Uranium Isotopes Research Articles

Determination of Uranium Isotopic Ratios by HRGS Using Various Efficiency Calibration Approaches

The effect of various efficiency calibration approaches on the value and source of the HRGS measurement uncertainty of 234U/238U, 235U/238U, 234U/235U isotopic ratios for the purposes of technological control, nuclear forensics, and environmental monitoring has been studied. The Canberra Broad Energy Germanium detector BEGe3830 and five samples of uranium certified reference materials CRM 969 and CRM 146 with a content of 235U/U ranging from 0.7 to 20.0 wt. % have been used. To calculate the uranium isotope ratios, the acquired gamma spectra were processed using: commercial MGAU (LLNL), FRAM (LANL), ISOCS software (Canberra/Mirion Technologies), based on intrinsic and absolute efficiency calibration approaches. It has been found that maximum relative biases, for the 234U/238U and 234U/235U isotopic ratios determined using the MGAU \ FRAM \ ISOCS software, are ~ 25 % \ ~ 10 % \ ~ 10 %, and the random uncertainty is varied within ± [18-25 % \ 2-15 % \ <=3 %], respectively. In the case of 234U/238U isotopic ratio determination using the MGAU \ FRAM \ ISOCS software, maximum relative biases come to ~ 3 % \ ~ 4 % \ ~ 1 %, and the random uncertainty values decrease to ± [1 % \ 1 % \ 1 %], respectively. In the present paper, we propose a combined intrinsic efficiency calibration approach with the use of the polynomial functions for the analytical description of the relation εrel.i(Ei). In this approach maximum relative biases, in the determination of the 234U/238U and 234U/235U isotopic ratios is 2.7 % at a random uncertainty of <= 1 %, and in the case of the 234U/238U ratio a maximum relative deviation is 0.5 % at a random uncertainty of <= 0.7 %.

Geochemical exploration for buried sandstone-hosted uranium mineralization using mobile U and Pb isotopes: case study of the REB deposit, Great Divide Basin, Wyoming

Mobile radiogenic lead isotopes ( 206 Pb, 207 Pb, 208 Pb and 210 Pb) represent products of radioactive decay of their parental uranium and thorium isotopes ( 238 U, 235 U, 232 Th), and are considered potential geochemical pathfinders of buried sandstone-type uranium deposits. Soil samples collected along a geochemical traverse intersecting buried uranium roll front mineralization at the REB deposit in the Great Divide Basin, Wyoming, USA were studied. Mineralization of this deposit is hosted in weakly lithified arkosic sands, at a depth of 120–200 m, without a strong surficial expression of its presence at depth, which makes discovery of this deposit type difficult, slow and expensive. All soil samples have been analysed for ratios of the mobile long-lived Pb isotopes and their parental U and Th isotopes, determined from partial leach products obtained using a weak acid leaching technique. The samples were also analysed for trace elements, assayed both in the partial leach products and using conventional whole-soil sample assays. Ratios of the mobile radiogenic Pb isotopes to their parental U and Th isotopes ( 206 Pb/ 238 U, 207 Pb/ 235 U and 208 Pb/ 232 Th) determined in the partial leach products exhibit anomalous contents in the soil samples collected above the uranium rolls. The anomalous values are several times greater than background values, to lateral distances of 350–400 m outside of the roll fronts. Notably, conventional whole-soil assays have failed to detect the anomalies that were detected using mobile Pb isotopes. Supplementary material : Minor and trace elements assays of the geochemical samples are available at https://doi.org/10.6084/m9.figshare.c.5610980

Variable local basin hydrography and productivity control the uranium isotope paleoredox proxy in anoxic black shales

