Anthropogenic 236U and 129I in the Mediterranean Sea: First comprehensive distribution and constrain of their sources

Sediments as sinks and sources of marine radionuclides: Implications for their use as ocean tracers

A Lagrangian transport model for the North Atlantic has been applied to simulate the historical releases of 137Cs, 129I and 236U from the European nuclear fuel reprocessing plants. Advection by currents, mixing and decay are included, as radionuclide interactions between water, sediments and suspended matter. The model was validated comparing predictions with measured radionuclide concentrations in water and sediments in several areas. 129I and 236U signals entering the Arctic Ocean have been compared with the input terms: the 236U signal is distorted, but the 129I signal preserves its shape. In the first moments after the releases, the sediments act as sinks for 236U, but not significantly for 129I and ultimately they become sources of 236U to the open sea. This results in a weaker correlation between input and output signals for 236U than for 129I. The same effects as for 236U have been found for 137Cs signal into the Arctic.

A review of anthropogenic radionuclide 236U: Environmental application and analytical advances

²³⁶U is an anthropogenic radionuclide that is produced from nuclear reactions of ²³⁵U(n, γ) and ²³⁸U(n, 3n). It has gained extensive attention in the field of environment, geology, nuclear emergency, and nuclear forensics. Due to the unique physical and chemical character and the distinct fingerprint character from different sources, ²³⁶U has been successfully applied in the environmental tracer, nuclear material source appointment, and environmental assessment. Until now, few reviews were published about the database, application, and the latest analytical technology development of ²³⁶U. In this review, the ²³⁶U concentration and ²³⁶U/²³⁸U isotope ratio were summarized, and the data were classified into four categories, including soil and seawater samples affected by global fallout and nuclear incidents. Furthermore, the development of environmental application and pretreatment methods were also summarized. The advanced pretreatment technology using alkali fusion and flow injection was especially discussed to introduce the development of a rapid analytical method. Finally, the research challenge and direction of ²³⁶U were proposed for further research, such as the tracer application combining ²³⁶U with other radionuclides in the terrestrial environment and the precise analysis of minor isotopes in ultra-trace uranium samples. We hope this review will help scholars to have a deep research on the analysis and application of ²³⁶U.

236U, 237Np and 239,240Pu as complementary fingerprints of radioactiveeffluents in the western Mediterranean Sea and in the Canada Basin (Arctic Ocean)

The aim of this study was to assess the potential of combining the conservatively behaving anthropogenic radionuclides ²³⁶U and ²³⁷Np to gain information on the origin of water masses tagged with liquid effluents from Nuclear Reprocessing Plants. This work includes samples collected from three full-depth water columns in two areas: i) the Arctic Ocean, where Atlantic waters carry the signal of Sellafield (United Kingdom) and La Hague (France) nuclear reprocessing facilities; and ii) the western Mediterranean Sea, directly impacted by Marcoule reprocessing plant (France). This work is complemented by the study of the particle-reactive Pu isotopes as an additional fingerprint of the source region. In the Canada Basin, Atlantic waters showed the highest concentrations and ²³⁷Np/²³⁶U ratios in agreement with the estimated values for North Atlantic waters entering the Arctic Ocean and tagged with the signal of European Nuclear Reprocessing Plants. These results may reflect the impact of the documented releases for the 1990s. In the Mediterranean Sea, an excess of ²³⁶U presumably caused by Marcoule is reflected in the lower ²³⁷Np/²³⁶U ratios compared to the Global Fallout signal in all the studied samples. On the contrary, the ^239,240Pu profiles were mainly governed by the Global Fallout. The impact of Marcoule as a local source is further corroborated when comparing the temporal evolution of these ratios between 2001 and 2013. The lowest ²³⁷Np/²³⁶U ratios observed in 2001 at the surface reflect a previous local input that is no longer observed in 2013 as it had been homogenized through the whole water column. This work presents the use of ²³⁷Np as a new ocean tracer. A more accurate characterization of the main sources is still needed to optimize the use of ²³⁶U-²³⁷Np as a new tool to understand transient oceanographic processes.

