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Effets de la température et de l'irradiation sur la mobilité du xénon dans UO$_2$ : étude profilométrique et microstructurale

Benoît Marchand 1
IP2I Lyon - Institut de Physique des 2 Infinis de Lyon
Abstract : In France, electricity is mainly produced (78%) through the operation of 58 PWRs (Pressurized Water Reactors). During reactor operation, many fission products (FP) are generated in the fuel which is, in most cases, UO2 enriched to about 4% in 235U. Among FPs, gaseous fission products as Xenon and Krypton, are abundantly produced (around 15% stable fission products). Because of their chemical nature, those two gases have a very low solubility in the fuel and therefore tend to form bubbles (to minimize surface tension) and can cause pellets swelling. The formed gas can also be released out of the pellet, and lead to a substantial increase in the pressure within the fuel cladding, thereby limiting the energy production. However, migration mechanisms, traditionally studied indirectly by measuring the amount of gas released after irradiation, are not yet fully understood. It is frequently assumed that atomic diffusion is the only mechanism that can lead to a migration of xenon. The objective of this thesis is to provide direct evidence of the different mechanisms controlling the behavior of Xenon during thermal annealing and irradiation. Therefore, we used ion implantation to introduce Xenon in uranium dioxide samples. After implantation, the Xenon distribution follows a quasi-Gaussian concentration profile (variation of the concentration regard to the depth) located in the first 300 nanometers of the sample. We have performed post-implantation annealing at 1400 °C and 1600 °C in order to study the impact of the temperature, and irradiation with ions to simulate the impact of fission products in the fuel. Subsequently, concentration depth profiles were measured by ion microprobe (SIMS). Although the feasibility of Xenon measurement has been demonstrated in several articles, no concentration profile had so far been presented in the literature because a classical data processing of SIMS data is not suitable in uranium dioxide. Therefore a new data processing software has been developed in the frame of this thesis to reconstruct depth profiles. From depth profile comparison, no significant Xenon mobility was observed during annealing at 1400 °C and 1600 °C. Further TEM studies highlighted bubble formation, which can trap the Xenon and thus explains its immobility. Studies of irradiation effects at high electronic stopping power ((dE / dx)electronic = 30 keV.nm-1) show diffusion and transport of Xenon if Uranium vacancy migration is activated (above ~ 800 °C). Below this threshold temperature, migration was only observed at the surface of the samples. Those results indicate that, with such an electronic stopping power, heterogeneous resolution of Xenon bubbles occurs and that Xenon migration is related to uranium vacancy mobility. Irradiation with low energy argon ions at the same temperature did not induce any modification and bubbles were observed, which implies that homogeneous Xenon resolution is not a relevant mechanism in our experimental conditions
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Benoît Marchand. Effets de la température et de l'irradiation sur la mobilité du xénon dans UO$_2$ : étude profilométrique et microstructurale. Matériaux. Université Claude Bernard - Lyon I, 2012. Français. ⟨tel-00830100⟩



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