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Rôle de l'acide nitreux sur la dissolution des oxydes à base d'uranium (IV) : Mécanisme et suivi operando de l'interface solide/solution

Abstract : The dissolution of spent nuclear fuel with higher plutonium contents in nitric acid appears as a key-step towards scaling a recycling process. The autocatalytic role of nitrogen based species, such as nitrous acid, has been suspected for many years. To assess the role of this species during dissolution, a multiparametric study was developed on several models compounds, with chemical compositions Th1-xUxO2 and UxCe1-xO2. Oxalic precipitation route was chosen to prepare compounds showing high cationic and microstructure homogeneity.The multiparametric study of the dissolution performed in various conditions (HNO3, NO3-, HNO2 concentration, chemical composition …) highlighted the dependence of the dissolution rate on HNO3 activity, which was defined as the main oxidant in the not catalyzed reaction. Although showing similar dissolution behavior, thorium incorporation in UO2 matrix led to slower dissolution rates than pure UO2. UxCe1-xO2 solid solutions exhibited a specific behavior, near to that reported for solid solutions doped with trivalent elements. Adding stable and fixed HNO2 concentration in the dissolution reactor induced significant increase of the dissolution rates of Th1-xUxO2 and UxCe1-xO2. Finally, the operando monitoring of the evolving solid/solution interface during dissolution was performed by ESEM. In a perfectly mixed reactor that limited the accumulation of autocatalytic species at the solid/liquid interface, the dissolution remained catalyzed in the porosity network, developed by the dissolution reaction itself.The results obtained in this work defined two dissolution reactions and improved the dissolution mechanism of uranium (IV) based mixed oxides. These results pave the way to study of the role of nitrogen based species upon the dissolution of UxPu1-xO2 solid solutions.
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Thomas Dalger. Rôle de l'acide nitreux sur la dissolution des oxydes à base d'uranium (IV) : Mécanisme et suivi operando de l'interface solide/solution. Autre. Université Montpellier, 2019. Français. ⟨NNT : 2019MONTS122⟩. ⟨tel-02939580⟩

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