Geochemical proxies—such as the isotopic compositions and abundances of redox-sensitive metals, organic biomarkers, and sedimentological indicators—can be used qualitatively and quantitatively to reconstruct marine redox landscapes through Earth history. Isotopic proxies with residence times longer than ocean mixing times, including uranium (238U/235U, commonly denoted as δ238U), are a promising but still developing approach to constrain global changes in redox conditions. Our current understanding of the controls on δ238U variability and associated isotopic fractionations is limited, complicating the interpretation of δ238U records through Earth history. With these gaps in mind, we investigate the major controls on the expression of uranium (U) isotope fractionation within two basins of the Miocene Monterey Formation (ca. 18–6 Ma) that represent contemporaneous deposition under different environmental conditions. Our finding that the isotopic offset in the productive and anoxic Santa Barbara Basin is similar to that in modern euxinic settings suggests that highly productive settings without persistent euxinia can also exert significant leverage on seawater δ238U. Distinct patterns in U concentrations and δ238U between basins demonstrate that local depositional controls can impart a strong influence on the U isotope offset in reducing settings, as has been observed in modern sediment datasets. High productivity and low sedimentation rates are predicted to result in a diagnostic inverse relationship between U enrichment and δ238U, which can partially explain the overall weak negative correlation between [U] and δ238U in the distal Naples Beach section in the Santa Barbara Basin. In a core from a more restricted proximal setting in the San Joaquin Basin, we propose that changes in basin hydrography led to a potential relationship between [U] and δ238U. These interpretations are consistent with the characteristic depositional conditions interpreted independently for each basin (e.g., persistence of anoxia, productivity, sedimentation rates, phosphate content, and basin hydrography) and are supported by modeling results of U enrichment and isotope fractionation under these conditions. In particular, our results suggest that the isotope offset of U in reducing sinks varies and has done so over Earth history and in different paleogeographic locations as a result of local productivity and basin restriction. These variations indicate the need to consider these uncertainties in quantitative models of global redox conditions from δ238U records—and specifically in using δ238U to distinguish between euxinic and oxic redox states without considering other oxygen-depleted environments as part of a continuum of settings that can impact δ238U if spatially widespread. This study also highlights the utility of using different lithologies to evaluate global versus local controls on δ238U records as well as multi-proxy approaches to constrain ancient redox landscapes.

SPATIAL NONUNIFORM DISTRIBUTION OF 235U ISOTOPE AT SUPERCRITICAL FLUID EXTRACTION WITH CARBON DIOXIDE IN A GRADIENT TEMPERATURE FIELD

The spatial redistribution of the 235U isotope of natural uranium in a gradient temperature field along the height of the reactor in supercritical carbon dioxide has been experimentally investigated. The scheme of the reactor is given and the principle of operation of the reactor is described. The method of preparation of initial samples from granite samples containing natural uranium and the procedure of extraction are described. The conclusion about the spatial redistribution of 235U isotopes in supercritical carbon dioxide is based on the analysis of gamma spectra of extracts. It is shown that the concentration of the 235U isotope in a supercritical fluid is maximal near the lower heated flange of the reactor, and decreases with approaching the upper, cooled flange. It was concluded that the separation factor of the 235U isotope in supercritical carbon dioxide can be about 1.2 ± 0.12.

The effects of low-[formula omitted] shielding on uranium isotope discrimination using the time-emission profiles of long-lived delayed neutrons

Characteristic radiation signals can often be obscured or distorted by relatively small amounts of shielding, as is the case for highly-enriched uranium (HEU). Methods that can detect and identify special nuclear material (SNM) even in the presence of shielding are therefore of high value in many nuclear security and nonproliferation applications. It has been previously shown that the buildup and decay time profiles of long-lived delayed neutron emission from induced fission are unique and can be used to successfully discriminate between uranium isotopes and infer enrichment without the need for a calibration standard. Because the long-lived delayed neutron precursors exhibit decay times on the order of tens of seconds, their time profiles are not sensitive to the delays associated with scattering or diffusion in shielding materials, which occur on much shorter time scales. The use of capture-based neutron detectors can also mitigate the effects of shielding, because unlike recoil-based detectors, they do not have a low-energy detection threshold, so that neutrons that lose their energy in the shielding are still detectable. In this work, the effects of low-Z shielding on the discrimination of uranium isotopes using delayed neutron time profile measurements are investigated for the first time. The delayed neutron buildup and decay time profiles of polyethylene-shielded HEU and depleted uranium objects are measured using two different capture-gated detector designs. The recorded profiles show good agreement with the shapes predicted from six-group delayed neutron models even with the shielding in place, and the enrichment of the HEU and DU objects was estimated accurately to within 5%. While this method shows potential for discriminating and characterizing extreme cases of enrichment when a moderate amount of shielding is present, simulation results suggest that significant limitations exist for intermediate enrichment levels due to distortions to the delayed neutron time profile caused by neutron reflection in the shielding.