Tracing the upwelling process in the northern Benguela upwelling system (nBUS) by 129I

New data on the presence of ¹²⁹I in seawater in the Southern Hemisphere measured by Accelerator Mass Spectrometry (AMS) is presented. The samples were collected in 2014 along the Namibian coast during a cruise organised by the National Marine Information and Research Centre (NatMIRC), the national laboratories of the Ministry of Fisheries and Marine Resources (MFMR) in Namibia, and the IAEA Environment Laboratories (IAEA NAEL) in Monaco. The Benguela upwelling system is known as one of the most important marine upwelling regions in the world. Strong winds induce an offshore transport of surface seawater which is substituted by cool subsurface water inshore. As this water is nutrient-rich, which leads to high primary productivity, the Benguela upwelling system has a very important role as a fishing production area. The ¹²⁹I concentrations in samples were between (0.66 ± 0.14) × 10⁷ and (1.45 ± 0.30) × 10⁷ atoms/kg. The highest ¹²⁹I concentrations were found in the offshore surface samples. Deep-sea and inshore samples contained lower ¹²⁹I concentrations, possibly as an effect of the upwelling process. A comparison with previously published studies suggests that the presence of ¹²⁹I in the northern Benguela upwelling system (nBUS), is mainly due to the impact of nuclear weapons global fallout, without any evident impact of nuclear fuel reprocessing.

A historical record of the impact of nuclear activities based on 129I in coral cores in Baler, Philippines: An update

In a previous study in 2016, we presented how ¹²⁹I in coral cores from the east (Baler) and west (Parola) sides of the Philippines recorded the impacts of human nuclear activities, including nuclear weapons testing, nuclear fuel reprocessing, and nuclear accidents. However, the 2016 Baler dataset only had a two-year time resolution and a crude age model based on growth band counting. Here we present a new 2020 Baler ¹²⁹I/¹²⁷I atomic ratio dataset that features at least annual time resolution and a more accurate age model constructed using 3D X-ray Computed Tomography. Results show that the bomb peaks in Baler primarily came from the Pacific Proving Grounds or PPG with a time lag of about 1.8 years (or more specifically, between 1.3 and 2.4 years). Moreover, a review of the Parola dataset shows that PPG signals may have been transported to Parola in the West Philippine Sea via two pathways: the northward and southward bifurcations of the North Equatorial Current, reaching Parola about 4.5 and 8.5 years after detonation, respectively. Moreover, a prominent peak in the year 2014.7 in Baler possibly came from the 2011 Fukushima Accident, transported by the Kuroshio Recirculation Gyre and the North Pacific Mode Waters with a 3.5-year time lag. This study contributes to the understanding of the impact and transport of human-made radionuclides to the Philippines and the relevant oceanographic processes in the Western Equatorial Pacific region.

Background and fingerprint characteristics of anthropogenic 236U and137Cs in soil and road dust samples collected from Beijing and Zhangjiakou, China

²³⁶U has attracted more attention as an environmental tracer in recent years. However, in-depth study of ²³⁶U in terrestrial environments is still rare in China. Data on ²³⁶U and ¹³⁷Cs concentrations in soil and road dust samples collected from Beijing and Zhangjiakou, China were obtained to demonstrate the background and distinct characteristics of anthropogenic ²³⁶U and ¹³⁷Cs. ²³⁶U and ¹³⁷Cs were detected in the range of (1.10-7.90) × 10⁷ atoms g^-1 and below the method limits of detection to 5.30 Bq kg^-1. A clear characteristic was observed in road dust, where ²³⁶U concentrations increased with decreasing of sample particle size. Soil samples showed an irregular characteristic, but the highest ²³⁶U concentrations were observed in particle size fraction of <0.053 mm in both samples. This phenomenon was caused by U chemical properties, higher specific surface areas and organic compounds in fine particles. Anthropogenic radionuclides fingerprint characteristics in <0.053 mm samples were specially discussed. ²³⁶U/²³⁸U atom ratios were detected in the range of (0.627-3.38) × 10^-8. A weak correlation between anthropogenic ²³⁶U and natural U isotopes were observed. The intermediate correlation between ²³⁶U and ¹³⁷Cs indicated somewhat distinct migration behavior of these two radionuclides in soil after release to the environment. The released amount of ²³⁶U from global fallout during the period of atmospheric nuclear weapons testing was roughly estimated to be 1300 ± 448 kg. These results could be used as fingerprint information for anthropogenic ²³⁶U migration behavior and tracer application in environment.

