Uranium Materials Research Articles

Potential of digital autoradiography for characterization of uranium materials

This study evaluates the capabilities of a commercial digital autoradiography set-up in characterizing the spatial distribution of radioactivity in U-bearing materials. Two different imaging plates (IPs) from various manufacturers, as well as three read-out granularities (25 µm, 50 µm, and 100 µm) of a high-resolution IP scanner, were tested. A custom software, developed using Python, was used for processing of the image raw data obtained from the IP scanner. The software extracted 25 intensity profiles from each autoradiographic image, allowing for a detailed analysis of the data. By applying various mathematical fitting functions, important metrics such as edge width, amplitude of the flat top, and central core width of the intensity profiles were determined. This provided valuable insights into the interaction between the U samples and IPs, particularly for samples of different geometry and thickness. Comparisons were made between samples embedded in epoxy resin or modelling clay and non-embedded U specimens. The results highlighted the superior performance of epoxy embedding in the characterization of U-bearing materials. The variation in signal intensity between runs for individual samples reached up to 20 %, showing consistent behavior across all samples in each measurement. This suggests that the inconsistency in signal response during repetitive analysis is likely due to the IP scanner rather than the IP itself. By using the analytical method of global spatial autocorrelation (Moran’s I), we were able to effectively detect enrichment inhomogeneity within low enriched UO2 pellets. The Moran’s I value quantified the levels of inhomogeneity, providing a numerical measure that was not visually apparent from the digital autoradiographic image.

Safeguards and Security for High-Burnup TRISO Pebble Bed Spent Fuel and Reactors

Several high-temperature thermal neutron–spectrum pebble bed reactors are being commercialized. China has started up two helium-cooled pebble bed high-temperature reactors. In the United States, the X-Energy helium-cooled and the Kairos Power salt-cooled pebble bed high-temperature reactors will produce spent nuclear fuel (SNF) with burnups exceeding 150 000 MWd per tonne. The reactor fuel in each case consists of small spherical graphite pebbles (4 to 6 cm in diameter) containing thousands of small TRISO (microspheric tri-structural isotropic) fuel particles embedded in the fuel of zone these pebbles. The unique isotopic, chemical, and physical characteristics of this high-burnup SNF create a technical case to eliminate safeguards based on the low risk for use in nuclear weapons, while maintaining safeguards in terms of risk for use in radiological weapons. These safeguards could be reduced to the simple counting and monitoring of pebbles in storage. Alternatively, there is the option to create a special category with reduced requirements for this SNF in storage, transport, and disposal. No safeguards would be required for a repository with only this type of SNF. Reactor safeguards are required for fresh fuel, partly burnt fuel, and to identify unconventional pebbles with depleted uranium or other materials that might be used to create weapons-useable materials.

HAXPES reference spectra of UO2 generated with Ga Kα x-ray source

HAXPES measurements were carried out using a Scienta Omicron HAXPES instrument to provide reference spectra for depleted uranium dioxide. High purity uranium dioxide, as confirmed by trace elemental analysis and x-ray diffraction, was synthesized via the integrated dry route from uranium hexafluoride. The material was fixed on double sided carbon tape for the analysis with charge control measures in place. The expanded energy range, using a Ga Kα x-ray source, presents core level photoelectrons not observed in traditional XPS. In addition, a region associated with the x-ray induced Auger transitions MNN is evident at binding energies only achievable with HAXPES. The reference spectra presented here act as the first in a line of proposed investigations into the comparison of XPS and HAXPES from surface to bulk as well as a fundamental understanding of the electronic structure of uranium materials.

