Isotopic Composition Of Uranium Research Articles

Application of ATONA Amplifiers to the Measurement of Uranium Isotopic Ratios by Thermal Ionization Mass Spectrometry.

Uranium isotopic composition can provide valuable information about the history and provenance of a nuclear material; therefore, uranium isotopic analyses are frequently made in the nuclear forensics, safeguards, and environmental monitoring communities. These measurements have always presented challenges due to the extreme variability in the relative abundance between the major (235U, 238U) and minor (233U, 234U, 236U) isotopes of uranium. The recently developed ATONA (Atto- to Nano-Amp) amplification system paired with Faraday cup detectors has a large dynamic range and low noise floor making it ideal for measuring uranium isotopic ratios in materials of both natural and anthropogenic origin. A wide variety of certified reference materials were analyzed to investigate the utility of the ATONA amplification system for determining uranium isotopic composition in samples ranging from depleted to highly enriched. The ATONA amplifiers provide nearly an order of magnitude improvement in external reproducibility over 1011 Ω amplifiers when measuring the minor 234U/238U ratio in isotopically natural and depleted samples and when paired with a secondary electron multiplier can measure very low relative abundance uranium isotopes (i.e., 236U).

URANIUM BEHAVIOR DURING THE FORMATION AND SUBSEQUENT STORAGE OF SOLID RADIOACTIVE WASTES FROM CONVERSION PRODUCTION

A complex of physicochemical studies of solid radioactive wastes (RW) of the near-surface storage facility from conversion uranium production was carried out. It has been established that the composition of the wastes is represented mainly by calcium compounds: gypsum, calcite, and calcium fluoride. Uranium is distributed throughout the entire volume of wastes. At the same time, uranium is unevenly concentrated along the depth of the sludge. The content of uranium in the wastes is 0.005-0.65 wt. %. Uranium was predominantly adsorbed on the surface of sludge by main components during the formation of the wastes solid phase. Uranium was partially precipitated in the form of calcium-containing weeksite Ca2(UO2)2(Si2O5)3·10H2O, uranophane-alpha Ca(UO2)2(SiO3OH)2·5H2O, and urankalcarite Ca(UO2)3CO3(OH)6·3H2O. The isotopic composition of uranium contained in the storage wastes corresponds, within the error, to the normal isotopic composition of natural uranium. Near-surface storage is constantly exposed to natural factors. Infiltration of atmospheric condensation leads to leaching of some components from solid RW. At the same time, uranium goes into solution in the form of [(UO2)2CO3(OH)3]-, [UO2(CO3)2]2-, [UO2(CO3)3]4- and Ca2UO2(CO3)3. The subsequent migration of uranium with infiltrating water contributes to its distribution within the entire sludge field, including the reclamation soil layer and the near-surface pond. In some areas of the storage facility, the uranium content in the reclamation soil layer reaches 0.043 wt.%. The concentration of uranium in the pond was 17.97 mg L-1. The chemical composition of the pond water was characterized by a high content of chloride-ions (1.29 g L-1), calcium (630 mg L-1), sulfate-ions (224 mg L-1), hydrocarbonates (122 mg L-1). The pond is connected to groundwater, which increases the risk of pollutants escaping beyond the storage facility. For citation: Skripchenko S.Yu., Titova S.М., Nalivaiko K.A., Semenishchev V.S. Uranium behavior during the formation and subsequent storage of solid radioactive wastes from conversion production. ChemChemTech [Izv. Vyssh. Uchebn. Zaved. Khim. Khim. Tekhnol.]. 2024. V. 67. N 5. P. 77-86. DOI: 10.6060/ivkkt.20246705.6925.

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.

Uranium isotopic analysis in unpurified solutions by ICP-MS

Uranium isotope analysis in unpurified samples using ICP-MS provides accurate results across platforms.

