Simple and fast method for the analysis of 236U, 237Np, 239Pu and 240Pu from seawater samples by Accelerator Mass Spectrometry

Depósito de investigación de la Universidad de Sevilla https://idus.us.es/ “This is an Accepted Manuscript of an article published by Elsevier in Talanta on 1 August 2019, available at: https://doi.org/10.1016/j.talanta.2019.03.036 ” 1 Simple and fast method for the analysis of 236U, 237Np, 239Pu and 240Pu from seawater samples by Accelerator Mass Spectrometry Mercedes López-Lora 1,2* – Isabelle Levy3 – Elena Chamizo 1 1Centro Nacional de Aceleradores (CNA). Universidad de Sevilla. Junta de Andalucía. Consejo Superior de Investigaciones Científicas. Parque científico y tecnológico Cartuja. Thomas Alva Edison 7, 41092, Sevilla, Spain 2Dpto. de Física Aplicada I, Escuela Politécnica Superior, Universidad de Sevilla, Virgen de África 7, 41011 Sevilla, Spain 3International Atomic Energy Agency, Environment Laboratories, MC 98000 Monaco *Corresponding author: E-mail address: [email protected] Abstract 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 236U, 237Np, 239Pu and 240Pu by Accelerator Mass Spectrometry (AMS). It is based on the use of 242Pu 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 242Pu as tracer for 237Np 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 (105-106 atoms/sample). The procedure is finally applied for the study by AMS of a reference seawater sample from the Mediterranean Sea (IAEA-418). Keywords: Accelerator Mass Spectrometry; 236U; 237Np; 239Pu; 240Pu; seawater 2 1. Introduction 236U (T1/2 = 2.34·107 y) and 237Np (T1/2 = 2.14·106 y) are both considered as ideal oceanographic tracers because of their conservative behavior in seawater, long half-life and mainly anthropogenic origin. These radionuclides were introduced into the general environment on a global scale during the period of atmospheric nuclear weapons testing (1945-1980), being the 236U and 237Np estimated inventories 900-1100 kg and 1500 kg, respectively [1–3]. Locally, the accidental and/or controlled discharges from nuclear fuel reprocessing plants, NRP, are the most outstanding sources. E.g., it is estimated that about 85 kg and 170 kg of 236U and 237Np, respectively, have been entered the North Atlantic from different European NRPs (i.e. 60 ± 30 kg and about 23.5 kg of 236U have been released from Sellafield (U.K) and La Hague (France), respectively, while 237Np discharges are coming mainly from Sellafield) [4,5]. Furthermore, the combined study of 236U and 237Np might be a potential tool in oceanography to identify water-mass sources and transient times, as it has been the case of 236U and 129I [6]. Besides, the 237Np/236U ratio can provide information on the contamination source in a specific environment. For instance, in surface soils samples solely influenced by global fallout, 237Np/236U expected atomic ratios are in the 1.7 -2.5 range in the North Hemisphere (estimations from the 237Np/239Pu and 236U/239Pu ratios reported in [7] and [8], respectively). In contrast to this, the measured ratio from the IAEA-381 (i.e. seawater from the Irish Sea collected in 1993, impacted by the discharges from the Sellafield NRP) is 42 ± 3 [9], and ratios at the 0.001 level are expected for the debris released as a consequence of the Chernobyl accident (former Soviet Union, now Ukraine, 1986) [10]. In the last years, an increasing number of studies have been focused on 236U in the oceans because of its potential as oceanographic tracer [11–15]; however, despite their similarities, the number of 237Np results reported in seawater samples [4,16–19] are very scarce. The most likely reason is the complexity associated to the radiochemical procedures aimed at the Np purification. On the other hand, 239Pu (T1/2 = 2.41·104 y) and 240Pu (T1/2 = 6.56·103 y), which are the most abundant Pu isotopes, present a very different geochemical behavior. Pu is a 3 particle-reactive element and it is exported from the surface seawater with sinking particles which subsequently suffer remineralization processes. Similar to 236U and 237Np, Pu has been also introduced in the oceans mainly by global fallout and the operation of NRP. However, there exists a large database on the presence of Pu isotopes in the general environment, and the source-dependency of the 240Pu/239Pu atom ratios has been widely documented. The minimum 240Pu/239Pu atomic ratio, i.e. 0.013 ± 0.003, has been reported for the mineral Trinitite, a green-glassy mineral formed during the testing of the first US nuclear atomic weapon, Trinity (20kT, New Mexico, 1945) [20]; and the maximum ones correspond to areas influenced by the Chernobyl accident, with values of 0.57 and above [21]. The concentrations in the open ocean of 236U, 237Np and 239,240Pu can be extremely low. For instance, surface seawater from the Eurasian basin at the Arctic Ocean , impacted by the releases from European NRP, presents 