Prediction of Europium Retention in Perovskite: Potential Candidates for an Engineering Barrier in the Disposal of Radioactive Waste

Perovskites, such as tausonite, are crystalline metal oxides with excellent optical and photocatalytic properties and have also been used successfully in the retention of metals, simulating the isotopes of uranium and plutonium. In this work, different pseudo-order and thermodynamic models were studied to achieve the prediction of the sorption of Eu3+ (chemical analogous for actinides) in tausonite. The effects of gamma irradiation and temperature on the structural characteristics of the material were determined, as an additional step in the evaluation of material as an engineering barrier in the disposal of radioactive waste. The results obtained show that the tausonite is resistant to the gamma irradiation and thermal energy. Likewise, it was possible to determine that europium sorption occurs through an exothermic and spontaneous reaction, as well as through the formation of surface complexes, where Eu3+ ions bind to sites on the tausonite by dipole-dipole interaction. Furthermore, it was shown that the sorption mechanism is influenced by diffusive phenomena, which participate in the formation of surface complexes. Additionally, a new sorption model with respect to pH was proposed, which allowed determining the physical parameter π. The evidence obtained suggests that π is a physical parameter that relates pH to an optimal value and could explain the equilibrium between the surface complexes that tausonite forms with europium. Likewise, the evidence suggests that 50 kg of tausonite would have the capacity to retain at least 26.59 g of alpha-emitting radionuclides, equivalent to a waste package (900 kg) with a maximum activity of 4000 Bq/g.

Uranium remobilisation in anoxic deep rock-groundwater system in response to late Quaternary climate changes – Results from Forsmark, Sweden

Unusually high uranium (U) concentrations (up to 175 μg/L) have been measured in groundwater at depths between 400 and 650 m at the Forsmark site, eastern Sweden. Since it is unlikely that such high concentrations formed under the stagnant and low redox groundwater conditions that currently prevail, this study employs U-series isotopes to understand how the recent evolution (<1 Ma) of the flow system has influenced the observed U distribution. Material from fractures as deep as 700 m along the assumed flow route was subject to U-series disequilibrium (USD) measurements, as well as sequential extractions (SE) and U redox-state analyses that revealed the U-series activity ratios in the bulk and soluble fraction of the fracture precipitates. Uranium isotope data collected over several years of annual groundwater monitoring were scrutinized to evaluate the U sources and U exchange in fractures located in high-U groundwater sections. Numerical simulations with the experimental data were used to study evolution of U-series isotope composition in a fracture in the highest U section at ~500 m depth under various U mobility scenarios. The results show that U redistribution in fractures with certain dissolution/deposition flux ratios during periodic water intrusions, driven by glaciation and deglaciation events during the last 120 ka, can explain the U anomaly in the groundwater.

New constraints on mid-Proterozoic ocean redox from stable thallium isotope systematics of black shales

