Thesis location: Radioelement Transfer Research Laboratory (LETR) - Cadarache (13)
Start: October 2021
Research Master's degree in physico-chemistry of materials with expertise in atomic scale simulation methods.
Age limit: 26 years old unless otherwise stated.
This thesis proposal covers the study of potential releases of radionuclides into the environment and the means implemented to improve their containment. The safety assessment of nuclear installations requires a detailed study of the risk of radioactive contaminants dispersing outside the installation with sufficient precision in terms of the associated consequences for the environment and humans. To carry out this analysis, it is first necessary to characterize the emission of radioactive contaminants from the fuel. In the United States and in Europe in particular, manufacturers are seeking to develop fuel elements known as E-ATF for “Enhanced Accident Tolerant Fuel”, having increased robustness in terms of safety. The objective is to obtain better behavior of the fuel in terms of degradation under heating, hydrogen production but also release of fission products (PFs) in accident situations. One concept for the UO2 fuel consists in increasing the size of the grains by doping to improve the retention capacity of the PFs in the matrix. Several dopants are being studied, including chromium oxide which is currently the standard solution. The work presented in this thesis therefore concerns the study of UO2 fuel doped with chromium oxide, and in particular the impact of this oxide on the behavior of structural defects and on the diffusion of fission products. The effect of chromium oxide on the release of rare gases has been studied experimentally, but the mechanisms at the origin of the retention of PFs and in particular the role of the dopant on the structure of defects and the diffusion of PFS has not been clearly identified. Modelling methods at the atomic scale (density functional theory and molecular dynamics methods) make possible the study of elementary mechanisms that are difficult to access by experiment by allowing the determination of the properties of interest in our study. The objective is therefore, through simulation, to study the migration mechanisms of PFs in chromium-doped UO2, so as to be able to compare them to an undoped UO2 fuel. It will be based in particular on existing work carried out in the laboratory on cesium and molybdenum FPs. In a first step, the aims should be to characterize, by calculation, the UO2 doped structure according to the chromium concentration in the matrix. In a second step, the student will have to carry out a detailed study of the structure of defects in order to understand how chromium affects the sites on which the fission products are incorporated. The third point will concern the study of the impact of chromium in the migration of FPs. These different points will be applied identically to the case of an over-stoichiometric fuel in order to measure its influence on the kinetic mechanisms by comparison with a non-doped fuel.