236U concentrations in the (1-4)·107 atoms L-1 range (i.e. from 8 to 30 nBq L-1, and 236U/238U atom ratios in the (1-4)·10-9 range), and 237Np in the (1-10)·107 atoms L-1 range (i.e. 0.1-1 µBq L-1) [4,6]. Regarding Pu isotopes, 239+240Pu activities ranging from 1 to 20 µBq L-1 (which means 239Pu and 240Pu concentrations at the level of 5·105-107 and 105-106 atoms L-1, respectively) have been reported in deep-water columns from the Arctic Ocean [8]. Even lower concentrations are expected in other areas only influenced by the global fallout. For example, in the South Pacific Ocean, surface seawater samples present concentrations in the (4-7)·106 atoms L-1 or 4-6 nBq L-1 range for 236U (i.e. 236U/238U atom ratios in the (4-7)·10-9 range) [22] and in the 0.5-1.4 µBq L-1 range for 239+240Pu activities (which means concentrations at the level of 105 and 104-105 atoms L-1 for 239Pu and 240Pu, respectively) [23]. Although there is no information about 237Np in this area, its concentrations are expected to be also very low, at the same level of 236U. The study of these radionuclides at environmental levels by using small-volume seawater samples (i.e. below 10L) has recently become possible as a result of the high sensitivity reached by modern Mass Spectrometry (MS) systems. Low detections limits for 237Np and 239,240Pu analysis are offered by Thermal Ionization Mass Spectrometry (TIMS) and High- 4 Resolution Inductively-Coupled-Plasma Mass Spectrometry (HR-ICP-MS) [24–26]. However, an effective U chemical separation is required in order to achieve the necessary purification levels [26] due to the presence of the naturally-occurring U isotopes (mainly 238U and 235U) results in the most important limitation for 237Np and 239Pu analysis. The presence of these U isotopes increase considerable the final backgrounds not only by tailing effect in the relative close mass of interest but also through the formation of molecular isobars (e.g. 238U1H+ in the case of 239Pu) [26]. Therefore, the introduction of additional purification steps becomes indispensable, becoming the radiochemical procedures more complex and time consuming. These problems caused by U molecular isobars can be overcome by using Accelerator Mass Spectrometry (AMS). This technique reaches the highest sensitivity mainly thanks to the destruction of the molecules in the stripping process, with occurs in the terminal of an electrostatic tandem accelerator. Hence, the use of AMS allows the simplification of the radiochemical procedures as U purification levels required are lower than for other MS techniques. Moreover, regarding 236U analysis, AMS also offer the highest sensitivities tanks to the destruction of the molecular isobar 235U1H [27]. In the recent years, many efforts have been focused on the development of methods for the sequential extraction of different actinides from the same seawater aliquot, given the high value and uniqueness of the samples involved in oceanographic expeditions [28–32]. Methods aimed at minimizing the sample volume are required in order to simplify the logistics of the sampling campaigns, and increase the number of samples. Usually, actinides are first pre-concentrated from the sample volume and the complexity of this step depends on the sample size (e.g. different co-precipitation stages are needed for large-volume samples, above 100 L, for 239+240Pu and 237Np determinations by alphaspectrometry (AS)). Then, actinides are purified using ion chromatography methods [33]. In the last decade, TEVA® and UTEVA® resins (Triskem Industries, Inc.), which are specifically designed for actinides, have demonstrated to be the most convenient ones because of their ease of use [34]. 5 On the other hand, most of the reported methods are not focused on the extraction of 237Np mainly due to the lack of a long-life Np isotope available to be used as a yield-tracer. The only choice would be 236Np (T1/2 = 1.54·105 y), but the stock solutions that are currently commercially available contain significant traces of 237Np. As for the 236U AMS analysis, its use won´t be convenient, since 236Np and 236U are isobars. 239Np (T1/2 = 2.35 d) has been also used as a yield-tracer for 237Np determinations [18,35], but due to its short half-life, it has important disadvantages: i) 239Np has to be previously milked from a parent 243Am solution [32]; ii) that parent solution might contain traces of 241Am, which subsequently decays to 237Np producing contamination; iii) 239Np recovery in each step has to be monitored by gamma spectrometry (GS); and iv) the whole processing of the samples has to be completed in a few days because of the fast decay of the tracer. To overcome all these problems, some authors have pointed out the possibility of using 242Pu (T1/2 = 3.73·105 y) instead, since both Pu and Np fractions can follow the same chemical behavior if their oxidations states are properly controlled [30,31,36]. In