Search

International audience; Atmospheric nuclear tests (1945-1980) have led to radioactive fallout across the globe. French tests in Polynesia (1966-1974) may influence the signature of fallout in South America in addition to those conducted by USA and former USSR until 1963 in the Northern hemisphere. Here, we compiled the $^{240}$Pu/$^{239}$Pu atom ratios reported for soils of South America and conducted additional measurements to examine their latitudinal distributions across this continent. Significantly lower ratio values were found in the 20-45° latitudinal band (0.04 to 0.13) compared to the rest of the continent (up to 0.20) and attributed to the contribution of the French atmospheric tests to the ultra-trace plutonium levels found in these soils. Based on sediment cores collected in lakes of Chile and Uruguay, we show the added value of measuring $^{240}$Pu/$^{239}$Pu atom ratios to refine the age models of environmental archives in this region of the world.

International audience; In 2011, the Fukushima Dai-Ichi Nuclear Power Plant (FDNPP) accident released significant quantities of radionuclides into the environment. Japanese authorities decided to progressively reopen the Difficult-To-Return Zone after the decontamination of priority reconstruction zones. These areas include parts of the initially highly contaminated municipalities located to the north of the FDNPP, including Namie Town, an area drained by the Ukedo and Takase Rivers. Eleven years after the accident, research focused on the spatial distribution of plutonium (Pu) and radiocesium (Cs) isotopes at contrasted individual locations. To complement previous results, the current research was conducted on flood sediment deposits collected at the same locations after major flooding events during eleven fieldwork campaigns organised between 2013 and 2020 at the outlet of the Ukedo and Takase Rivers (n = 22).The results highlighted a global decrease of the Pu and 137Cs contents in sediment with time during the abandonment phase in the region, from 2013 (238.20 fg g−1) to 2020 (4.28 fg g−1). Furthermore, based on the analysis of the 240Pu/239Pu isotopic ratios, the plutonium transiting these rivers (range: 0.166 – 0.220) essentially originated from the global fallout (0.180 ± 0.014 (Kelley et al., 1999)). Sediment showed contrasted properties in the two investigated rivers, which is likely mainly the result of the occurrence of Ogaki Dam on upper sections of the Ukedo River as it strongly impacts the material supply from this river to the Pacific Ocean. A statistical analysis highlighted the strong correlation between Pu activity concentrations and 137Cs activities in both rivers, confirming that both radionuclides are transported with a similar pathway. Despite it was detected early after the accident (2011–2013), the current research demonstrates that plutonium originating from FDNPP is no longer detected in these rivers draining the Difficult-To-Return Zone at the onset of the reopening of the area to its former inhabitants.

In 2017, the Nuclear Forensics International Technical Working Group organized their fifth Collaborative Materials Exercise (CMX-5). The exercise samples were two uranium dioxide fuel pellets manufactured from the same starting materials by different processes to have similar bulk isotopic composition, but different spatial uranium isotopic distributions. Sets of identical materials were sent to all participating laboratories, who then utilized their existing nuclear forensic capabilities to independently analyse fuel pellets and identify similarities and differences of the materials’ characteristics. Here we present and compare the ability of different analytical techniques to spatially resolve uranium isotopic heterogeneity in the uranium oxide fuel pellets.

A highly-enriched (244)Pu isotope dilution reference material has been prepared and characterized for metrologically traceable measurements of very small quantities of plutonium. The amount of plutonium in samples associated with nuclear safeguards and nuclear forensic measurements can be significantly less than 1 ng. Accordingly, the ability to quantify the amount and isotopic composition of plutonium from a single mass-spectrometric analysis is particularly desirable. The highly-enriched (244)Pu reference material, described here, will minimize the magnitude of spike corrections necessary to obtain accurate information on plutonium isotopic composition from isotope dilution measurements.

A new determination of the (229)Th half-life was made based on measurements of the (229)Th massic activity of a high-purity solution for which the (229)Th molality had previously been measured. The (229)Th massic activity was measured by direct comparison with SRM 4328C using 4παβ liquid scintillation counting, NaI counting, and standard addition liquid scintillation counting. The massic activity was confirmed by isotope dilution alpha spectrometry measurements. The calculated (229)Th half-life is (7825 ± 87) years (k = 2), which is shorter than the three most recent half-life determinations but is consistent with these values within uncertainties.