Stable thallium (Tl) isotope data from organic-rich siliciclastic sedimentary rocks have the potential to track ocean redox state on a broad scale. Here, we report new Tl isotope data from the Mesoproterozoic Velkerri Formation (Roper Group) and the Paleoproterozoic Wollogorang Formation (Tawallah Group), McArthur Basin, Northern Territory, Australia, and interpret these in the context of rhenium-osmium (Re-Os) geochronometry on the same sample suite. Previous work has shown that marine black shales from the Velkerri Formation provide evidence for closed-system Re-Os systematics, yielding a precise isochron with an age of 1361 ± 21 Ma that agrees well with independent age constraints for the unit. The isotopic composition of authigenic Tl in euxinic black shales from the upper Velkerri Formation (ε205Tl = −2.4 ± 0.8, 2SD) indicates that the Tl isotope composition of local seawater at 1.36 Ga was within a plausible range for Tl inputs to the ocean. Isotope mass-balance modeling of the Tl isotope system within a Monte Carlo framework suggests that the Tl isotopic composition of seawater at 1.36 Ga was homogenous on a global scale and that the burial of Mn-oxides exerted minimal isotopic leverage on the Tl isotope composition of seawater at 1.36 Ga. Taken together with existing Mo, Cr, and U isotope data from the same samples, these observations are consistent with a low-O2 ocean-atmosphere system during this period of the Mesoproterozoic.Previous work has shown that the Re-Os systematics of black shales from the older (1.73 Ga) Wollogorang Formation are scattered and yield an erroneously young isochron age of 1359 ± 150 Ma, which has been attributed to post-depositional hydrothermal alteration at ∼1640 Ma. We observe no systematic relationship between stable Tl isotope compositions and the extent of alteration as gauged by open-system Re-Os behavior (−4.7 ± 1.4 for the upper Wollogorang Formation and −4.8 ± 0.4 for the lower Wollogorang Formation), in marked contrast to previous observations for the molybdenum (Mo) and uranium (U) isotope systems. The invariant signature of the Tl isotope data suggests the Tl isotope system was largely unperturbed during hydrothermal alteration. However, it remains difficult to definitively rule out the possibility that authigenic Tl isotope signatures have been overprinted by later localized hydrothermal fluid alteration in the Wollogorang Formation shales. These observations highlight the potential insights afforded by evaluating open-system behavior via radiogenic isotope systems together with other stable isotope tracers in efforts to reconstruct the redox landscape of Earth’s oceans over time.

Special Nuclear Material Identification Through One-Minute Measurement with a New Backpack Radiation Device in Real Scenario Conditions

The constant concerns in global nuclear safety, aimed at deterring and combating the illicit trafficking of Material Out of Regulatory Control (MORC) and its possible use in criminal acts has raised the necessity of new detection solutions with higher efficiency and resolution to provide a high level of accuracy in the report to the authorities. Today’s radioactive isotope identifiers perform gamma spectroscopy identification and, sometimes, neutron counting. This paper presents the results of measurements performed in real-scenario conditions with a new type of portable radioactive isotope identifier for the detection and identification of both gamma and neutron sources. Its singular features are: 1) the capability to identify sources through the detection of neutrons, discriminating spontaneous fission sources (Cf-252), α-n sources (Am/Be, Am/Li) and nuclear material containing mix of isotopes of plutonium or uranium 2) the capability to make cross correlation between gamma and neutron measurements to achieve a higher level of accuracy in the identification of SNM that emits both neutrons and characteristics gammas. The test results are compared with international standards. The device exceeds the standard performance by triggering a neutron alarm for a 20.000 n/s Cf-252 source at a five times greater distance than the ANSI N42.34 one.

Investigations on the Synthesis of Different Isotopes of Uranium Using Fusion Reactions

Investigations on the Synthesis of Different Isotopes of Uranium Using Fusion Reactions

Application of HRGS for forensic characterization of uranium oxides, pure uranium metals and uranium alloys

A nondestructive iterative method for uranium-bearing material characterization with HRGS developed earlier in Burdeinyi et al. (2020) is applied to determine matrix densities, uranium mass fraction and uranium isotope masses of uranium ore, UO2 and U3O8 powders, fuel elements in the form of UO2 microspheres, uranium metal and uranium alloys. It is shown that U3O8 powders with uranium mass fraction of about 84% can be distinguished from the powders of UO2 with uranium mass fraction of about 87%; uranium products in the form of liquid or loose powder with matrix density of 0.5–2.0g/cm3 can be distinguished from uranium products in the form of compacted fuel elements with matrix density of 6.0–10.0g/cm3 and from pure metal uranium and uranium alloys with matrix density of 14.0–19.0g/cm3. In fuel microspheres based on UO2 the uranium mass fraction 88.02% measured by HRGS is consistent, within the measurement uncertainties, with the results of isotope dilution mass spectrometry 87.76±0.64% and also is confirmed by X-ray diffraction technique. The uranium mass fraction of the uranium ore estimated as 0.08% by HRGS is consistent, within the measurement uncertainties, with the value 0.09±0.01% determined with WDXRF. Densities of two different uranium metal samples, estimated as 18.42g/cm3 and 19.33g/cm3 by HRGS are consistent with values 18.24±0.55g/cm3 and 18.86±0.59g/cm3, respectively, obtained by the gas pycnometry technique.