any case, most of these methods have been developed for conventional MS techniques, and complex and time-consuming extraction chromatography procedures are needed in order to ensure a reliable purification of the different fractions [19,37]. In this work, we present a simple and fast sequential method to separate U, Np and Pu from smallvolume seawater samples for the final analysis of 236U, 237Np, 239Pu and 240Pu by AMS. The method is based on the use of 242Pu as the yield tracer of both 237Np and Pu isotopes, which are recovered in the same fraction (from now on, Pu+Np fraction), and is an extension of the procedure previously reported by our group for the sequential extraction of U and Pu [29]. Samples are pre-concentrated by only one Fe(OH)2 coprecipitation step and TEVA® and UTEVA® resins are used in a very simplified way for the final purification of the U and Pu+Np fractions. Radiometric techniques (AS and GS) have been used to assess and optimize the performance of the method, and for its final validation by analyzing a reference sample provided by the International Atomic Energy Agency (IAEA) (i.e. IAEA-443, Irish Sea seawater). The method is finally applied for the analysis of the IAEA-418 (Mediterranean seawater sample) on the 1 MV AMS system at 6 the CNA following the recently developed technique [38]. Laboratory background levels have been also optimized in order to take advantage of the low detection limits of the AMS technique for the study of environmental samples. A description of the method and its performance, and the different validation exercises carried out are described in the following sections. 2. Experimental 2.1 Reagents, materials and spikes During the sample processing, M-RO water (i.e. Milli-RO® water, pure distilled water) and acids of the highest purity were used in every stage to prevent contamination in the AMS analysis. The different reagents used throughout the procedure were supplied by Panreac, Honeywell Fluka, Acros Organics and Labosi. Besides, all these reagents were studied in order to evaluate possible sources of contamination (see section 3.4). Ion chromatography separations were performed using TEVA®, UTEVA® and Pre-filter resins (2 mL columns of 100-150 µm particle size; Triskem Industries, Inc.). For the AMS cathode preparation step, a Fe(III) solution provided by High Purity Standards (HPS, England) was used, with a certified 238U concentration below the 0.5 ppb level. The Nb powder was provided by Sigma-Aldrich® (purity of 99.8%). During the testing of the procedure by radiometric techniques, the following spikes were used: 232U (T1/2 = 68.9 y), provided by National Physical Laboratory (NPL, England); and 239Np (T1/2 = 2.35 d), milked from a 243Am (T1/2 = 7.37·103 y) solution provided by Harwell Laboratory (Atomic Energy Research Establishment, U.K.). 239Np, in secular equilibrium with its 243Am parent, was separated and purified according to the method described in [32,39]. For the 236U, 237Np and 239,240Pu AMS measurements, the following spikes were used: 233U (T1/2 = 1.59·105 y), provided by CEA (French Alternative Energies and Atomic Energy Commission), and 242Pu (T1/2 = 3.73·105 y), provided by NPL. 7 2.2 Samples Two sets of samples were used for method testing purposes: i) seawater samples collected at the Monaco coast (2.5 and 10 L aliquots), and ii) some mL of diluted HNO3 spiked with 232U, 239Np and 242Pu (i.e. called in-house column-blank samples). For the validation of the proposed procedure, two IAEA reference seawater samples were used: IAEA-443, Irish Sea seawater influenced by Sellafield NRP [40]; and IAEA-418, surface seawater collected in 2011 at the DYFAMED station, in the north-western Mediterranean Sea [41]. 239Pu and 240Pu reported activities from the IAEA-443 are (8.2 ± 0.8) mBq kg-1 and (7.0 ± 0.6) mBq kg-1, respectively, and 238Pu certified activity is (3.1 ± 0.1) mBq kg-1[40]. IAEA-443 aliquots come from the same original sample than IAEA-381 aliquots [40], which certified 237Np activity is (8.7 ± 0.5) mBq kg-1 [9]. All these uncertainties are expressed at the k=2 (95%) confidence level. The IAEA-418 is only certified for 129I. However, based on the reported information on samples collected at the same station, it can be expected 237Np and 239+240Pu activities of (0.223 ± 0.071) µBq kg-1 and (14 ± 2) µBq kg-1, respectively [18,42]. Every seawater sample was filtered with a 0.45 µm pore-size filter to remove coarse particles before applying the chemical procedure. 2.3 Techniques AS determinations were performed in the IAEA Environment Laboratories (IAEA-NAEL, Monaco) by using low background ion-implanted silicon detectors (Ultra-AS Ortec) with an active area of 450 mm2 according to [43]. For the evaluation of the spectra, Alphavision 6 software (Ortec) was used. 239Np analysis by GS were developed at IAEA-NAEL following the method described in [43]. The measurements were performed by using a High Purity Ge detector with 44% of relative efficiency (Eurisys); and, for the evaluation of the spectra, Gammavision 6 software (Ortec) was used. 8 AMS determinations were performed on the 1 MV AMS system at the CNA. Details about the current status of the facility are given in [44,45]. The setup for the 236U measurements is discussed in [46,47]. 