Multiactinide Analysis with Accelerator Mass Spectrometry for

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Multiactinide Analysis with Accelerator Mass Spectrometry for Ultratrace Determination in Small Samples: Application to an in Situ Radionuclide Tracer Test within the Colloid Formation and Migration Experiment at the Grimsel Test Site (Switzerland) Francesca Quinto,*,† Ingo Blechschmidt,‡ Carmen Garcia Perez,† Horst Geckeis,† Frank Geyer,† Robin Golser,§ Florian Huber,† Markus Lagos,†,∥ Bill Lanyon,⊥ Markus Plaschke,† Peter Steier,§ and Thorsten Schaf̈ er†,# †

Karlsruhe Institute of Technology (KIT), Institute for Nuclear Waste Disposal (INE), Hermann-von-Helmholtz-Platz 1, D-76344 Eggenstein-Leopoldshafen, Germany ‡ NAGRA (National Cooperative for the Disposal of Radioactive Waste), Hardstrasse 73, CH-5430 Wettingen, Switzerland § VERA Laboratory, Faculty of Physics-Isotope Research and Nuclear Physics, University of Vienna, Währinger Straße 17, A-1090 Vienna, Austria ∥ Steinmann-Institut für Geologie, Mineralogie und Paläontologie, University of Bonn, Poppelsdorfer Schloss, 53115 Bonn, Germany ⊥ Fracture Systems Ltd, Tregurrian, Ayr, TR26 1EQ St. Ives, United Kingdom # Friedrich-Schiller-University Jena (FSU), Institute of Geosciences, Applied Geology, Burgweg 11, D-07749 Jena, Germany ABSTRACT: The multiactinide analysis with accelerator mass spectrometry (AMS) was applied to samples collected from the run 13-05 of the Colloid Formation and Migration (CFM) experiment at the Grimsel Test Site (GTS). In this in situ radionuclide tracer test, the environmental behavior of 233U, 237 Np, 242Pu, and 243Am was investigated in a water conductive shear zone under conditions relevant for a nuclear waste repository in crystalline rock. The concentration of the actinides in the GTS groundwater was determined with AMS over 6 orders of magnitude from ∼15 pg/g down to ∼25 ag/g. Levels above 10 fg/g were investigated with both sector field inductively coupled plasma mass spectrometry (SF-ICPMS) and AMS. Agreement within a relative uncertainty of 50% was found for 237Np, 242Pu, and 243Am concentrations determined with the two analytical methods. With the extreme sensitivity of AMS, the long-term release and retention of the actinides was investigated over 8 months in the tailing of the breakthrough curve of run 13-05 as well as in samples collected up to 22 months after. Furthermore, the evidence of masses 241 and 244 u in the CFM samples most probably representing 241Am and 244Pu employed in a previous tracer test demonstrated the analytical capability of AMS for in situ studies lasting more than a decade.

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material could be generated.5,6 In such groundwater, bentonite colloids show high stability and mobility.7 Colloid mediated transport of Pu(IV) and Am(III) and negligible interaction of U(VI) and Np(V) have been observed in the first two in situ tracer tests at the GTS, namely, the Colloids and Radionuclides Retention (CRR)8 experiments runs 1 and 2, performed in 2002.1,9 In the CRR radionuclide tracer test run 2 with smectite colloid addition,9 especially, the recovery of tri- and tetravalent An was significantly increased. High mobility was observed also for Np(V) and U(VI) and attributed to the low residence time, due to a high flow velocity that had to be chosen because of radiation protection constraints. In parallel performed laboratory studies on cores from the GTS under variation of the residence time, it could be undoubtedly shown that sorption/

ontinuous progress in understanding the geochemical behavior of the actinides (An), e.g., uranium (U), neptunium (Np), plutonium (Pu), and americium (Am), is of great relevance for the safe geological disposal of nuclear waste. The Grimsel Test Site (GTS), a generic underground research laboratory (URL), located in the Swiss Alps, offers with its controlled zone the unique opportunity to perform in situ tracer tests investigating the migration and retention of radionuclides in a fracture of a shear zone in a crystalline rock with the natural overburden. The GTS groundwater is of meteoric origin and characterized by the high pH value equal to 9.6 and low ionic strength, I = 1.2 × 10−3 M.1 In view of future predicted climate changes with a return to glacial and interglacial periods,2,3 the GTS geochemical conditions are considered appropriate for studying the possible scenario of glacial meltwater intrusion into a repository for high-level radioactive waste in granitic host rock.4 Upon contact with water, the engineered compacted bentonite barrier surrounding the nuclear waste canisters will form a gel, from which colloidal © XXXX American Chemical Society