Exploring uranium isotopes in shark teeth as a paleo-redox proxy

The uranium isotope composition (δ238U) of seawater is a powerful proxy for the extent of marine anoxia. For paleoredox reconstructions, carbonates are the most popular U isotope archive, but they have recently come under increased scrutiny as their δ238U values are subject to diagenetic alteration after deposition. Therefore, there is a need to investigate other archives that may record and preserve the original seawater δ238U signal. In this study, we explore whether shark teeth provide such an archive. Shark teeth enameloid consisting of crystalline fluorapatite is more resistant to post-depositional alteration and less sensitive to isotopic exchange than marine carbonates due to the lower solubility of the crystalline fluorapatite. Since U is readily incorporated into phosphate, shark teeth could incorporate and preserve the original δ238U signature of seawater.To assess whether U isotopes in shark teeth can record seawater signatures, we measured the U isotopes (both δ238U and δ234Usec) in 39 fossil shark teeth from various locations, including Banks Island (Arctic), the Gulf of Mexico (GOM), and Pisco Basin (Peru), and ranging in age from modern to Cretaceous. Our results show that U concentrations are negligible in modern shark teeth (<1 ppb) but elevated in fossil samples (up to several hundred ppm), indicating that U is incorporated into shark teeth postmortem during burial. The δ238U values range from −0.72 to +0.57 ‰, and the δ234U values from −162.1 to +969.7 ‰. The data indicate that (i) diagenetic overprinting of seawater U isotope ratios is common among shark teeth, and (ii) δ238U data are influenced by local depositional environments. Nonetheless, the U isotope variations observed in shark teeth are comparable to those seen in marine carbonates, indicating that the samples with less diagenetic alterations might offer useful insight into the past extent of ocean anoxia.

СРАВНЕНИЕ РАДИАЦИОННОЙ И ХИМИЧЕСКОЙ ТОКСИЧНОСТИ СОЕДИНЕНИЙ УРАНА НА ОСНОВЕ РАСЧЕТА ПО НОВЫМ БИОКИНЕТИЧЕСКИМ МОДЕЛЯМ МКРЗ

Purpose: Harmonization and improvement of the system for regulating the internal radiation exposure of workers and the basic requirements for ensuring radiation safety, taking into account the application of new international requirements and recommendations. Material and methods: The article presents a comparison of the radiation and chemical toxicity of uranium compounds, obtained on the basis of calculating the levels of inhalation intake and committed effective dose depending on the types of compounds F, M, F/M and M/S in the AMAD range from 0.3 to 20 μm for typical isotopic compositions of natural (NU), depleted (DU), low enriched (LEU) and highly enriched (HEU) uranium, which lead to the maximum permissible concentration of uranium in the kidneys. The calculations were carried out using new ICRP biokinetic models, which give more physiologically realistic representations of uptake and retention in organs and tissues, and excretion. Results: The dynamics of uranium activity in the kidneys was calculated for constant chronic inhalation intake over a 50-year period and for acute intake. It was shown that in case of chronic intake, the rate of accumulation of uranium in the kidneys, expressed in relative units, does not depend on the AMAD in the range from 0.3 to 20 μm and slightly depends on the types of compounds F, F/M, M and M/S, which include almost all chemical compounds of uranium. In case of acute intake, there is a rapid, within 1–3 days, an increase of uranium in the kidneys to a maximum value and then a gradual decrease to a value of 20 % of the maximum value in 20–60 days, depending on the type of compound F, M, F/M, M/S and AMAD in a wide range of values from 0.3 to 20 µm. To compare the radiation and chemical toxicity of uranium, the values of the committed effective dose were calculated, which is formed after intake of uranium aerosols of the types F, M, F/M and M/S and AMADs from 0.3 to 20 µm in an amount that creates the maximum concentration of uranium in the kidneys 0.3 µg/g for chronic intake and 3 µg/g for acute intake. The values of uranium intake per year in milligrams, which form the maximum concentration of uranium in the kidneys of 0.3 µg/g, in case of constant chronic intake of uranium aerosols, as well as the values of uranium intake in milligrams, which form the maximum concentration of uranium in the kidneys of 3 μg/g after a single intake of uranium aerosols in both case of the types F, M, F/M and M/S and AMAD in the range from 0.3 to 20 µm were calculated, which are evidently independent of the considered isotopic composition of the uranium. Conclusion: It is shown that chemical toxicity prevails over radiation toxicity for the types of uranium compounds F and F/M for all considered uranium isotopic composition, except for HEU; for the type of compound M it is the same for mixtures of NU and DU, and for the type M/S radiation toxicity prevails for all considered uranium isotopic composition. In case of chronic intake at committed effective dose exposure rate of several mSv per year, workers can suffer from the chemical toxicity of uranium when working with F and F/M compounds and isotope compositions of natural (NU), depleted (DU) and low enriched (LEU) uranium already after 1–2 years of work. In case of acute intake, the chemical toxicity of uranium should be taken as a criterion for limiting exposure for compounds F and F/M, and also partially M (for uranium isotope compositions of NU, DU and LEU), which can significantly, by tens and hundreds of times, reduce the permissible limit of uranium intake.

