Molybdenum behaviour during U-Al RRSF dissolution

In the frame of Research Reactors Spent Fuel (RRSF) treatment by hydrometallurgy, the dissolution in nitric acid of irradiated U-Al, is a key issue because of the low solubility of molybdenum fission product in presence of high concentration of aluminium. In this study, the values of molybdenum solubility have been accurately measured in different operating conditions. Studies have carried out with non-active materials. To be more representative of metallic fuel, uranium-molybdenum alloy powder and molybdenum metal have been dissolved in aluminium nitrate solutions at high temperature. In order to be sure that molybdenum solubility has been reached, experiments have been carried out with an excess of molybdenum metal. In spite of this excess addition, metallic elements have been dissolved completely after stirring time of thirty minutes with a magnetic stirrer. Shortly after this total dissolution, a slow molybdenum precipitation has been observed for almost 15 hours. An experimental protocol has been developed to properly wash precipitates in order to determine their elemental composition. No uranium has been detected in the washed precipitate by ICP-AES measurements performed after redissolution of solids in aluminium free nitric acid solutions. Further analyses by Scanning Electron Microscope have shown a needle-like morphology. Energy-dispersive X-ray spectroscopy analyses on several selected areas have confirmed the absence of uranium in precipitates. EDX semi-quantification has been carried out on ionically polished particles. They are composed of 75% oxygen and 25% molybdenum, suggesting MoO3 compounds. X-ray diffraction spectra of powders have confirmed this result all samples matched the crystallographic form of MoO3.