237Np and Pu isotopes are analyzed jointly from the same cathode (i.e. the final sample adapted to the optimal matrix for the actinides extraction in the ionsource) following the recently developed techniques detailed in [38]. All uncertainties presented in this article are expressed at the k = 1 (68%) confidence level, except where specified otherwise. 2.4 Proposed radiochemical method for 236U, 237Np and 239,240Pu determinations The proposed method is schematized in Fig. 1 and explained in detail below. 2.4.1 Sample pre-treatment Filtered seawater samples are acidified by adding concentrated HCl until pH 2. About 3 pg of 233U and 242Pu are added as spikes to control the radiochemical yield of U and Np+Pu fractions, respectively, and to quantify their concentrations in the sample matrix following the isotope-dilution method. They are also used as the reference isotopes during the corresponding AMS measurement of 236U and 237Np+239,240Pu [38]. 2.4.2 Co-precipitation of the actinides with Fe(OH)2 Actinides are pre-concentrated from the bulk sample by Fe(OH)2 co-precipitation following different steps: (i) potassium metabisulphite (K2S2O5) is added to the samples; (ii) once dissolved, Fe(II) is added as iron sulphate (FeSO4) salt and mixed; and (iii) finally, the Fe(OH)2 precipitate (i.e. blue-greenly particles, see Fig. 2) is obtained by increasing the pH to 9 with concentrated ammonia (NH4OH). For 10 L samples, 5 g of K2S2O5 and 500 mg of Fe(II) are used, which means that 0.5 g of K2S2O5 and 50 mg of Fe(II) are added for each liter of the processed sample. In the case of seawater aliquots below 5 L, these concentrations are increased in order to ensure that co-precipitation process works properly: 2.5 g of K2S2O5 and 250 mg of Fe(II) are used for 2.5 L samples, and 1.25 g of K2S2O5 and 125 mg of Fe(II) are used for 1 L samples. After about 1-2 hours of settling of 15 Regarding the U results, the recoveries were high (> 80%) in all cases, both in the 2.5 L and in the 10 L aliquots. The average recovery was 93% (8% SD). Np and Pu recoveries were similar and over 80% in all the samples except for Sample 2, which presents lower recoveries but also similars for both Np and Pu. The average recoveries were (80 ± 10)% for Np and (80 ± 10)% for Pu. Np/Pu ratios from the obtained radiochemical yields for each sample are shown in the last column of Table 3. The average value for this Np/Pu yield ratios is 0.98 with a standard deviation of 0.04. 3.4 Background optimization AMS offers detection limits at the 105-106 atoms/sample level for every actinide. These levels are similar to the 236U, 237Np and 239,240Pu concentrations in general seawater samples so, in principle, their measurement using small-volume aliquots (i.e. below 10 L) should be possible. In practice, the achievement of such background levels entails a careful control of the reagents used throughout the chemical procedure. In order to study the background introduced by the reagents used in the proposed procedure, different tests were conducted. Negligible amounts of 237Np and Pu isotopes were detected in every studied reagent, with processed blank levels below 105 atoms. However, significant traces of 236U were identified in one of the three studied Fe(II) salts used as a carrier in the first co-precipitation stage, at the level of 107 atoms·g-1. This level is too high to process environmental samples, which could present 236U concentrations at the level of 106 atoms·L-1 or even lower. Therefore, it is strongly recommended to investigate the 236U background coming from the Fe(II) salt whenever the procedure is applied for the first time in a laboratory or when a new one is used, even if it comes from the same supplier. 4.4 Validation of the proposed procedure by radiometric techniques: IAEA-443 The necessary validation exercise for the determination of 237Np using 242Pu as the yield tracer, which is the most critical stage of the proposed procedure, was carried using a reference seawater sample with a known 237Np activity (i.e. IAEA-443) that is measurable 16 by AS (see section 2.5.3). The obtained results for the 3 studied samples are shown in Table 4. The average radiochemical yields were (69 ± 2)% and (70 ± 5)% for Np and Pu, respectively. Although these recoveries are slightly lower than the obtained ones during the testing stage with Monaco seawater (section 3.3), it is once more proved the same chemical behavior of Np and Pu in every processed sample. Two different estimations of the 237Np activity concentration were obtained in this case (section 2.5.3): i) from the 239Np added as a spike, and ii) using the 238Pu present in the original sample as the yield tracer. The obtained average result from the first approach agrees with the one from the second and both match the certified value within errors, which validates the proposed procedure. 