Concurrent determination of U, Np, Pu, Am, and Cm in clay systems at ultra-trace levels with accelerator mass spectrometry

The geochemistry of actinides under reducing conditions in a deep geological nuclear waste repository is characterized by low solubility and strong sorption to mineral surfaces. The quantification of actinide migration (i.e. diffusion) at resulting concentration levels requires an analytical method able to determine actinide concentrations down to ultra-trace levels (≈fg g$^{-1}$). In the actual study, such an analytical procedure was tested by using accelerator mass spectrometry (AMS), one of the very few analytical techniques that can presently meet those requirements. Specimens simulating the sample matrix representative for a diffusion experiment in natural clay rock were produced by spiking clay rock powders (Opalinus Clay or Callovo-Oxfordian Clay) and aliquots of corresponding pore waters with the actinide nuclides $^{233}$U, $^{237}$Np, $^{244}$Pu, and $^{248}$Cm in amounts ranging from approximately 3 × 10$^{-19}$ to 5 × 10$^{-15}$ mol per sample (≈0.07–1000 fg per sample). The actinide nuclides were separated as group via Fe(OH)$_{3}$ co-precipitation and then analyzed sequentially with AMS. During such analysis a decrease in count rates by up to a factor of 6 was observed in high-matrix clay rock samples compared to low-matrix standard solutions. Since the chemical yield of the actinides in the Fe(OH)$_{3}$ co-precipitation step prior to analysis turned out to be quantitative, this observation must originate from a reduction of the sputter rate of the actinide nuclides in the AMS ion source, which can be described partly as a dilution effect. By determining chemical-ionization-yield factors, suitable non-isotopic tracers were identified for $^{237}$Np and $^{243}$Am. This allowed for the concurrent determination of all actinide nuclides at levels down to 3 × 10−19 mol per sample. Different actinides in a deep geological formation may be present in concentration ranges differing by orders of magnitude depending on their chemical form and solubility. Such concentration spreads were simulated by preparing clay rock/pore water samples where each individual sample contained $^{233}$U, $^{237}$Np, $^{244}$Pu, $^{243}$Am, and $^{248}$Cm at quantities ranging from approximately 3 × 10$^{-19}$ to 4 × 10$^{-15}$ mol per sample. The presented sample preparation procedure, in combination with the extraordinary detection sensitivity of AMS allows for the simultaneous determination of diffusion profiles of several actinides at ultra-trace levels within one experiment

Detection of 135Cs & 137Cs in environmental samples by AMS

The detection of low abundances of 135Cs in environmental samples is of significant interest in different fields of environmental sciences, especially in combination with its shorter-lived sister isotope 137Cs. The method of Ion–Laser InterAction Mass Spectrometry (ILIAMS) for barium separation at the Vienna Environmental Research Accelerator (VERA) was investigated and further improved for low abundance cesium detection. The difluorides BaF2- and CsF2- differ in their electron detachment energies and make isobar suppression with ILIAMS by more than 7 orders of magnitude possible. By this method, samples with ratios down to the order of 135,137Cs/133Cs ˜10-11 are measurable and the 135Cs/137Cs ratios of first environmental samples were determined by AMS

Radionuclides in surface waters around the damaged Fukushima Daiichi NPP one month after the accident : Evidence of significant tritium release into the environment

Following the Fukushima nuclear accident (2011), radionuclides mostly of volatile elements (e.g., 131I, 134,137Cs, 132Te) have been investigated frequently for their presence in the atmosphere, pedosphere, biosphere, and the Pacific Ocean. Smaller releases of radionuclides with intermediate volatility, (e.g., 90Sr), have been reported for soil. However, few reports have been published which targeted the contamination of surface (fresh) waters in Japan soon after the accident. In the present study, 10 surface water samples (collected on April 10, 2011) have been screened for their radionuclide content (3H, 90Sr, 129I, 134Cs, and 137Cs), revealing partly unusually high contamination levels. Especially high tritium levels (184 ± 2 Bq·L−1; the highest levels ever reported in scientific literature after Fukushima) were found in a puddle water sample from close to the Fukushima Daiichi nuclear power plant. The ratios between paddy/puddle water from one location only a few meters apart vary around 1% for 134Cs, 12% for 129I (131I), and around 40% for both 3H and 90Sr. This illustrates the adsorption of radiocesium on natural minerals and radioiodine on organic substances (in the rice paddy), whereas the concentration differences of 3H and 90Sr between the two waters are mainly dilution driven.Peer reviewe

Pego do Diabo (Loures, Portugal): Dating the Emergence of Anatomical Modernity in Westernmost Eurasia

Neandertals and the Middle Paleolithic persisted in the Iberian Peninsula south of the Ebro drainage system for several millennia beyond their assimilation/replacement elsewhere in Europe. As only modern humans are associated with the later stages of the Aurignacian, the duration of this persistence pattern can be assessed via the dating of diagnostic occurrences of such stages