International audience; The Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident released limited amounts of actinides on soils of Japan. Characterisation of these particles is essential to determine the fate of actinides in the environment. The method presented in this paper, based on α-tracks detections, microscope observations and mass-spectrometry measurements, was designed to identify and characterize actinide-bearing particles in soil samples. The method was tested on a road dust sample collected in the main radioactive plume of the Fukushima region. Accordingly, α-tracks detection was demonstrated to provide a powerful technique to localise these particles and prepare their morphological, elemental and isotopic characterization.

The

The Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident in Japan resulted in a major release of radionuclides into the environment. Compared to other radionuclides, few studies have investigated the fate of actinides in the environment. Accordingly, this research investigates the Pu composition in soil samples collected in paddy fields before and after the accident. Furthermore, the vertical distributions of Pu and U isotopic signatures, along with 137Cs activities, were measured in a sediment core collected in the Mano Dam reservoir, in the Fukushima Prefecture. Changes in the relative contributions of the major actinide sources (global fallout or FDNPP derived fallout) were investigated in sediment deposited in the reservoir. The distinct peak observed for all Pu isotope ratios (240Pu/239Pu, 241Pu/239Pu and 242Pu/239Pu) and for 137Cs concentrations in the sediment core was attributed to the Fukushima fallout, and coincided with the maximum atomic contribution of only 4.8 ± 1.0% of Pu from the FDNPP. Furthermore, 236U/238U ratios measured in the sediment core remained close to the global fallout signature indicating there was likely no U from the FDNPP accident detected in the sediment core. More research is required on the environmental dynamics of trace actinides in landscapes closer to the FDNPP where there are likely to be greater abundances of FDNPP-derived Pu and U.

Numerous applications require the precise analysis of U isotope relative enrichment in sample amounts in the sub-nanogram to picogram range, among those are nuclear forensics, nuclear safeguards, environmental survey and geosciences. However, conven-tional thermal ionization mass spectrometry (TIMS) yields U combined ionization and transmission efficiencies (i.e ratio of ions detected to sample atoms loaded) less than 0.1 or 2% depending on the loading protocol, motivating the development of sources capable of enhancing ionization. The new prototype cavity source TIMS at ETH offers improvements from 4 to 15 times in com-bined ionization and transmission efficiency compared to conventional TIMS, yielding up to 5.6 % combined efficiency. Uranium isotope ratios have been determined on reference standards in the 100 pg range bound to ion-exchange or extraction resin beads. For natural U standards, n(235U)/n(238U) ratios are measured to relative external precisions of 0.5 to 1.0 % (2RSD, 2

A new determination of the

International audience; The $^{236}$U/$^{238}$U isotope ratio is a widely used tracer, which provides information on source identification forsafeguard purposes, nuclear forensic studies and environmental monitoring. This paper describes an originalapproach to determine $^{236}$U/$^{238}$U ratios, below 10$^{−8}$, in environmental samples by combination of ICP-MS/MS for $^{236}$U/$^{238}$U ratio and multiple collector ICPMS measurements for $^{235}$U/$^{238}$U and $^{234}$U/$^{235}$U isotope ratios.Since the hydride form of U$^+$ (UOH$^+$) is less prone to occur than UH$^+$, we were focused on the oxidisedforms of uranium in order to reduce hydride based-interferences in ICP-MS/MS. Then, in-cell ion-moleculereactions with O$_2$ and CO$_2$ were assessed to detect the uranium isotopes in mass-shift mode (Q1: U$^+$ → Q2:UO$^+$). The performances in terms of UO$^+$ sensitivity and minimisation of hydride form of UO$^+$ were evaluatedusing five different desolvating systems. The best conditions, using an Apex $\Omega$ or an Aridus system, produceduranium oxide hydride rate ($^{235}$U$^{16}$O1H$^+$/$^{235}$U$^{16}$O+) of about 10$^{-7}$ with O$^2$ in the collision cell.The method was validated through measurements of two certified IRMM standards with $^{236}$U/$^{238}$U isotoperatio of 1.245×10$^{17}$ and 1.052×10$^{-8}$, giving results in agreement with certified reference values. The relativestandard deviations on seven independent measurements for each standard were respectively of 1.5% and6.2%. Finally, environmental samples corresponding to sediments from the radioactive contamination plumeemitted by the Fukushima Daiichi Nuclear Power Plant accident were analysed after a well-established uraniumchemical separation procedure. $^{236}$U/$^{238}$U atomic ratios between 1.5×10$^{-8}$ and 7×10$^{-9}$ were obtained with a level accuracy lower than 20%.