4.5 AMS results: IAEA-418 In order to test the performance of the procedure for the AMS analysis of 239,240Pu + 237Np, and 236U in seawater samples, two aliquots of the IAEA-418 reference seawater sample were processed. The study of the IAEA-418 is interesting because corresponds to surface seawater collected at DYFAMED site in 2001 (north-western Mediterranean Sea), where results on 239+240Pu and 237Np activities have been also reported. Those values can be considered for comparison purposes [18,42]. On the other hand, 236U and 240Pu/239Pu atom ratios have been obtained for the first time in this specific reference material in this work. It is important to notice that, following the proposed procedure, in the final purified Np+Pu fractions traces of 238U at the 0.1 µg level or lower might be present (estimations from different tests performed by AS). During the AMS analysis on the 1 MV CNA system, the related 238U background effects can be corrected if a fine tuning is applied, as detailed in [38]. This might not be the case in conventional MS instruments, where the abundance sensitivity (i.e. mass suppression factor between neighboring masses) that can be achieved is usually much worse. A further adaptation of the method might be necessary in those cases. 17 The final values for the IAEA-418 are shown in Table 5. The results from the two aliquots are in agreement for all the studied radionuclides. The uncertainties can be considered acceptable (relative uncertainties ranging from 9% to 17% ) taking to account that only 1 L samples were processed for availability reasons. 239+240Pu obtained activities agree with the value reported in [42], (14 ± 2) µBq·L-1, and 237Np results are also in agreement with the previous one, i.e. (0.22 ± 0.07) µBq·kg-1 or (22 ± 7) at·kg-1 [18]. It is important to remark that, in contrast with the 1 L aliquots processed in this work, 60 L samples were used in the case of the results reported in the previous works [18,42]. 240Pu/239Pu atom ratios are in agreement with the expected value for global fallout (i.e. 0.18 ± 0.02 for the North Equatorial region [7]). Finally, 237Np/236U ratios were lower than the expected one for global fallout in this area (i.e. 1.7-2.5 [7,8]). This could indicate the presence of an excess of 236U coming from a local source, in agreement with the reported studies of 236U in the Mediterranean Sea [14,53]. 5. Conclusions A new radiochemical method for a rapid sequential extraction of U, Np and Pu from smallvolume seawater samples (<10 L) is presented. The method is optimized for the final 236U, 237Np, 239Pu and 240Pu determinations by AMS and is based on the use of 242Pu as tracer for both 237Np and Pu isotopes. High radiochemical yields (>80%) were obtained for the three elements and the ratios between the Np and Pu radiochemical yields were, on average 0.98 ± 0.04. Besides, the reliability of the use of 242Pu as 237Np tracer was validated by analysing the reference seawater sample IAEA-443 by radiometrics techniques. Because of the very low concentrations that can be expected from seawater samples, laboratory background was optimized by an in-depth study of the different reagents. This study reveals Fe(SO)4 comercial salts as a possible source of 236U background. The method is finally apply for the study of the reference sampe IAEA-418 (Mediterranean seawater sample). Results were consistent for 236U and 240Pu/239Pu, which were reported for the first time in this reference material, and 237Np and 239+240Pu activities agreed with the reported values. Therefore, the method has been sucesfully validate and inplemented for 18 AMS analys at the CNA, opening new possibilities for the determination of actinides in oceanographic campaigns, where the simplification of the procedures and the optimization of the sample volumes are key points. Furthermore, the possibility of studying 237Np by using a simple radiochemical method allows us to increase the very limited 237Np dataset available from oceanographic samples. Acknowledgments This work has been financed from the project FIS2015-69673-P, provided by the Spanish Ministry of Economy. This work was partially funded by Fundación Cámara Sevilla through a Grant for Graduate Studies. The IAEA is grateful to the Government of the Principality of Monaco for the support provided to its Environment Laboratories. Data availability The data that support the findings of this study are openly available at the following URL: https://github.com/AMS-CNA/Method-U-Np-Pu-Seawater 19 References [1] A. Sakaguchi, K. Kawai, P. Steier, F. Quinto, K. Mino, J. Tomita, M. Hoshi, N. Whitehead, M. Yamamoto, First results on236U levels in global fallout, Sci. Total Environ. 