A multi-collector ICP-MS method for quantification of plutonium, uranium, and americium in hair and nails of occupationally or medically exposed individuals

The 239Pu, 238U, and 241Am concentrations and 239Pu/240Pu, 235U/238U, and 236U/238U atom ratios were measured in the hair and nail samples using a new method utilized TEVA, UTEVA, and DGA extraction chromatography and multi-collector ICP-MS. Samples were collected from individuals who donated their bodies to the United States Transuranium and Uranium Registries. The concentration of 239Pu ranged from 0.22 to 15.8 ng/kg. The 240Pu/239Pu isotopic ratios ranged from 0.026 to 0.127 which is consistent with weapons-grade plutonium. Concentration of uranium fell between 1.84 μg/kg and 29.5 μg/kg and 235U/238U ratios ranged from 4.8 × 10−3 to 7.6 × 10−3. Elevated 236U/238U atom ratios were measured in two cases and ranged from 5.0 × 10−6 – 2.4 × 10−5 indicating exposure to spent or reprocessed uranium material. The concentration of 241Am was measured in four hair samples and ranged from 0.02 to 0.21 ng/kg.

Morphology of uranium oxides reduced from magnesium and sodium diuranate

Abstract Morphological analysis of uranium materials has proven to be a key signature for nuclear forensic purposes. This study examines the morphological changes to magnesium diuranate (MDU) and sodium diuranate (SDU) during reduction in a 10 % hydrogen atmosphere with and without steam present. Impurity concentrations of the materials were also examined pre and post reduction using energy dispersive X-ray spectroscopy combined with scanning electron microscopy (SEM-EDX). The structures of the MDU, SDU, and UO x samples were analyzed using powder X-ray diffraction (p-XRD). Using this method, UO x from MDU was found to be a mixture of UO2, U4O9, and MgU2O6 while UO x from SDU were combinations of UO2, U4O9, U3O8, and UO3. By SEM, the MDU and UO x from MDU had identical morphologies comprised of large agglomerates of rounded particles in an irregular pattern. SEM-EDX revealed pockets of high U and high Mg content distributed throughout the materials. The SDU and UO x from SDU had slightly different morphologies. The SDU consisted of massive agglomerates of platy sheets with rough surfaces. The UO x from SDU was comprised of massive agglomerates of acicular and sub-rounded particles that appeared slightly sintered. Backscatter images of SDU and related UO x materials showed sub-rounded dark spots indicating areas of high Na content, especially in UO x materials created in the presence of steam. SEM-EDX confirmed the presence of high sodium concentration spots in the SDU and UO x from SDU. Elemental compositions were found to not change between pre and post reduction of MDU and SDU indicating that reduction with or without steam does not affect Mg or Na concentrations. The identification of Mg and Na impurities using SEM analysis presents a readily accessible tool in nuclear material analysis with high Mg and Na impurities likely indicating processing via MDU or SDU, respectively. Machine learning using convolutional neural networks (CNNs) found that the MDU and SDU had unique morphologies compared to previous publications and that there are distinguishing features between materials created with and without steam.

Uranium age dating measurements by laser ablation multi-collector ICP-MS in uranium materials

The aim of the present work was to develop a direct method for age dating (production date measurement) of uranium samples by laser ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS) by the measurement of the 230Th/234U ratio. The major instrumental conditions and sample characteristics affecting the accuracy and precision were investigated in this systematic study. By comparing the obtained LA-MC-ICP-MS results with those obtained after chemical separation measurements, it shows that the LA-MC-ICP-MS method is capable to produce accurate results for pure highly enriched uranium. Natural and low-enriched uranium, however, needs a higher mass resolution to remove the identified interferences, which can lead to erroneous results.

Ab Initio Molecular Dynamics Study of Electron Excitation Effects on UO2 and U3Si.

In this study, an ab initio molecular dynamics method is employed to investigate how the microstructures of UO2 and U3Si evolve under electron excitation. It is found that the U3Si is more resistant to electron excitation than UO2 at room temperature. UO2 undergoes a crystalline-to-amorphous structural transition with an electronic excitation concentration of 3.6%, whereas U3Si maintains a crystalline structure until an electronic excitation concentration reaches up to 6%. Such discrepancy is mainly due to their different electronic structures. For insulator UO2, once valence U 5f electrons receive enough energy, they are excited to the conduction bands, which induces charge redistribution. Anion disordering is then driven by cation disordering, eventually resulting in structural amorphization. As for metallic U3Si, the U 5f electrons are relatively more difficult to excite, and the electron excitation leads to cation disordering, which eventually drives the crystalline-to-amorphous phase transition. This study reveals that U3Si is more resistant to electron excitation than UO2 under an irradiation environment, which may advance the understanding of related experimental and theoretical investigations to design radiation-resistant nuclear fuel uranium materials.

