Thermoanalytical study on the oxidation of uranium dioxides derived from uranyl nitrate, uranyl acetate and ammonium diuranate

Abstract

The oxidation of UO 2 was investigated by TG, DSC and X-ray diffraction . UO 2 samples were prepared by the reduction of UO 3 at P H 2 + P N 2 = 100 + 50 mm Hg and 5°C min −1 up to 800°C. In order to obtain six UO 2 samples with different preparative histories, UNH, UAH and ADU were used as starting materials and their thermal decomposition was carried out at 450–625°C for 0–9 h at an air flow rate of 100 ml min −1. α-UO 3, γ-UO 3, UO 3 - 2 H 2O, and their mixtures were obtained. The reduction of UO 3 gave β-UO 2+ x with different x values from 0.030 to 0.055. The oxidation carried out at P O 2 = 150 mm Hg was found to consist of oxygen uptake at room temperature. UO 2 - U 3O 7 (Step I) and U 3O 7 → U 3O 8 (Step II). TG and DSC curves of the oxidation showed two plateaus and two exothermic peaks corresponding to Steps I and II. In the case of two of the samples, the DSC peak of Step II split into two substeps, which were assumed to be due to the different reactivities of U 3O- formed from α-CO 3 and that from other types of UO 3. The increase in O/U ratio due to the oxygen uptake at room temperature changed from 0.010 to 0.042 except for a sample prepared from ADU which showed an extraordinarily large value of 0.445. TG curves showed an increase in O/U from room temperature to near 250°C for Step I and the plateau at 250–350°C where O/U was about 2.42, and showed a sharp increase in O/U above 350°C for Step II and the plateau above 100°C where O/U was 2.72–2.75. It is thought that the prepared UO 2 had a defective structure with a large interstitial volume to accommodate the excess oxygen.