Capabilities of laser ablation coupled to a time of flight ICP-MS (‘icpTOF’, Tofwerk) for direct isotopic analysis of U micrometric reference particles deposited on carbon disks were demonstrated. Thanks to the very high temporal resolution which allows quasi simultaneous measurement of all nuclides, the icpTOF is well-suited for the acquisition of the very short transient signals produced by laser ablation of μm-sized particles. With appropriate corrections of the background due to high peak tailing of the 238U isotope and selection of signals within the detector's linear range, excellent internal and external precisions (0.2–0.4%) for measurement of the 235U/238U ratios were obtained in the most favorable cases (same abundances for the two isotopes). These precisions are better than the values reported until now in the literature, even with laser ablation coupled with multi-collector ICP-MS (MC-ICP-MS). For particles from other reference materials with low abundances of 235U, equal or close to the one of natural U, precisions for 235U/238U ratio measurements (4–5%) were not as good as the ones reported for MC-ICP-MS, but better than the ones obtained with single collector ICP-MS. Minor isotope ratios (234U/238U and 236U/238U) were also successfully measured, although with limited precision and accuracy when the abundances of the minor isotopes are very low, in the 10−5 range. This work shows that laser ablation coupled to ICP-TOF-MS has a great potential for ultra-precise isotope ratio measurements provided that the ablated area contains enough analyte and that abundances between isotopes are of the same order of magnitude.

International audience; The Fukushima Daiichi Nuclear Power Plant (FDNPP) accident resulted in a significant release of radionuclides that were deposited on soils in Northeastern Japan. Plutonium was detected at trace levels in soils and sediments collected around the FDNPP. However, little is known regarding the spatial-temporal variation of plutonium in sediment transiting rivers in the region. In this study, plutonium isotopic compositions were first measured in soils (n = 5) in order to investigate the initial plutonium deposition. Then, plutonium isotopic compositions were measured on flood sediment deposits (n = 12) collected after major typhoon events in 2011, 2013 and 2014. After a thorough radiochemical purification, isotopic ratios ($^{240}$Pu/$^{239}$Pu, $^{241}$Pu/$^{239}$Pu and $^{242}$Pu/$^{239}$Pu) were measured with a Multi-Collector Inductively Coupled Mass Spectrometer (MC ICP-MS), providing discrimination between plutonium derived from global fallout, from atmospheric nuclear weapon tests, and plutonium derived from the FDNPP accident. Results demonstrate that soils with the most Fukushima-derived plutonium were in the main radiocaesium plume and that there was a variable mixture of plutonium sources in the flood sediment samples. Plutonium concentrations and isotopic ratios generally decreased between 2011 and 2014, reflecting the progressive erosion and transport of contaminated sediment in this coastal river during flood events. Exceptions to this general trend were attributed to the occurrence of decontamination works or the remobilisation of contaminated material during typhoons. The different plutonium concentrations and isotopic ratios obtained on three aliquots of a single sample suggest that the Fukushima-derived plutonium was likely borne by discrete plutonium-containing particles. In the future, these particles should be isolated and further characterized in order to better understand the fate of this long-lived radionuclide in the environment.

