Investigation of Uranium Polymorphs

Abstract

Executive SummaryThe UO 3 -water system is complex and has not been fully characterized, even though these species are common throughout the nuclear fuel cycle. As an example, most production processes for UO 3 result in a mixture of up to six or more different polymorphic phases, and small differences in these conditions will affect phase genesis that ultimately result in measureable changes to the end product. As a result, this polymorphic feature of the UO 3 -water system may be useful as a means for determining process history. This research effort attempts to better characterize the UO 3 -water system with a variety of optical techniques for the purpose of developing some predictive capability for estimating process history in polymorphic phases of unknown origin. Three commercially relevant production methods for the production of UO 3 were explored. Previously unreported low temperature routes to β-and γ-UO 3 were discovered. Raman and fluorescence spectroscopic libraries were established for pure and mixed polymorphic forms of UO 3 in addition to the common hydrolysis products of UO 3 . An advantage of the sensitivity of optical fluorescence microscopy over x-ray diffraction has been demonstrated. Preliminary aging studies of the α and γ forms of UO 3 have been conducted. In addition, development of a 3-D phase field model used to predict phase genesis of the system was initiated. Thermodynamic and structural constants that will feed the model have been gathered from the literature for most of the UO 3 polymorphic phases.v

Density Functional Theory Study of the Thermodynamic and Raman Vibrational Properties of γ-UO3 Polymorph

“…The Raman spectrum of γ-UO3 was observed by Armstrong et al 51 and Sweet et al [1][2] . A strong band was observed at 766 cm -1 , a medium strength band at 335 cm -1 , and weaker bands appear at 690, 483, 235, 103 and 51 cm -1 .…”

“…Fig. 6 shows the experimental Raman spectrum of γ-UO3 obtained by Sweet et al, [1][2] together with the calculated one. A summary of the experimental and calculated spectra is shown in Table 6, as well as the assignments of the vibrational modes derived from the calculations.…”

“…5 Since production processes of uranium trioxide result in mixtures of the different polymorphic phases and small changes in the conditions of these processes affect the phase genesis giving rise to differences in the final products, the study of UO3 system components is very useful in order to understand the process history. Sweet et al [1][2] studied this system through the combined application of traditional (X-Ray diffraction) and a variety of optical techniques. These authors developed Raman and fluorescence spectroscopic libraries for pure and mixed polymorphic forms of UO3 in addition to the common hydrolysis products of UO3.…”

Short communication on further elucidating the structure of amorphous U2O7 by extended X-ray absorption spectroscopy and DFT simulations

“…This procedure returned a density closer to experimental observation, 7.40 gcm -3 , and a greater number of uranyl bonds in the final relaxed structure (Figure 2b). 10 There appeared to be, therefore, a negative correlation between the fully relaxed supercell density and propensity for uranyl bonding in this simulation study. This can be explained by the nature of the uranyl bonds observed in the simulated structures: the structure can be described as uranyl units with a variable degree of interconnection through shared equatorial oxygens, similar to the description of U2O7 by Odoh et al The uranyl bonds are observed where the oxygen is non-bridging to another monomer unit -a more prevalent observation in the lower density, lower network connectivity structure.…”

“…Initial densities of 6.80 g/cm 3 and 1.60 g/cm 3 were trialled; while the former is the experimental density for this compound, it has previously been reported that lower densities, such as the latter, are required when modelling amorphous structures. [10][11][12] 20 different randomised structures were produced to ensure that the amorphous nature of the system is adequately captured. 12,13 Trials with 72-atom supercells were also carried out to understand the impact of system size on the final relaxed structure of the compound.…”