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Mécanismes d’incorporation et de migration du molybdène dans le dioxyde d’uranium stoechiométrique et sur-stoechiométrique

Abstract : The fission products (FP) migration in oxide nuclear fuels are of primary importance in order to control the performance of nuclear power plants and their safety during operation. Molybdenum is one the most abundant fission products and has a very complex chemistry. Indeed, its chemical form depends strongly on the atmosphere conditions and the fuel stoichiometry.The aim of this work is to study the migration of molybdenum in uranium dioxide stoichiometric and hyper-stoichiometric at high temperature and under irradiation under several energy conditions in order to vary the damage regimes (ballistic or electronic). The first step was to prepare UO2+x samples by two oxidation methods, with a dry gas or under wet conditions. Then Mo was introduced in polycrystalline UO2 and UO2+x pellets by ion implantation at a maximum concentration of 0.8 at%. Irradiation conditions were chosen in order to understand, from a fundamental point of view, the effect of ballistic or electronic effects. The thermal Mo behaviour was also investigated by high temperature annealing (1600°C). The Mo concentration profiles before and after treatments were measured by SIMS (Secondary Ion Mass Spectrometry). We show that the sole effect of temperature is not efficient to involve a Mo migration in UO2 and UO2+x. This result is in agreement with ab-initio calculation results showing that Mo is well integrated in the uranium dioxide structure where its position is stabilized by the surrounding oxygen atoms. The combined effect of irradiation and temperature leads to the migration of Mo, especially in high electronic stopping regime. This can be relied to the thermal spike mechanism. Raman spectroscopy and DRX were used to probe the defect creation or annealing in the material microstructure, showing that the Mo mobility can be relied to the matrix restructuration
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Lola Sarrasin. Mécanismes d’incorporation et de migration du molybdène dans le dioxyde d’uranium stoechiométrique et sur-stoechiométrique. Matériaux. Université de Lyon, 2017. Français. ⟨NNT : 2017LYSE1284⟩. ⟨tel-01743739⟩

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