Received: April 11, 2017 Accepted: June 9, 2017 Published: June 9, 2017 A

DOI: 10.1021/acs.analchem.7b01359 Anal. Chem. XXXX, XXX, XXX−XXX

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Analytical Chemistry

shear zone previously employed for the CRR experiments. The remaining fraction was kept in order to perform analysis of the starting concentration of the injected An and colloids. Groundwater samples eluted from the extraction point of the dipole were continuously collected in HDPE bottles for a period of 236 d over the BTC. The concentrations of injected An were quantified with SF-ICPMS, and the sampling interval was chosen to be as short as one sample per hour representing 190 mL with an extraction flow rate of 5 mL/min. This resulted in 55 samples describing the peak of the BTC and covering ∼25 d of experimental duration. Samples further in the tailing of the BTC showed An concentrations below the DL of SFICPMS equal to ∼2 × 107 atoms/g. Therefore, multi-An analysis with AMS was carried out for a total of 20 samples chosen in order to represent the entire BTC. In particular, 5 samples collected before the arrival of the migration front between 0.01 and 0.4 d (14.4 min and 9.6 h, respectively), 9 samples taken from the peak between 0.7 (17 h) and 25 d, and 8 specimens from the tailing between 33 and 236 d were investigated with AMS. Furthermore, two additional groundwater samples eluted from the fracture after 518 and 660 d from the starting of run 13-05 were analyzed. Both latter samples were collected at the “Pinkel” surface packer, the main outflow from the shear zone under the controlled conditions maintained by the mega-packer system. This extraction point is different from that used for the previous samples and is located at the tunnel wall. The samples, previously investigated with SF-ICPMS, presented An concentrations ranging from ∼9 × 107 atoms/g of 237Np in the sample collected after 0.3 d to ∼4 × 1010 atoms/g of 242Pu in the sample collected after 2 d, as depicted in Figure 1. Such data offered a guideline for the selection of

reduction kinetics are significantly controlling the transport of aqueous An species.10 In order to address the redox kinetics in situ on the decameter scale and under near natural repository postclosure hydraulic conditions, a mega-packer system was installed in 2009 at the GTS. Such a system allowed one to control the groundwater flow and gave the opportunity for one to compare laboratory derived smectite colloid desorption kinetics11 with field data. Under such conditions, two new in situ tests were performed in 2012 and 2013 in the frame of the Colloid Formation and Migration (CFM)12 Project, namely, runs 12-02 and 13-05. In both the CFM tests, sector field inductively coupled plasma mass spectrometry (SF-ICPMS) and accelerator mass spectrometry (AMS) have been used as complementary analytical techniques in order to monitor the An tracers in the groundwater samples eluted from the fracture over a wide range of concentrations. In fact, in run 12-02, An levels from ∼10 pg/g (∼2 × 1010 atoms/g) and down to the detection limit (DL) of ∼10 fg/g (∼2 × 107 atoms/g) were determined with SF-ICPMS. AMS was capable of accessing concentrations below 10 fg/g and down to ∼10 ag/g (∼2 × 104 atoms/g).13 The reason for the high sensitivity of AMS for An nuclides is the complete suppression of polyatomic isobaric background and tailing interferences,14−16 providing low DL and high abundance sensitivity (AS) equal to ∼104 atoms per sample and ≤10−15, respectively. These instrumental features also enabled us to develop a novel analytical method consisting of the simultaneous determination of several An nuclides without previous chemical separation from each other and with the use of nonisotopic tracers for the determination of 237Np and 243 Am. Such ultratrace multi-An analysis has been applied to seawater and surface and groundwater samples and has proven to be suited for the investigation of An originating both from global fallout and in situ radionuclide tracer tests.13 In the CFM run 12-02 experiment, AMS allowed for the determination of the injected An tracers in the tailing of the breakthrough curve (BTC) providing in this way information on the retention and release of the An from the granitic fracture even six months after the tracer injection.13 In the actual study, we present the recent findings from the application of the multi-An analysis with AMS to the following in situ experiment, CFM run 13-05, which was performed in the same dipole as the CRR runs 1 and 2 but under longer residence time. (In this context, a dipole is the region of the shear zone fracture defined by injection and extraction borehole intervals between which the tracer test is performed.) A direct comparison between the results obtained with AMS and SFICPMS for 237Np, 242Pu, and 243Am is also discussed. The novelty and importance of the actual work consist of the investigation of the long-term migration behavior of U, Np, Pu, and Am at the GTS under groundwater flow close to repository relevant conditions.

