The ammonium uranyl carbonate, (NH4)4UO2(CO3)3, is an important material used in UO2 and U3O8 ceramics production for the nuclear fuel fabrication. Thermal study and kinetic analysis of ammonium uranyl carbonate conversion under isothermal conditions has been studied in air atmosphere to obtain the tri-uranium octoxide (U3O8), using muffle furnace equipment, UV–visible spectrophotometer, gas adsorption, Hg porosimetry, laser granulometry, and optic spectroscopy. The textural properties (specific surface area, morphology, pore size, grain size, inter-particular porosity, and intra-particular porosity) and characteristics (uranium content and stoichiometry) of the prepared samples were estimated from the physical–chemical characterization. The kinetic parameters were estimated by a fitting of the experimental data. The activation energy Ea, frequency factor A, and reaction rate constants k were calculated from the conventional and iso-conversion kinetic models and were within the range of literature values. The activation energy average values are 36.69 and 30.36 kJ mol−1 by conventional and iso-conversion models, respectively.

The work describes the wet chemical route process to recover and recycle a defective un-irradiated uranium dioxide sintered pellets. The optimal conditions of the un-irradiated UO 2 sintered pellets dissolution in Nitric acid solutions has been examined. The effects of acid concentration, contact time and temperature on dissolution action have been studied. The samples were exposed to leaching over a range of nitric acid concentrations 2-8 mol/L and temperatures from 20 to 70 °C. The stoichiometry of the dissolution reaction depends on the acidity of the solution. The shrinking core model (product diffusion control model and chemical reaction control model) was used to correlate the experimental results. Kinetics study shows that the chemical reaction model controls the dissolution of UO 2 sintered pellets in nitric acid solution. The average value of apparent activation energy estimated from the temperature effect of the chemical reaction was found to be about 101.40 kJ/ mol. The optimal conditions to dissolve the UO 2 sintered pellets in nitric acid solution have been proposed.