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 A degrees 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)center dot NH3 center dot 3H(2)O 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 A degrees C, and denitration at about 450 A degrees C. After cooling from heat treatment at 600 A degrees 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 A degrees 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 A degrees 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.