The thermal decomposition of studtite: analysis of the amorphous phase

Abstract

Studtite is known to exist at the back-end of the nuclear fuel cycle as an intermediate phase formed in the reprocessing of spent nuclear fuel. In the thermal decomposition of studtite, an amorphous phase is obtained at calcination temperatures between 200 and 500 °C. This amorphous compound, referred to elsewhere in the literature as U2O7, has been characterised by analytical spectroscopic methods. The local structure of the amorphous compound has been found to contain uranyl bonding by X-ray absorption near edge (XANES), Fourier transform infrared and Raman spectroscopy. Changes in bond distances in the uranyl group are discussed with respect to studtite calcination temperature. The reaction of the amorphous compound with water to form metaschoepite is also discussed and compared with the structure of schoepite and metaschoepite by X-ray diffraction. A novel schematic reaction mechanism for the thermal decomposition of studtite is proposed.

Structural incorporation of lanthanides (La, Eu, and Lu) into U3O8 as a function of the ionic radius

“…The XRD pattern shows the formation of a phase mixture of α-UO 3 and β-UO 3 after calcination at 520 °C and of a pure orthorhombic α-U 3 O 8 phase with the space group C2mm after calcination at 700 °C, which is stable at room temperature [15]. Both the formation of the phase mixture of α-UO 3 and β-UO 3 and the formation of orthorhombic U 3 O 8 were to be expected for the undoped material in this temperature range and correspond to the literature [13,16,17].…”

“…However, this swelling cannot be detected for the Lu-doped U 3 O 8 . This deviation indicates that Lu, due to the smaller ionic radius compared to La and Eu, occupies U-sites that apparently leads to a volume contraction, most probably since the ionic radius of Lu is roughly in the range of uranium (r(U 6+ ) = 0.87 Å; r(U 5+ ) = 0.9 Å) [16].…”

The effect of oxygen concentration on the speciation of laser ablated uranium

“…These spectral elements are consistent with am-UO x from literature (Fig. 6 , blue and red traces) 41 , 42 . As with the in-situ results, a high power HeNe produces ex-situ spectra similar to α-U 3 O 8 (Fig.…”

“…2 includes a spectrum from Hoekstra and Siegel (red trace), which is attributed to amorphous UO 3 15 . This spectrum shows symmetric and asymmetric 16 , 41 , 47 , 48 uranyl stretching features at 765 and 910 cm −1 , respectively. This is in good agreement with the in-situ frequencies measured in this study at 790 and 910 cm −1 (Fig.…”

“…Spectra are offset for clarity.

Figure 6 Ex-situ Raman spectra of collected particulates using a low HeNe power (black) plotted along with am-UO x spectra from Thompson et al 41 (blue) and Spano et al 42 (red). Comparisons give support for the am-UO x assignment of particulates.

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