Figure 1. Count rates of the internal tracers 239Pu and 248Cm normalized to the corresponding added number of atoms (counts × s−1 × atom−1) in the CFM samples (blue diamonds for 239Pu and orange squares for 248Cm) and in the calibration samples (green triangles for 239Pu and black circles for 248Cm) as a function of sample size.

the sample amount for AMS analysis. AMS measurements of samples containing 1010 atoms or more of a certain An nuclide may overload the detector and contaminate the ion source possibly producing a memory effect on the following measurement of less concentrated samples. Therefore, only 0.1 g of a sample was employed for An concentrations in the order of 1010 atoms/g. Higher amounts of sample, namely 1, 10, and 100 g, were taken for the analysis of accordingly lower concentrations and up to 250 g for those specimens with An levels lower than the DL of SF-ICPMS. Calibration Samples. Like in our previous study,13 233U, 237 Np, and 242Pu were determined by using 239Pu as yield tracer, while for 243Am, 248Cm was used. In order to normalize the measured concentrations relative to the corresponding yield

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EXPERIMENTAL SECTION CFM Samples Run 13-05. A radionuclide cocktail containing 233U, 237Np, 242Pu, and 243Am pre-equilibrated with a mixture of synthetic Ni-montmorillonite17 colloids with concentration of 13.9 ± 0.1 mg/L and Febex smectite colloids18 was prepared in Grimsel groundwater resulting in a total colloid concentration of 99.7 ± 1.0 mg/L. The cocktail was also spiked with a fluorescence compound, Amino-G, that was used as conservative (nonreactive) tracer. A fraction of this cocktail was injected into the same dipole, ∼2.3 m long, of the B


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Analytical Chemistry tracer, the chemical and the ionization yields (CIY) of the five An in the Cs sputtering source of AMS have been determined. CIY factors were obtained by preparing and measuring five calibration samples consisting of Grimsel groundwater spiked with known amounts of 233U, 237Np, 239Pu, 243Am, and 248Cm together with the unknown samples, according to the procedure described in Quinto et al.13 In brief, the procedure involved the evaporation of the groundwater sample to dryness, the leaching of the so-obtained residue with conc. HNO3, and the successive fuming off with conc. HCl. To the sample, 1.5 mg of Fe was added and then dissolved in 20 mL of 0.2 M HCl, obtaining a solution with Fe concentration higher than 70 mg/ L. The An were coprecipitated with Fe(OH)3, and after a waiting time of 12 h, the suspension was centrifuged and the precipitate converted to Fe-oxides and pressed into the sputter cathodes suited for the VERA AMS. To evaluate a possible influence of the sample size on the CIY factors, five calibration samples were prepared from different amounts of Grimsel groundwater, namely, 250, 100, 10, 1, and 0.1 g corresponding to the analyzed sample sizes. With in-house solutions of 237Np and 248Cm and the reference solutions of 233U (IRM040-1), 239Pu (Isotope Products Laboratories), and 243Am (P624735, Eckert & Ziegler Nuclitec GmbH), two tracer solutions were prepared. The first one containing 239Pu and 248Cm to be used as internal spikes for the CFM samples and the second one containing the five An nuclides, namely, 233U, 237Np, 239Pu, 243Am, and 248Cm, to be employed in the calibration samples. Approximately the same amount of tracer solution was added gravimetrically to each CFM sample so that the number of atoms of 239Pu and 248Cm would be ∼108 for each nuclide. Similarly, in the calibration samples, the added number of 233U, 237Np, 239Pu, 243Am, and 248 Cm was ∼108 atoms for each nuclide. Four procedural blanks, named, B1, B2, B3, and B4, were prepared employing the same reagents and kind of glassware used for the CFM and the calibration samples and measured following the same procedure.13 The AMS measurements were carried out at the VERA laboratory (University of Vienna) with the instrument setup described in Winkler et al.19 The ICPMS analysis was performed with a SF-ICPMS (Thermo Element XR/2) in low resolution mode in order to maximize the transmission of the An nuclides.

Pu presents an average normalized count rate of (5.3 ± 0.2) × 10−8, (5.3 ± 0.8) × 10−8, and (4.5 ± 2.1) × 10−8 counts × s−1 × atom−1, while 248Cm average values of (9.6 ± 0.8) × 10−8, (9.5 ± 0.2) × 10−8, and (7.9 ± 3.7) × 10−8 counts × s−1 × atom−1, respectively. In the calibration samples, the normalized count rates of the other three An nuclides, namely, 233U, 237Np, and 243Am, also decrease with increasing sample size following the same behavior of 239Pu and 248Cm (Figure 1). Such a phenomenon can be interpreted as an increase of both the chemical yield and/or the ionization yields of 239Pu, 248Cm, and the other An nuclides in the AMS ion source. Both these effects can be dependent on the amount of sample matrix that increases with the sample size. The ratio between the normalized count rates of 248Cm and 239Pu represents the chemical and ionization yield of 248Cm relative to 239Pu, namely, CIY 248Cm/239Pu. The CIY 248Cm/239Pu serves as a comparison parameter on the behavior of the two internal tracers, 239Pu and 248Cm, between the CFM and the calibration samples, allowing the use of the calibration samples in the evaluation of the results as described in the following text. In Figure 2, the CIY 248Cm/239Pu are depicted as a function of the sample size. Similarly, like for the normalized count rates 239

Figure 2. CIY factors of 248Cm relative to 239Pu measured for the CFM samples (blue squares) and the calibration samples (red circles) as a function of sample size. The average of the CIY factors of 248Cm relative to 239Pu for the CFM samples is also depicted (black circles).