Rationale A multi-collector inductively coupled plasma (MC-ICP) mass spectrometer coupled to a UV ns-laser ablation (LA) system was used to measure uranium isotopic ratios (234 U/238 U, 235 U/238 U and 236 U/238 U) in single uranium particles of various sizes and isotopic compositions, including home-made sub-micrometric natural uranium particles of narrow size distribution (415 ± 60 nm). Methods The LA-ICP mass spectrometer was operated in wet plasma conditions thanks to simultaneous injection of the laser aerosol and water vapor through a desolvating nebulizer. The isotopic ratios were corrected for mass bias and gain factors between detectors. The 236 U/238 U ratios were also corrected for the presence of 235 U hydrides and tailing of the 238 U+ peak. Results 236 U/238 U ratios were successfully measured in micrometer-sized particles from the NBS U050 certified standard material with a 236 U/238 U ratio of ~5 × 10-4 . The analysis of 77 natural uranium sub-μm-sized particles yielded a very good trueness with respect to the expected 234 U/238 U and 235 U/238 U ratios, while the measurement errors for single particles ranged from -2.7% to +2.1% for 235 U/238 U and from -17% to +33% for the 234 U/238 U ratios. Their relative combined standard uncertainties ranged from 3.3% to 32.8% and from 0.4% to 4.0% for 234 U/238 U and 235 U/238 U ratios, respectively. In addition, extremely low detection limits, in the attogram range, were achieved. Conclusions This study demonstrates that coupling of a ns-laser ablation system with a MC-ICP mass spectrometer allows measurements of the isotopic composition in natural uranium particles of a few hundreds of nm with very good trueness, average combined standard uncertainties of ~1% for 235 U/238 U ratios and 12% for 234 U/238 U ratios, and detections limits of a few ag for minor isotopes.

The design of a prototype thermal ionization cavity (TIC) source attached to the mass analyser of a MAT262 thermal ionization mass spectrometer is presented. In addition to a detailed calculation and experimental verification of the ion optics, the new design includes several innovative features that set it apart from previously reported TIC sources. The goal is to employ this new ion source for high-efficiency isotope ratio analysis. For uranium adsorbed to a single resin bead, an overall efficiency gain of about a factor of 10 compared to the same sample type analysed by state-of-the-art conventional thermal ionization mass spectrometry (TIMS) on a Triton instrument is reported (ions detected per atom loaded). First attempts at isotope ratio analysis of micrometre sized uranium oxide particles, as is commonly done for nuclear safeguards, seem to confirm the enhancement in overall efficiency and could help with the analysis of the minor uranium isotopes on such samples. (C) 2018 Elsevier B.V. All rights reserved.

The results of a joint effort by expert nuclear forensic laboratories in the area of age dating of uranium, i.e. the elapsed time since the last chemical purification of the material are presented and discussed. Completely separated uranium materials of known production date were distributed among the laboratories, and the samples were dated according to routine laboratory procedures by the measurement of the 230Th/234U ratio. The measurement results were in good agreement with the known production date showing that the concept for preparing uranium age dating reference material based on complete separation is valid. Detailed knowledge of the laboratory procedures used for uranium age dating allows the identification of possible improvements in the current protocols and the development of improved practice in the future. The availability of age dating reference materials as well as the evolvement of the age dating best-practice protocol will increase the relevance and applicability of age dating as part of the tool-kit available for nuclear forensic investigations.

In an inter-laboratory measurement comparison study, four laboratories determined 230Th–234U model ages of uranium certified reference material NBL U050 using isotope dilution mass spectrometry. The model dates determined by the participating laboratories range from 9 March 1956 to 19 October 1957, and are indistinguishable given the associated measurement uncertainties. These model ages are concordant with to slightly older than the known production age of NBL U050.

A new reference material was characterized for (229)Th molality and thorium isotope amount ratios. This reference material is intended for use in nuclear forensic analyses as an isotope dilution mass spectrometry spike. The reference material value and expanded uncertainty (k = 2) for the (229)Th molality is (1.1498 ± 0.0016) × 10(−10) mol g(−1) solution. The value and expanded uncertainty (k = 2) for the n((230)Th)/n((229)Th) ratio is (5.18 ± 0.26) × 10(−5) and the n((232)Th)/n((229)Th) ratio is (3.815 ± 0.092) × 10(−4).