407 (2009) 4238– 4242. doi:10.1016/j.scitotenv.2009.01.058. [2] S.R. Winkler, P. Steier, J. Carilli, Bomb fall-out 236U as a global oceanic tracer using an annually resolved coral core, Earth Planet. Sci. Lett. 359–360 (2012) 124–130. doi:10.1016/J.EPSL.2012.10.004. [3] T.M. Beasley, J.M. Kelley, T.C. Maiti, L.A. Bond, 237Np239Pu atom ratios in integrated global fallout: a reassessment of the production of 237Np, J. Environ. Radioact. 38 (1998) 133–146. doi:10.1016/S0265-931X(97)00033-7. [4] T. Beasley, L.W. Cooper, J.M. Grebmeier, K. Aagaard, J.M. Kelley, L.R. Kilius, 237Np/129I atom ratios in the Arctic Ocean: Has 237Np from Western European and Russian fuel reprocessing facilities entered the Arctic Ocean?, J. Environ. Radioact. 39 (1998) 255–277. doi:10.1016/S0265-931X(97)00059-3. [5] M. Christl, N. Casacuberta, C. Vockenhuber, C. Elsaasser, Pascal Bailly du Bois, J. Herrmann, H.-A. Synal, Reconstruction of the 236U input function for the Northeast Atlantic Ocean: Implications for 129I/236U and 236U/238U-based tracer ages, J. Geophys. Res. Ocean. 120 (2015) 7282–7299. doi:doi:10.1002/ 2015JC011116. [6] N. Casacuberta, P. Masqué, G. Henderson, M. Rutgers van-der-Loeff, D. Bauch, C. Vockenhuber, A. Daraoui, C. Walther, H.-A.A. Synal, M. Christl, First 236U data from the Arctic Ocean and use of 236U/238U and 129I/236U as a new dual tracer, Earth Planet. Sci. Lett. 440 (2016) 127–134. doi:http://dx.doi.org/10.1016/j.epsl.2016.02.020. [7] J.M. Kelley, L.A. Bond, T.M. Beasley, Global distribution of Pu isotopes and 237Np, Sci. Total Environ. 237–238 (1999) 483–500. doi:http://dx.doi.org/10.1016/S0048-9697(99)00160-6. [8] E. Chamizo, M. López-Lora, M. Villa, N. Casacuberta, J.M. López-Gutiérrez, M.K. Pham, Analysis of 236U and plutonium isotopes, 239,240Pu, on the 1 MV AMS system at the Centro Nacional de Aceleradores, as a potential tool in oceanography, Nucl. Instruments Methods Phys. Res. Sect. B Beam Interact. with Mater. Atoms. 361 (2015) 535–540. doi:10.1016/j.nimb.2015.02.066. [9] P.P. Povinec, C. Badie, A. Baeza, Et al., Certified reference material for radionuclides in seawater IAEA-381 (Irish Sea Water), J. Radioanal. Nucl. Chem. 251 (2002) 369–374. doi:10.1023/A:1014861620713. [10] U.N.S.C. on the E. of A. Radiation, Exposures and effects of the Chernobyl accident, in: UNSCEAR 2000 Rep. Vol. II, 2000: p. 115. [11] A. Sakaguchi, A. Kadokura, P. Steier, Y. Takahashi, K. Shizuma, M. Hoshi, T. Nakakuki, M. Yamamoto, Uranium-236 as a new oceanic tracer: A first depth profile in the Japan Sea and comparison with caesium-137, Earth Planet. Sci. Lett. 333–334 (2012) 165–170. doi:10.1016/j.epsl.2012.04.004. [12] N. Casacuberta, M. Christl, J. Lachner, M.R. van der Loeff, P. Masqué, H.A. Synal, A first 20 transect of 236U in the North Atlantic Ocean, Geochim. Cosmochim. Acta. 133 (2014) 34– 46. doi:http://dx.doi.org/10.1016/j.gca.2014.02.012. [13] M. Christl, J. Lachner, C. Vockenhuber, I. Goroncy, J. Herrmann, H.-A. Synal, First data of Uranium-236 in the North Sea, Nucl. Instruments Methods Phys. Res. Sect. B Beam Interact. with Mater. Atoms. 294 (2013) 530–536. doi:10.1016/j.nimb.2012.07.043. [14] E. Chamizo, M. López-Lora, M. Bressac, I. Levy, M.K. Pham, Excess of 236 U in the northwest Mediterranean Sea, Sci. Total Environ. 565 (2016) 767–776. doi:10.1016/j.scitotenv.2016.04.142. [15] R. Eigl, P. Steier, K. Sakata, A. Sakaguchi, Vertical distribution of 236U in the North Pacific Ocean, J. Environ. Radioact. 169–170 (2017) 70–78. doi:10.1016/J.JENVRAD.2016.12.010. [16] R.J. Pentreath, B.R. Harvey, The presence of 237Np in the Irish Sea, Mar. Ecol. Prog. Ser. 6 (1981) 243–247. [17] P. Lindahl, P. Roos, E. Holm, H. Dahlgaard, Studies of Np and Pu in the marine environment of SwedisheDanish waters and the North Atlantic Ocean, J. Environ. Radioact. 82 (2005) 285–301. doi:10.1016/j.jenvrad.2005.01.011. [18] M. Bressac, I. Levy, E. Chamizo, J.J. La Rosa, P.P. Povinec, J. Gastaud, B. Oregioni, Temporal evolution of 137 Cs, 237 Np, and 239+240 Pu and estimated vertical 239+240 Pu export in the northwestern Mediterranean Sea, Sci. Total Environ. 595 (2017) 178–190. doi:10.1016/j.scitotenv.2017.03.137. [19] M. Castrillejo Iridoy, Sources and distribution of artificial radionuclides in the oceans: from Fukushima to the Mediterranean Sea, n.d. http://www.tdx.cat/bitstream/handle/10803/457438/mci1de1.pdf?sequence=1 (accessed December 28, 2017). [20] P.P. Parekh, T.M. Semkow, M.A. Torres, D.K. Haines, J.M. Cooper, P.M. Rosenberg, M.E. Kitto, Radioactivity in Trinitite six decades later, J. Environ. Radioact. 85 (2006) 103–120. doi:10.1016/J.JENVRAD.2005.01.017. [21] T. Bisinger, S. Hippler, R. Michel, L. Wacker, H.