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Studsvik, a Swedish nuclear research facility, has been releasing aquatic radioactive discharges in the Baltic Sea, through the bay Tvären, since 1959. The permissible discharge levels are regulated by the Swedish Radiation Safety Authority (SSM) but only information about 60 Co, 137 Cs, 152 Eu, total alpha and beta activities were reported up to 2002. Since then, the reports cover most a more comprehensive set of radionuclides. In this context, the seabed can be utilized as a chronological archive to investigate historical Studsvik releases. To this end, 23 sediment cores covering the whole area of the bay were studied and 5 of them were dated using 210 Pb-dating methods. Since the discharges from Studsvik contain both plutonium and caesium, neither can be used to validate the 210 Pb-dating method. Instead, stable lead with maximum deposition, known to be dated to 1970, was used. Cobalt-60, 137 Cs, and 152 Eu depth distributions were studied from the dated sediment cores and compared with reported levels of aquatic discharges. The expected sediment 137 Cs-peak from the fallout from the Chornobyl accident was however smeared out, indicating an ongoing inflow of 137 Cs with the Baltic seawater. Our findings show that reported releases of aquatic discharges of 60 Co and 152 Eu are consistent with measured sediment activity distribution. The sediments from the deepest parts of Tvären, with intact chronology and with a high time resolution, are ideal for investigating historical nuclear discharges and can be a tool to investigate unreported radionuclide releases. Dated sediment can in this way be a tool for nuclear safeguards to evaluate past and present activities in nuclear facilities., Competing Interests: Declaration of competing interest Mats Eriksson reports financial support was provided by Swedish Radiation Safety Authority. The authors declare that they have no competing interests., (Copyright © 2023 The Authors. Published by Elsevier Ltd.. All rights reserved.)

Plutonium distribution was studied in an undisturbed sediment core sampled from the Tvären bay in the vicinity of the Studsvik nuclear facility in Sweden. The complete analysis, including minor isotopes, of the Pu isotope composition ( 238 Pu, 239 Pu, 240 Pu, 241 Pu, 242 Pu, and 244 Pu) allowed us to establish the Pu origin in this area of the Baltic Sea and to reconstruct the Studsvik aquatic release history. The results show highly enriched 239 Pu, probably originating from the Swedish nuclear program in the 1960s and 1970s and the handling of high burn-up nuclear fuel in the later years. In addition, the 244 Pu/ 239 Pu atomic ratio for the global fallout period between 1958 and 1965 is suggested to be (7.94 ± 0.31)·10 -5 . In the bottom layer of the sediment, dated 1953-1957, we detected a higher average 244 Pu/ 239 Pu ratio of (1.51 ± 0.11)·10 -4 , indicating the possible impact of the first US thermonuclear tests (1952-1958).

The aim of this study was to assess the potential of combining the conservatively behaving anthropogenic radionuclides 236 U and 237 Np 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 237 Np/ 236 U 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 236 U presumably caused by Marcoule is reflected in the lower 237 Np/ 236 U ratios compared to the Global Fallout signal in all the studied samples. On the contrary, the 239,240 Pu 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 237 Np/ 236 U 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 237 Np as a new ocean tracer. A more accurate characterization of the main sources is still needed to optimize the use of 236 U- 237 Np as a new tool to understand transient oceanographic processes., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2020 Elsevier B.V. All rights reserved.)

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, 236 U and 237 Np were collected off the coast of Namibia within the northern BUS. Surface seawater exhibited similar 236 U and 237 Np concentrations, ranging from 3.9·10 6 to 5.6·10 6 atoms kg -1 and from 4.6·10 6 to 8.5·10 6 atoms kg -1 , respectively. The observed inventories in a water column from the continental margin, of (2.10 ± 0.11)·10 12 atoms m -2 for 236 U and (3.48 ± 0.13)·10 12 atoms m -2 for 237 Np, 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 237 Np 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 236 U within the data uncertainties. An explanation based on the larger particle reactivity of GF 237 Np compared to GF 236 U 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 237 Np/ 236 U atom ratio for the GF source term in the marine environment., (Copyright © 2019 Elsevier B.V. All rights reserved.)

A simple and fast radiochemical procedure for the sequential extraction of U, Np and Pu from small-volume seawater samples (<10 L) is presented. The method has been developed and optimized for the final determination of 236 U, 237 Np, 239 Pu and 240 Pu by Accelerator Mass Spectrometry (AMS). It is based on the use of 242 Pu as tracer for both Np and Pu isotopes. Samples are pre-concentrated by Fe(OH) 2 co-precipitation. TEVA ® and UTEVA ® resins are used in a very simplified way for the final purification of the Pu+Np and U fractions, respectively. The radiochemical yields of the three elements have been investigated in detail by alpha spectrometry (AS) and gamma spectrometry (GS). The obtained results indicate high and robust chemical yields for the three elements and similar ones for Pu and Np. Furthermore, the use of 242 Pu as tracer for 237 Np is validated by analyzing a reference seawater sample (IAEA-443) by radiometric techniques. We demonstrated that, if chemicals are properly chosen, processed blank levels can be kept at the same level of the extremely low detection limits that can be achieved by AMS (10 5 -10 6 atoms/sample). The procedure is finally applied for the study by AMS of a reference seawater sample from the Mediterranean Sea (IAEA-418)., (Copyright © 2019 Elsevier B.V. All rights reserved.)

In this work we present and evaluate a radiochemical procedure optimised for the analysis of 236 U and 239,240 Pu in seawater samples by Accelerator Mass Spectrometry (AMS). The method is based on Fe(OH) 3 co-precipitation of actinides and uses TEVA® and UTEVA® extraction chromatography resins in a simplified way for the final U and Pu purification. In order to improve the performance of the method, the radiochemical yields are analysed in 1 to 10L seawater volumes using alpha spectrometry (AS) and Inductively Coupled Plasma Mass Spectrometry (ICP-MS). Robust 80% plutonium recoveries are obtained; however, it is found that Fe(III) concentration in the precipitation solution and sample volume are the two critical and correlated parameters influencing the initial uranium extraction through Fe(OH) 3 co-precipitation. Therefore, we propose an expression that optimises the sample volume and Fe(III) amounts according to both the 236 U and 239,240 Pu concentrations in the samples and the performance parameters of the AMS facility. The method is validated for the current setup of the 1MV AMS system (CNA, Sevilla, Spain), where He gas is used as a stripper, by analysing a set of intercomparison seawater samples, together with the Laboratory of Ion Beam Physics (ETH, Zürich, Switzerland)., (Copyright © 2017 Elsevier B.V. All rights reserved.)