Spatially Resolved Raman Spectroscopic Investigation of Uranyl Fluoride: A Case Study in the Importance of Instrument Optimization.

Raman spectroscopy is an emerging technique for rapid and nondestructive analysis of nuclear materials for forensic and nonproliferation applications as it is a powerful tool for distinguishing multiple chemical forms of materials with similar stoichiometries. Recent developments in spectroscopic software have enabled rapid data collection with high-speed Raman spectroscopic mapping capabilities. However, some uranium-rich materials are susceptible to degradation in humid air and/or laser-induced phase transformations. To mitigate environmental or measurement-related sample degradation of potential samples of interest, we have taken a systematic approach to define optimized data collection parameters for high-throughput measurements of uranyl fluoride (UO2F2), which is an important intermediate material in the nuclear fuel cycle. First, we systematically describe the influence of optical magnification (5× to 100×), laser power, and exposure time on obtained signal for identical particles of UO2F2 and find that at low laser power and exposure times, comparable signal is obtained regardless of optical magnification. Second, we ensure sample integrity during data collection, and third, collect spectroscopic maps that employ optimized parameters to reduce the time required to obtain spatially resolved spectroscopic information. Reductions of 90% and 99% in measurement times are discussed as they relate to differences in resolving spectroscopic features of particles in identical mapping areas. During this work, we found that additional data processing options were needed and thus developed a customized Python script for importing, processing, analyzing, and visualizing Raman spectroscopic map data.

Uranium Single Particle Analysis for Simultaneous Fluorine and Uranium Isotopic Determinations via Laser-Induced Breakdown Spectroscopy/Laser Ablation-Multicollector-Inductively Coupled Plasma-Mass Spectrometry.

Uranyl fluoride (UO2F2) particles (<20 µm) were subjected to first-of-its-kind analysis via simultaneous laser-induced breakdown spectroscopy (LIBS) and laser ablation multi-collector inductively coupled plasma-mass spectrometry (LA-MC-ICP-MS). Briefly, a nanosecond pulsed high-energy laser was focused onto the sample (particle) surface. In a single laser pulse, the UO2F2 particle was excited/ionized within the microplasma volume, and the emission of light was collected via fiber optics such that emission spectroscopy could be employed for the detection of uranium (U) and fluorine (F). The ablated particle was simultaneously transported into the MC-ICP-MS for high precision isotopic (i.e., 234U, 235U, and 238U) analysis. This method, LIBS/LA-MC-ICP-MS was optimized and employed to rapidly measure 80+ UO2F2 particles, which were subjected to different calcination processes, which results in varying degrees of F loss from the individual particles. In measuring the particles, the average F/U ratios for the populations treated at 100 and 500 °C were 2.78 ± 1.28 and 1.01 ± 0.50, respectively, confirming loss of F through the calcination process. The average 235U/238U on the particle populations for the 100 and 500 °C were 0.007262 (22) and 0.007231 (23), which was determined to be <0.2% from the expected value. The 234U/238U ratios on the same particles were 0.000053 (11) and 0.000050 (10) for the 100 and 500 °C, respectively, <10% from the expected value. Notably, each population was analyzed in under 5 min, demonstrating the truly rapid analysis technique presented here.

Elucidating the Composition and Structure of Uranium Oxide Powders Produced via NO2 Voloxidation.

Voloxidation is a potential alternative reprocessing scheme for spent nuclear fuel that uses gas-solid reactions to minimize aqueous wastes and to separate volatile fission products from the desired actinide phase. The process uses NO2(g) as an oxidant for uranium dioxide (UO2) fuel, ideally producing soluble uranium powders which can then be processed for full recycle. To continue development of the process flowsheet for voloxidation, ongoing examination of the process chemistry and associated process materials is required: discrepancies in the proposed chemical reactions that occur when spent nuclear fuel is exposed to NO2(g) atmospheres must be addressed. The objective of this work is to analyze the intermediate solid phases produced during voloxidation to support verification of the proposed NO2(g) voloxidation reaction mechanisms. This objective was achieved through using (1) powder X-ray diffraction and Raman spectroscopy to identify bulk uranium phases and (2) scanning electron microscopy to describe the morphology and microstructure of the powders at each reaction stage. The initial oxidation of UO2 under NO2(g) reactions produced ε-UO3. Further exposure to NO2(g) did not nitrate the solid to produce uranyl nitrate, as reported in some literature. However, after the powder was hydrated with steam and then further exposed to NO2(g), some traces of uranyl nitrate hexahydrate were found. The results of this study suggest that surface hydration of powders plays a vital role in uranyl nitrate formation under voloxidation conditions and raises questions about the kinetics of the oxide-to-nitrate voloxidation conversion process. Future chemical and engineering design decisions for the voloxidation process may benefit from an improved understanding of these chemical mechanisms.

Mapping of uranium particles on J-type swipes with microextraction-ICP-MS.

