Determination of uranium isotopic composition and 236U content of soil samples and hot particles using inductively coupled plasma mass spectrometry

As a result of the accident at the Chernobyl nuclear power plant (NPP) the environment was contaminated with spent nuclear fuel. The ²³⁶U isotope was used in this study to monitor the spent uranium from nuclear fallout in soil samples collected in the vicinity of the Chernobyl NPP. Nuclear track radiography was applied for the identification and extraction of hot radioactive particles from soil samples. A rapid and sensitive analytical procedure was developed for uranium isotopic ratio measurement in environmental samples based on double-focusing inductively coupled plasma mass spectrometry (DF–ICP–MS) with a MicroMist nebulizer and a direct injection high-efficiency nebulizer (DIHEN). The performance of the DF–ICP–MS with a quartz DIHEN and plasma shielded torch was studied. Overall detection efficiencies of 4×10^–4 and 10^–3 counts per atom were achieved for ²³⁸U in DF–ICP–QMS with the MicroMist nebulizer and DIHEN, respectively. The rate of formation of uranium hydride ions UH⁺/U⁺ was 1.2×10^–4 and 1.4×10^–4, respectively. The precision of short-term measurements of uranium isotopic ratios (n = 5) in 1 μg L^–1 NBS U-020 standard solution was 0.11% (²³⁸U/²³⁵U) and 1.4% (²³⁶U/²³⁸U) using a MicroMist nebulizer and 0.25% (²³⁵U/²³⁸U) and 1.9% (²³⁶U/²³⁸U) using a DIHEN. The isotopic composition of all investigated Chernobyl soil samples differed from those of natural uranium; i.e. in these samples the ²³⁶U/²³⁸U ratio ranged from 10^–5 to 10^–3. Results obtained with ICP–MS, α- and γ-spectrometry showed differences in the migration properties of spent uranium, plutonium, and americium. The isotopic ratio of uranium was also measured in hot particles extracted from soil samples.     

Rapid separation and purification of uranium and plutonium from dilute-matrix samples

This work describes a streamlined approach to the separation and purification of trace uranium and plutonium in environmental swipe samples that contain a small amount of collected bulk material. We describe key modifications to conventional techniques that result in a relatively rapid, safe, cost-effective, and efficient U and Pu separation process. Simulated samples were produced by loading appropriate ²³⁵U, ²³⁸U, and ²⁴⁰Pu onto high purity cotton swipes. Uranium concentration and isotopic composition were measured by multi-collector inductively coupled mass spectrometry. Corresponding plutonium measurements were conducted with a three stage thermal ionization mass spectrometer. Quantitative U and Pu recoveries were observed with this method.     

Determination of rare earth elements, thorium and uranium by inductively coupled plasma mass spectrometry and strontium isotopes by thermal ionization mass spectrometry in soil samples of Bryansk region contaminated due to Chernobyl accident

The present paper describes the inductively coupled plasma mass spectrometric (ICP-MS) determination of rare earth elements (REEs), thorium and uranium in forest, pasture, field and kitchen garden soils from a Russian territory and in certified reference materials (JLK-1, JSD-2 and BCR-1). In addition to concentration data, strontium isotopic composition of the soil samples were measured by thermal ionization mass spectrometry. The measurements contributed to the understanding of the background levels of these elements in an area contaminated due to Chernobyl accident. There was not a significant variation in the concentration of REEs at different depth levels in forest soil samples, however, the ratio of Th/U varied from 3.32 to 3.60. Though concentration of U and Th varied to some extent, the ratio did not show much variation. The value of ⁸⁷Sr/⁸⁶Sr ratio, was in the top layer soil sample relatively higher than in the lower layers.     

A combined method of neutron activation analysis and radiometric measurements for <Superscript>234</Superscript>U and <Superscript>238</Superscript>U determination in soil samples of low uranium concentration

A method that combines the use of non-destructive neutron activation analysis and high-resolution α spectrometry has been developed for determination of the activities of ²³⁴U and ²³⁸U in geological samples of low uranium content. The ²³⁸U content is determined by k₀-based neutron activation analysis, whereas the ²³⁴U/²³⁸U relationship is measured by α spectrometry after isolation and electrodeposition of the uranium extracted from a lixiviation with 6 M HCl. The main advantage of the method is the simplicity of the chemical operations, including the fact that the steps destined to assure similar chemical state for the tracer and the uranium species present in the sample are not necessary. The method was applied to soil samples from sites of the North Peru Coast. Uranium concentration range 3–40 mg/kg and the isotopic composition correspond to natural uranium, with about 10% uncertainty.     