Unravelling 5 decades of anthropogenic 236U discharge from nuclear reprocessing plants

Marine biogenic materials such as corals, shells, or seaweed have long been recognized as recorders of environmental conditions. Here, the bivalve Cerastoderma edule is used for the first time as a recorder of past seawater contamination with anthropogenic uranium, specifically ²³⁶U. Several studies have employed the authorized radioactive releases, including ²³⁶U, from nuclear reprocessing plants in La Hague, France, into the English Channel, and Sellafield, England, into the Irish Sea, to trace Atlantic waters and to understand recent climate induced circulation changes in the Arctic Ocean. Anthropogenic ²³⁶U has emerged over recent years as a new transient tracer to track these changes, but its application has been challenged owing to paucity of fundamental data on the input (timing and amount) of ²³⁶U from Sellafield. Here, we present ²³⁶U/²³⁸U data from bivalve shells collected close to La Hague and Sellafield from two unique shell collections that allow the reconstruction of the historical ²³⁶U contamination of seawater since the 1960s, mostly with bi-annual resolution. The novel archive is first validated by comparison with well-documented ²³⁶U discharges from La Hague. Then, shells from the Irish Sea are used to reconstruct the regional ²³⁶U contamination. Apart from defining new, observationally based ²³⁶U input functions that will allow more precise tracer studies in the Arctic Ocean, we find an unexpected peak of ²³⁶U releases to the Irish Sea in the 1970s. Using this peak, we provide evidence for a small, but significant recirculation of Irish Sea water into the English Channel. Tracing the 1970s peak should allow extending ²³⁶U tracer studies into the South Atlantic Ocean.

Presence of 236U and 237Np in a marine ecosystem: The northern Benguela Upwelling System, a case study

The Benguela Upwelling System (BUS), off the south-western African coast, is one of the four major eastern boundary upwelling ecosystems in the oceans. However, this area has been overlooked in the field of environmental radioactivity. In this work, ²³⁶U and ²³⁷Np were collected off the coast of Namibia within the northern BUS. Surface seawater exhibited similar ²³⁶U and ²³⁷Np concentrations, ranging from 3.9·10⁶ to 5.6·10⁶ atoms kg^-1 and from 4.6·10⁶ to 8.5·10⁶ atoms kg^-1, respectively. The observed inventories in a water column from the continental margin, of (2.10 ± 0.11)·10¹² atoms m^-2 for ²³⁶U and (3.48 ± 0.13)·10¹² atoms m^-2 for ²³⁷Np, were in agreement with the global fallout (GF) source term in the Southern Hemisphere that was the main source of actinides to the region. A pattern was observed in the surface samples, with ²³⁷Np concentrations that decreased by 25-30% when moving from inshore to offshore stations, but such an effect could not be clearly discerned in the case of ²³⁶U within the data uncertainties. An explanation based on the larger particle reactivity of GF ²³⁷Np compared to GF ²³⁶U was proposed. Such an effect would have been important at the studied site due to the enhance presence of particles in the continental shelf triggered by the upwelling phenomenon. A value of 1.77 ± 0.20 was obtained for the ²³⁷Np/²³⁶U atom ratio for the GF source term in the marine environment.

First report on global fallout 236U and uranium atom ratios in soils from Hunan Province, China