Uranium isotopes in non-euxinic shale and carbonate reveal dynamic Katian marine redox conditions accompanying a decrease in biodiversity prior to the Late Ordovician Mass Extinction

The Late Ordovician mass extinction (LOME) is the first major mass extinction event in the Phanerozoic. However, global biodiversity started to decline during the Katian (prior to the LOME) and coeval global ocean redox conditions are not well understood. The deposition of Katian organic-rich sedimentary rocks, namely the calcareous Collingwood Member and overlying siliciclastic Rouge River Member, was triggered by the concurrent Taconic Orogeny in southern Ontario (Canada) near the interface between the epicratonic Appalachian and Michigan basins. In this study, we used geochemical elemental proxies and uranium isotope compositions (δ238U) to reconstruct local and global ocean redox states during deposition of both units at seven drill core localities throughout southern Ontario. Local redox conditions are revealed by several proxies (redox sensitive trace metals, Fe speciation, and Corg:P ratios). Collectively, these proxies suggest spatiotemporal redox variations for the Collingwood Member: O2-deficient conditions (non-euxinic) for the deep waters of the Appalachian and Michigan basins and mostly oxic-suboxic conditions for the shallow waters. In contrast, predominantly oxic-suboxic conditions are suggested for the Rouge River Member.Global ocean redox conditions are inferred from the δ238U of both units. The coeval seawater δ238U value during Collingwood Member deposition is estimated to be from ∼−0.87‰ to ∼−0.64‰, based on both carbonate δ238U (samples leached with 1 N HCl) and δ238Ubulk-non-detrital data (corrected from the bulk δ238U). Particularly, the δ238Ubulk-non-detrital data of the Collingwood Member from multiple cores (both shallow and deep waters) are positively correlated with elemental redox proxies, suggesting a local redox control on δ238U offsets between sediments and water columns. In contrast, the δ238Ubulk-non-detrital of the suboxic Rouge River Member (average = −0.32 ± 0.12‰) from several cores do not correlate with local redox proxies. The coeval seawater δ238U during Rouge River Member deposition is estimated between −0.62 ± 0.12‰ and −0.42 ± 0.12‰ assuming a δ238U offset of 0.1–0.3‰ as observed between modern suboxic sediments and seawater. For this range of seawater δ238U, a three-sink U isotope mass balance model suggests a contraction of global euxinic seafloor area from deposition of the Collingwood Member (0.5–31.6%; median = 1.0–14.2%) to the Rouge River Member (0.2–2.0%; median = 0.3–1.0%), which could have been associated with the concurrent Taconic Orogeny. Collectively, global marine redox proxy data (i.e., δ98Mo, δ238U) from this and previous studies imply dynamic Katian ocean redox conditions during the decrease of metazoan biodiversity prior to the LOME. As a similar extent of global ocean euxinia during the LOME2 also occurred episodically during the Katian but did not result in LOME2-like mass extinctions, it is suggested that other factors (e.g., climate), besides expanded ocean euxinia, could have contributed to the second phase of the LOME.

Faraday detector uranium isotope ratio measurement: Insights from solution- and laser ablation-based sampling methodologies on the Neoma MC-ICP-MS

In this study we utilize an all-faraday cup detector configuration on the Neoma MC-ICP-MS to perform uranium isotope ratio determinations from certified reference materials and a nuclear fuel precursor material via solution and laser ablation based sampling methodologies. The goal in performing these measurements is primarily to demonstrate that the all-faraday cup method can produce highly precise and accurate isotope ratios even at relatively low signal intensity levels while also developing a preliminary understanding of how the complex interplay between different amplifier resistor levels (e.g. 1011 vs 1013 Ω) and integration times impacts the precision and accuracy of different types of measurements. Our results show that the faraday cups with 1013 Ω resistors in their amplifier feedback loops can be used to produce accurate and relatively precise isotope ratios for ion beams down to ∼5 K cps. However, at the lower end of the signal intensities observed in this study, longer integration times are necessary to mitigate the impact of ion beam instability that can occur during laser ablation based sampling. Despite this potential limitation, the all-faraday cup method is highly versatile as exemplified by the analysis of a collection of uranium reference materials with highly variable uranium isotopic compositions.

