More precisely the FPs can contribute to an increase of the fuel volume, commonly referred to as fission product swelling, and lead to a chemical as well as a mechanical interaction between the ceramic fuel pellets and the metallic cladding material constituting the first barrier against the release of radioactive FP in the environment. A precise prediction of the moment at which such an interaction is established during the lifetime of a fuel rod in the reactor, or the moment at which the cladding material may fail as a result of such an interaction is strongly affected by the capability to predict the fuel swelling. The contribution of the various classes of fission products to the swelling of the fuel depends strongly on their physical and chemical properties. Because of their virtual negligible solubility, inert gas atoms for instance, have a tendency to precipitate and form bubbles, whereas the soluble fission products will remain within the matrix during normal operating conditions. Accurately predicting the contribution of each (type of) fission product is required for the accurate forecast of the fuel swelling. Nevertheless, there are many uncertainties pertaining to the contribution from each class of FP. The present report aims at assessing the contribution from the soluble FPs by means of an atomistic simulation of the effect of each element on the lattice parameter of the fuel crystal. In the first part a review of previous simulation studies of FPs is provided. In this report, results for the defect volumes associated with the introduction of FP ions in UO2 and uranium dioxide lattice volume swelling as a function of FP concentration are presented, which have not been considered before. In the following section, a review is given of the experimental data available in the open literature for each fission product separately. The details of the fabrication as well as the experimental analysis are provided since they are essential for a comparison of our computations with the experimental data. The theoretical assessment of the effect of each FP on the lattice parameter of UO2 is provided in the third section, and is followed by a direct successful comparison with the experimental results for individual FPO2/FPO/FP2O3-UO2 systems. The models should therefore be reliable for FPs where no or little experimental data exists such as for Sm accommodation in UO2.In the subsequent section, a linear model is developed that combines the isolated defect cluster predictions in the Mott-Littleton technique for predicting the total contribution of all soluble FP on the fuel swelling in UO2 as a function of the burnup. The comparison with experimental volume change data is excellent for annealed irradiated UO2 up to approximately 3% burnup. Understanding swelling in uranium dioxide at larger burnup levels requires models accounting for FP-FP interactions beyond the cluster level considered here. This model for swelling behaviour of UO2 at low burnups is, however, suitable for inclusion in fuel performance codes such as TRANSURANUS. (The present document is extracted from the dissertation presented at Imperial College London for obtaining the degree of Doctor of Philosophy by K.H. Desai in September 2008).