Résumé
The synthesis conditions of mixed uranium and thorium oxides by hydroxide precipitation (wet process) have been investigated and optimized. This mainly consists of controlling the order and the rate of addition of the reagents, which allows better reproducibility during the precipitation stage. Two synthesis methods were compared: Direct Droplet Adding (DDA, gradual addition of the ammonia solution) and Indirect Droplet Adding (IDA, gradual addition of the cation solution), with all other parameters held constant. The solids produced by these two methods were converted to oxide by calcination and then sintered at 1600 degrees C in a Ar-4 %H2 atmosphere to produce dense pellets. Although the pellets produced by the two synthesis methods did not differ at the macroscopic scale, differences in microstructure and chemical durability were observed. DDA induced cationic heterogeneity, which limits the densification of the material and leads to the formation of thorium-rich agglomerates. This resulted in significant open porosity at the end of the sintering stage. Conversely, IDA makes it possible to obtain a very homogeneous cationic distribution within the pellet as well as an increased density after sintering. These microstructural differences also influence the chemical durability of the pellets during dissolution tests. In fact, the presence of cationic heterogeneities in the samples prepared by DDA significantly affects the dissolution kinetics and requires an adaptation of the reprocessing conditions due to the refractory character of such heterogeneities. These results therefore highlight the importance of the synthesis protocol on the microstructure and chemical durability of sintered uranium-thorium mixed oxides.