Assessing the oxygen stoichiometry during the sintering of (U, Pu)O2 fuel

International audienceDiffusion phenomena occurring in ceramics such as (U, Pu)O2 during sintering are affected by the oxygen content in the atmosphere. The latter sets the nature and the concentration of point defects which govern diffusion mechanisms in the bulk of the material. The oxygen partial pressure, pO2, of the sintering gas in equilibrium with mixed oxide (MOX) pellets needs to be precisely controlled; otherwise it may induce a large dispersion in the critical parameters for fuel manufacturing (Gauche, 2013; Matzke, 1987). It is crucial to understand the relation between the sintering atmosphere and the fuel throughout the thermal cycle. In this study, the oxygen potential of the sintering gas was monitored by measuring the oxygen partial pressure (pO2) at the outlet of a dilatometer by means of a zirconia probe. Coupling the thermal cycle with an outlet gas pO2 measurement makes it possible to identify different redox phenomena. Variations in the oxygen stoichiometry can be determined during the sintering of (U, Pu)O2, as well as can its final O/M. Our results make it possible to recommend a sintering atmosphere and sintering thermal cycle in order to obtain an O/M ratio that is as close as possible to the target value

Controlling the oxygen potential to improve the densification and the solid solution formation of uranium-plutonium mixed oxides

Diffusion mechanisms occurring during the sintering of oxide ceramics are affected by the oxygen content of the atmosphere, as it imposes the nature and the concentration of structural defects in the material. Thus, the oxygen partial pressure, p(O2), of the sintering gas has to be precisely controlled, otherwise a large dispersion in various parameters, critical for the manufacturing of ceramics such as nuclear oxides fuels, is likely to occur. In the present work, the densification behaviour and the solid solution formation of a mixed uranium-plutonium oxide (MOX) were investigated. The initial mixture, composed of 70% UO2 + 30% PuO2, was studied at p(O2) ranging from 10−15 to 10−4 atm up to 1873 K both with dilatometry and in situ high temperature X-ray diffraction. This study has shown that the initial oxides UO2+x and PuO2−x first densify during heating and then the solid solution formation starts at about 200 K higher. The densification and the formation of the solid solution both occur at a lower temperature when p(O2) increases. Based on this result, it is possible to better define the sintering atmosphere, eventually leading to optimized parameters such as density, oxygen stoichiometry and cations homogenization of nuclear ceramics and of a wide range of industrial ceramic materials