Determination of uranium isotopic composition and 236U content of soil samples and hot particles using inductively coupled plasma mass spectrometry

As a result of the accident at the Chernobyl nuclear power plant (NPP) the environment was contaminated with spent nuclear fuel. The 236U isotope was used in this study to monitor the spent uranium from nuclear fallout in soil samples collected in the vicinity of the Chernobyl NPP. Nuclear track radiography was applied for the identification and extraction of hot radioactive particles from soil samples. A rapid and sensitive analytical procedure was developed for uranium isotopic ratio measurement in environmental samples based on double-focusing inductively coupled plasma mass spectrometry (DF-ICP-MS) with a MicroMist nebulizer and a direct injection high-efficiency nebulizer (DIHEN). The performance of the DF-ICP-MS with a quartz DIHEN and plasma shielded torch was studied. Overall detection efficiencies of 4 x 10(-4) and 10(-3) counts per atom were achieved for 238U in DF-ICP-QMS with the MicroMist nebulizer and DIHEN, respectively. The rate of formation of uranium hydride ions UH+/U+ was 1.2 x 10(-4) and 1.4 x 10(-4), respectively. The precision of short-term measurements of uranium isotopic ratios (n = 5) in 1 microg L(-1) NBS U-020 standard solution was 0.11% (238U/235U) and 1.4% (236U/238U) using a MicroMist nebulizer and 0.25% (235U/238U) and 1.9% (236U/P38U) using a DIHEN. The isotopic composition of all investigated Chernobyl soil samples differed from those of natural uranium; i.e. in these samples the 236U/238U ratio ranged from 10(-5) to 10(-3). Results obtained with ICP-MS, alpha- and gamma-spectrometry showed differences in the migration properties of spent uranium, plutonium, and americium. The isotopic ratio of uranium was also measured in hot particles extracted from soil samples.     

A comparative study of234U/238U alpha-activity ratios determined by alpha-spectrometry and calculated from mass spectrometric data

²³⁴U/²³⁸U α-activity ratios determined by α-spectrometry (AS) and those calculated from the atom ratio data using the half-life values are compared in some of the isotopic reference materials of uranium and a few other uranium samples. For α-spectrometry, electrodeposited sources were prepared and a large area passivated ion implanted (IPE) detector (450 mm²) was used for recording the α-spectra. The isotopic composition of U was determined by thermal ionisation mass spectrometry (TIMS) and the recommended half-life values of²³⁴U and²³⁸U were used to calculate the α-activity ratio. It is observed that²³⁴U/²³⁸U α-activity ratios calculated from the atom ratio data are consistently high, with a mean difference of about 5%, when compared to the α-spectrometry results. This discrepancy warrants confirmation by a few more laboratories and suggests redetermination of the half-life values of²³⁴U and²³⁸U.     

Effect of humic acid,pH and ionic strength on the sorption of Eu(III) on red earth and its solid component

A highly sensitive separation procedure has been developed to investigate uranium and thorium activities and their isotopic ratios in environmental water samples in Tokushima, Japan. Uranium and thorium isotopes in environmental water samples were simultaneously isolated from interfering elements with extraction chromatography using an Eichrom UTEVA™ resin column. After the chemical separation, activities of U and Th isotopes coprecipitated with samarium fluoride (SmF₃) were measured by α-spectrometry. It has been confirmed that uranium isotopes are isolated successfully from thorium decay chains by analyzing a test aqueous solution as a simulation of an environmental water sample. The separation procedure has been first applicable to the determination of U and Th activities and their isotopic ratios in a drinking well water named “Kurashimizu” in Tokushima City, Japan. The specific activities of ²³⁸U and ²³²Th in “Kurashimizu” were deduced to be within the upper limits of <0.31 and <0.19 mBq/l, respectively.     

Simultaneous analysis of uranium and plutonium using thermal ionization mass spectrometry

Simultaneous isotopic analysis of uranium and plutonium using thermal ionization mass spectrometer coupled to a multi-collector detection assembly with 9 Faraday cups has been reported earlier. Subsequently investigations have been carried out (1) to understand the applicability of correction methodologies available to account for the contribution of²³⁸Pu at²³⁸U and (2) to evaluate the effectiveness of these methodologies on the accuracy of²³⁵U/²³⁸U atom ratio being determined, particularly when samples containing different U/Pu atom ratios. Isotopic fractionation for both U and Pu in the simultaneous isotopic analysis has been compared with the results of the individual analysis of these elements. The different isotopic fractionation factors observed for U were attributed to different conditions of analysis. There was no significant difference in the isotopic fractionation patterns for Pu. The consideration to extend this method to actual samples from our observations on synthetic samples with diferent U/Pu atom ratios containing U and Pu isotopic reference standards is described.     