Application of peak based-Bayesian statistical method for isotope identification and categorization of depleted, natural and low enriched uranium measured by LaBr3:Ce scintillation detector

Todays, medium energy resolution detectors are preferably used in radioisotope identification devices(RID) in nuclear and radioactive material categorization. However, there is still a need to develop or enhance « automated identifiers » for the useful RID algorithms. To decide whether any material is SNM or NORM, a key parameter is the better energy resolution of the detector. Although masking, shielding and gain shift/stabilization and other affecting parameters on site are also important for successful operations, the suitability of the RID algorithm is also a critical point to enhance the identification reliability while extracting the features from the spectral analysis. In this study, a RID algorithm based on Bayesian statistical method has been modified for medium energy resolution detectors and applied to the uranium gamma-ray spectra taken by a LaBr3:Ce detector. The present Bayesian RID algorithm covers up to 2000 keV energy range. It uses the peak centroids, the peak areas from the measured gamma-ray spectra. The extraction features are derived from the peak-based Bayesian classifiers to estimate a posterior probability for each isotope in the ANSI library. The program operations were tested under a MATLAB platform.The present peak based Bayesian RID algorithm was validated by using single isotopes(241Am, 57Co, 137Cs, 54Mn, 60Co), and then applied to five standard nuclear materials(0.32–4.51% at.235U), as well as natural U- and Th-ores. The ID performance of the RID algorithm was quantified in terms of F-score for each isotope. The posterior probability is calculated to be 54.5–74.4% for 238U and 4.7–10.5% for 235U in EC-NRM171 uranium materials. For the case of the more complex gamma-ray spectra from CRMs, the total scoring (ST) method was preferred for its ID performance evaluation. It was shown that the present peak based Bayesian RID algorithm can be applied to identify 235U and 238U isotopes in LEU or natural U–Th samples if a medium energy resolution detector is was in the measurements.

Evidence of a Uranium‐Paddlewheel Node in a Catecholate‐Based Metal–Organic Framework

AbstractThe interactions between uranium and non‐innocent organic species are an essential component of fundamental uranium redox chemistry. However, they have seldom been explored in the context of multidimensional, porous materials. Uranium‐based metal–organic frameworks (MOFs) offer a new angle to study these interactions, as these self‐assembled species stabilize uranium species through immobilization by organic linkers within a crystalline framework, while potentially providing a method for adjusting metal oxidation state through coordination of non‐innocent linkers. We report the synthesis of the MOF NU‐1700, assembled from U4+‐paddlewheel nodes and catecholate‐based linkers. We propose this highly unusual structure, which contains two U4+ ions in a paddlewheel built from four linkers—a first among uranium materials—as a result of extensive characterization via powder X‐ray diffraction (PXRD), sorption, transmission electron microscopy (TEM), and thermogravimetric analysis (TGA), in addition to density functional theory (DFT) calculations.

Trace Actinide Signatures of a Bulk Neptunium Sample.

In nuclear forensic analyses, measurements of actinide elements in a sample can assist with identifying interdicted or unknown materials. While these radiochemical signatures have been extensively investigated in uranium materials, less is known about bulk neptunium samples. This paper describes the measurement of trace actinide concentrations and isotopic profiles in a 237Np oxide sample. Uranium, plutonium, americium, and curium concentrations and isotopic profiles in the sample were determined and deemed potentially useful for distinguishing different sources of 237Np. Several different potential radiochronometry systems were also investigated; discordant results indicate that the Np sample was never completely purified of other actinide elements, or that subsequent contamination of the sample occurred. Few prior studies of neptunium materials have been reported, and these data suggest that trace actinide constituents could provide unique signatures to identify material out of regulatory control.