Carbonate uranium isotopes across Cretaceous OAE 2 in southern Mexico: New constraints on the global spread of marine anoxia and organic carbon burial

Oceanic anoxic events (OAEs) represent discrete intervals of decreased marine oxygen concentrations often associated with volcanism, enhanced organic carbon burial coupled with positive δ13C excursions, and significant biotic turnover. Cretaceous OAE 2 (ca. 94 Mya) is especially notable for globally-distributed changes in calcareous invertebrate and plankton populations. While the presence of organic-rich facies is consistent with locally anoxic environments in many cases, determining the global extent of anoxia is more problematic. To address this issue, we investigate uranium isotope (δ238U) compositions of upper Cretaceous (Cenomanian-Turonian) open marine platform carbonates from southern Mexico as a proxy for global seawater redox conditions. These data are complementary to previous δ238U studies across OAE 2 in both black shales and pelagic carbonates, which have yielded variable results that reflect both global redox and local depositional processes. In Morelos Formation carbonates, a significant and well-defined negative δ238U excursion down to a nadir of −0.6‰ is recorded over an ∼40 m interval. This is consistent with the expansion of marine anoxia, pointing to an areal extent of anoxic seawater of about 1–10% of the global seafloor (or ∼5 to 50 times the modern value). Importantly, based on biostratigraphically-controlled estimates of sediment accumulation rates, the δ238U anomaly precedes the δ13C excursion by a median of ∼45 to 51 kyr (95th percentile confidence interval, CI) or ∼105 to 120 kyr (95th percentile CI) depending on how the onset of the δ238U anomaly is estimated. These results, along with previously reported thallium isotope and trace metal data, suggest that anoxic expansion preceded carbon cycle perturbation. This observation further increases estimates of the duration of OAE 2, implying that widespread ocean anoxia may have lasted >900 kyr.

The large decline in carbonate δ238U from a PETM section at Tingri (South Tibet) was driven by local sea-level changes, not global oceanic anoxia

Uranium isotope compositions (δ238U) and Th/U in carbonates are being explored as paleoredox proxies to study global oceanic anoxia. However, the impact of changes to local depositional environments that might cause a decline in δ238U and an increase in Th/U has not been thoroughly investigated. Here we report a decline in δ238U (from +0.1 to −0.3‰) and an increase in Th/U from shallow-marine carbonates during the Paleocene-Eocene thermal maximum at Tingri (south Tibet). Changes of δ238U and Th/U are not fully coupled. The decline in δ238U coincided with a local sea-level fall, whereas the increase in Th/U occurred mainly during a sea-level rise. The decline in δ238U likely results from changes in redox conditions of pore waters and in primary carbonate mineralogy. The increase in Th/U is ascribed to reduced authigenic U(IV) accumulation. Our results suggest that changes in local depositional environments can cause behavior of δ238U and Th/U resembling that induced by expanded global oceanic anoxia. This finding calls for caution to consider local factors before applying δ238U and Th/U as global paleoredox proxies.

The importance of uranium isotope composition for gross alpha activity regulation

AbstractUranium is a regulated contaminant (maximum contaminant level [MCL] 30 μg/L) that contributes to the gross alpha activity of groundwater. The adjusted gross alpha activity (MCL 15 pCi/L) is determined by subtracting the total uranium activity from the measured gross alpha activity. U.S. water utilities can use mass‐based and radiochemical analysis methods for compliance monitoring of uranium. Mass‐based measurements use a conversion factor of 0.67 pCi/μg of uranium to calculate the adjusted gross alpha activity. This conversion factor assumes that the activity of 234U equals 238U. Here, we present two decades of uranium isotope data measured by alpha spectrometry that shows 234U activity typically exceeds 238U. Using mass‐based measurements, the total uranium activity is biased low causing artificial exceedances of the adjusted gross alpha activity. Therefore, water utilities with gross alpha activities &gt;15 pCi/L should utilize radiochemical analyses for uranium for the most accurate calculation of adjusted gross alpha activity.

A new milestone for ultra-low 236U/238U isotope ratio measurements by ICP-MS/MS

A new limit of detection for 236U/238U isotope ratios was achieved with ICP-MS/MS technology.