of 239Pu and 248Cm, their ratios show a dependency on the sample size and a scattering around the corresponding average values. The CIY 248Cm/239Pu increases with decreasing sample size. Also, in this case, this trend can be attributed both to an increase in the preferential ionization of Cm relative to Pu in the AMS ion source and to a decrease in the chemical yield of Pu relative to Cm with decreasing sample size. As it can be seen in Figure 2 and in Table 1, the CIYs 248 Cm/239Pu measured in the calibration samples are slightly higher than the average of values for the CFM samples with 250, 100, 10, and 1 g. The opposite is observed for the samples with 0.1 g. However, the average value of the CIY 248Cm/239Pu over the five calibration samples is equal to 1.7 ± 0.3 and is consistent with the average value for all the CFM samples (1.6 ± 0.2) (Table 1). We assume that the average value of the CIY 248 Cm/239Pu for the calibration samples can be taken as a good representative for the CIY 248Cm/239Pu in the CFM sample, independent of the sample size. The concentration of 243Am in the CFM samples is determined by using 248Cm as internal tracer and normalizing the chemical and ionization yield of 243Am relative to 248Cm with the CIY 248Cm/243Am obtained from the calibration samples. Similarly, the concentrations of 237Np, 233U, and 242Pu are determined by using 239Pu as internal tracer and the CIY 239 Pu/237Np, CIY 239Pu/233U, and CIY 239Pu/242Pu, respec-

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RESULTS AND DISCUSSION CIY Factors. To all the CFM and calibration samples, approximately the same amount of the internal yield tracers 239 Pu and 248Cm was added before starting the chemical preparation. However, the count rates of such spikes were different in several samples, increasing with decreasing sample size with a certain scattering around the corresponding average values. In Figure 1, the count rates of 239Pu and 248Cm, each normalized to the corresponding number of atoms introduced in the CFM and calibration samples, are depicted as a function of the employed sample size. In general, higher normalized count rates are observed for lower sample sizes. At the sample size of 250 g, slightly lower normalized count rates are detected compared to the sample size of 100 g. In particular, 239Pu presents an average normalized count rate of (1.5 ± 0.6) × 10−8 and (2.3 ± 0.9) × 10−8 counts × s−1 × atom−1 and 248Cm average values of (1.9 ± 0.8) × 10−8 and (3.2 ± 1.3) × 10−8 counts × s−1 × atom−1 in the samples with masses equal to 250 and 100 g, respectively. For the sample sizes of 10, 1, and 0.1 g, C


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Table 1. Chemical and Ionization Yield (CIY) Factors of 239Pu Relative to 233U, 239Pu Relative to 237Np, 248Cm Relative to 243 Am, and 248Cm Relative to 239Pu Measured in the 5 Calibration Samples in Grimsel Groundwater with Masses of 250, 100, 10, 1, and 0.1 ga CIY factors 250 g 100 g 10 g 1g 0.1 g μ±σ rel. σ

239

Pu/233U calibration samples 1.50 ± 0.05 2.22 ± 0.08 2.56 ± 0.04 3.78 ± 0.08 1.62 ± 0.02 2.3 ± 0.9 0.39

239

Pu/237Np calibration samples

248

Cm/243Am calibration samples

0.97 ± 0.02 1.12 ± 0.02 1.20 ± 0.01 1.29 ± 0.01 0.99 ± 0.01 1.1 ± 0.1 0.12

1.14 ± 0.03 1.18 ± 0.03 1.31 ± 0.01 1.20 ± 0.01 1.44 ± 0.02 1.3 ± 0.1 0.10

248

Cm/239Pu calibration samples 1.47 ± 0.05 1.52 ± 0.04 1.93 ± 0.02 2.06 ± 0.03 1.60 ± 0.02 1.7 ± 0.3 0.15

248

Cm/239Pu CFM samples 1.28 ± 0.11 1.35 ± 0.20 1.80 ± 0.09 1.69 ± 0.22 1.80 ± 0.06 1.6 ± 0.2 0.16

The average CIYs for 248Cm relative to 239Pu over the CFM samples of similar mass are listed as well. The average of the five calibration samples, μ ± σ, was used for the normalization of the data obtained from the CFM samples. a