Limits on Supernova-Associated Fe 60/Al 26 Nucleosynthesis Ratios from Accelerator Mass Spectrometry Measurements of Deep-Sea Sediments

We searched for the presence of Al26 in deep-sea sediments as a signature of supernova influx. Our data show an exponential dependence of Al26 with the sample age that is fully compatible with radioactive decay of terrigenic Al26. The same set of samples demonstrated a clear supernova Fe60 signal between 1.7 and 3.2 Myr ago. Combining our Al26 data with the recently reported Fe60 data results in a lower limit of 0.18-0.08+0.15 for the local interstellar Fe60/Al26 isotope ratio. It compares to most of the ratios deduced from nucleosynthesis models and is within the range of the observed average galactic Fe60/Al26 flux ratio of (0.15±0.05).This work was funded in part by the Austrian Science Fund (FWF), Projects No. P20434 and No. I428 (EUROCORES project EuroGENESIS, subproject CoDustMas), by BMBF Project No. 05K2016, DAAD (56266169), and by the University of Vienna

Anthropogenic ²³⁶U in Danish Seawater: Global Fallout versus Reprocessing Discharge

This work focuses on the occurrence of ²³⁶U in seawater along Danish coasts, which is the sole water-exchange region between the North Sea−Atlantic Ocean and the Baltic Sea. Seawater collected in 2013 and 2014 were analyzed for ²³⁶U (as well as ²³⁸U and ¹³⁷Cs). Our results indicate that ²³⁶U concentrations in Danish seawater are distributed within a relatively narrow range of (3.6–8.2) × 10⁷ atom/L and, to a certain extent, independent of salinity. ²³⁶U/²³⁸U atomic ratios in Danish seawater are more than 4 times higher than the estimated global fallout value of 1× 10^–9. The levels of ²³⁶U/²³⁸U atomic ratios obtained are comparable to those reported for the open North Sea and much higher than several other open oceans worldwide. This indicates that besides the global fallout input, the discharges from the two major European nuclear reprocessing plants are dominating sources of ²³⁶U in Danish seawater. However, unexpectedly high ²³⁶U/²³⁸U ratios as well as high ²³⁶U concentrations were observed at low-salinity locations of the Baltic Sea. While this feature might be interpreted as a clue for another significant ²³⁶U input in the Baltic Sea, it may also be caused by the complexity of water currents or slow turnover rate

Developing Accelerator Mass Spectrometry Capabilities for Anthropogenic Radionuclide Analysis to Extend the Set of Oceanographic Tracers

Recent major advances in Accelerator Mass Spectrometry (AMS) at the Vienna Environmental Research Accelerator (VERA) regarding detection efficiency and isobar suppression have opened possibilities for the analysis of additional long-lived radionuclides at ultra-low environmental concentrations. These radionuclides, including $^{233}$U, $^{135}$Cs, $^{99}$Tc, and $^{90}$Sr, will become important for oceanographic tracer application due to their generally conservative behavior in ocean water. In particular, the isotope ratios $^{233}$U/$^{236}$U and $^{137}$Cs/$^{135}$Cs have proven to be powerful fingerprints for emission source identification as they are not affected by elemental fractionation. Improved detection efficiencies allowed us to analyze all major long-lived actinides, i.e., $^{236}$U, $^{237}$Np, $^{239,240}$Pu, $^{241}$Am as well as the very rare $^{233}$U, in the same 10 L water samples of a depth profile from the northwest Pacific Ocean. For this purpose, a simplified and very flexible chemical purification procedure based on extraction chromatography (a single UTEVA® column) was implemented which can be extended by a DGA® column for Am purification. The procedure was validated with the reference materials IAEA-381/385. With the additional increase in ionization efficiency expected for the extraction of actinides as fluoride molecules from the AMS ion source, a further reduction of chemical processing may become possible. This method was successfully applied to an exemplary set of air filter samples. In order to determine the quantitative $^{237}$Np concentration reliably, a $^{236}$Np spike material is being developed in collaboration with the University of Tsukuba, Japan. Ion-Laser Interaction Mass Spectrometry (ILIAMS), a novel technique for the efficient suppression of stable isobaric background, has been developed at VERA and provides unprecedented detection sensitivity for the fission fragments $^{135}$Cs, $^{99}$Tc, and $^{90}$Sr. The corresponding setup is fully operational now and the isobar suppression factors of >10$^{5}$ achieved, in principle, allow for the detection of the mentioned radionuclides in the environment. Especially for $^{90}$Sr analysis, this new approach has already been validated for selected reference materials (e.g., IAEA-A-12) and is ready for application in oceanographic studies. We estimate that a sample volume of only (1-3) L ocean water is sufficient for $^{90}$Sr as well as for $^{135}$Cs analysis, respectively