Investigation of ammonium diuranate calcination with high-temperature X-ray diffraction

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 2UO 3 ·NH 3 ·3H 2 O undergoes a process involving several endothermic and exothermic reactions. In situ HT-XRD shows that amorphous UO 3 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 UO 3 phase appears, as displayed by ex situ XRD. The self-reduction of UO 3 into orthorhombic U 3 O 8 takes place at about 600 °C, but a long heat treatment or higher temperature is required to stabilise the structure of U 3 O 8 at room temperature. U 3 O 8 remains stable in air up to 850 °C. Above this temperature, oxygen losses lead to the formation of U 3 O 8− x , as demonstrated by subtle changes in the diffraction pattern and by a mass loss recorded by TGA.