Gamma spectrometric measurement of uranium isotopic composition and mass in sintered UO2 pellets using the efficiency transfer method

Gamma spectrometric measurements to determine the isotopic composition and total uranium mass in UO2 pellets (D = 7.5 mm; H = 3.5 mm, ρ = 10 g/cm3) were carried out. The required efficiency curve was obtained by applying the efficiency transfer method from a calibration standard (D = 65 mm; H = 20 mm) of a slightly acidified water solution. The average isotopic composition of ten UO2 pellets was consistent with values of natural uranium given by IUPAC. The average relative bias for the 235U/238U amount ratio was −0.73% using the 1001 keV gamma line for 238U and 0.50% using the 63 keV gamma line (186 keV was always used for 235U). For the total uranium mass, the mean deviation as compared to mass determinations using a balance was 5.5% using the 1001 keV gamma line for 238U and 4.3% using the 63 keV gamma line.

No evidence for expansion of global ocean euxinia during the base Stairsian mass extinction event (Tremadocian, Early Ordovician)

A Tremadocian (Early Ordovician, base Stairsian North American Stage) mass extinction event is recorded globally in rocks from several ancient continents and is accompanied by a globally correlated positive carbon isotope excursion (CIE; the largest during the Early Ordovician). In this study, elemental concentrations and uranium isotope compositions (δ238U) were measured for carbonate samples from three sections (along a proximal-to-distal transect: Ibex area, Shingle Pass, Meiklejohn Peak, respectively) in the Great Basin to test the role of ocean anoxia/euxinia on the base Stairsian mass extinction event. Dysoxic/suboxic to oxic water columns and non-sulfidic porewaters are inferred for these sections based on multiple local redox proxies (e.g., Mo/Al, U/Al, Mo and U concentrations, Mo/U ratios).Carbonate δ238U data exhibit different δ238U trends in the three sections. The proximal Ibex area exhibits a small negative δ238U excursion (a magnitude of 0.2–0.3‰) and the Shingle Pass section has one sample with unusually low δ238U, in both cases within the middle of the CIE. Samples with the lowest δ238U in each section also have the highest Mn/Sr ratios, suggesting diagenetic alteration associated with adsorption of isotopically light U onto Mn oxides, followed by dissolution of Mn oxides by reducing porewaters and capture of the isotopically light U by sediments. The distal Shingle Pass and Meiklejohn Peak sections better preserve global ocean signals and the limited δ238U variations in these sections imply no significant changes in global seawater δ238U and thus no significant changes in the extent of global ocean euxinia. Given the non-sulfidic porewaters inferred for these carbonates, we apply a sediment–seawater δ238U offset of 0–0.27‰ (the 0.27 ± 0.14‰ isotopic offset observed for modern Bahamas carbonates with sulfidic porewaters is used as a maximum constraint) to the average carbonate δ238U from the two distal sections (−0.48 ± 0.09‰) to derive a coeval seawater δ238U of between −0.75 ± 0.09‰ and −0.48 ± 0.09‰. A three-sink U isotope mass balance model suggests 0.2–15.8% global euxinic seafloor area existed during deposition of these Tremadocian carbonates. Our study implies that a small biotic crisis during a small CIE was not associated with an expansion of global ocean euxinia, in contrast to major Phanerozoic mass extinctions that were commonly associated with significant expansion of ocean euxinia during large carbon perturbations (e.g., the Late Devonian). In addition, we propose that the post-SPICE Cambrian and Early Ordovician oceans were characterized by limited changes in ocean euxinia and only a few episodes of expanded ocean suboxia-anoxia during carbon perturbations. The “biomere” events within this period, including the base Stairsian event, were probably related to these episodes of expanded suboxic-anoxic oceans and/or other ecological stresses.