Determination of uranium isotopic composition in aqueous solutions by combined gamma-spectrometry and X-ray fluorescence

The possibility of use of combined gamma spectrometry and X-ray fluorescence for uranium isotopic analysis in dilute aqueous solutions has been studied. Uranium K X-rays were excited by gamma-rays of⁵⁷Co radionuclide source and were measured together with 185.7 keV gamma-rays of²³⁵U using a planar pure Ge detector. Uranium K_α1 fluorescent X-rays were used for the evaluation. The method was proved to be independent of total uranium concentration in the region of 8–20 mg U/ml. The procedure making use of measurement of 7 ml samples and allowing to achieve an accuracy better then 1% in the region of 0.4–4.5 at. % of²³⁵U is proposed.     

Determination of U isotope ratios in sediments using ICP-QMS after sample cleanup with anion-exchange and extraction chromatography

The determination of uranium is important for environmental radioactivity monitoring, which investigates the releases of uranium from nuclear facilities and of naturally occurring radioactive materials by the coal, oil, natural gas, mineral, ore refining and phosphate fertilizer industries, and it is also important for studies on the biogeochemical behavior of uranium in the environment. In this paper, we describe a quadrupole ICP-MS (ICP-QMS)-based analytical procedure for the accurate determination of U isotope ratios (²³⁵U/²³⁸U atom ratio and ²³⁴U/²³⁸U activity ratio) in sediment samples. A two-stage sample cleanup using anion-exchange and TEVA extraction chromatography was employed in order to obtain accurate and precise ²³⁴U/²³⁸U activity ratios. The factors that affect the accuracy and precision of U isotope ratio analysis, such as detector dead time, abundance sensitivity, dwell time and mass bias were carefully evaluated and corrected. With natural U, a precision lower than 0.5% R.S.D. for ²³⁵U/²³⁸U atom ratio and lower than 2.0% R.S.D. for ²³⁴U/²³⁸U activity ratio was obtained with less than 90 ng uranium. The developed analytical method was validated using an ocean sediment reference material and applied to an investigation into the uranium isotopic compositions in a sediment core in a brackish lake in the vicinity of U-related nuclear facilities in Japan.     

Soil and sediment sample analysis for the sequential determination of natural and anthropogenic radionuclides

A new sequential method for the determination of both natural (U, Th) and anthropogenic (Sr, Cs, Pu, Am) radionuclides has been developed for application to soil and sediment samples. The procedure was optimised using a reference sediment (IAEA-368) and reference soils (IAEA-375 and IAEA-326). Reference materials were first digested using acids (leaching), ‘total’ acids on hot plate, and acids in microwave in order to compare the different digestion technique. Then, the separation and purification were made by anion exchange resin and selective extraction chromatography: transuranic (TRU) and strontium (SR) resins. Natural and anthropogenic alpha radionuclides were separated by uranium and tetravalent actinide (UTEVA) resin, considering different acid elution medium. Finally, alpha and gamma semiconductor spectrometer and liquid scintillation spectrometer were used to measure radionuclide activities. The results obtained for strontium-90, cesium-137, thorium-232, uranium-238, plutonium-239+240 and americium-241 isotopes by the proposed method for the reference materials provided excellent agreement with the recommended values and good chemical recoveries. Plutonium isotopes in alpha spectrometry planchet deposits could be also analysed by ICPMS.     

Isotopic uranium analysis in urine samples by alpha spectrometry

Urine assay is the preferred method for monitoring accidental or chronic internal intake of uranium into the human body. A new radiochemical separation procedure has been developed to provide isotopic uranium analysis in urine samples. In the procedure, uranium is co-precipitated with hydrous titanium oxide (HTiO) from urine matrix, and is then purified by anion exchange chromatographic column. Alpha spectrometry is used for isotopic uranium analysis after preparation of a thin-layer counting source by cerium fluoride micro-precipitation. Replicate spike and procedural blank samples were prepared and measured to validate the procedure. The ²³²U tracer was utilized for chemical recovery correction, and an average recovery of 76.2 ± 8.1% was found for 1400 mL urine samples. With 48 h of counting, the minimum detectable activity concentrations were determined to be 0.43, 0.21 and 0.42 mBq/L for ²³⁸U, ²³⁵U and ²³⁴U, respectively.     