-A. Synal, Determination of plutonium from different sources in environmental samples using alpha-spectrometry and AMS, Nucl. Inst. Methods Phys. Res. B. 268 (2009) 1269–1272. doi:10.1016/j.nimb.2009.10.150. [22] M. Villa-alfageme, E. Chamizo, T.C. Kenna, M. López-Lora, N. Casacuberta, P. Masqué, M. Christl, Distribution of 236U in the GEOTRACES EPZT and its use as a water mass tracer (In press), Chem. Geol. (n.d.). [23] K. Hirose, M. Aoyama, M. Fukasawa, C.S. Kim, K. Komura, P.P. Povinec, J.A. Sanchez-Cabeza, Plutonium and 137Cs in surface water of the South Pacific Ocean, Sci. Total Environ. 381 (2007) 243–255. doi:10.1016/J.SCITOTENV.2007.03.022. [24] J. Zheng, M. Yamada, Inductively coupled plasma-sector field mass spectrometry with a high-efficiency sample introduction system for the determination of Pu isotopes in settling particles at femtogram levels, Talanta. 69 (2006) 1246–1253. doi:10.1016/j.talanta.2005.12.047. 21 [25] V.N. Epov, K. Benkhedda, R.J. Cornett, R.D. Evans, Rapid determination of plutonium in urine using flow injection on-line preconcentration and inductively coupled plasma mass spectrometry, J. Anal. At. Spectrom. 20 (2005) 424. doi:10.1039/b501218j. [26] C.S. Kim, C.K. Kim, K.J. Lee, Simultaneous analysis of 237Np and Pu isotopes in environmental samples by ICP-SF-MS coupled with automated sequential injection system, J. Anal. At. Spectrom. 19 (2004) 743. doi:10.1039/b400034j. [27] L.K. Fifield, Accelerator mass spectrometry of the actinides, Quat. Geochronol. 3 (2008) 276–290. doi:10.1016/J.QUAGEO.2007.10.003. [28] I. Levy, Détermination des radionucléides anthropiques plutonium, neptunium, américium, césium et strontium dans l’environnement marin Méditerranéen., 2004. [29] M. López-Lora, E. Chamizo, M. Villa-Alfageme, S. Hurtado-Bermúdez, N. Casacuberta, M. García-León, Isolation of 236U and 239,240Pu from seawater samples and its determination by Accelerator Mass Spectrometry, Talanta. 178 (2018). doi:10.1016/j.talanta.2017.09.026. [30] J. Qiao, X. Hou, P. Steier, R. Golser, Sequential Injection Method for Rapid and Simultaneous Determination of 236 U, 237 Np, and Pu Isotopes in Seawater, Anal. Chem. 85 (2013) 11026–11033. doi:10.1021/ac402673p. [31] S.L. Maxwell, B.K. Culligan, D.R. Mcalister, Rapid determination of actinides in seawater samples, (2014) 1175–1189. doi:10.1007/s10967-014-3079-0. [32] J. La Rosa, L. Gastaud, L. Lagan, S.-H. Lee, I. Levy-Palomo, P.P. Povinec, E. Wyse, Recent developments in the analysis of transuranics (Np, Pu, Am) in seawater, J. Radioanal. Nucl. Chem. 236 (2005) 9. [33] P. Thakur, G.P. Mulholland, Determination of 237Np in environmental and nuclear samples: A review of the analytical method, Appl. Radiat. Isot. 70 (2012) 1747–1778. doi:10.1016/j.apradiso.2012.02.115. [34] E.P. Horwitz, M.L. Dietz, R. Chiarizia, H. Diamond, S.L. Maxwell, M.R. Nelson, Separation and preconcentration of actinides by extraction chromatography using a supported liquid anion exchanger: application to the characterization of high-level nuclear waste solutions, Anal. Chim. Acta. 310 (1995) 63–78. doi:10.1016/0003-2670(95)00144-O. [35] M.J. Keith-Roach, J.P. Day, F.R. Livens, L.K. Fifield, Measurement of 237Np in environmental water samples by accelerator mass spectrometry, Analyst. 126 (2001) 58–61. doi:10.1039/b007493o. [36] Q. Chen, H. Dahlgaard, S.P. Nielsen, A. Aarkrog, Pu as tracer for simultaneous determination of 237 Np and 239,240 Pu in environmental samples, J. Radioanal. Nucl. Chem. 253 (2002) 451–458. https://link.springer.com/content/pdf/10.1023%2FA%3A1020429805654.pdf (accessed December 22, 2017). [37] S.L. Maxwell, B.A. Culligan, V.D. Jones, S.T. Nichols, G.W. Noyes, Rapid determination of 237Np and Pu isotopes in water by inductively-coupled plasma mass spectrometry and alpha spectrometry, J. Radioanal. Nucl. Chem. 287 (2011) 223–230. doi:10.1007/s10967- 22 010-0825-9. [38] M. López-Lora, E. Chamizo, Acelerator Mass Spectrometry of 237Np 239Pu and 240Pu for environmental aplications at the Centro Nacional de Aceleradores (CNA) (Submitted), Nucl. Instruments Methods Phys. Res. Sect. B Beam Interact. with Mater. Atoms. (n.d.). [39] J. La Rosa, I. Outola, E. Crawford, S. Nour, H. Kurosaki, K. Inn, Radiochemical measurement of 237Np in a solution of mixed radionuclides: Experiences in chemical separation and alpha-spectrometry, J. Radioanal. Nucl. Chem. 277 (2008) 11–18. doi:10.1007/s10967-008- 0702-y. [40] CERTIFIED REFERENCE MATERIAL IAEA-443 NATURAL AND ARTFICIAL RADIONUCLIDES IN SEA WATER FROM THE IRISH SEA, (n.d.). https://nucleus.iaea.org/rpst/referenceproducts/referencematerials/radionuclides/IAEA- 443/RS_IAEA_443_final.pdf (accessed December 13, 2017). [41] IAEA-418 IODINE-129 IN MEDITERRANEAN SEA WATER, (n.d.). https://nucleus.iaea.org/rpst/ReferenceProducts/ReferenceMaterials/Radionuclides/IAEA- 418/RS_IAEA_418.pdf (accessed December 13, 2017). [42] S.-H. Lee, J.J. La Rosa, I. Levy-Palomo, B. Oregioni, M.K. Pham, P.P. Povinec, E. Wyse, Recent inputs and budgets of 90Sr, 137Cs, 239,240Pu and 241Am in the northwest Mediterranean Sea, Deep Sea Res. Part II Top. Stud. Oceanogr. 