More nuclear power plants continue to be built in China. Due to its long half-life, radiotoxicity and potential application as an environmental tracer, ²³⁶U is one of the most important artificial radionuclides deserving more study since activity data are important for risk assessment. However, the ultra-trace activity of ²³⁶U and its dilution by natural uranium isotopes make it difficult to distinguish its sources and there are only limited global fallout ²³⁶U data for present in Chinese environmental samples. In order to understand the background levels for uranium isotopes, especially ²³⁶U, and clarify their sources, inductively coupled plasma tandem mass spectrometry (ICP-MS/MS) was applied to analyze uranium isotopes in 48 soil samples from Hunan Province, China. The ²³⁴U, ²³⁵U, ²³⁸U and ²³⁶U concentrations were measured as 9.91-33.7, 0.312-1.43, 6.63-28.7 Bq kg^-1 and (1.61-21.3) × 10⁷ atoms g^-1, while, the ²³⁶U/²³⁸U, ²³⁴U/²³⁸U and ²³⁵U/²³⁸U atom ratios were (0.470-4.91) × 10^-8, (5.10-9.31) × 10^-5, and (7.11-7.82) × 10^-3, respectively. The uranium isotopic fractionation may be due to irrigation of the agricultural lands where the samples were collected. Considering the facts that neither previous nuclear tests nor nuclear accidents had occurred in Hunan Province and the present ²³⁶U/²³⁸U atom ratios were included in the range of global fallout values in other areas, it may be concluded that ²³⁶U in soils from Hunan Province is mainly from global fallout. To the best of the authors' knowledge, the presence of global fallout ²³⁶U in soil samples from China has been confirmed for the first time, and these values may be useful as background data for risk assessment in the future.

Natural and artificial radionuclides in a marine core. First results of 236U in North Atlantic Ocean sediments

There are very few data available of ²³⁶U in marine sediment cores. In this study we present the results from the first oceanic depth profile of ²³⁶U in a sediment core sampled in the North Atlantic Ocean, at the PAP site (4500 m depth, Porcupine Abyssal Plain (PAP) site, 49°0' N, 16°30' W). Additionally, the sediment core was radiologically characterized through the measurement of anthropogenic ¹³⁷Cs, ²³⁹Pu, ²⁴⁰Pu, ¹²⁹I and ¹⁴C and natural ²¹⁰Pb, ⁴⁰K and ²²⁶Ra. The measured ²³⁶U concentrations decrease from about 90·10⁶ at g^-1 at the seafloor down to 0.5·10⁶ at g^-1 at 6 cm depth. They are several orders of magnitude lower than the reported values for soils from the Northern Hemisphere solely influenced by global fallout (i.e. from 2700·10⁶ to 7500·10⁶ at g^-1). ²³⁶U/²³⁸U atom ratios measured are at least three orders of magnitude above the estimated level for the naturally occurring dissolved uranium. The obtained inventories are 1·10¹² at m^-2 for ²³⁶U, 80 Bq m^-2 for ¹³⁷Cs, 45 Bq m^-2 for ^239+240Pu and 2.6·10¹² at m^-2 for ¹²⁹I. Atomic ratios for ²³⁶U/²³⁹Pu, ¹³⁷Cs/²³⁶U and ¹²⁹I/²³⁶U, obtained from the inventories are 0.036, 0.11 and 2.5 respectively. Concentration profiles show mobilization probably due to bioturbation from the abundant detritivore holothurian species living at the PAP site sea-floor. The range of ²³⁶U, ¹³⁷Cs, ^239+240Pu and ¹²⁹I values, inventories and ratios of these anthropogenic radionuclides are more similar to the values due to fall-out than values from a contribution from the Nuclear Fuel Reprocessing Plants dispersed to the south-west of the North Atlantic Ocean. However, signs of an additional source are detected and might be associated to the nuclear wastes dumped on the Eastern North Atlantic Ocean.

Recent evolution of 129I levels in the Nordic Seas and the North Atlantic Ocean

Most of the anthropogenic radionuclide ¹²⁹I released to the marine environment from the nuclear fuel reprocessing plants (NFRP) at Sellafield (England) and La Hague (France) is transported to the Arctic Ocean via the North Atlantic Current and the Norwegian Coastal Current. ¹²⁹I concentrations in seawater provides a powerful and well-established radiotracer technique to provide information about the mechanisms which govern water mass transport in the Nordic Seas and the Arctic Ocean and is gaining importance when coupled with other tracers (e.g. CFC, ²³⁶U). In this work, ¹²⁹I concentrations in surface and depth profiles from the Nordic Seas and the North Atlantic (NA) Ocean collected from four different cruises between 2011 and 2012 are presented. This work allowed us to i) update information on ¹²⁹I concentrations in these areas, required for the accurate use of ¹²⁹I as a tracer of water masses; and ii) investigate the formation of deep water currents in the eastern part of the Nordic Seas, by the analysis of ¹²⁹I concentrations and temperature-salinity (T-S) diagrams from locations within the Greenland Sea Gyre. In the Nordic Seas, ¹²⁹I concentrations in seawater are of the order of 10⁹ at·kg^-1, one or two orders of magnitude higher than those measured at the NA Ocean, not so importantly affected by the releases from the NFRP. ¹²⁹I concentrations of the order of 10⁸atoms·kg^-1 at the Ellet Line and the PAP suggest a direct contribution from the NFRP in the NA Ocean. An increase in the concentrations in the Nordic Seas between 2002 and 2012 has been detected, which agrees with the temporal evolution of the ¹²⁹I liquid discharges from the NFRPs in years prior to this. Finally, ¹²⁹I profile concentrations, ¹²⁹I inventories and T-S diagrams suggest that deep water formation occurred in the easternmost area of the Nordic Seas during 2012.

