Kinetic studies of isothermal decomposition of (NH₄)4UO₂(CO₃)₃ to uranium oxide

Abstract

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.

Uranyl ammonium carbonate precipitation and conversion into triuranium octaoxide

“…By analyzing this mass variation, the material balance calculations unequivocally indicated that the AUC compound was (NH 4 ) 4 UO 2 (CO 3 ) 3 . Consequently, our study successfully identified the conditions conducive to the formation of AUC precipitates, which are very similar to the findings reported in the literature [ [37] , [38] , [39] , [40] , [41] , [42] ]. Therefore, to achieve the precipitation of free AUF, a C/U molar ratio greater than six is required.…”

“…8 . The typical dTG and TG curves of the AUC decomposition reaction obtained under these conditions are similar to those described in the literature [ [37] , [38] , [39] , [40] , [41] , [42] ]. The dTG and TG curves obtained during AUC decomposition ( Fig.…”

“…The decomposition reaction involves the formation of intermediate compounds, specifically, UO 3 . When AUC is treated in air, the final product is U 3 O 8 , as reported previously [ [37] , [38] , [39] , [40] , [41] , [42] ].…”

“…The decomposition reaction of AUC in air is summarized as follows (Eq. (19) ) [ [37] , [38] , [39] , [40] , [41] , [42] ]: (NH 4 ) 4 UO 2 (CO 3 ) 3 = UO 3 + 4NH 3 + 3CO 2 + 2H 2 O …”

“…Numerous previous studies [ [37] , [38] , [39] , [40] , [41] , [42] ] explored the decomposition of AUC in different environments, such as O 2 , Ar, N 2 , and 90% Ar–10% H 2 mixtures. Our TG–dTG curves obtained at a heating rate of 10 °Cmin -1 in oxygen and inert gases (argon and nitrogen) were similar to those reported in the literature.…”