Structural characterization of M(IV)1-xLn(III)xO2-x/2 (M = Ce, Th) mixed-oxides prepared from oxalate precursors. Multi-parametric study of dissolution and microstructural evolution

In the framework of Gen IV program development, several physico-chemical properties of some foreseen fuels, including the chemical durability, have to be evaluated. In this aim, a study was undertaken on M(IV)1-xLn(III)xO2 (M=Ce,Th) model compounds prepared from oxalate precursors. The fluorite-type structure of CeO2 and ThO2 remains stable up to x ≅ 0.4, the substitution of M(IV) by Ln(III) occurring simultaneously to the formation of oxygen vacancies. For higher x values, a cubic superstructure is formed as a result of oxygen vacancies ordering. The normalized dissolution rates of such solids were found to be strongly enhanced by the Ln(III) fraction. On the contrary, the nature of the M(IV) and Ln(III) elements did not modify significantly the normalized dissolution rates. The effect of temperature and acid concentration suggested the existence of surface-controlling dissolution reactions. Simultaneously, the microstructural evolution of both powdered and sintered samples revealed some important changes in the reactive surface during dissolution tests. ESEM images allowed observing the existence of preferential dissolution sites located at grains boundaries and around crystalline defects, leading to the formation of corrosion pits. In addition, the formation of gelatinous phases, acting as diffusion barriers (thus slowing down the dissolution process) was also evidenced. (author)