In this article, the coupling of a UV-fs-LA system with a sector-field ICP-MS for isotopic analysis of individual sub micrometric and micrometric uranium particles, especially for the measurement of minor isotopes (234U and 236U), is discussed. Ablation of uranium particles results in short (a few seconds) and highly noisy signals with sudden and extremely brief (

In this paper, we will describe and indicate the performance of a new method based on the use of femtosecond laser ablation (fs-LA) coupled to a quadrupole-based inductively coupled plasma mass spectrometer (ICP-QMS) for analyzing the isotopic composition of micrometer-size uranium particles. The fs-LA device was equipped with a high frequency source (till 10 kHz). We applied this method to 1–2 μm diameter-uranium particles of known isotopic composition and we compared this technique with the two techniques currently used for uranium particle analysis: Secondary Ionization Mass Spectrometry (SIMS) and Fission Track Thermal Ionization Mass Spectrometry (FT-TIMS). By optimizing the experimental conditions, we achieved typical accuracy and reproducibility below 4% on 235 U/ 238 U for short transient signals of only 15 s related to 10 to 200 pg of uranium. The detection limit (at the 3 sigma level) was ~ 350 ag for the 235 U isotope, meaning that 235 U/ 238 U isotope ratios in natural uranium particles of ~ 220 nm diameter can be measured. We also showed that the local contamination resulting from the side deposition of ablation debris at ~ 100 μm from the ablation crater represented only a small percentage of the initial uranium signal of the ablated particle. Despite the use of single collector ICP-MS, we were able to demonstrate that fs-LA-ICP-MS is a promising alternative technique for determining uranium isotopic composition in particle analysis.

International audience; The isotope drift encountered on short transient signals measured by multicollector inductively coupled plasma mass spectrometry (MC-ICPMS) is related to differences in detector time responses. Faraday to Faraday and Faraday to ion counter time lags were determined and corrected using VBA data processing based on the synchronization of the isotope signals. The coefficient of determination of the linear fit between the two isotopes was selected as the best criterion to obtain accurate detector time lag. The procedure was applied to the analysis by laser ablation-MC-ICPMS of micrometer sized uranium particles (1–3.5 μm). Linear regression slope (LRS) (one isotope plotted over the other), point-by-point, and integration methods were tested to calculate the 235U/238U and 234U/238U ratios. Relative internal precisions of 0.86 to 1.7% and 1.2 to 2.4% were obtained for 235U/238U and 234U/238U, respectively, using LRS calculation, time lag, and mass bias corrections. A relative external precision of 2.1% was obtained for 235U/238U ratios with good accuracy (relative difference with respect to the reference value below 1%).

An in-house developed code, Sofie, and the methods used for numerical determination of the ionization efficiency of thermal ionization cavity (TIC) sources up to intermediate temperatures (\textless2500 K for cavity dimensions of a few millimetres) are presented, thus, creating a tool to identify cavity geometries offering better ionization efficiencies and to aid in matching the ion beam produced by a TIC source to the ion optics of a beam line or a mass analyser as used in conventional thermal ionization mass spectrometry (TIMS). Essential is the code's ability to approximate the solution of Poisson's equation, in which the space charge predominantly originates from electrons thermally emitted from the inner cavity walls. The assumption of a quasi-neutral plasma forming in the volume of the cavity is no longer valid at lower temperatures and/or cavity dimensions, and charged particles can be regarded as moving freely throughout the cavity only interacting via the electrostatic potential shaped by the particles' space charge. Under such conditions the geometry of the cavity has got a large effect on the ionization efficiency of sample atoms loaded into the source. The code also allows the simulation of surface adsorption and re-emission of various particle species simultaneously, making the calculation of ionization efficiencies possible by determining the currents of sample ions and neutral atoms out of the cavity. The validity of the code is successfully demonstrated by simulating problem sets, for which analytical solutions exist, and also by reproducing experimental data from TIC sources found in the literature. The necessity of including space charge in such simulations is demonstrated. A non-exhaustive search for a best cavity geometry has been performed and a geometry has been identified that could enhance the ionization efficiency by up to a factor of 100 over the ionization efficiency of the Saha-Langmuir equation, which corresponds to the efficiency achievable with conventional flat filament TIMS sources. (C) 2016 Elsevier B.V. All rights reserved.

