Environmental monitoring as an important tool for safeguards of nuclear material and nuclear forensics

Summary The use of environmental monitoring as a technique to identify activities related to the nuclear fuel cycle has been proposed by international safeguards organizations. The elements specific for each kind of nuclear activity, or “nuclear signatures”, inserted in the ecosystem can be intercepted by different live organisms. This work demonstrates the technical viability of using pine needles as bioindicators of nuclear signatures associated with uranium enrichment activities. Additionally, it proposes the use of HR-ICP-MS to identify the signature corresponding to that kind of activities in the ecosystem. Nitric acid solutions, used to wash pine needles sampled near nuclear facilities and containing only 0.1 mg ^. kg^-1 of uranium, exhibit a n(²³⁵U)/n(²³⁸U) isotopic abundance ratio of 0.0092±0.0002, while solutions originated from samples collected at places located more than 200 km far from activities related to the nuclear fuel cycle exhibit a value of 0.0074±0.0002. Similar results were obtained for sample solutions prepared using the acid leaching process. The different values of n(²³⁵U)/n(²³⁸U) isotopic abundance ratio obtained permit to confirm the presence of anthropogenic uranium and demonstrate the viability of using the methodology proposed in this work.     

Reagent grade uranium salts: Isotopic composition by neutron activation

A rapid non-destructive neutron activation technique for the determination of the²³⁸U/²³⁵U ratio is described. Reagent grade uranium salts from commercial sources have a widely variable²³⁸U/²³⁵U ratio. The isotopic composition of uranium found in such salts is quite different from the natural value. This difference is largely due to the use of by-product uranium depleted in²³⁵U.     

Measurements of238U,234U and230Th in impure carbonates for age determination

The carbonate cements of conglomeritic deposits of late Pleistocene age have been leached with 0.2N hydrochloric acid and analyzed radiochemically. The leachate and the residue fractions were separately measured for²³⁸U,²³⁴U and²³⁰Th, using isotope-dilution and alpha-spectrometric techniques. The data are used to estimate the isotopic activities of uranium and thorium in the carbonate phase. These activities give age information for the carbonate cementation. Ages in the range of 185–320·10³ years were obtained for the samples studied.     

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.     

New methodology for uranium analysis in swipe samples for nuclear safeguards purposes

Environmental swipe sampling for safeguards purpose has been used by International Atomic Energy Agency since 1997, being a powerful tool to detect undeclared materials and activities. This work describes a new methodology for swipe samples analysis based on ultrasound-assisted acid leaching and compares it with traditional total digestion bulk analysis. The proposed method requires few preparation steps, decreasing the risk of contamination, reduced amounts of reagents and a good option to extract uranium from swipe sample. In a real case study, the swipe samples were collected in a conversion plant at IPEN/CNEN, Brazil. The measurements were carried out by ICP-MS and the results showed relative error lower than 0.96 % for uranium isotopic ratios for the certified reference material (NBS U200). The uncertainties were estimated by following the ISO GUM, with a confidence level of 95 %. The uncertainties percentage for n(²³⁵U)/n(²³⁸U) ratio of the samples ranged from 2.5 to 4.3 %. The values of uranium isotopic ratio obtained for each method demonstrate the viability of using the methodology proposed in this work.     

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.     

Determination of isotopic ratios of uranium samples using passive gamma spectroscopy with multiple detectors

Uranium samples of various enrichments have been passively counted on the University of Texas detector gamma–gamma coincidence system. By observing gamma rays emitted from ²³⁵U and its daughters compared to gamma rays emitted by ²³⁸U daughters and comparing the data to standards of known enrichments, a technique has been developed to take a uranium sample of unknown enrichment and passively count it to determine its uranium isotopic concentration. Because the gamma rays from ²³⁵U are generally in the low-energy regime, there is a strong susceptibility to background interferences, especially from the Compton background produced from higher energy gamma rays. Other interferences, such as those from the decay series of uranium also exist for ²³⁵U gamma rays. In this light, we have collected data using list-mode to produce two-dimensional gamma–gamma coincidence spectra, which allows us to gate the low-energy gamma rays from ²³⁵U with gamma rays that are in coincidence. In doing this, much of the low energy interferences are reduced, and one can analyze the ²³⁵U gamma rays with high precision. Because of the high density of uranium, self-shielding has significant effects especially in the low-energy regime. To correct for this attenuation the detector system has been modeled by MCNP and self-shielding factors have been calculated across the energy spectrum. A big advantage to this method is the capability of performing this analysis with small (<1 g) samples in a non-destructive and relatively inexpensive manner. If necessary, this analysis can be performed within 24 h if an urgent nuclear forensics scenario arises.     

