Matthew Stephen Wellons

International nuclear safeguards require microparticle reference materials (RMs) with complex uranium (U) and plutonium (Pu) compositions. RMs are needed for quality control within the international nuclear safeguard community governed by the International Atomic Energy Agency, and as qualification specimens for new microanalytical methods. Prior to this effort, no combined U/Pu microparticle specimens existed with sufficient characterization and corresponding qualified analytical measurands for U and Pu isotopic and elemental composition to meet this need. Savannah River National Laboratory (SRNL) produced homogeneous mixed U/Pu, monodisperse, micrometer-scale particles (denoted RM UPu-100A) using the SRNL THermally Evaporated Spray for Engineered Uniform particulateS (THESEUS) particle production platform. The targeted attributes for RM UPu-100A were a monodisperse, spherical particle size distribution centered at 1 μm diameter, n(²³⁸U)/n(²³⁹Pu) = 100; n(²⁴⁰Pu)/n(²³⁹Pu) = 0.30, and n(²³⁵U)/n(²³⁸U) = 0.05. After identifying, purifying, and mixing three uranium and two plutonium endmembers, monodisperse U/Pu microparticulate of the desired elemental and isotopic distributions was produced and electrostatically collected onto silicon planchets. Characterization of the produced microparticulate at SRNL and Los Alamos National Laboratory (LANL) used large geometry secondary ion mass spectrometry (LG-SIMS), multicollector-inductively coupled plasma-mass spectrometry (MC-ICP-MS), scanning electron microscopy (SEM), and alpha spectroscopy. Analysis results demonstrate that a compositionally homogeneous particle population was achieved. Key characteristics of RM UPu-100A include an approximate particle diameter of 1.08 μm and n(²³⁸U)/n(²³⁹Pu) = 103.3 ± 2.8; n(²⁴⁰Pu)/n(²³⁹Pu) = 0.2838 ± 0.0032; and n(²³⁵U)/n(²³⁸U) = 0.05221 ± 0.00024. A SIMS relative sensitivity factor, RSF_Pu:U, was calculated from n(²³⁸U)/n(²³⁹Pu); RSF_Pu:U = 2.282 ± 0.064, and n(²³⁵U + ²³⁸U)/n(²³⁹Pu + ²⁴⁰Pu); RSF_Pu:U = 2.242 ± 0.063.

The THermally Evaporated Spray for Engineered Uniform particulateS (THESEUS) production platform was developed to generate highly uniform mixed actinide oxide particles. The particulate synthesis platform builds on previous efforts and utilizes an aerosol-based technology to generate, calcine, characterize, and aggregate a monodisperse oxide phase particle product. In this study, particles comprised of uranium oxide, incorporated with varying compositions of thorium, were produced. Th/U test materials with ²³²Th concentrations between 1 ppm and 10%, ratioed to ²³⁸U, were successfully generated with in situ calcination at 600 °C and characterized by in situ aerodynamic particle size spectrometry and ex situ microanalytical methods. Populations of monodisperse particulates (geometric standard deviation – GSD < 1.15) with an average diameter near 1 μm were generatated and micro-Raman spectroscopy of individual particles identified U₃O₈ as the primary material phase for the range of Th/U samples analyzed. Single particle measurements and automated particle analyses by secondary ion mass spectrometry (SIMS) were performed. Uniform inter-particle elemental and isotopic homogeneity for uranium and thorium isotopes was characterized by SIMS, and a ²³²Th/²³⁸U relative sensitivity factor of 0.53 was determined. SIMS results demonstrated differences in the ²³²Th/²³⁸U profiling behavior for Th/U particulates with increased Th content (>1%). Despite the observed profiling behavior, single particle measurements of the 10% Th sample indicate inter-particle homogeneity. This work represents the first systematic study of Th/U microparticulate reference materials generated and intended for nuclear safeguards applications and serves as a demonstration of THESEUS to support a sustained capability for the production mixed-element particulate reference materials.

Anhydrous UF₄ stirred in water for 1–6 days forms UF₄(H₂O)_2.5 and a minor amount of uranyl fluoride. This hydrolysis results in morphological and spectroscopic changes.