In this study we describe a method for uranium dating (i.e. determination of the date of the last chemical purification undergone by the material) by measurement of the 230Th/234U ratio, applicable to sub-microgram quantities. The chosen protocol (AG1x8 resin in hydrochloric acid medium) has been tested on separation microcolumns (100 μl). This ‘microchemistry’ technique considerably limits the risks of contamination by reagents or the environment. Thorium extraction efficiencies were greater than 90 % and reproducible. The quantities of 230Th introduced by the chemical purification procedure were negligible. Using an ultra-sensitive inductively coupled plasma mass spectrometry measurement technique, detection limits of the order of femtograms (10−15 g) of 230Th were obtained. The complete procedure, chemical separation and isotope measurement, was successfully tested and validated on a few micrograms of uranium.

In this paper we describe the application of laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) coupling to particle analysis, i.e., the determination of the isotopic composition of micrometric uranium particles. The performances of this analysis technique are compared with those of the two reference particle analysis techniques: secondary ion mass spectrometry (SIMS) and fission track-thermo-ionization mass spectrometry (FT-TIMS), based on the measurement of the isotopic ratios of 235U/238U in particles present in an inter-comparison particulate sample. The agreement of the results obtained using LA-ICP-MS with target values and with the results obtained using FT-TIMS and SIMS was good. Accuracy was equivalent to that of the other two techniques (±3 % deviation). However, relative experimental uncertainties present with LA-ICP-MS (7 %) were higher than those present with FT-TIMS (4.5 %) and SIMS (3 %). Furthermore, measurement yield of LA-ICP-MS coupling was close to that obtained with the same quadrupole ICP-MS for the measurement of a liquid sample (~10−4), but lower than that obtained with FT-TIMS and SIMS, respectively, by a factor of 10 and 20, although the particles analyzed using LA-ICP-MS were most likely smaller (diameter ~0.6 μm, containing 4–7 fg of 235U). Nevertheless, thanks to the brevity of the signals obtained, the detection capacity for low isotopic concentrations by LA-ICP-MS coupling is equivalent to that of FT-TIMS, although it remains well below that of SIMS (×15). However, with more sensitive double focusing ICP-MS, performances equivalent to those achieved using SIMS could be obtained.

Formation of a polyatomic species made of an atom of a heavy element like lead, mercury or iridium, and atoms abundant in plasma (argon, nitrogen, oxygen, and hydrogen) when using an inductively coupled plasma-sector-field mass spectrometer (ICP-SFMS) may lead to false detection of femtograms (fg) of plutonium or bias in the measured concentrations. Mathematical corrections, based on the measurement of heavy element concentrations in the sample solutions and determination of the extents of formation of the polyatomic interferences, are efficient but time-consuming and degrade detection limits. We describe and discuss a new method based on the combination of, on the one hand, medium mass resolution (MR) of the ICP-SFMS to separate plutonium isotopes physically from interfering polyatomic species, and, on the other, use of a desolvation introduction system (DIS) to enhance sensitivity, thus partly compensating for the loss of transmission due to use of a higher resolution. Plutonium peaks are perfectly separated from the major interfering species (PbO2, HgAr, and IrO3) with a mass resolution of ~ 4000. The resulting nine-fold transmission loss is partly compensated by a five-fold increase in sensitivity obtained with the DIS and a lower background. The instrumental detection limits for plutonium isotopes, calculated for measurements of pure synthetic solutions, of the new method (known as MR-DIS method) and of the one currently used in the laboratory (LR method), based on a low mass resolution equal to 360, a microconcentric nebulizer and two in-line cooled spray chambers, are roughly equivalent, at around 0.2 fg ml− 1. Regarding the measurement of real-life samples, the results obtained with both methods agree and the corresponding analytical detection limits for plutonium isotopes 239Pu, 240Pu and 241Pu are of a few fg·ml− 1 of sample solution, slightly lower with the MR-DIS method than with the current LR method. Although less sensitive than other plutonium ICP-MS measurement methods described in the literature, the main advantage of the MR-DIS method is that it is robust against the risk of false plutonium detection, even in the case of relatively high concentrations of heavy elements like lead, mercury or iridium.