238U/235U in calcite is more susceptible to carbonate diagenesis

The uranium isotopic composition (δ238U) of bulk marine calcium carbonates has been extensively explored as a promising paleoredox proxy to track the extent of global oceanic anoxia in deep time. Multiple studies have examined whether primary calcium carbonates can directly capture seawater δ238U and whether bulk measurements of recent and ancient carbonates preserve seawater U isotope signatures. Here we assess the role of diagenesis in altering δ238U signatures in carbonates sediments that have a primary calcitic mineralogy at the Paleocene-Eocene Thermal Maximum (PETM), an interval with rapid global warming and oceanic deoxygenation at ∼56 million years ago.Although primary abiotic and biogenic calcium carbonates (aragonite and calcite) can directly capture seawater δ238U with small offsets (<0.1‰) relative to modern seawater, diagenetic alteration of Bahamian shallow-water platform carbonate sediments that have a predominantly primary aragonitic mineralogy resulted in significantly larger offsets (up to 0.6‰). Since U concentration in primary aragonite is at least one order of magnitude higher than primary calcite (>1 ppm vs. <0.1 ppm), δ238U in calcite should be even more susceptible to diagenesis than that in aragonite.We find strong evidence of this effect in analysis of δ238U in PETM shallow-water carbonate sediments from Drilling Project (ODP) Hole 871C (Limalok Guyot, Pacific Ocean). Our results reveal large fluctuations in bulk carbonate δ238U from −0.69 to +0.71‰ around the PETM boundary but consistently heavier δ238U (between −0.14 and +0.47‰) than modern seawater outside of this interval. The significantly lighter δ238U values than modern seawater were interpreted to result from the operation of a Mn oxide shuttle. The heavier δ238U values are most likely caused by authigenic reductive accumulation of U(IV) in pore waters below the sediment-water interface. We found that carbonate δ238U values higher than modern seawater tend to increase with increasing U/Ca. This relationship is well-explained by an authigenic reductive accumulation model that simply assumes addition to primary calcite during diagenesis of calcitic cements containing isotopically heavier U(IV).Our work confirms expectations that δ238U in primary calcite is more susceptible to the amount of diagenetic cementation compared to primary aragonite, and that variations of δ238U in carbonate sediments with a primary calcitic mineralogy would more dominantly reflect the local redox state of depositional and early diagenetic environments. It is essential to identify the original carbonate mineralogy, the diagenetic history, and constrain the redox state of local deposition environments of sedimentary carbonate rocks when applying bulk carbonate δ238U as a global proxy for oceanic anoxia in deep time.

Uranium Isotopic Composition and Constraints on the Provenance of the Qinghai‐Tibet Plateau's Surface Dust

AbstractThe production and emission of aeolian dust can provide key information on the contemporary and historical links between geological evolution and climate. Here we use 234U/238U isotope ratios and the uranium (U) comminution age method on samples collected from Qinghai‐Tibet Plateau (TP) surface dust and glacier snowpack/cryoconites to determine the erosion and potential transport provenance of the dust, regardless of petrological origin. The spatial variation of the (234U/238U) isotope ratios of TP dust showed relatively high values in the Songpan‐Ganzi‐Hoh Xil, Himalayan and the Kunlun‐Qaidam‐Qilian terranes, while showing relatively low (234U/238U) values in Lhasa and Qiangtang terranes, but among all samples the highest value appeared in the glaciation zone. Dust collected from nearby glacial areas with rugged terrain (e.g., the Qilian and Himalayan Mountains) have higher (234U/238U) values than those collected in the topologically flat non‐glacial areas. These differences are consistent with elevated erosion rates and dust comminution‐transport processes in glacial areas, which yield a major source for large quantities of fresh comminuted particles. Compared to sediments from other regions of the globe, the (234U/238U) values of the typical aeolian sinks (such as loess on the Loess Plateau and ice core dust, etc.) are generally lower in comparison to areas with high elevations where erosion rates are high. The (234U/238U) isotope values of TP dust tend to fall between the high‐elevation sites and the dust sinks, indicating the combined influence of short comminution times (tectonics, landscape dynamics, and glacial erosion) and long residence time on the TP. When compared with other central Asian dust sources, U‐Nd‐Sr isotope signatures in TP dust are distinct and can be thus used as an effective tracer of dust provenance. Using U isotopes, we show that dust originating from the TP undergoes long‐range transport and constitutes potentially significant component of the Asian and Northern Hemisphere atmospheric dust load.

Cell design for laser-induced breakdown spectroscopy measurements on reactive gas samples

Uranium hexafluoride (UF6) is the primary material used for the enrichment of uranium in the production of light water nuclear reactor fuels worldwide. Accurate, rapid quantification of uranium isotopic composition in nuclear materials is required for safeguards programs and nonproliferation purposes. One potential technique for isotopic measurements in uranium species in the field is laser induced breakdown spectroscopy (LIBS). Safe and effective application of LIBS to UF6 for enrichment measurements is uniquely challenging due to the chemical and physical properties of UF6, which necessitate specific handling procedures. The objective of this work is to design a cell for isotopic analysis of UF6 that is (1) compatible with chemical and physical properties of UF6, (2) compatible with LIBS laser, and (3) portable-sized for nuclear safeguards applications. Along with cell design, initial testing of the cell for basic performance and chemical compatibility is performed. As designed and constructed, the portable gas cell was gas-tight, chemically compatible with UF6, and withstood long-duration laser exposure. The cell has proven capability for handling reactive gasses, such as UF6, with specification application to isotopic analysis.