A microextraction liquid sampling system coupled to a quadrupole inductively coupled plasma-mass spectrometer (ICP-MS) was utilized to spatially discern uranium particles, isotopically, on a cellulose-based swipe material (i.e., J-type swipe). These types of swipes are often used by the International Atomic Energy Agency (IAEA) as part of their environmental sampling program. A grid was created such that extraction locations covered the center circle (n = 34 without overlapping). Uranium (U) particulates (<20 µm) of varying U isotopic abundance and chemical form (i.e., uranyl fluoride and uranyl nitrate hexahydrate) were mechanically placed on the swipes in random locations and detected via the microextraction-ICP-MS methodology. Heat maps were subsequently generated to show the placement of the particulate with their respective intensity and isotopic determination. This detection of the uranium particulates, via isotopic determination, agreed with reference values for these materials. Additionally, depleted (235U/238U = 0.002) uranium particulates were placed directly within a clay matrix, on the swipe surface, and subjected to analysis by microextraction-ICP-MS. The mapping of the swipe demonstrated, for the first time, the employment of the microextraction-ICP-MS method for extracting sample from a complex matrix, and correctly identifying the uranium isotopic composition. This example ultimately demonstrates the utility of the methodology for detecting particles of interest in complex matrices.

Analysis of solid uranium particulates on cotton swipes with an automated microextraction-ICP-MS system.

An automated microextraction method coupled to an inductively coupled plasma - mass spectrometer (ICP-MS) was developed for the direct analysis of solid uranium particulates on the surface of cotton swipes. The microextraction probe extracts particulates from the sample surface, in a flowing solvent, and directs the removed analyte to an ICP-MS for isotopic determination. The automated system utilizes a mechanical XY stage that is software controlled with the capability of saving and returning to specific locations and a camera focused to the swipe surface for optimal viewing of the extracted locations (i.e., material present). Here, particulates (n = 135) were extracted and measured by ICP-MS, including 35 depleted uranyl nitrate hexahydrate (UN) (used for mass bias corrections), 50 uranyl fluoride (UO2F2), and 50 uranyl acetate (UAc) particulates. Blank extractions were performed on the cotton swipes between triplicate sample analyses. Between each swipe extraction, the probe was sent between two wells containing 10% and 5% HNO3 to clean the probe head and to eliminate any analyte carryover between particulates. The measured 235U/238U and 234U/238U isotope ratios for the UO2F2 particulates were 0.00725(8) and 0.000054(4), a percent relative difference (% RD) of -0.041% and -1.7% from the reference isotope ratios determined in-lab through multi-collector ICP-MS analysis of dissolved aliquots of the U material. The UAc samples had a measured 235U/238U isotope ratio of 0.00206(7), a -0.96% relative difference from the reference value of 0.00208(1). The 234U/238U and 236U/238U isotope ratios were 0.000008(1) and 0.000031(4), -5.1% RD and -4.3% RD, respectively. The automated sample stage enabled seamless and rapid particle analysis, leading to a significant increase in throughput versus what was previously possible. Additionally, the saved location capability reduced user sampling error as sampling locations were easily stored and recalled. Analysis of U particles on the swipe surface - including blanks, mass bias, and triplicate extractions - was completed in less than an hour without any sample preparation necessary.

Direct isotopic analysis of solid uranium particulates on cotton swipes by microextraction-ICP-MS.

Direct isotope ratio analysis of solid uranium particulates on cotton swipes was achieved using a solution-based microextraction technique, coupled to a quadrupole inductively coupled plasma - mass spectrometer (ICP-MS). This microextraction-ICP-MS methodology provides rapid isotopic analysis which could be applicable to nuclear safeguards measurements. Particulates of uranyl nitrate hexahydrate (UO2(NO3)2·6H2O) and uranyl fluoride (UO2F2) ranging from 6 µm to 40 µm in length were transferred to cotton swipes with a particle manipulator. The microextraction probe then delivers a 5% nitric acid (HNO3) solvent onto the swipe surface to extract the uranium species. The extracted sample is then delivered to the ICP-MS for isotopic determination. The majority of uranium signal (∼99% and ∼94% for UO2(NO3)2·6H2O and UO2F2, respectively) was detected in the first 15 s extraction, while subsequent extractions on the same location had low or no U signal, suggesting near complete removal of the solid uranium compounds from the swipe surface. Ten samples (for each of the uranium compounds), were analyzed for their isotopic composition. For UO2(NO3)2·6H2O, the determined isotope ratios resulted in a % relative difference (% RD) from the referenced isotope ratios of 0.97, 1.0, and 7.3% for 234U/238U, 235U/238U, and 236U/238U, respectively. The % RD of the UO2F2 isotope ratios were 1.9 and 0.60% for 234U/238U and 235U/238U, respectively. The preliminary limits of detection were determined to be 0.002, 0.4, and 60 pg for 234U, 235U and 238U, respectively This work demonstrates that microextraction ICP-MS is a rapid and sensitive method that could directly determine uranium isotope ratios of UO2(NO3)2·6H2O and UO2F2 particulates on cotton swipes.