Challenges in correlating oxygen stable isotope ratios of hydrates on uranium ore concentrates to process waters

Abstract Exchange of oxygen stable isotopes (δ18O values) between precipitation waters and uranium oxides is governed by thermodynamics or kinetics. It has been assumed that meteoric waters can be related to precipitation waters in uranium ore concentrates and their calcined and reduced uranium oxide products. With this assumption, the δ18O values of uranium materials could provide forensic signatures that identify the production history and geolocation of nuclear materials. To further exploit the potential of δ18O values in nuclear material analysis, this study examines the oxygen stable isotope exchange in two UOCs, magnesium diuranate (MDU) and sodium diuranate (SDU). MDU and SDU were synthesized from solutions of uranyl nitrate hexahydrate using precipitation waters with unique oxygen isotope compositions. The structures of the MDU and SDU were analyzed using powder X-ray diffraction (p-XRD) and thermal mass loss curves, while the δ18O values of waters generated during thermal decomposition were analyzed using a thermogravimetric analyzer coupled to an isotope ratio infrared spectrometer (TGA-IRIS). By p-XRD, the MDU was uniform and amorphous across all syntheses with residual crystalline material incorporated as a minor component. Combined with the TGA results, all of the MDU is likely amorphous MgU2O7·3H2O with MgO impurities present throughout. In contrast, the SDU synthesis resulted in multiple phases with many samples exhibiting crystalline phases including a combination of Na(UO2)4O2(OH)5·5H2O and Na2(UO2)6O4(OH)6·8H2O with a Na2U2O7 minor phase. A small fraction of the SDU samples were amorphous with no crystalline XRD peaks observed. Mass loss curves of the SDU samples revealed that the amorphous samples contained inclusions of similar crystalline phases compared to the crystalline materials. The uniformity of the MDU samples enabled highly reproducible measurements of δ18O values of the water vapor yielded from two dehydration events at 170 °C and 500 °C. In contrast, the multiphase composition of the SDU samples resulted in poor reproducibility in δ18O values. Neither system revealed any correlation between the δ18O values of precipitation water and the waters released during dehydration of the UOCs.

Simulation-Based Characterization of Tube-Cap Resistance Butt Welding of Nuclear Fuel for Light Water Reactors and Development of Melted-Volume Prediction Models

The most important process in the manufacture of light-water reactor nuclear fuel rods is tube-cap welding, which serves to seal radioactive uranium materials. Tube-cap welding is a method commonly used in production by sealing various materials into tubes and caps, and is used in the manufacture of fuel rods in nuclear power plants. In this study, the simulation-based characterization of tube-cap welding was carried out to understand the underlying mechanisms that govern the welding process and to use a model that can predict the melted volume during the welding process. The simulation results were then used to analyze the effects of various welding parameters on the melted volume during welding process. To analyze the mechanism of welding phenomena in the tube-cap welding, various weld variables such as welding current, overlap length, and force were used. Those are the main variables in the tube-cap welding used in nuclear fuel rods manufacturing thought simulation. To predict the melted volume of the weld, a regression model and a neural network were used to predict the amount of melt using the process with the main process variables as input variables. The study quantitatively determined the effects of each welding variable on the melted volume and analyzed the correlation between two factors.

Study of the effect of photon self-absorption on the detection efficiency of HPGe detector in measurements of an activated uranium sample

The photon self-absorption effect of uranium on the full-energy peak (FEP) efficiency calibration of high purity germanium (HPGe) detector was studied as a function of energy using an activated uranium sample has a thickness of 1 mm. The absolute efficiencies of the detector were obtained with the use of gamma standard sources and gamma rays of radionuclides created by irradiating the uranium sample with secondary neutrons. The measurements were carried out with the use of a Planar HPGe detector. MCNP simulation code was used to verify the experimental results. The comparison of the empirical results with the simulation results showed a good agreement. To interpret the efficiencies, the reaction rates were calculated using two methods: (1) - the use of the efficiency curve from the standard sources combined with the self-absorption correction of the uranium material and (2) - the use of the efficiency curve obtained from gamma rays in the sample.