tively, estimated from the calibration samples. In Table 1, the above-mentioned CIY factors for each sample size are listed. The CIY 239Pu/242Pu is equal to 1, since 239Pu is an isotopic tracer for 242Pu. In agreement with previous studies,13,20−23 the CIY of the An increases with the atomic number. Their values increase also with decreasing sample mass, except for the CIY 248 Cm/243Am in the sample with mass equal to 1 g and for the CIY 239Pu/237Np and CIY 239Pu/233U in the 0.1 g mass sample. Considering the comparison between the CIY 248Cm/239Pu in the CFM and calibration samples and the observed scattering of the data (Figure 2 and Table 1), it is reasonable to use the average values of the CIY over the five calibration samples in order to normalize the concentration of the investigated An nuclides in the CFM samples, independent of the sample size. In this way, the average values, μ, of the CIY 248Cm/243Am, CIY 239 Pu/237Np, and CIY 239Pu/233U equal to 1.3 ± 0.1, 1.1 ± 0.1, and 2.3 ± 0.9, respectively, have been employed in the determination of 243Am, 237Np, and 233U in the CFM samples. As it can be seen in Table 1, the CIY 239Pu/233U varies significantly according to the sample size, compared to the CIY 239 Pu/237Np and CIY 248Cm/243Am. The relative standard deviation, rel. σ, of the average value of the CIY 239Pu/233U, CIY 239Pu/237Np, and CIY 248Cm/243Am corresponds to 39%, 12%, and 10%, respectively. This indicates a higher precision in the determination of 237Np and of 243Am using 239Pu and 248 Cm, respectively, as yield tracer, compared to that of 233U using 239Pu as nonisotopic tracer. AMS Results for CFM Run 13-05 Samples. In Figure 3, the concentrations measured with AMS (blue circles) of 243Am (a), 242Pu (b), 237Np (c), and 233U (d) are expressed as atoms/ g of the groundwater sample as a function of the sampling time after the injection of the An tracers. The concentrations of the investigated An measured with SF-ICPMS are also depicted (red squares). The comparison between the results of the two employed analytical methods is discussed in a following paragraph. As it can be seen in Figure 3, the determination of the injected An with AMS was possible over a concentration range of 6 orders of magnitude, in particular from the maximum value of the 242Pu concentration equal to (3.71 ± 0.04) × 1010 atoms/g (∼15 pg/g) down to the minimum value of the 243Am concentration of (6.2 ± 0.6) × 104 atoms/g (∼0.025 fg/g). The values of the first five samples depicted in Figure 3, collected between 0.01 d (14 min) and 0.4 d (9 h), represent the An concentration levels in the groundwater before the first arrival of the injected tracers at the extraction point. In particular, almost constant concentrations of 243Am in the range

Figure 3. Concentrations of 243Am (a), 242Pu (b), 237Np (c), and 233U (d) measured with AMS (blue circles) in chosen samples from the effluent of run 13-05 until 236 d from the start of the experiment. The first five samples account for the levels of the An in the Grimsel groundwater before the arrival of the tracers injected in run 13-05. The red square represents the values of the investigated An measured with SF-ICPMS.

of ∼(2.0; 2.4) × 106 atoms/g (Figure 3a), of 242Pu in the range of ∼(2.2; 3.0) × 107 atoms/g (Figure 3b), of 237Np in the range of ∼8.2 × 107 and ∼1.3 × 108 atoms/g (Figure 3c), and of 233U in the range of ∼(4.3; 8.5) × 106 atoms/g (Figure 3d) have been determined. Those results were unexpected since no An tracers should be present in those first samples. In order to D


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Analytical Chemistry investigate the origin of this finding, we have performed further investigations that are discussed in a following paragraph. The arrival of the groundwater containing the injected cocktail was recognized via the detection of the conservative fluorescent tracer Amino-G after 0.56 d (13.4 h) at the extraction point. The concentrations of the colloid associated An nuclides in the eluted samples increase rapidly until the maximum is attained after 1.5 d for 243Am and 242Pu with levels of (5.2 ± 0.5) × 109 and (3.71 ± 0.04) × 1010 atoms/g, respectively. This is slightly before the peak maximum of the conservative tracer Amino-G at 1.8 d. The maximum concentrations of 237Np and 233U are observed slightly later, at ∼3 d with (1.3 ± 0.2) × 1010 atoms/g and at ∼2 d with (1.4 ± 0.5) × 109 atoms/g, respectively. After 236 d (∼8 months), the levels of the An nuclides in the groundwater samples decrease then down to (2.1 ± 0.2) × 105, (2.10 ± 0.05) × 106, (1.4 ± 0.2) × 107, and (1.4 ± 0.5) × 107 atoms/g for 243Am, 242 Pu, 237Np, and 233U, respectively. As it can be seen in Figure 3, the An tracers could be quantified with AMS until the last sample taken 8 months after the start of the experiment. In two further samples collected after 518 and 660 d at the “Pinkel” surface packer, still the injected An nuclides were detected with concentrations significantly lower than those found in sample collected after 236 d. In particular, (6.2 ± 0.6) × 104 and (8 ± 1) × 104 atoms/g for 243Am, (3.42 ± 0.07) × 105 and (4.3 ± 0.2) × 105 atoms/g for 242Pu, (1.1 ± 0.1) × 106 and (1.6 ± 0.2) × 106 atoms/g for 237Np, and (8 ± 3) × 105 and (1.0 ± 0.4) × 106 atoms/g for 233U were determined in sample at 518 and 660 d, respectively. The relative uncertainties associated with the measured concentrations change substantially depending on the nuclide. With the sensitivity of AMS, An nuclides can be determined at levels down to 105 atoms in a sample with a relative statistical uncertainty of ∼32%. Similarly, the detection of 107 atoms is associated with an error of ∼0.02%. In the actual study, the use of nonisotopic tracers leads to a higher uncertainty in the determination of 233U, 237Np, and 243Am than of 242Pu. While the concentration data for 242Pu have a relative error between 1% and 5% mainly due to counting statistics, the other nuclides have higher uncertainties due to the use of nonisotopic tracers. As discussed previously, the variability of CIY of Np relative to Pu and Am relative to Cm has resulted in uncertainties of ∼10% and ∼12%, respectively. On the other hand, the variability of the CIY of U relative to Pu has been significantly higher in the actual experiment, so that for the concentrations of 233U, a relative uncertainty of up to ∼39% is obtained. Comparison between the Results Obtained with AMS and SF-ICPMS. In the ten samples belonging to the peak of the BTC curve chosen for the direct comparison between the two employed analytical methods, the concentrations of 243Am, 242 Pu, and 237Np measured with the multi-An method and AMS mirror the behavior of those measured with SF-ICPMS, as it can be seen in Figure 3a−c. In Figure 4, the ratio between the AMS and the corresponding SF-ICPMS values is plotted as a function of the sampling time. It can be seen that the results of the two analytical techniques mostly are not consistent within the related analytical uncertainties and the majority of the AMS values lie below the SF-ICPMS ones. However, considering increased relative uncertainties on both the AMS and SFICPMS values up to ∼50%, the two analytical techniques agree for 243Am, 242Pu, and 237Np. The 233U concentrations determined with SF-ICPMS (not presented in the actual manuscript) are most likely affected by a

