Abstract
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.
Highlights
Ammonium diuranate (NH4)2U2O7 (ADU), once used to create coloured glazes in ceramics, is the most prominent chemical compound among the uranium ore concentrates, often referred to as ‘‘yellow cake’’: ammonium diuranate (ADU) plays an important role in the fabrication of uranium oxide fuel
The thermal decomposition of ammonium diuranate (ADU) in air is investigated using in situ hightemperature X-ray diffraction (HT-XRD), thermogravimetry and differential thermal analysis
ADU is of great interest for nuclear forensics, a relatively young discipline that aims at providing hints on the history and intended use of nuclear materials of unknown origin, on the basis of certain measurable parameters that, as fingerprints, are characteristics of the material, of the place where it was produced and of the route followed for its transformation [1,2,3,4,5,6]
Summary
Ammonium diuranate (NH4)2U2O7 (ADU), once used to create coloured glazes in ceramics, is the most prominent chemical compound among the uranium ore concentrates, often referred to as ‘‘yellow cake’’: ADU plays an important role in the fabrication of uranium oxide fuel For this reason, ADU is of great interest for nuclear forensics, a relatively young discipline that aims at providing hints on the history and intended use of nuclear materials of unknown origin, on the basis of certain measurable parameters that, as fingerprints, are characteristics of the material, of the place where it was produced and of the route followed for its transformation [1,2,3,4,5,6].
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