Investigation of UO2 fuel efficiency for ABV small modular reactor

ABSTRACT In recent years, small modular reactors have received considerable attention in various countries due to having some capabilities such as small dimensions. The ABV reactor is a small modular reactor with a power of 45 MWth. For this reactor, various applications such as use as a desalination plant, source of electrical and thermal energy, etc. have been considered. In this reactor, an unconventional metal fuel called silumin with 16.5% enrichment and a cycle length of 3000 days has been used. The main purpose of this research is to investigate the performance of conventional UO2 fuel while maintaining the initial cycle length as the main fuel of ABV reactor. To achieve this purpose, the performance of UO2 fuel with enriches of 8%, 10%, 12%, 14%, and 16% with different percentages of Gd2O3 has been investigated. To perform the calculations, the RELAP5 thermohydraulic code and the PARCS neutronic code have been coupled by using a Parallel Virtual Machine, and the reactor core quantities included: effective multiplication factor, relative power distribution, feedback coefficients, plutonium and uranium isotope concentrations, axial and radial temperature distribution of fuel rod, heat flux and DNBR have been calculated and is within an acceptable range for this design.

Development of computer code ADWITA and data library for the solution of transmutation chain equations and application to the analysis of nuclear fuel cycles

A fuel cycle analysis code, ADWITA (Activation, Decay, Waste Incineration and Transmutation Analysis) has been developed which can generate isotopic inventory, calculate radioactivity, and decay heat based on irradiation history. The code can be used for fuel cycle design and transmutation studies. The system of linearized transmutation chain equations is solved by application of Bateman series solution method and the matrix exponential method in which the matrix exponential is approximated by the exponential power series. The required data libraries for Pressurised Heavy Water Reactor (PHWR) and Advanced Heavy Water Reactor (AHWR) have also been developed. The code and the data have been validated using the analytical decay of 237Np, the PHWR fuel cycle benchmark data available in IAEA TECDOC and experimental measurement of isotopic composition and decay heat in AECL report. The code has been applied for the analysis of fuel cycle of AHWR which has been designed for the utilisation of thorium as nuclear fuel on large scale and envisages fuel reprocessing. The application of code ADWITA and its library to AHWR has helped to understand the unique features of AHWR fuel cycle. It has been found that in AHWR the activity and decay heat due to actinides and fission product (FP) in both (Th, 233U) MOX and (Th, Pu) MOX fuel is similar at the discharge but the activity and decay heat due to actinide takes over that due to FP in case of (Th, Pu) MOX earlier. The accumulation of uranium isotopes at the end of burnup in individual (Th, 233U) and (Th, Pu) MOX fuel pin has been estimated which is an important input to the reprocessing of these fuels. The rise in activity after reprocessing due to presence of 232U has also been estimated.

New Aspects of Uranium Ore Concentration by using Acidic Media Solutions

AbstractThe present investigation aims to compare the leaching characteristics of uranium isotopes in acidic and alkaline dissolving solutions with different concentrations. The leachate of uranium is prepared from sedimentary rocks collected from SW Sinai, Egypt. The uranium isotope activity concentration was measured using gamma counting recorded by the HPGe detector. Additionally, activity ratios as well as the leaching efficiency of different uranium isotopes were calculated. Results have shown that sulfuric acid at 40 % has been able to outpace other concentrations in dissolving a higher ratio of uranium isotopes with a 1 : 4 solid to acid ratio and 2 days as soaking time.234Th contributes to the 234U activity causes a significant efficiency difference as opposed to 235U &amp; 238U in prepared bearing solutions. The recorded pH values in a bearing solution of different samples at investigated leaching conditions were oscillated from −1.2 to 4 due to lead impurities diffusion to bearing solutions.