CURIES: Compendium of uranium Raman and infrared experimental spectra

Abstract Identification of radioactive materials is a critical goal of resource exploration, basic actinide science, and nuclear forensics, and we provide here new insights toward rapid, nondestructive analysis of uranium-containing minerals and technogenic phases. Raman and infrared spectroscopic data are powerful indicators of solid-phase U(VI) coordination chemistry. In addition, U(VI) minerals exhibit high chemical and structural diversity as artifacts of geochemical processes leading to ore formation. Spectral signals of axial UO22+ (U-Oyl) bond lengths and the influences of additional oxyanions on these values are well documented for uranium oxide and oxysalt minerals and technogenic phases. Additional insight regarding the underlying crystallographic structure and chemical composition of uranium materials can be extracted through a survey of all available Raman spectroscopic data for these phases. To this end, we have developed the Compendium of Uranium Raman and Infrared Experimental Spectra (CURIES). CURIES was compiled via a thorough review of literature and databases, and for mineral species that lack measured and recorded spectra, data were obtained either from museum and academic collections or by direct syntheses. Characteristic Raman spectroscopic features for subgroups of uranyl minerals within CURIES were elucidated using multivariate statistical analyses. In addition, average spectra for groups of uranyl minerals were determined, providing insight into common spectroscopic characteristics that are indicative of the structural origins from which they arise. As of publication, 275 mineral species and technogenic phases have been entered in CURIES, and of these, 83 phases have published spectra that have been included in the CURIES database. Data collection is ongoing, and we have triaged missing data sets to assess CURIES for completion and to identify mineral groups that lack representation and should therefore be prioritized for data acquisition and inclusion in the database.

Study on the Equivalence of Metallic-Cerium-Simulated Uranium-Aerosol Generation under Fire

Uranium aerosols are released from uranium-containing materials in high-temperature environments caused by nuclear accidents or other processes. Research on the generation characteristics of uranium aerosols under such conditions is an important part of nuclear-safety analysis. In this experiment, the similarity between metal cerium aerosols and uranium material aerosols was evaluated from the aspects of particle size distribution and source term. Combined with the experiment data, the effect of air flow rate and sampling time is discussed. The calculation result of the air release fraction (ARF) is 6.07 × 10−3–4.8 × 10−2, and the respirable fraction (RF) is 0.810–0.978, respectively, showing that the size distribution of particles and ARF of the cerium aerosol are different from the results of the uranium aerosols in the literature, while the RF is similar to the results obtained by using the uranium–niobium alloy in the literature.

Investigation of the possibility of a new detector based on SiPM in nuclear forensics

The development of nuclear technologies, the production and active use of radioisotopes, and the production of radiopharmaceuticals, medical isotopes and other radioactive materials are increasing every year. Therefore, the importance of ensuring the safety of highly active isotopes, as well as providing the necessary instruments for measuring and identifying radioactive materials, must be taken into account. Modern equipment such as high purity germanium detectors (HPGe) is costly and requires specialized staff skills as well as special operating conditions such as low temperatures and high voltages. It is proposed to explore the possibilities of using a silicon photomultiplier (SiPM) with a deep pixel structure in nuclear gamma spectrometry, which will make it possible to increase the efficiency of scintillation detectors. The paper presents the results of a study of the newest silicon photomultipliers MAPD-3NM II assembled in a 16-element matrix, which was the detector part of the proposed LaBr3(Ce) scintillation spectrometer. The study was carried out using radioisotopes of uranium. The aim of the research is to reveal the possibility of differentiating depleted and natural uranium materials from each other without using special software by means of the proposed set of equipment.

