Many commercial processes exist for converting uranium from ore to the desired uranium compound for use in nuclear power or nuclear weapons. Accurately determining the processing history of the uranium ore concentrates (UOCs) and their calcination products, can greatly aid a nuclear forensics investigation of unknown or interdicted nuclear materials. In this study, two novel forensic signatures, based on nuclear materials synthesis, were pursued. Thermogravimetric analysis – mass spectrometry (TGA-MS) was utilized for its ability to discern UOCs based on mass changes and evolved gas species; while scanning electron microscopy (SEM), in conjunction with particle segmentation, was performed to identify microfeatures present in the calcination and reduction products (i.e. UO3, U3O8, and UO2) that are unique to the starting UOC. In total, five UOCs from common commercial processing routes including: ammonium diuranate (ADU), uranyl peroxide (UO4), sodium diuranate (SDU), uranyl hydroxide (UH), and ammonium uranyl carbonate (AUC), were synthesized from uranyl nitrate solutions. Samples of these materials were calcined in air at 400 °C and 800 °C. The 800 °C calcination product was subsequently reduced with hydrogen gas at 510 °C. The starting UOCs were investigated using TGA-MS; while SEM quantitative morphological analysis was used to identify signatures in the calcination products. Powder X-ray diffractometry (p-XRD) was used to identify the composition of each UOC and the subsequent calcination products. TGA-MS of the starting UOCs indicate temperature-dependent dehydration, volatilization, and reduction events that were unique to each material; thus making this a quantifiable signature of the initial material in the processing history. In addition, p-XRD, in conjunction with quantitative morphological analysis, was capable of discriminating calcination products of each processing history at the 99 % confidence level. Quantifying these nuclear material properties, enables nuclear forensics scientists to better identify the origin of unknown or interdicted nuclear materials.
Morphological changes in UO based on calcination temperature have been quantified enabling a morphological feature to serve as a signature of processing history in nuclear forensics. Five separate calcination temperatures were used to synthesize α-UO, and each sample was characterized using powder X-ray diffraction (p-XRD) and scanning electron microscopy (SEM). The p-XRD spectra were used to evaluate the purity of the synthesized U-oxide; the morphological analysis for materials (MAMA) software was utilized to quantitatively characterize the particle shape and size as indicated by the SEM images. Analysis comparing the particle attributes, such as particle area at each of the temperatures, was completed using the Kolmogorov-Smirnov two sample test (K-S test). These results illustrate a distinct statistical difference between each calcination temperature. To provide a framework for forensic analysis of an unknown sample, the sample distributions at each temperature were compared to randomly selected distributions (100, 250, 500, and 750 particles) from each synthesized temperature to determine if they were statistically different. It was found that 750 particles were required to differentiate between all of the synthesized temperatures with a confidence interval of 99.0%. Results from this study provide the first quantitative morphological study of U-oxides, and reveals the potential strength of morphological particle analysis in nuclear forensics by providing a framework for a more rapid characterization of interdicted uranium oxide samples.
The analytical techniques typically utilized in a nuclear forensic investigation often provide limited information regarding the process history and production conditions of interdicted nuclear material. In this study, scanning electron microscopy (SEM) analysis of the surface morphology of amorphous-UO samples calcined at 250, 300, 350, 400, and 450°C from uranyl peroxide was performed to determine if the morphology was indicative of the synthesis route and thermal history for the samples. Thermogravimetic analysis-mass spectrometry (TGA-MS) and differential scanning calorimetry (DSC) were used to correlate transitions in the calcined material to morphological transformations. The high-resolution SEM images were processed using the Morphological Analysis for Material Attribution (MAMA) software. Morphological attributes, particle area and circularity, indicated significant trends as a result of calcination temperature. The quantitative morphological analysis was able to track the process of particle fragmentation and subsequent sintering as calcination temperature was increased. At the 90% confidence interval, with 1000 segmented particles, the use of Kolmogorov-Smirnov statistical comparisons allowed discernment between all calcination temperatures for the uranyl peroxide route.
This study aims to determine forensic signatures for processing history of UO2 based on modifications in intermediate materials within the uranyl peroxide route. Uranyl peroxide was calcined to multiple intermediate U-oxides including Am-UO3, α-UO3, and α-U3O8 during the production of UO2. The intermediate U-oxides were then reduced to α-UO2 via hydrogen reduction under identical conditions. Powder X-ray diffractometry (p-XRD) and X-ray photoelectron spectroscopy (XPS) were used to analyze powders of the intermediate U-oxides and resulting UO2 to evaluate the phase and purity of the freshly synthesized materials. All U-oxides were also analyzed via scanning electron microscopy (SEM) to determine the morphology of the freshly prepared powders. The microscopy images were subsequently analyzed using the Morphological Analysis for Materials (MAMA) version 2.1 software to quantitatively compare differences in the morphology of UO2 from each intermediate U-oxide. In addition, the microscopy images were analyzed using a machine learning model which was trained based on a VGG 16 architecture. Results show no differences in the XRD or XPS spectra of the UO2 produced from each intermediate. However, results from both the segmentation and machine learning proved that the morphology was quantifiably different. In addition, the morphology of UO2 was very similar, if not identical, to the intermediate material from which it was prepared, thus making quantitative morphological analysis a reliable forensic signature of processing history.
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