Carbonate associated uranium isotopes as a novel local redox indicator in oxidatively disturbed reducing sediments

The interpretation of local redox indicators in marine sediments relies on their preservation. By their nature, such biogeochemical tracers are susceptible to diagenetic alteration, particularly through post-depositional re-oxidation of the sediment. This can result in the mobilization and loss of distinctive redox dependant signatures, leading to the classical problem that the presence of oxygenated conditions cannot be robustly inferred by the absence of evidence for anoxia. Authigenic uranium enrichments, and their isotope signatures (δ238U), are widely used to infer bottom-water and pore-water redox conditions but are susceptible to later oxidative diagenetic disturbance. Here we explore the preservation of authigenic δ238U signatures in sediment samples from Sapropel S1 at Site 64PE406-E1 in the Eastern Mediterranean Sea (~1760 m water depth), which was originally deposited under reducing conditions but severely affected by later oxidative diagenesis and the variable loss of authigenic U(IV) from the upper sapropel. To this end, we compare U isotope signatures from bulk measurements (δ238Ubulk) with detrital corrected authigenic U (δ238Uauth) and carbonate associated U (δ238UCAU). In contrast to open ocean carbonates deposited under fully oxic conditions, sapropelic carbonate leachates yield δ238UCAU similar to calculated δ238Uauth, thus recording (predominantly) redox dependant isotope fractionation. There is no evidence for significant isotope fractionation resulting from post-depositional oxidative diagenesis, but associated authigenic U(IV) removal results in larger relative detrital contributions to δ238Ubulk, and hence large uncertainties on δ238Uauth estimates. By contrast, δ238UCAU successfully avoids detrital phases, and the uncertainty associated with detrital corrections, providing a primary record of changes in local redox conditions. Thus, we propose that δ238UCAU can be used for accurate reconstructions of benthic de-oxygenation in sediments with low U enrichments, including sediments with post-depositional U(IV) loss and less reducing environments.

High-precision measurement of U-Pu-Np-Am concentrations and isotope ratios in environmental reference materials by mass spectrometry

We report results of precise and sensitive mass spectrometric measurements of uranium, plutonium, neptunium, and americium concentrations and isotope ratios in a variety of environmental reference materials. Most of our work has been done on NIST SRM 4350b, River Sediment, but we also present results for NIST SRM 4354, Lake Sediment; NIST SRMs 4355 and 4355a, Peruvian Soil; NIST SRM 4357, Ocean Sediment; NIST SRM 1648a, Urban Particulate Material; NIST SRM 1649b, Urban Dust; IAEA CRM 385, Ocean Sediment; USGS BCR-2, Columbia River Basalt; and USGS BHVO-2, Hawaiian Volcanic Observatory Basalt. These materials reflect a wide range in long-lived actinide concentrations (e.g. 1E4 to 1E10 atoms 239Pu/g) and isotope ratios. Measurements were performed in a clean laboratory by isotope dilution, multi-collector thermal ionization and multi-collector inductively coupled plasma mass spectrometry. In general, our results are in agreement with, but lower the uncertainty of, literature or certificate values for these reference materials. Our uranium results for the basalts also confirm previously reported high-precision mass spectrometric results from our laboratory. In many cases our measurements of U-Pu-Np-Am nuclides appear to be novel.Extensive results for NIST SRM 4350b, River Sediment, indicate that this material is heterogeneous for Pu-Np-Am concentrations and isotope ratios at a sample size of 5 g or lower. Pu–Np isotope ratios and a241Pu-241Am model age of 1954 reflect a mix of plutonium production operations at the nearby Hanford, Washington site, and global atmospheric fallout from nuclear weapons testing. Results for the oceanic sediment materials (NIST SRM 4357 and IAEA 385) collected near Sellafield, U.K. vary but are also indicative of local anthropogenic sources of varying Pu isotopic composition and a mean 241Pu-241Am model age of 1964. Large environmental fractionation between Pu and Np is observed for the ocean, river, and lake sediment reference materials. Novel measurements for the two air particulate SRMs indicate high U, Pu and Np concentrations for these collections in 1976–1977 with an anomalous regional fallout Pu isotopic signature. Results for BHVO-2 and other Hawaiian basalts indicate that those which erupted before or during the period of abundant atmospheric nuclear weapons testing (1950–1970) contain significant levels of Pu (on the order of 1E7 atoms 239Pu/g) with a global fallout Pu isotopic composition, compared to more recent eruptions which incorporated less Pu. Hence, Hawaiian basalts may provide an integrated temporal record of anthropogenic actinide fallout deposition from the atmosphere since eruption.

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.

Anoxic depositional overprinting of 238U/235U in calcite: When do carbonates tell black shale tales?