Anthropogenic 236U in the North Sea - A Closer Look into a Source Region

In this study we present new seawater data of ²³⁶U and ²³⁸U sampled in the North Sea in 2010. The North Sea has been and is still receiving a considerable input of anthropogenic radionuclides from nuclear reprocessing facilities located in La Hague (France) and Sellafield (Great Britain). It therefore represents an important source region for oceanographic tracer studies using the transient signal of anthropogenic ²³⁶U. A proper knowledge of the sources of ²³⁶U is an essential prerequisite for such tracer studies. The ²³⁶U data set presented in this study covers the transition regions of the North Sea to the Atlantic Ocean, to the Baltic Sea, and upstream the Elbe River. It is discussed in the context of available ²³⁶U data from the literature. Our results show that both ²³⁶U concentrations and ²³⁶U/²³⁸U ratios in surface waters of the North Sea can be explained by simple binary mixing models implying that ²³⁶U behaves conservatively in seawater. We further show that the input of ²³⁶U by the Elbe River is negligible, while there might be a maximum input of 12 g/yr via the Baltic Sea. The results of the mixing models suggest that this still unidentified ²³⁶U contamination could be supplied by fresh water input.

U isotopes distribution in the Lower Rhone River and its implication on radionuclides disequilibrium within the decay series

The large rivers are main pathways for the delivery of suspended sediments into coastal environments, affecting the biogeochemical fluxes and the ecosystem functioning. The radionuclides from ²³⁸U and ²³²Th-series can be used to understand the dynamic processes affecting both catchment soil erosion and sediment delivery to oceans. Based on annual water discharge the Rhone River represents the largest river of the Mediterranean Sea. The Rhone valley also represents the largest concentration in nuclear power plants in Europe. A radioactive disequilibrium between particulate ²²⁶Ra_(p) and ²³⁸U_(p) was observed in the suspended sediment discharged by the Lower Rhone River (Eyrolle et al. 2012), and a fraction of particulate ²³⁴Th was shown to derive from dissolved ²³⁸U_(d) (Zebracki et al. 2013). This extensive study has investigated the dissolved U isotopes distribution in the Lower Rhone River and its implication on particulate radionuclides disequilibrium within the decay series. The suspended sediment and filtered river waters were collected at low and high water discharges. During the 4-months of the study, two flood events generated by the Rhone southern tributaries were monitored. In river waters, the total U_(d) concentration and U isotopes distribution were obtained through Q-ICP-MS measurements. The Lower Rhone River has displayed non-conservative U-behavior, and the variations in U_(d) concentration between southern tributaries were related to the differences in bedrock lithology. The artificially occurring ²³⁶U was detected in the Rhone River at low water discharges, and was attributed to the liquid releases from nuclear industries located along the river. The (²³⁵U/²³⁸U)_(d) activity ratio (=AR) in river waters was representative of the ²³⁵U natural abundance on Earth. The (²²⁶Ra/²³⁸U)_(p) AR in suspended sediment has indicated a radioactive disequilibrium (average 1.3 ± 0.1). The excess of ²³⁴Th in suspended sediment =(²³⁴Th_xs(p)) was apparent solely at low water discharges. The activity of ²³⁴Th_xs(p) was calculated through gamma measurements and ranged from unquantifiable to 56 ± 14 Bq kg^-1. The possibility of using ²³⁴Th as a tracer for the suspended sediment dynamics in large Mediterranean river was then discussed.