Study of crystallization and morphology of ammonium diuranate and uranium oxide

We have developed a lexicon to consistently and objectively describe morphological features observed in scanning electron microscopy (SEM) images. Here we provide the lexicon flowsheet, define the terminology, and detail step-by-step characterization of SEM images collected from a set of actinide oxides. We conclude that this lexicon can be used to characterize texture and surface features, particle structure and size, and grain boundaries in an image of a material. The lexicon should be applicable to characterization of images collected from other techniques for measuring morphology, as well. LA-UR-15-26746

A lexicon for consistent description of material images for nuclear forensics

Study of calcinations of ammonium diuranate at different temperatures

Forensic Signatures of Chemical Process History in Uranium Oxides

The thermal decomposition of ammonium diuranate (ADU) in air is investigated using in situ high-temperature X-ray diffraction (HT-XRD), thermogravimetry and differential thermal analysis. Data have been collected in the temperature range from 30 to 1000 °C, allowing the observation of phase transformation and the assessment of the energy changes involved in the calcination of ADU. The starting material 2UO3·NH3·3H2O undergoes a process involving several endothermic and exothermic reactions. In situ HT-XRD shows that amorphous UO3 is obtained after achieving complete dehydration at 300 °C, and denitration at about 450 °C. After cooling from heat treatment at 600 °C, a crystalline UO3 phase appears, as displayed by ex situ XRD. The self-reduction of UO3 into orthorhombic U3O8 takes place at about 600 °C, but a long heat treatment or higher temperature is required to stabilise the structure of U3O8 at room temperature. U3O8 remains stable in air up to 850 °C. Above this temperature, oxygen losses lead to the formation of U3O8−x, as demonstrated by subtle changes in the diffraction pattern and by a mass loss recorded by TGA.

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J.L. Woolfrey

Papers

The preparation of UO2 powder: Effect of ammonium uranate properties