Figure 4. Comparison of the concentrations of 237Np (blue circles), 242 Pu (red squares), and 243Am (green diamonds) measured with AMS and SF-ICPMS in samples collected within the peak of the BTC curve of run 13-05. Data are expressed as ratio of AMS and SF-ICPMS values (AnAMS/AnICPMS). The yellow horizontal line marking the value 1 indicates the case of complete coincidence of the AMS and SFICPMS values.

polyatomic isobaric interference. According to Pointurier et al.,24 there are indeed several polyatomic species in environmental sample solutions which can interfere with the determination of An nuclides with masses in the range of 233 to 247. In particular, the species 185Re16O3, 193Ir40Ar, 197Au36Ar, 198 Hg35Cl, 201Hg16O2, 200Hg16O2H, and 203Tl14N16O can be responsible for contributing to the count rate of mass 233 during ICPMS measurements. Fifield et al.20 presented a first direct comparison between data obtained with AMS and ICPMS on measurements of 237Np in three independent preparations of mud from a saltmarsh near Sellafield. The authors found agreement between the results of the two techniques, but with AMS values ∼9% higher than the corresponding ICPMS ones, and ascribed such difference to unidentified systematic uncertainties in both the ICPMS and the AMS measurements. More investigations are needed in order to identify the source of the differences observed in the actual study between the AMS and SF-ICPMS results. Discussion on 243Am, 242Pu, 237Np, and 233U Concentrations in Samples Collected before the Arrival of the Migration Front and Further Investigations in This Matter. In order to explain the origin of the unexpected signal of 243Am, 242Pu, 237Np, and 233U in the first five samples before the peak arrival of the injected An with the eluted groundwater, we have considered the hypothesis of a background level coming from the previous in situ tracers experiments. In fact, 237Np, 242Pu, and 243Am were injected as An tracers in the granodiorite fracture in the frame of both the CFM run 12-02 and the CRR run 1, while 237Np, 233U, 241Am, and 244Pu were employed in the CRR run 2.9,13 In particular, the CRR experiments took place in the same dipole subsequently used for CFM run 13-05. The only two An nuclides exclusively injected in those previous experiments and not employed in CFM run 13-05 were 241Am and 244Pu. Therefore, we have performed further AMS investigations with a group of selected samples obtained from the actual experiment in order to measure at masses 241 and 244 u. The levels of 242Pu and 243Am and masses 241 and 244 u measured in the procedural blanks B1, B2, B3, and B4 are in some cases zero and in other cases significantly lower than the corresponding values measured in the CFM samples. No counts were detected for masses 241 and 244 u during a measuring time equal to 3582 s per selected mass. Similarly, E


