Abstract
Recent experiences allowed to identify the UO2 fuel fragmentation mechanisms during a LOCA type thermal transient. This PhD thesis aims to study these mechanisms to enhance the understanding and the modeling of fragmentation.The first mechanism which is studied is the effect of the pressure of fission gas bubbles that are present in the material. To achieve this, the stress induced by the bubble pressurisation is estimated and the uncertainty associated are quantified. This study draws upon (1) an analysis of the equation of state that allows the determination of the cavity pressure, (2) an assessment of material state after irradiation (porosity, gas quantity, ...) from the results of simulation and post-irradiation examinations (3) an estimate of the stress taking into account the non-spheric bubble shape observed in 3D by MEB-FIB. The second mechanism studied is the effect of a drop of the pressure applied to the fuel due to a loss of sealing. To do so, the fuel fissuration during a transient is simulated by a model coupling the discret element method (DEM) and a cohesive zone model (CZM). The parametrical analysis of the DEM- CZM model allows the identification of the parameters representative of the fracture behavior of the fuel during an annealing test at controlled pressure (pression uniform). Based ont the results obtained, a crack initiation criteria is established.The fracture behavior of the fuel during the loss of sealing (axial pressure gradient along the fissile stack) is simulated with the DEM-CZM model. The results uphold the conclusion that the proposed crack initiation criteria can be applied to the case of a non-uniform pressure drop. The model is then coupled to a immersed granular flow modeling and the fragments dispersal out of the cladding is studied. The results suggest a strong influence of the fragments shape (polyhedric or spherical) on the dispersed quantity.