50 (2003) 2817–2834. doi:http://dx.doi.org/10.1016/S0967-0645(03)00144-9. [43] I. Levy, P.P. Povinec, M. Aoyama, K. Hirose, J.A. Sanchez-Cabeza, J.-F. Comanducci, J. Gastaud, M. Eriksson, Y. Hamajima, C.S. Kim, K. Komura, I. Osvath, P. Roos, S.A. Yim, Marine anthropogenic radiotracers in the Southern Hemisphere: New sampling and analytical strategies, Prog. Oceanogr. 89 (2011) 120–133. doi:10.1016/j.pocean.2010.12.012. [44] E.C. Calvo, F.J. Santos, J.M. López-Gutiérrez, S. Padilla, M. García-León, J. Heinemeier, C. Schnabel, G. Scognamiglio, Status report of the 1 MV AMS facility at the Centro Nacional de Aceleradores, Nucl. Instruments Methods Phys. Res. Sect. B Beam Interact. with Mater. Atoms. 361 (2015) 13–19. doi:10.1016/j.nimb.2015.02.022. [45] G. Scognamiglio, E. Chamizo, J.M. López-Gutiérrez, A.M. Müller, S. Padilla, F.J. Santos, M. López-Lora, C. Vivo-Vilches, M. García-León, Recent developments of the 1 MV AMS facility at the Centro Nacional de Aceleradores, Nucl. Instruments Methods Phys. Res. Sect. B Beam Interact. with Mater. Atoms. 375 (2016) 17–25. doi:10.1016/j.nimb.2016.03.033. [46] E. Chamizo, M. Christl, L.K. Fifield, Measurement of 236U on the 1 MV AMS system at the Centro Nacional de Aceleradores (CNA), Nucl. Instruments Methods Phys. Res. Sect. B Beam Interact. with Mater. Atoms. 358 (2015) 45–51. doi:http://dx.doi.org/10.1016/j.nimb.2015.05.008. [47] E. Chamizo, M. López-Lora, Accelerator mass spectrometry of 236 U with He stripping at the Centro Nacional de Aceleradores, Nucl. Instruments Methods Phys. Res. Sect. B Beam Interact. with Mater. Atoms. (2018). doi:10.1016/j.nimb.2018.04.020. [48] E.P. Horwitz, M.L. Dietz, R. Chiarizia, H. Diamond, A.M. Essling, D. Graczyk, Separation and preconcentration of uranium from acidic media by extraction chromatography, Anal. Chim. 23 Acta. 266 (1992) 25–37. doi:10.1016/0003-2670(92)85276-C. [49] L. Hallstadius, A method for the electrodeposition of actinides, Nucl. Instruments Methods Phys. Res. 223 (1984) 266–267. doi:http://dx.doi.org/10.1016/0167-5087(84)90659-8. [50] J. Qiao, X. Hou, P. Steier, S. Nielsen, R. Golser, Method for 236 U Determination in Seawater Using Flow Injection Extraction Chromatography and Accelerator Mass Spectrometry, Anal. Chem. 87 (2015) 7411–7417. doi:10.1021/acs.analchem.5b01608. [51] C.W. Sill, F.D. Hindman, J.I. Anderson, Simultaneous determination of alpha-emitting nuclides of radium through californium in large environmental and biological samples, Anal. Chem. 51 (1979) 1307–1314. doi:10.1021/ac50044a043. [52] M. Mendes, J. Aupiais, C. Jutier, F. Pointurier, Determination of weight distribution ratios of Pa(V) and Np(V) with some extraction chromatography resins and the AG1-X8 resin, Anal. Chim. Acta. 780 (2013) 110–116. doi:10.1016/J.ACA.2013.04.019. [53] M. Castrillejo, N. Casacuberta, M. Chirstl, C. Vockenhuber, P. Masqué, J. García-Orellana, Mapping of 236U and 129I in the Mediterranean Sea, Annu. Rep. 2014, Ion Beam Physics, ETH Zurich. (2014). 24 Tables Table 1- U and Np co-precipitation yields obtained from 10 L seawater samples increasing the pH to different values for the formation of the Fe(OH)2 precipitate. In samples A, B and C, Np yields were quantified separately from a first precipitate (1st ppt) and a second one obtained from the supernatant (2nd ppt); both precipitated were finally combined (final). U results from these samples were obtained by the analysis of the combined precipitates. In samples D, E and F, only one precipitate was recovered. The average results (Avg.) together with the Standard Deviation (SD) are shown for each set of samples. Sample (10 L seawater) pH Co-precipitation yield (%) U Np (1st ppt) Np (2nd ppt) Np (final) A 8 - 8.5 80 ± 10 69 ± 2 16.2 ± 0.6 86 ± 2 B 8 - 8.5 25 ± 4 69 ± 3 11.0 ± 0.7 80 ± 3 C 8 - 8.5 70 ± 10 67 ± 2 12.5 ± 0.6 80 ± 2 Avg. 60 68 13 82 SD 30 1 3 3 D 9 - 9.5 83 ± 9 96 ± 2 - 96 ± 2 E 9 - 9.5 110 ± 10 88 ± 2 - 88 ± 2 F 9 - 9.5 100 ± 10 94 ± 4 - 94 ± 4 Avg. 98 93 - 93 SD 9 4 - 4 Table 2- Results obtained from two set of column-blank samples processed with different oxidation state adjustments in order to study the radiochemical yield during the extraction chromatography step. The average results (Avg.) together with the Standard Deviation (SD) are shown for each set of samples. In the last column the ratios between the Np and Pu yields are calculated. Oxidation state adjustment Sample Extraction Chromatography Yield (%) Np/Pu (yield ratios) U Np Pu - Dissolution in 17 mL of 1 M HNO3 - 200 mg of Mohr’s Salt - 2.75 mL HNO3 (70%) Blk-1 100 ± 10 90 ± 4 82 ± 8 1.1 ± 0.1 Blk-2 110 ± 10 92 ± 5 91 ± 9 1.0 ± 0.1 Blk-3 100 ± 10 92 ± 5 93 ± 9 1.0 ± 0.1 Avg. 103 91.2 89 1.03 SD 6 1.1 6 0.06 - Dissolution in 20 mL of 1 M HNO3 - 200 mg of Mohr’s Salt - 3.25 mL HNO3 (70%) Blk-4 93 ± 9 94 ± 5 100 ± 10 0.9 ± 0.1 Blk-5 66 ± 7 98 ± 4 97 ± 9 1.0 ± 0.1 Blk-6 100 ± 10 95 ± 3 100 ± 10 0.9 ± 0.1 Avg. 90 96 99 0.9 SD 20 2 2 0.1