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.     

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.     

Isotopic and ultratrace analysis of uranium by double-focusing sector field ICP mass spectrometry

An analytical method for the ultratrace and isotopic analysis of uranium in radioactive waste samples using a double-focusing sector field ICP mass spectrometer is described. In high-purity water a detection limit for uranium in the lowest fg/mL range has been achieved. Under optimum experimental conditions (²³⁵U/²³⁸U ≈ 1), the precision in ²³⁵U/²³⁸U isotopic ratio determinations has been determined as 0.07% RSD. With the isotopic standard U-020 (²³⁵U/²³⁸U = 0.0208) a precision of 0.23% RSD at the 100 pg/mL level using ultrasonic nebulization has been achieved. With ²³⁴U/²³⁸U isotopic ratios of down to 10^–5, the values obtained by double-focusing sector field ICP-MS and alpha spectrometry were in agreement. Received: 27 February 1997 / Revised: 10 Juni 1997 / Accepted: 12 June 1997     

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.     

High precision instrumental neutron activation analysis of uranium by multiple gamma-ray peak ratio determination

The method of multiple γ-ray peak ratio determination has been applied to the nondestructive neutron activation analysis of uranium in rocks and ores. The photopeaks of²³⁹Np gamma-rays produced by the activation of²³⁸U and those of the fission products of²³⁵U are a measure of the quantity of uranium in the irradiated sample, provided that the uranium is of natural isotopic composition. The ratios between the integrated areas of the different photopeaks are calculated and compared with those obtained for a uranium standard. The uranium concentration in the sample is calculated from the photopeaks whose ratios correspond, within the error limits to those of pure natural uranium. High accuracy better than ±2% has been obtained.     

Uranium isotope dilution mass spectrometry using NBL certified reference materials as spikes

The isotope dilution mass spectrometry method of analysis is used to determine the elemental uranium contents in a wide variety of uranium bearing materials. The method is based on the mass spectrometric analysis of a mixture prepared by diluting the sample to be analyzed with a spike of distinctly different isotopic composition to that of the sample. In this work, a beginning is made to identify suitable candidates among the multitude of certified reference materials (CRMs) available at the New Brunswick Laboratory to supplant the use of ²³³U which remains now as the preferred spike nuclide. The results of the study presented here identify CRM 112-A (of normal isotopic composition) and CRM 115 (depleted uranium composition) as suitable candidates to replace ²³³U as spike material for determining uranium in high enriched uranium materials, and CRM 116 (²³⁵U mass fraction of >90 %) for determining uranium in materials of low enrichment.     

Accurate measurement of uranium isotope ratios in soil samples using thermal ionization mass spectrometry equipped with a warp energy filter

A chemical and mass-spectrometric procedure for uranium isotopic analysis using a thermal ionisation mass spectrometer equipped with a Wide Aperture Retardation Potential energy filter has been developed and applied to uranium isotopic measurements for various soil samples. Soil samples were digested using a microwave digestor. Uranium was isolated from soil samples by the chemical separation procedure based on the use of anion-exchange resin and UTEVA extraction chromatography column. The isotope ratios were measured for two certified reference materials by using a VG Sector 54-30 thermal ionisation mass spectrometer in dynamic mode with Faraday cup and Daly ion counting system. Replicates of standard reference materials showed excellent analytical agreement with established values supporting the reliability and accuracy of the method. Precision of the ²³⁵U/²³⁸U ratio was achieved by a correction factor of 0.22% amu as a function of ion-beam intensity with sample loads of around 250?ng of U. The resulting reproducibility for standards and soil samples was better than 0.2% at two standard deviations (SD). Uranium isotopic compositions have been determined in several reference soil samples such as Buffalo river sediment, NIST 2704, river sediment SRM 4350b and ocean sediment NIST-4357 and a Chernobyl soil sample. There was a significant deviation from the natural uranium in comparison with Chernobyl soil samples.     

