Impact of the atmosphere on the sintering capability and chemical durability of Nd-doped UO2+x mixed oxides

In this study, the influence of the working atmosphere on the sinterability and chemical durability of Nd-doped UO2 mixed oxides was investigated. To this end, the starting powder was first prepared by a hydroxide co-precipitation route, resulting in a nano-sized granulometry combined with a high specific surface area. The powders were then converted to oxides by heating and sintered in pellet form at 1600°C under an argon or reducing (Ar-4 %H2) atmosphere. The use of argon or reducing atmosphere resulted in very different densification pathways and final microstructures. The reducing sintering atmosphere hindered the uranium (IV) oxidation that could occur at high temperature, leading to the formation of U3O8, as was the case when working under argon atmosphere. Regarding the microstructure of the sintered pellets, the use of an argon sintering atmosphere resulted in an average grain size ten times larger than that of a reducing sintering atmosphere, while macroscopic properties such as relative density, porosity and homogeneity of cation distribution at the pellet scale remained the same. Nevertheless, a slight local enrichment of Nd at the grain boundaries was observed for the pellet sintered under Ar-4 %H2. In a second step, the study of the chemical durability of these sintered samples showed a significant influence of the sintering atmosphere on the dissolution kinetics and mechanism. These differences could be related to the microstructural properties of the pellets, i.e. the average grain size and the occurrence of grain boundaries. The cation distribution in the pellets could also influence their chemical durability, such as local Nd enrichment, the formation of defects in the oxygen sublattice and the presence of a different fraction of U(V) depending on the sintering atmosphere, as shown by HERFD-XANES measurements. The use of reducing or argon sintering atmospheres could even direct the charge compensation mechanisms that occur into the solid, thereby simultaneously affecting the sinterability and chemical durability of the samples.