Abstract The fidelity of uranium isotopes (δ238U) in marine carbonates as a paleoredox proxy relies on whether carbonates can record and preserve seawater δ238U. Although modern carbonate sediments deposited under oxic conditions have been shown to track seawater δ238U, it remains unknown whether this is true for carbonates deposited under anoxic conditions. This is a crucial question because many ancient carbonates were likely deposited or reworked under anoxic bottom waters. To better understand the behavior of uranium isotopes under this scenario, we investigated U isotope geochemistry in the meromictic Fayetteville Green Lake (FGL; New York, USA), where primary calcite is precipitated from oxic surface waters, sinks past the chemocline, and is deposited under anoxic bottom waters. We observed significant depletions of dissolved U concentration (from 2.7 to 0.9 ppb) and δ238U (from –0.55‰ to –0.96‰) below the chemocline in FGL. Parallel with these depletions, δ238U of sediment traps increased progressively from –0.51‰ to –0.16‰, suggesting that U(VI) reduction was occurring in the anoxic water column. Carbonate sediments deposited under anoxic bottom waters were enriched in U by 6–18× compared to primary calcite. Our data suggest that such significant authigenic U enrichments resulted from U(VI) reduction in the anoxic water column and below the sediment-water interface. The δ238U value in the top 0.25 cm of sediments was –0.29‰ ± 0.10‰, overprinting original δ238U in primary calcite (–0.51‰ ± 0.02‰). Future applications of carbonate δ238U as a paleoredox proxy should consider depositional environments (oxic vs. anoxic) of carbonates.

An improved method for determination of isotope ratios in individual uranium particles by thermal ionization mass spectrometry

The determination of isotope ratios of individual uranium particles in environmental swipe samples is very important for nuclear safeguards. A new technique to measure isotope ratios for individual uranium particle was developed in this work, which was a combination of particle transfer by scanning electron microscope (SEM) and isotope ratios analysis by thermal ionization mass spectrometry (TIMS). The technique was verified by measuring isotope ratios in individual micrometer-size uranium particles of known isotopic abundance. The particles were identified by SEM combined with energy dispersive X-ray spectrometer (EDX) and transferred by micromanipulator, which could improve the efficiency of sample preparation and simplify the procedure. The experimental condition was optimized by using a new kind of thermal ion emitter to enhance the ionization efficiency of uranium. Several individual uranium particles from certified reference materials were successfully measured by SEM-TIMS. The results show that the measured isotope ratios were in good agreement with the reference values. The relative errors of 13 particles were within 2.7% for 234U/238U, 1.1% for 235U/238U and 4.5% for 236U/238U isotope ratios, respectively, and the relative standard deviation (RSD) were within 1.6% for 234U/238U, 0.5% for 235U/238U and 3.3% for 236U/238U, respectively. It is expected that this method will become a promising alternative technique for determining uranium isotope ratios in particle analysis. The particles used in this study had sizes between 1.3 and 4.7 μm.

Direct Uranium Isotopic Analysis of Swipe Surfaces by Microextraction-ICP-MS.

The ability to directly measure uranium isotope ratios on environmental swipes has been achieved through a solution-based microextraction process and represents a significant advancement toward the development of a rapid method to analyze international nuclear safeguard samples. Here, a microextraction probe is lowered and sealed onto the swipe surface, and analytes within the sampling site (∼8 mm2) are dissolved and extracted into a flowing solvent of 2% nitric acid (HNO3). The mobilized species are subsequently directed into an inductively coupled plasma-mass spectrometer (ICP-MS) for accurate and precise isotope ratio determination. This work highlights the novelty of the sampling mechanism, particularly with the direct coupling of the microextraction probe to the ICP-MS and measurement of uranium isotope ratios. The preliminary method detection limit for the microextraction-ICP-MS method, utilizing a quadrupole-based MS, was determined to be ∼50 pg of 238U. Additionally, precise and accurate isotope ratio measurements were achieved on uranium reference materials for both the major (235U/238U) and minor (234U/238U and 236U/238U) ratios. While the present work is focused on directly measuring uranium isotopic systems on swipe surfaces for nuclear safeguards and verification applications, the benefits would extend across many applications in which direct solid sampling is sought for elemental and isotopic analysis.