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.     

Use of chromatographic pre-concentration for routine uranium bioassay analysis by ICP-MS

Routine monitoring of urine is an effective way to detect occupational intake of radioactive material. Historically, determinations of uranium isotopic ratios have been performed by radiochemical separation followed by alpha spectrometry. With recent advancements in technology, inductively coupled plasma-mass spectrometry (ICP-MS) has become widely available for the determination of trace metals as well as radioactive nuclides with long half-lives, such as ²³⁸U in urine. Furthermore, ICP-MS measurements of ²³⁸U do not require radiochemical separation since the number of atoms in the sample is determined instead of the number of alpha particles emitted. However, this method does not provide good sensitivity for the determination of ²³⁵U due to its shorter half-life. An improved procedure using pre-concentration of uranium and determination by ICP-MS decreases the detection limit by a factor of ten or greater with only slight increase in total analysis time. The method also has the capability of accurately determining the isotopic ratio of the sample, which is very important in cases where enriched or depleted uranium is involved.     

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.     

Anomalous behaviour of uranium isotopes in backwater sediments of Zuari river

The surface leaching of the labile component of uranium has been carried out in estuarine sediments of Zuari river in Goa. The measurements of alpha activities of²³⁸U,²³⁵U and²³⁴U in the leachates indicated a remarkable anomaly between the activities of²³⁸U and²³⁴U. The activity ratios of²³⁴U/²³⁸U in these leachates have been found to be in the range of 1.10 to 1.14. However, the activity ratios of²³⁵U/²³⁸U have been found to be 0.045 which is close to that in natural uranium. It has also been observed that the anomaly between²³⁸U and²³⁴U exists only on the surface organic layers of the backwater sediments of the Zuari river.     

Analysis of Ores and its Purified Constituents by γ‐Spectrometry with Calculation of Uranium Isotopic Atom,Mass, and Activity Ratios

The physical verifications, that the national and international inspectors carry out in order to perform a credibility control, often consist in the measurement of physical quantities, related to the declared nuclear material properties, by Non‐destructive Assay (NDA). Analysis of ores and its purified constituent's samples has been carried out in this work using non‐destructive gamma assay technique. The spectrometer based on HpGe detector and its electronics was calibrated using standard IAEA multi‐lines gamma sources. The efficiency calibration curve was plotted for broad gamma energies; 50–2600 keV. The gamma transition of ²³⁵U (143.7, 163.3, 185.7, and 205.3 keV) and ²³⁸U (63, 766.3, and 1001.03 keV) were used for qualitative and quantitative assay of the samples. The specific activities of the samples were calculated based on the determined efficiency, branching ratio (emission probability per disintegration), mass of sample and count rate of the characteristics gamma transitions of uranium isotopes at fixed geometrical conditions. A simplified equation was derived for calculation of ²³⁵U atom ratios. The results of calculation show natural origin of the analyzed samples; around 0.72 %. Where, the anthropogenic ²³⁶U was not detected at all in the spectra. The uranium activity ratios (²³⁵U/²³⁸U) were calculated based on the measured activity. The uranium isotopic mass and total uranium content of the investigated samples were also calculated. The results obtained are depicted, tabulated and discussed in comparison with recent published national and international works.     

Determination of uranium isotopes in environmental samples

The determination of isotopes of uranium by alpha spectrometry in different environmental components (sediments, soil, water, plants and phosphogypsum) is presented and discussed in this paper. The alpha spectrometry is a very convenient and good technique for activity concentration of natural uranium isotopes (²³⁴U, ²³⁵U, ²³⁸U) in environmental samples and provides the most accurate determination of isotopic activity ratios between ²³⁴U and ²³⁸U. The analysis were provided information about possible sources of high concentrations of uranium in the examined sites determined by anthropogenic sources. The calculation of values ²³⁴U/²³⁸U in all analyzed samples was applied to identifying natural or anthropogenic uranium origin. Activity concentration of uranium isotopes in analyzed environmental samples shows that measurement of uranium levels is of great importance for environmental and safety assessment especially in contaminated areas (phosphogypsum waste heap).