This study investigates U-series, Sr isotopes, major and trace elements in a chalk aquifer system located in Eastern France. Soil and rock samples were collected along depth profiles down to 45 m in four localities as an attempt to investigate the weathering processes in the soil, the unsaturated zone and the saturated zone of the aquifer. Interstitial water was extracted from soils and rocks by a centrifugation technique. U-series offer a powerful tool to calculate weathering rates because the relative mobility of the U- and Th-isotopes can be precisely measured and it does not require the determination of a reference state as in other approaches. As expected, the data show very large mobile element depletion in the soil with large 230Th excess relative to 238U, while the rocks show more limited but not insignificant mobile element depletion. The U-series data have been used to constrain weathering rates based on a 1-D reactive transport model. Weathering rates in the near surface are about 10–100 times faster than at depth. However, when integrated over the depth of the cores, including the unsaturated and the saturated zones, this underground weathering represents more than 30% of the total weathering flux, assuming congruent dissolution of carbonates. The (234U/238U) ratios in interstitial water are consistent with solid samples showing 234U depletion near the surface and an excess 234U at depth. A leaching experiment performed on chalk shows that the excess 234U in natural waters percolating through carbonate rocks results both from preferential 234U leaching and direct recoil in the interstitial water. A new approach was used to derive the recoil ejection factor based on BET measurements and the fractal dimension of chalk surface. Consideration of preferential leaching and recoil allows a more accurate modeling of weathering rates.

We compare the analytical performance of a modern quadrupole-based ICP-MS (“X-Series”, Thermo-Electron, Winsford, UK) with a single-collector double-focusing sector-field ICP-MS (“Axiom”, VG Elemental, Winsford, UK) for uranium isotopic measurements in environmental samples. We focus on the precision and accuracy obtained with both instruments for the 235U/238U isotopic ratios and on the abundance sensitivity that is a key parameter for low 236U/238U isotopic ratios measurements. We observe that isotopic measurements are more precise accurate with the “X-Series” than with the “Axiom”. Besides, we demonstrate that the “Axiom’s” higher abundance sensitivity limits its capability to measure 236U/238U ratio below a few ppm.

In the frame of the low level plutonium measurements performed by ICP-MS on environmental samples in support of the International Atomic Energy Agency’s safeguards program, we observe that background in the actinide mass range for the sample solutions is always higher than for blanks made of deionised water and ultra-pure grade nitric acid. This additional background is due to polyatomic species including heavy metals like Pb, Hg, Ir, etc., which remain in sample solutions despite the purification chemistry that is systematically carried out. As Pu content in the samples may be extremely low—in the femtogram (10–15 g) range—it is of uttermost importance to estimate detection limits as accurately as possible and to minimize risks of false detections. We propose an approach that consists of systematically correcting for contributions of polyatomic species to the background. To accomplish this, it is necessary to estimate all the formation rates of polyatomic species consisting of a “heavy metal” (from Hf to Bi) and one or more atoms, such as O, N, Ar, Cl, etc., that have total masses corresponding to those of Pu isotopes. Although the formation rates can vary largely with settings and analytical conditions, this study allows identification of the higher formation rates. The analytical procedure must also include, in addition to Pu and background masses, the measurement of one isotope for each “heavy metal”. Starting from examples of real sample measurements, we compare various calculation models for background and detection limits and prove that the one taking into account corrections from polyatomic interferences gives the most accurate results and minimises the risk of false detection for ultra low-level Pu measurements in the fg range.