Isotopic analysis of single uranium and plutonium particles by chemical treatment and mass spectrometry

The isotopic composition of single uranium and plutonium particles was measured with an inductively coupled plasma mass spectrometer (ICP-MS) and a thermal ionization mass spectrometer (TIMS). Particles deposited on a carbon planchet were first analyzed with an energy dispersive X-ray spectrometer (EDX) attached to a scanning electron microscope (SEM) and then transferred on to a silicon wafer using a manipulator. The particle on the silicon wafer was dissolved with nitric acid and the isotopic ratios of U and Pu were measured with ICP-MS and TIMS. The results obtained by both methods for particles of certified reference materials showed good agreement with the certified values within the expected uncertainty. The measurement uncertainties obtained in this study were similar for both mass spectrometric methods. This study was performed to establish the method of particle analysis with SEM, EDX, the particle manipulation and chemical preparation technique, and the measurement of isotopic ratios of U and Pu in a single particle by mass spectrometry.     

Chemical and radiochemical characterization of depleted uranium in contaminated soils

The main results of chemical and radiochemical characterization and fractionation of depleted uranium in soils contaminated during the Balkan conflict in 1999 are presented in the paper. Alpha-spectrometric analysis of used depleted uranium material has shown the presence of man-made radioisotopes ²³⁶U, ²³⁷Np, and ^{239, 240}Pu traces. The fractionation in different soil types was examined by the application of a modified Tessier’s five-step sequential chemical extraction procedure, specifically selective to certain physical/chemical associations. After ion-exchange-based radiochemical separation of uranium, depleted uranium is distinguished from naturally occurring uranium in extracts on the basis of the isotopic activity ratios ²³⁴U/²³⁸U and ²³⁵U/²³⁸U and particular substrates for recently present uranium material in soils are indicated. The text was submitted by the authors in English.     

Establishing the traceability of radioactivity standards to the National Institute of Standards and Technology (NIST)

Uranium content of soil samples collected from different states of India, namely Uttar Prades, Rajasthan and Kerala has been estimated. The areas cover the normal background area, high background area and dumpyards of fertilizer factory and thermal power plant. Plastic sheets were used as a detector for the registration of fission tracks resulting from the (n, f) reaction on²³⁵U present in the sample due to the thermal neutrons from a nuclear reactor. The uranium concentration has been found to vary from 0.24 to 9.20 μg/g in various soil samples. Higher levels of uranium were found in the vicinity of a coal fired power plant. The present results may be useful for the radiation hygiene.     

Isotopic “fingerprints” for natural uranium ore samples

A set of six samples, collected worldwide from various uranium ore mining facilities, was analysed for uranium isotopic composition by high accuracy isotope mass spectrometry. The goal of this article was twofold: to measure isotopic variations between samples of different geographical origin and to produce calibrated isotope ratios with the smallest achievable uncertainty (as defined according to the ISO Guide to the Expression of Uncertainty in Measurement). In the first step, the molar ratio of the isotopes ²³⁵U and ²³⁸U, n(²³⁵U)/n(²³⁸U), was measured using a UF₆-gas-inlet isotope mass spectrometer (VARIAN MAT 511). This instrument was calibrated against gravimetrically prepared synthetic isotope mixtures thus allowing SI-traceable measurements to be made. The ratios of the “minor isotopes” to ²³⁸U [n(²³⁴U)/n(²³⁸U) and n(²³⁶U)/n(²³⁸U)] were determined in a second step using a thermal ionisation mass spectrometer with high abundance sensitivity (Finnigan MAT262-RPQ-PLUS). The mass-fractionation correction was done internally using the result of the n(²³⁵U)/n(²³⁸U) measurement. As a result, the complete measured uranium isotopic composition is traceable to the SI system. For all ratios n(²³⁴U)/n(²³⁸U), n(²³⁵U)/n(²³⁸U), and n(²³⁶U)/n(²³⁸U) significant differences for samples of different origin were found. Regarding the n(²³⁶U)/n(²³⁸U) results, only two samples, one of them from the Oklo reactor in Gabon, showed significant presence of ²³⁶U. For all other samples an upper limit for n(²³⁶U)/n(²³⁸U) of about 6 × 10⁻¹⁰, mainly dependent on the instrumentation, was found. As a result of this study we propose values for the isotope abundances of natural uranium for the “Best Measurement from a Single Terrestrial Source” and the “Range of Natural Variations” in the IUPAC-table of the “Isotopic Composition of the Elements.”     