Matrix-assisted ionization (MAI) demonstrates high sensitivity for a variety of organic compounds; however, few studies have reported the application of MAI for the detection and characterization of inorganic analytes. Trace-level uranium analysis is important in the realms of nuclear forensics, nuclear safeguards, and environmental monitoring. Traditional mass spectrometry methods employed in these fields require combinations of extensive laboratory chemistry sample preparation and destructive ionization methods. There has been recent interest in exploring ambient mass spectrometry methods that enable timely sample analysis and higher sensitivity than what is attainable by field-portable radiation detectors. Rapid characterization of uranium at nanogram levels is demonstrated in this study using MAI techniques. Mass spectra were collected on an atmospheric pressure mass spectrometer for solutions of uranyl nitrate, uranyl chloride, uranyl acetate, and uranyl oxalate utilizing 3-nibrobenzonitrile as the ionization matrix. The uranyl complexes investigated were detectable, and the chemical speciation was preserved. Sample analysis was accomplished in a matter of seconds, and limits of detection of 5 ng of uranyl nitrate, 10 ng of uranyl oxalate, 100 ng of uranyl chloride, and 200 ng of uranyl acetate were achieved. The observed gas-phase speciation was similar to negative-ion electrospray ionization of uranyl compounds with notable differences. Six matrix-derived ions were detected in all negative-ion mass spectra, and some of these ions formed adducts with the uranyl analyte. Subsequent analysis of the matrix suggests that these molecules are not matrix contaminants and are instead created during the ionization process.

Rationale: The ability to detect and quantify the presence of specific inorganic elements and complexes is essential for environmental monitoring and nuclear safeguards applications. In this work, paper spray ionization mass spectrometry was used for the rapid chemical and isotopic characterization of trace inorganic species collected on cotton swipe substrates. The direct analysis of cotton swipes using this ambient ionization technique led to fast sample analysis that retained original chemical information of the source material with minimal sample preparation. Methods: Mass spectra were collected with an atmospheric pressure ionization, high‐resolution mass spectrometer for solutions containing uranyl acetate, uranyl chloride, uranyl nitrate, and uranyl tri‐ n ‐butylphosphate complexes. Gadolinium nitrate was used as an internal standard for the quantitative analysis of uranium. To demonstrate the ability to characterize inorganic contaminants in the presence of uranium, a multi‐element inorganic standard containing U, Bi, Pb, Cd, Fe, and Zn was deposited onto cotton substrates and directly analyzed without purification. Results: All elements doped on the cotton substrate were detected with strong signal‐to‐noise ratios ( ca 1000 for UO ₂ ⁺ on multi‐element doped swipes) and high integrated intensities (>10 ⁵ counts) from collection periods of approximately 1 min. Limits of detection were determined to be approximately 94 ng for UO ₂ ⁺ and uranyl acetate through the measurement of ppb level solutions. Conclusions: The rapid analysis of uranium and other inorganic‐containing samples while still retaining original chemical information (e.g. uranyl complexation) was demonstrated. Qualitative detection and speciation were achieved in less than 1 min of analysis. For uranium isotopic quantitation, longer accumulations (>15 min) can be sustained to improve the accuracy of minor ²³⁵ U isotopic abundance measurements to approximately 1% error.

High-resolution 19F magic angle spinning (MAS) NMR spectra were obtained for the uranium-bearing solid, uranyl fluoride (UO2F2 ·1.57H2O). While there are seven distinct crystallographic fluorine sites, the 19F NMR spectrum reveals six peaks at -33.3, 9.1, 25.7, 33.0, 39.0, and 48.2 ppm, with the peak at 33.0 ppm twice the intensity of all the others and therefore corresponding to two sites. To assign the peaks in the experimental spectra to crystallographic sites, 19F chemical shifts were calculated by using the GIPAW (Gauge Including Projector Augmented Waves) plane-wave pseudopotential approach for a DFT-optimized crystal structure. The peak assignments from DFT are consistent with 2D double-quantum (DQ) 19F MAS NMR experiments.