Impact of Controlled Storage Conditions on the Hydrolysis and Surface Morphology of Amorphous-UO3

Abstract

The hydration and morphological effects of amorphous (A)-UO₃ following storage under varying temperature and relative humidity have been investigated. This study provides valuable insight into U-oxide speciation following aging, the U-oxide quantitative morphological data set, and, overall, the characterization of nuclear material provenance. A-UO₃ was synthesized via the washed uranyl peroxide synthetic route and aged based on a 3-factor circumscribed central composite design of experiment. Target aging times include 2.57, 7.00, 14.0, 21.0, and 25.4 days, temperatures of 5.51, 15.0, 30.0, 45.0, and 54.5 °C, and relative humidities of 14.2, 30.0, 55.0, 80.0, and 95.8% were examined. Following aging, crystallographic changes were quantified via powder X-ray diffraction and an internal standard Rietveld refinement method was used to confirm the hydration of A-UO₃ to crystalline schoepite phases. The particle morphology from scanning electron microscopy images was quantified using both the Morphological Analysis of MAterials software and machine learning. Results from the machine learning were processed via agglomerative hierarchical clustering analysis to distinguish trends in morphological attributes from the aging study. Significantly hydrated samples were found to have a much larger, plate-like morphology in comparison to the unaged controls. Predictive modeling via a response surface methodology determined that while aging time, temperature, and relative humidity all have a quantifiable effect on A-UO₃ crystallographic and morphological changes, relative humidity has the most significant impact.

Figure 1

Comparison of p-XRD diffraction spectra of samples aged at varying times, temperatures, and relative humidities. (A) Unaged A-UO₃; (B,C) center points aged at 14.0 d, 30.0 °C, 55.0% RH; (D) factorial point aged at 21.0 d, 45.0 °C, 80.0% RH; (E) factorial point aged at 21.0 d, 45.0 °C, 30.0% RH; (F) axial point aged at 14.0 d, 54.5 °C, 55.0% RH; (G) axial point aged at 14.0 d, 30.0 °C, 14.2% RH; (H) Cr₂O₃ reference pattern. ICSD #90158; and (I) dehydrated schoepite, α-UO₂(OH)₂, reference pattern. ICSD #9116 (J) metaschoepite, UO₃·2H₂O, reference pattern. ICSD #76895.

Figure 2

Graphical comparison of Rietveld refinement results using the internal standard method. (B,C) Center points aged at 14.0 d, 30.0 °C, 55.0% RH; (D) factorial point aged at 21.0 d, 45.0 °C, 80.0% RH; (E) factorial point aged at 21.0 d, 45.0 °C, 30.0% RH; (F) axial point aged at 14.0 d, 54.5 °C, 55.0% RH; (G) axial point aged at 14.0 d, 30.0 °C, 14.2% RH. Samples with high humidity represent hydration to metaschoepite, UO₃·2H₂O, while samples with high temperatures and lower humidity represent hydration to dehydrated schoepite, UO₂(OH)₂.

Figure 3

Comparison of SEM imagery of samples aged at varying times, temperatures, and relative humidities. (A) Unaged A-UO₃; (B,C) Center point aged at 14.0 d, 30.0 °C, 55.0% RH; (D) Factorial point aged at 21.0 d, 45.0 °C, 80.0% RH; (E) factorial point aged at 21.0 d, 45.0 °C, 30.0% RH; (F) axial point aged at 14.0 d, 54.5 °C, 55.0% RH; and (G) axial point aged at 14.0 d, 30.0 °C, 14.2% RH.

Figure 4

HCA heat map of Cluster 0 fractions for each aging temperature and RH combination. As the RH increases, the fraction of particles in Cluster 0 increases. Supporting Information Figures S-85 and S-86 show a similar trend in which longer aging times and highly RH corresponds to more particles in Cluster 0. These results prove that morphological changes are more dependent on aging time and RH than temperature, corroborating the earlier XRD results which found similar conclusions.

Figure 5

Response surface plot illustrating the effect of aging time and RH on the increase of metaschoepite from the hydration of A-UO₃. The response was held at the center point aging conditions of 14.0 d, 30.0 °C, and 55.0% RH.

Figure 6

Response surface plot illustrating the effect of aging time and temperature on the increase of metaschoepite from the hydration of A-UO₃. The response was held at the center point aging conditions of 14.0 d, 30.0 °C, and 55.0% RH.

Figure 7

Response surface plot illustrating the effect of RH and temperature on the increase of metaschoepite from the hydration of A-UO₃. The response was held at center point aging conditions of 14.0 d, 30.0 °C, and 55.0%.