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Analytical Chemistry during a measurement time of 5970 s, no counts were collected for 243Am. The level of 242Pu in the procedural blanks is not zero but referred to a measuring time of 5970 s, corresponding to count rates of 0.0014 cps (7 counts), 0.0048 cps (23 counts), 0.0008 cps (4 counts), and 0.0046 cps (22 counts) for B1, B2, B3, and B4, respectively. The count rates of 242Pu detected in the two samples presenting the lowest concentrations and also the lowest count rates among the CFM samples, namely, the samples at 518 and 660 d, are equal to 0.83 cps (1607 counts and 2388 s measurement time) and 0.25 cps (493 counts and 2388 s measurement time). Comparing such values to those of B1, B2, B3, and B4, we can conclude that the 242Pu levels determined in the four procedural blanks are negligible. We have also considered that masses 241 and 244 u may be present as minor nuclides in both the 242Pu and 243Am solutions used for the preparation of the injection cocktail and in the 239Pu and 248Cm solutions used as internal tracers in the actual experiment. The isotopic composition of the internal tracers were determined with SF-ICPMS before starting the experiment. The isotopic ratios 241Pu/239Pu, 242Pu/239Pu, and 244 Pu/239Pu in the 239Pu (Isotope Products Laboratories) reference solution are equal to (2.1 ± 0.1) × 10−4, (8.7 ± 0.8) × 10−6, and ≤8.2 × 10−6, respectively. The 248Cm solution is characterized by the 244Cm/248Cm isotopic ratio equal to (1.19 ± 0.05) × 10−5. Therefore, spiking the actual samples with ∼1 × 108 atoms of 239Pu and 248Cm, we have introduced into each sample ∼2.1 × 104 atoms of 241Pu, 8.7 × 102 atoms of 242 Pu, and 2 × 103 atoms with the mass 244 u (244Pu + 244Cm). As it can be seen in Figure 5, the levels of masses 241 and 244 u

Figure 6. Concentrations of nuclides with masses 241 u (a) and 244 u (b) (orange triangles) in 11 chosen samples from run 13-05 and in two samples collected after 518 and 660 d from the start of the experiment. The concentrations of 243Am (a) and 242Pu (b) (green squares), the atomic ratio of 241/243 (a) as well as of 244/242 (b) (black diamonds) in the same samples and in the injection cocktail (blue circles) of run 13-05 are also depicted.

injection cocktail for samples taken during the BTC peak. In particular, the 244/242 atomic ratio before the BTC peak is constantly equal to (1.6 ± 0.1) × 10−2 until 0.25 d, decreases to (2 ± 1) × 10−5 at 1.5 d, and increases again then up to (1.34 ± 0.09) × 10−2 at 236 d where it remains. At the peak maximum of 242Pu and mass 244 u, the 244/242 atomic ratio corresponds to the value for the injection cocktail. A similar trend is observed for the 241/243 atomic ratio: in the first samples, the ratio remains constant at (4.9 ± 0.2) × 10−2 until 0.25 d and decreases over the BTC peak for 243Am and mass 241 u down to (2.5 ± 0.6) × 10−3 between 0.7 and 116 d. Also, in this case, the minimum value is consistent with the value in the injection cocktail. Values increase then again up ∼1.5 × 10−2 at 660 d. Considering such results, we can conclude that there are two sources of nuclides with masses 241 and 244 u in the samples: one of these is likely the injection cocktail itself, with the respective 241/243 and 244/242 atomic ratio signatures appearing in samples taken at the BTC maximum. Previous in situ tracer tests, where 241Am and 244Pu had been injected, are possibly the second source. In CRR run 1, 110 ± 13% 237 Np, ∼43% 243Am, and ∼29% 242Pu were recovered. In run 2, ∼82% 237Np, 102 ± 5% 233U, ∼70% 241Am, and 86 ± 9% 244Pu were found.9 Considering those recoveries and for those nuclides where more or less quantitative elution was found (237Np in run 1 and 233U in run 2), a recovery of 99% (corresponding to the lower boundary of the analytical uncertainty range), we can estimate the amount of the An nuclides left in the granodiorite fracture as follows: ∼5.2 × 1014 atoms of 233U, ∼1.2 × 1016 atoms of 237Np, ∼1.2 × 1013 atoms of 241Am, ∼4.2 × 1014 atoms of 242Pu, ∼2.0 × 1014 atoms of 243 Am, and ∼5.7 × 1013 atoms of 244Pu. Considering the series of in situ tests performed at the GTS in the period between the CRR tests and the CFM run 1305,8,12 we can furthermore estimate a total volume of ∼1 × 105 L of groundwater having passed through the fracture. Such a value has to be taken as an upper limit. If we assume within a very simplistic hypothesis that all the An left in the fracture from previous experiments before the start of the CFM run 1305 is entirely released into the aforementioned volume of

Figure 5. Number of atoms of 241Pu (green line) and 244Cm + 244Pu (black line) introduced into the samples after spiking with the 239Pu and 248Cm tracer solution. The number of atoms with mass 241 u (assumed to be 241Pu) and with mass 244 u (assumed to be 244Cm + 244 Pu) determined in the samples are depicted as orange squares and blue circles, respectively.

