An original precipitation route toward the preparation and the sintering of highly reactive uranium cerium dioxide powders

Abstract

The preparation of dense U1−xCexO2 mixed dioxides pellets was achieved from the initial precipitation of highly reactive precursors. In a first step, a wet chemistry route, based on the mixture of U4+ and Ce4+ in acidic solution with large excess of NH4OH, was set up to reach the precipitation of the cations. The solid phase was then dried under vacuum to avoid aggregation phenomena. Further characterization of the powders by XRD, EDS and TEM revealed the formation of hydrated U1−xCexO2⋅nH2O that probably resulted from the aging of hydroxide compounds. Also, microscopy investigations evidenced the nanosized character of the powder which was associated to high values of specific surface area, typically in the 100–150m2g−1 range.The behavior of U1−xCexO2⋅nH2O versus temperature was investigated in a second part. If the increase of the heat temperature allowed one to observe an improvement of the crystallization state linked with the growth of crystallites, it was also accompanied by a strong decrease of the powders reactivity. On this basis, sintering tests were conducted in reducing atmosphere on the compounds as prepared. Dilatometry experiments indicated a low densification temperature compared to other ways of preparation reported in the literature. Also, the pellets prepared after firing at different temperatures (1350–1550°C) showed that a wide range of microstructures was achievable. Particularly, bulk materials with densities of 90–95% of the calculated value could be prepared with average grain size ranging from around 100nm to more than 5μm. This simple process of elaboration of dense materials from highly reactive hydrated oxide precursor thus appears as a very interesting way to prepare actinide oxides materials.