A preliminary investigation into the use of molecular oxide and hydride secondary ion relationships for improvement of the 236U/238U determination on a NanoSIMS 50L.

A NanoSIMS 50L is used to investigate uranium molecular (235U16O, 236U16O, 238U16O, 235U1H, 238U1H, 236U16O1H, and 238U16O1H) and elemental (235U, 236U, and 238U) secondary ion production during sputtering of synthetic UO2 and the NIST-610 standard to determine if: (1) the 236U16O/238U16O molecular oxide ratio performs better than the 236U/238U elemental ratio, and (2) there is co-variance between the molecular hydrides and oxides. Despite an order of magnitude greater abundance of 236U16O secondary ions (compared to 236U), the 236U16O/238U16O ratios are less accurate than the 236U/238U ratios. Further work is needed before the higher count rate of the 236U16O secondary ion can be used to obtain a better 236U/238U ratio. The second objective was undertaken because correction for the interference of 235U1H on the 236U secondary ion species typically utilizes the 238U1H/238U ratio. This becomes problematic in samples containing 239Pu, so our aim was to understand if the hydride formation rate can be constrained independently of having to measure the 238U1H. We document correlations between the hydride (238U1H and 238U16O1H) and oxide (236U16O) secondary ions, suggesting that pursuing an alternative correction regime is worthwhile.

Computationally Guided Investigation of the Optical Spectra of Pure ß-UO3.

Single-phase ß-UO3 is synthesized by flash heating UO2(NO3)·6H2O in air to 450 °C and annealing for 60 h under the same conditions. For the first time, we report the Raman spectra of pure ß-UO3. To facilitate the assignment of Raman and infrared vibrational modes, we use density functional theory with density functional perturbation theory. By employing a novel analysis scheme that includes the mode frequencies as well as a quantitative analysis of the mode eigenvectors, we assign the observed spectral features to individual chemical modes. In particular, the density functional theory optimized structure, observed Raman spectrum, and eigenvector analysis suggest the presence of four crystallographically distinct uranyl ions, one more than has previously been suggested.

Formation of a uranyl hydroxide hydrate via hydration of [(UO2F2)(H2O)]7·4H2O.

Hydrated uranyl fluoride, [(UO2F2)(H2O)]7·4H2O, is not stable at elevated water vapor pressure, undergoing a complete loss of fluorine to form a uranyl hydroxide hydrate. Powder X-ray diffraction data of the resultant uranyl hydroxide species is presented for the first time, along with Raman and infrared (IR) spectra. The new uranyl hydroxide species is structurally similar to the layered uranyl hydroxide hydrate minerals schoepite and metaschoepite, but has a significantly expanded interlayer spacing (c = 15.12 vs. 14.73 Å), suggesting that additional H2O molecules may be present between the uranyl layers. Comparison of the Raman and IR spectra of this new uranyl hydroxide hydrate and synthetic metaschoepite ([(UO2)4O(OH)6]·5H2O) suggests that the equatorial environment of the uranyl ion may differ and that H2O molecules in the new species participate in stronger hydrogen bonds. In addition, the interlayer spacing of both this new uranyl hydroxide species and synthetic metaschoepite is shown to be sensitive to the environmental humidity, contracting and re-expanding with desiccation and rehydration. Structural distinction between the new uranyl hydroxide species and synthetic metaschoepite is confirmed by a comparison of the thermal behavior; unlike metaschoepite, the new hydrate does not form α-UO2(OH)2 upon dehydration.

Sulfate-Centered Sodium-Icosahedron-Templated Uranyl Peroxide Phosphate Cages with Uranyl Bridged by µ-η1:η2 Peroxide.

Two novel hybrid uranyl peroxide phosphate cage clusters, designated U20P6 and U20P12, contain peroxide bridges between uranyl in an unusual µ-η1:η2 configuration, as well as the common µ-η2:η2 configuration. These appear to be the only high-nuclearity metal peroxide complexes containing µ-η1:η2 peroxide bridges, and they are unique among uranyl peroxide cages. Both clusters contain 20 uranyl polyhedra, and U20P6 and U20P12 contain 6 and 12 phosphate tetrahedra, respectively. The 20 uranyl polyhedra in both cages are arranged on the vertices of distorted topological dodecahedrons (20 vertex fullerenes). Each cage is completed by phosphate tetrahedra and is templated by a sulfate-centered Na12 cluster with the Na cations defining a regular convex isocahedron. Whereas µ-η2:η2 peroxides are essential features of uranyl peroxide cages, where they form equatorial edges of uranyl hexagonal bipyramids, the µ-η1:η2 peroxide groups in U20P6 and U20P12 are associated with strong distortions of the uranyl polyhedra. Formation of U20P6 and U20P12 is a further demonstration of the pliable nature of uranyl polyhedra, which contributes to the tremendous topological variability of uranyl compounds. Despite the unusual structure and highly distorted polyhedral geometries of U20P6, small-angle X-ray scattering and Raman spectra suggest its stability in the aqueous solution and solid state.