The relative chronology and detailed chemistry of paleofluids circulating at the base of the Kombolgie Sub-basin were investigated in the East Alligator River district (Northern Territory, Australia), where world-class unconformity-type uranium deposits are located. The chemistry of fluid inclusions was determined using in-situ analysis (Raman microprobe and laser-induced breakdown spectroscopy [LIBS]) and by observing the melting sequences by microthermometry. This study revealed the occurrence of three distinct fluids: (i) a sodium-rich brine that corresponds to a diagenetic fluid percolating at the bottom of the Kombolgie sandstones at a temperature close to 150±15 °C; (ii) a calcium-rich brine, probably corresponding to a residual brine in evaporitic environment that has evolved by fluid–rock interactions with the basement lithologies; and (iii) a low salinity fluid, heated in the basement, injected into the base of the sandstone cover. H2 and O2 and/or traces of CH4 were detected in the vapor phase of some fluid inclusions, especially in the low salinity ones in quartz breccia samples taken above mineralized areas. Hydraulic brecciation of the sandstone was associated with a pressure decrease favoring fluid mixing and the subsequent cementation of breccias. According to the fluid inclusion study and other geologic constrains, the minimum thickness of the Sub-Kombolgie Basin is estimated at 4 km. Drusy quartz breccias with evidence of fluid mixing are quite common at the base of the Kombolgie Basin, but not necessarily linked to U-mineralization. However, it is proposed that the presence of gases such as H2 and O2 in fluid inclusions, which results from water radiolysis, constitutes an indicator of gas linked to significant U concentrations deeper in the basement rocks.

International audience; The Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident led to important releases of radionuclides into the environment, and trace levels of plutonium (Pu) were detected in northeastern Japan. However, measurement of Pu isotopic atom and activity ratios is required to differentiate between the contributions of global nuclear test fallout and FDNPP emissions. In this study, we used a double-focusing sector field ICP-MS to measure Pu atom and activity ratios in recently deposited sediment along rivers draining the most contaminated part of the inland radioactive plume. Results showed that plutonium isotopes (i.e., 239 Pu, 240 Pu, 241 Pu, and 242 Pu) were detected in all samples, although in extremely low concentrations. The 241 Pu/ 239 Pu atom ratios measured in sediment deposits (0.0017-0.0884) were significantly higher than the corresponding values attributed to the global fallout (0.00113±0.00008 on average for the Northern Hemisphere between 31°-71°N, Kelley, J.M.; Bond, L.A.; Beasley, T.M., Global distribution of Pu isotopes and 237 Np, Sci. Total. Env., 1999, 237/238, 483-500). The results indicated the presence of Pu from FDNPP, in slight excess compared to the Pu background from global fallout that represented up to ca. 60% of Pu in the analyzed samples. These results demonstrate that this radionuclide has been transported relatively long distances (∼45 km) from FDNPP and been deposited in rivers representing a potential source of Pu to the ocean. In future, the high 241 Pu/ 239 Pu atom ratio of the Fukushima accident sourced-Pu should be measured to quantify the supply of continental-originating material from Fukushima Prefecture to the Pacific Ocean.

The determination of very low concentrations of plutonium, in the femtogram per millilitre range, in environmental samples using ICP-MS can be interfered by polyatomic species containing heavy elements, from hafnium to bismuth. Mass-to-charge ratios of these polyatomic species are very close to the ones of the plutonium isotopes and cannot be separated from them in the low resolution mode currently used for trace analysis. In this paper, we present an evaluation of the extent and impact of these interferences on the background at masses of plutonium isotopes 239Pu and 240Pu, based on measurement of environmental samples for three years using a double-focusing sector-field ICP-MS. We demonstrate that these molecular interferences, especially the ones involving lead, mercury, and iridium through species PbO2+, ArHg+, IrO3+, must be considered as the additional background induced by these interferences (on average for all interferences respectively ∼1.2 and ∼1.9 counts s−1 at 239 and 240 atomic mass units) is often higher than the instrumental background measured with deionised water acidified at 2% by ultra-pure grade HNO3 (∼0.4 counts s−1), and are generally of the same order as other usual sources of background for low-level plutonium analysis by ICP-MS, i.e.242Pu tracer isotopic impurities, 238U hydrides and peak tail. Thus, if not corrected, these polyatomic interferences may lead to false detection of femtogram amounts of plutonium or overestimation of results. The extents of formation of the main polyatomic species are highly variable from one analysis to the other, over one or two orders of magnitude, depending on instrumental settings. Chemical purification procedures are very efficient in eliminating the greater part of heavy elements present in the initial samples. However, these procedures also bring heavy elements into the final sample solutions through contamination by the atmosphere, glassware and reagents.