Investigations of the Cobalt Hexamine Uranyl Carbonate System: Understanding the Influence of Charge and Hydrogen Bonding on the Modification of Vibrational Modes in Uranyl Compounds.

Hydrogen bonding networks within hexavalent uranium materials are complex and may influence the overall physical and chemical properties of the system. This is particularly true if hydrogen bonding takes places between the donor and the oxo group associated with the uranyl cation (UO22+). In the current study, we evaluate the impact of charge-assisted hydrogen bonding on the vibrational modes of the uranyl cation using uranyl tricarbonate [UO2(CO3)3]4– interactions with [Co(NH3)6]3+ as the model system. Herein, we report the synthesis and structural characterization of five novel compounds, [Co(NH3)6]Cl(CO3) (Co_Cl_CO3), [Co(NH3)6]4[UO2(CO3)3]3(H2O)11.67 (Co4U3), [Co(NH3)6]3[UO2(CO3)3]2Cl (H2O)7.5 (Co3U2_Cl), [Co(NH3)6]2[UO2(CO3)3]Cl2 (Co2U_Cl), and [Co(NH3)6]2[UO2(CO3)3]CO3 (Co2U_CO3), which contain differences in the crystalline packing and extended hydrogen bonding networks. We show that these slight changes in the supramolecular assembly and hydrogen bonding networks result in the modification of modes as observed by infrared and Raman spectroscopy. We use density functional theory calculations to assign the vibrational modes and provide an understanding about how uranyl bond perturbation and changes in hydrogen bonding interactions can impact the resulting spectroscopic signals.

Laser ablation inductively coupled plasma mass spectrometry analysis of isotopically heterogeneous uranium materials

A reliable and accurate laser ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS) method was developed for analysis of inhomogeneous samples containing uranium particles or aggregates of various enrichments. For the method development, a mixed solid standard was prepared using 1% and 3% 235U enriched certified reference materials. After screening and localization of the particles of interest, the U isotopic composition was analysed for a 5-µm sample spot determining accurately and precisely the various constituents. Besides the LA-MC-ICP-MS, the standard was also measured by large-geometry secondary ion mass spectrometry (LG-SIMS) for additional verification.

Modeling dose rate increase from the addition of Uranium-232 for use as a tracer in uranium fuels

The isotope 232U is being considered for additive to uranium fuel for use as a tracer. The 232U decay chain has high energy gammas that can be used for tracer purposes. However, the presence and intensity of such gammas may increase effective dose rates to workers around such materials This study examines the dose rate from different uranium materials that varying amounts of 232U has been added to. Several materials and their respective storage geometries are modeled for particle transport calculations. For each material and 232U concentration, gamma source terms were generated using SCALE 6.2 ORIGEN. These source terms were then used in MCNP for each material and geometry. Upon analysis, it was determined that the baseline dose from enriched uranium dominates until the 232U concentration reaches about 10–100 parts per trillion (ppt), after which the dose rate increases linearly.

The modified wear resistance of uranium induced by ultrasonic surface rolling process

With the development of the nuclear industry, uranium materials with better wear resistance are demanded. This paper aims to demonstrate the effects of ultrasonic surface rolling process (USRP) on the tribological properties of uranium. Surface characteristics, i.e., surface topography, residual stress, microstructure, hardness and wear behavior, were examined. The results indicated that nanostructure, deformation twins and texture were generated on the uranium surface after USRP treatment. Surface roughness Ra, Rp and Rv showed a remarkable decline by 85.6%, 85.5% and 74.8% compared to the untreated specimen, respectively. Moreover, increased microhardness and enhanced residual compressive stress were obtained by USRP treatment. According to the wear tests against silicon carbide counterball in dry sliding conditions, the friction coefficient was decreased to 0.66 and the wear volume was also drastically reduced, which prolonged the friction and wear life of uranium materials. Additionally, it was concluded that abrasion wear, adhesion wear and oxidation wear played the dominant roles in the USRP treated specimens and the silicon carbide counterball showed the characteristics of abrasion wear.