A response surface model of morphological changes in UO₂ and U₃O₈ following high temperature aging

Abstract

The morphological changes that take place during the processing and storage of uranium oxides can provide valuable information on the processing history and storage conditions of an interdicted sample. In this study microstructural changes in two uranium oxides (UO2 and U3O8) due to changes in the aging conditions at elevated temperatures were quantified and modeled using a response surface methodology approach. This allowed the morphological changes to be used as a signature for the aging conditions for nuclear forensic analysis. A Box-Behnken design of experiment was developed using the independent variables: temperature from 100 to 400 °C, aging times from 2 to 48 h, and partial pressure of ${{\rm{O}}_2}({{\rm{P}}_{{{\rm{O}}_{\rm{2}}}}})$ between ~0.0 kPa and 21.3 kPa. The design of experiment consisted of 54 samples per uranium oxide. Each aged sample was characterized using scanning electron microscopy (SEM) for image analysis. Utilizing the Morphological Analysis for Materials (MAMA) software package, particle size and shape were quantified using the acquired SEM images. Analysis of the particle attributes was completed using the Kolmogorov–Smirnov two sample test (K–S test) to determine if the particle size and shape distributions were statistically distinct. This data was then used to create response surfaces of the quantitative morphological changes based on the developed design of experiment. The U3O8 samples showed no statistically quantifiable differences due to the aging conditions. However, the UO2 samples had distinct morphological changes due to the experimental aging conditions. Specifically, the temperature factor had an increasing effect on the particle area, and a decreasing effect on particle circularity.

Progress in Uranium Chemistry: Driving Advances in Front-End Nuclear Fuel Cycle Forensics

“…Building significantly on this potential, several recent studies investigated the morphology of various UOCs using a software program, morphological analysis for materials ( MAMA ), to quantitatively characterize the particles from SEM images. − ,− Olsen et al studied the differences in the morphological features of U 3 O 8 materials produced by calcining α-UO 3 at various temperatures, 600, 650, 700, 750, and 800 °C . While they did observe some qualitative variations among the U 3 O 8 samples produced at the various calcining temperatures, processing the SEM images through the MAMA particle segmentation software enabled a quantitative determination of the morphological statistics for the material.…”

“…In particular, recent research has examined the morphology of various uranium oxides, with a focus on discerning process history or process conditions for a particular sample of material. [20][21][22][23][24][25][26][27][28][29][30][31][32]46 One outcome of these efforts was a lexicon to standardize descriptions of material images for nuclear forensics, indicating a likely increasing role for morphology within nuclear forensics. 33 Recent research has also investigated elemental and chemical impurities present in process samples of uranium compounds, also with a focus of discerning process history as well as the origin of the uranium.…”

“…Over the past decade, a significant research effort has focused on several of these key uranium compounds with the aim of advancing the science of these compounds and the field of nuclear forensics. In particular, recent research has examined the morphology of various uranium oxides, with a focus on discerning process history or process conditions for a particular sample of material. − , One outcome of these efforts was a lexicon to standardize descriptions of material images for nuclear forensics, indicating a likely increasing role for morphology within nuclear forensics . Recent research has also investigated elemental and chemical impurities present in process samples of uranium compounds, also with a focus of discerning process history as well as the origin of the uranium. − ,,,− For example, the concentrations of rare-earth elements (REEs) have been found to be potentially useful for identifying the origin of UOCs .…”