This work presents a detailed study of hyperstoichiometric UO (0 < x < 0.25) oxides and an assessment of the structural evolution taking place as oxidation proceeds. For this purpose, different UO powder samples with controlled degree of non-stoichiometry have been identified by thermogravimetric analysis and characterized by X-ray diffraction (XRD) and Raman spectroscopy. XRD analysis reflects that the commonly assumed Vegard's law is not applicable over the whole hyperstoichiometry range, since a slight increase of the lattice constant is observed for 0.13 < x < 0.20. A quantitative Raman analysis of the UO spectra as a function of the oxidation degree is also shown. A new method to characterize any UO sample (for x < 0.20), based on the shift of the 630 cm band observed in the Raman spectrum, is proposed here for the first time. Moreover, three structure transitions have been detected at x = 0.05, 0.11 and 0.20, giving rise to four distinct regions associated with consecutive structural rearrangements over the hyperstoichiometry range: x < 0.05, 0.05 < x < 0.11, 0.11 < x < 0.20 and 0.20 < x < 0.25.

No abstract

A reliable method to probe and characterise the oxidation of actinide oxides by means of Raman spectroscopy is introduced. The present so‐called Raman laser heating method enables studying the behaviour of various compounds at high temperatures and under a given atmosphere with the unique alteration of a small amount of sample, which is certainly advantageous in terms of safety when handling hazardous or radioactive materials. The approach is based on a dual use of the laser beam, which is at the same time employed as excitation source for the Raman analysis and as heating source, by exploiting the possibility to vary the beam power density reaching the sample surface. A high laser power density can lead to a significant increase of the analyte surface temperature by up to several hundred of degrees. A sufficiently low power density allows us to subsequently acquire the corresponding Raman spectrum at the same point without distorting the measurement. In this work, UO2 powder has been subjected to Raman laser heating in air as a proof of this method's applicability, attaining a sequential acquisition of the characteristic Raman spectra of the different oxides involved in the oxidation from UO2 to U3O8. The temperature at which such sequence of phase transformations started to occur was estimated to be around (560 ± 40) K. The temperature at the sample surface was estimated from the Stokes/anti‐Stokes intensities ratio, using a similar set‐up to that used in the Raman laser heating experiments. These results are particularly appealing in remote analyses, like those required in the study of nuclear fuel and nuclear waste.

No abstract

No abstract