High resolution characterization of uranium in sediments by DGT and DET techniques ACA-S-12-2197

Diffusive equilibrium (DET) and diffusive gradient in thin film (DGT) techniques with an inductively coupled plasma mass spectrometry detection of elements were applied to characterize uranium, manganese, iron and ²³⁸U/²³⁵U isotopic ratio depth profiles in sediment pore water at high spatial resolution and to monitor uranium uptake/remobilization processes in uranium spiked sediment core samples under laboratory, well controlled conditions. Modified constrained sediment DGT probes, packed with Spheron-Oxin^® resin gel, were employed for selective uranium measurements. Spatially resolved DET and DGT responses were indicative of local redistribution of uranium in naturally uranium poor and rich sediments.     

Determining the isotopic compositions of uranium and fission products in radioactive environmental microsamples using laser ablation ICP–MS with multiple ion counters

This paper presents the application of a multicollector inductively coupled plasma mass spectrometer (MC–ICP–MS)—a Nu Plasma HR—equipped with three ion-counting multipliers and coupled to a laser ablation system (LA) for the rapid and sensitive determination of the ²³⁵U/²³⁸U, ²³⁶U/²³⁸U, ¹⁴⁵Nd/¹⁴³Nd, ¹⁴⁶Nd/¹⁴³Nd, ¹⁰¹Ru/(⁹⁹Ru+⁹⁹Tc) and ¹⁰²Ru/(⁹⁹Ru+⁹⁹Tc) isotope ratios in microsamples collected in the vicinity of Chernobyl. Microsamples with dimensions ranging from a hundred μm to about 1 mm and with surface alpha activities of 3–38 mBq were first identified using nuclear track radiography. U, Nd and Ru isotope systems were then measured sequentially for the same microsample by LA–MC–ICP–MS. The application of a zoom ion optic for aligning the ion beams into the ion counters allows fast switching between different isotope systems, which enables all of the abovementioned isotope ratios to be measured for the same microsample within a total analysis time of 15–20 min (excluding MC–ICP–MS optimization and calibration). The ¹⁰¹Ru/(⁹⁹Ru+⁹⁹Tc) and ¹⁰²Ru/(⁹⁹Ru+⁹⁹Tc) isotope ratios were measured for four microsamples and were found to be significantly lower than the natural ratios, indicating that the microsamples were contaminated with the corresponding fission products (Ru and Tc). A slight depletion in ¹⁴⁶Nd of about 3–5% was observed in the contaminated samples, but the Nd isotopic ratios measured in the contaminated samples coincided with natural isotopic composition within the measurement uncertainty, as most of the Nd in the analyzed samples originates from the natural soil load of this element. The ²³⁵U/²³⁸U and ²³⁶U/²³⁸U isotope ratios were the most sensitive indicators of irradiated uranium. The present work yielded a significant variation in uranium isotope ratios in microsamples, in contrast with previously published results from the bulk analysis of contaminated samples originating from the vicinity of Chernobyl. Thus, the ²³⁵U/²³⁸U ratios measured in ten microsamples varied in the range from 0.0073 (corresponding to the natural uranium isotopic composition) to 0.023 (corresponding to initial ²³⁵U enrichment in reactor fuel). An inverse correlation was observed between the ²³⁶U/²³⁸U and ²³⁵U/²³⁸U isotope ratios, except in the case of one sample with natural uranium. The heterogeneity of the uranium isotope composition is attributed to the different burn-up grades of uranium in the fuel rods from which the microsamples originated. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

The determination of uranium in biological materials by neutron activation analysis using the fission product134I

A method for the determination of uranium based on²³⁵U thermal neutron fission, has been developed and employed on samples of ashed fish tissue and seaweed. The method involves a post-irradiation ion exchange separation of iodine isotopes. The 884 keV photopeak of¹³⁴I is used for measurement. Uranium detection limits in the samples concerned have been estimated to be 1·10⁻⁸g in terms of natural uranium. The precision achieved in analysing several series of 3–5 samples was 4–10 per cent. The accuracy of the method was tested by employing an independent neutron activation procedure based on²³⁹U measurement. The accuracy of both methods was checked by analysing NBS SRM 1571 ‘Orchard Leaves’.     

The determination of plutonium by mass spectrometry using a [242]-plutonium tracer

An isotopic dilution method is described for the determination ot plutonium in samples of irradiated uranium using a ²⁴²Pu tracer. An aliquot of tracer is added to the sample and the mixture treated to ensure isotopic exchange; plutonium is then separated by an ion exchange procedure and an isotopic analysis made using an M.S.5. mass spectrometer. The precision (3 σ) for an aliquot containing 0.1 μg plutonium is 0.6%. A possible application of the method would be its use for control analyses of the feed solution in a chemical plant processing natural uranium fuel elements as, for example, the Windscale primary separation plant.