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

Abstract

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 O 2 ( P O 2 ) ${{\rm{O}}_2}({{\rm{P}}_{{{\rm{O}}_{\rm{2}}}}})$ 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.