2, CENBG, Gradignan, , France
3, CEMHTI, Orléans, , France
Significant quantities of xenon and krypton are produced in nuclear fuels under irradiation and the release of these fission gases induces an increase of the pressure inside the fuel pin and fuel swelling due to gas bubble formation. To prevent cladding failure, a better understanding of the fission gas release process is essential. The purpose of this work is to get further insight into the diffusion mechanisms of these fission gases and their interaction with defects in uranium dioxide. To do this, separated effects studies coupling ion irradiations/implantations and fine characterizations using Thermal Desorption Spectrometry (TDS), Positron Annihilation Spectroscopy (PAS) and Transmission Electronic Microscopy (TEM) have been performed.
TDS characterizations of UO2 discs implanted with various xenon concentrations were performed on the PIAGARA (Plateforme Interdisciplinaire pour l’Analyse des GAz Rares en Aquitaine) platform at CENBG in Bordeaux to evaluate its thermal diffusion coefficient. The results show that xenon release decreases when the quantity of xenon implanted increases. This proves that fission gases are trapped in radiation induced defects. From the released fraction, we determined a model of gas release taking into account the initial burst and the diffusion in the bulk affected by the trapping effect. The fraction of gas trapped into UO2 during annealing experiment determined by the model could be related afterwards with microstructure characterizations by PAS (vacancy defect characterization) and MET (cavities observation). Doppler broadening spectroscopy is performed using a slow positron accelerator on UO2 samples with various xenon concentrations in as implanted state and after annealing at different temperatures. The results suggest that the main defect created after irradiation is the Schottky defect [1-2]. For a higher implantation dose, however, larger defects are detected probably due to vacancies aggregation. After annealing experiments, positrons probe large defects associated to cavities considering annihilation characteristics which is confirmed by TEM observation performed on those same samples.
The fraction of gas trapped determined by TDS measurements coupled to defect characterization by PAS experiments and TEM observations allows us to determine intrinsic diffusion coefficient of xenon and to identify the favorite trap sites related either to the Schottky defect or to the cavities depending on the fluence and annealing temperature. These results will enable us to describe the thermal behaviour of rare gases in UO2 fuel starting from the atomic scale.
 H. Labrim et al. NIMB 261 (2007)
 T. Belhabib, PhD Thesis, Université d’Orléans (2012)