Non-Equilibrium Uranium as an Indicator of Global Climate Variations—The World Ocean and Large Lakes

In natural water, as a rule, there is a violation of radioactive equilibrium in the chain 238U … → 234U → 230Th →. Groundwater usually has a 234U/238U ratio in the range of 0.8–3.0 (by activity). However, in some regions, the 234U/238U ratio reaches &gt;10 and up to 50. Ultrahigh excesses of 234U can be explained by climatic variations. During a cold period, minerals accumulate 234U as a normal component of the radioactive chain, and after the melting of permafrost, it is lost from the mineral lattice faster than 238U due to its higher geochemical mobility. This hypothesis was tested using data on the isotopic composition of uranium in the chemo- and bio-genic formations of the World Ocean and large lakes, which are reservoirs that accumulate continental runoff. The World Ocean has the most significant 234U enrichments in the polar and inland seas during periods of climatic warming in the Late Pleistocene and Holocene. In the bottom sediments of Lake Baikal, the 234U/238U ratio also increases during warm periods and significantly exceeds the 234U excess of the World Ocean. Furthermore, the 234U/238U ratio in the water of Lake Baikal and its tributaries increases from north to south following a decrease in the area of the continuous permafrost and has a seasonal variation with a maximum 234U/238U ratio in summer. The behavior of 234U in large water reservoirs is consistent with the hypothesis about the decisive influence of permafrost degradation on the anomalies in 234U/238U ratios in groundwater.

30\xa0years of IRMM-1027 reference materials for fissile material accountancy and control: development, production and characterisation

The IRMM-1027 Large-sized dried (LSD) spikes are certified reference materials applied to measure the uranium and plutonium content of dissolved fuel solutions using isotope dilution mass spectrometry. High quality starting metals of uranium and plutonium are dissolved to produce a stock solution, which is dispensed into individual vials and dried down. The present spikes are mixtures of typically 50 mg 20% enriched U and 2 mg enriched 239Pu as dried nitrates, conditioned in an organic substance for stability. Each vial of an IRMM-1027 LSD spike comes with certified masses of uranium (235U and 238U) and 239Pu and isotopic composition with associated uncertainty. This article will discuss major developments since the production of the first batch of LSD spikes and will reflect on the current preparation and certification approaches.

Uranium isotopes as a possible tracer of terrestrial authigenic carbonate

The concentration and isotopic composition of uranium (δ238U, 234U/238U activity ratio) in combination with traditional isotopes (δ18O, δ13C) were examined as potential tracers of authigenic carbonate formation in a karst aquifer. The U concentration and 234U/238U activity ratios in the tufa-precipitating sections of two connected karst rivers (Krka and Zrmanja, Croatia) decreased downstream in water and in precipitated carbonate due to active self-purification processes, i.e. adsorption of isotopically lighter U(VI) on mineral particles, sedimentation and co-precipitation with carbonate. The isotopic composition of carbonate in tufa mostly resembled the 234U/238U activity ratio and the δ238U values of dissolved U in water but was also affected by the presence of detrital carbonate flushed into the river from soil and weathered bedrock. This interpretation was supported by the δ18O and δ13C values of tufa, which were shifted out of equilibrium with river water and dissolved in organic carbon and in their isotopic signature, which showed the presence of lithic carbonate. Large fluctuations of the δ238U values of water, leachable U (eluted in acetic acid buffered with Na-acetate) and residual U fraction could not be fully explained by available data due to the overlapping U isotopic signatures of leachable (mainly carbonate) and residual fractions of soil, bedrock and tufa. Therefore, a long-term, systematic, seasonal and event-based observation of the isotopic composition of dissolved and suspended particulate U in water is necessary. Nevertheless, the U isotopes were found to have the potential to be used as identifiers of authigenic carbonate and the storage of CO2 in terrestrial river sediments, to improve knowledge on fluxes within local and global biogeochemical carbon cycle.