(assumed to be 241Pu and 244Cm + 244Pu, respectively) determined in the samples are always significantly higher than those introduced into the samples by spiking the internal standards. These results suggest that the measured levels at masses 241 and 244 u are inherent to the CFM samples. We have then compared the isotopic distribution determined in the samples with that of the injection cocktail of run 13-05. Also, in this case, the isotopic composition of the injection cocktail was investigated with SF-ICPMS and the atomic ratios of mass 241 u nuclides/243Am and mass 244 u nuclides/242Pu in the injection cocktail were found to be equal to (2.6 ± 0.6) × 10−3 and (3 ± 4) × 10−5, respectively. As it can be seen in Figure 6, the corresponding atomic ratios measured in the chosen samples of CFM run 13-05 are significantly higher than those in the injection cocktail both before and after the peak of the BTC curve, while they are consistent with the values of the F



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groundwater, we obtain concentrations equal to ∼5.3 × 106 atoms/mL of 233U, ∼1.3 × 108 atoms/mL of 237Np, ∼1.2 × 105 atoms/mL of 241Am, ∼4.3 × 106 atoms/mL of 242Pu, ∼2.1 × 106 atoms/mL of 243Am, and ∼5.8 × 105 atoms/mL of 244Pu. Such a scenario does certainly not represent the complexity of the sorption/desorption reactions of the various An within the system of groundwater−bentonite colloids−fault gouge minerals present in the granodiorite fracture at the GTS over a decade. However, those values are in the range of what is measured with AMS in the fracture effluent before the tracer BTC of CFM run 13-05. A further important conclusion of this finding is that apparently the An undergo reversible reactions within the fracture and desorption from fracture surfaces obviously occurs. The results show in addition that, by taking advantage of the sensitivity of AMS, one is given the opportunity to extend in situ studies to investigate the longterm behavior of An over a time span of a decade or more.

Article

AUTHOR INFORMATION

Corresponding Author

*Phone: + 49 721 608-22233. Fax: +49 721 608-23927. E-mail: [email protected] or [email protected]. ORCID

Francesca Quinto: 0000-0001-6535-9258 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are indebted to Stephanie Heck (KIT-INE) for preparation of the radionuclide cocktail and groundwater electrolyte analysis and Karam Kontar (SolExperts) for many fruitful discussions during the CFM field team meetings and substantial support at the CFM site. The work has received funding by the Federal Ministry of Economics and Technology (BMWi) under the joint KIT-INE, GRS research project “KOLLORADO-e” (Grant agreement no. 02E11203B) and “KOLLORADO-e2” (Grant agreement no. 02E11456A), through the Collaborative Project BELBaR (Grant Agreement no. 295487) by the European Commission (7th Euratom Framework Programme for Nuclear Research & Training Activities; FP7/2007−2013), and through the HGF PoF-III program “NUSAFE”. Furthermore, the authors would like to acknowledge the support of the CFM Project partners and the support of the local Grimsel Test Site/NAGRA staff.

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CONCLUSIONS AND PERSPECTIVES An nuclides were determined with AMS in GTS groundwater samples over 6 orders of magnitude in concentrations ranging from ∼15 pg/g down to ∼25 ag/g in sample sizes of 0.1 to 250 g. Agreement within a 50% margin is found between the values of 237Np, 242Pu, and 243Am determined with SF-ICPMS and the multi-An analysis with AMS. Further studies are needed to identify a possible polyatomic isobaric background in the ICPMS measurements of 233U. An additional consideration can be made concerning the shape of the BTC curves of the injected actinides. While the concentrations of 237Np, 242Pu, and 243Am in the samples after the peaks are significantly lower than those determined before the arrival of the injected cocktail (see Figure 3), the levels of 233 U after the peak are higher than those preceding it. These different trends could reflect a preferential stripping of Pu and Am from the fracture via binding to bentonite colloids, while Np(V) and U(VI) preferentially exist as noncolloidal aquo species. The shape of the BTC of Np could be interpreted as an indication of a partial reduction and adsorption/precipitation of Np(IV) in the shear zone, which also might be (partially) desorbed by bentonite colloids. We will in future studies investigate the hypothesis of the potential stripping of fracture surface associated actinides by a colloid source as well as the redox speciation of Np leading to the observed differences in the BTC’s of the four investigated actinides (Figure 3). With the extreme sensitivity of AMS, the long-term release and retention of the actinides have been investigated up to 8 months in the tailing of the BTC curve of run 13-05 as well as in samples collected up to 22 months after at the “Pinkel” surface packer. Furthermore, considering the detection of actinides with masses 241 and 244 u in the investigated CFM samples, we assume that they can be attributed to a release from the fracture of 241Am and 244Pu injected within the previous in situ test CRR run 2. It is important to note that such signals from previous experiments are detectable not only at the extraction point of the dipole where the CRR tests and the CFM run 13-05 were performed but also further downstream into the granodiorite fracture at the “Pinkel” surface packer. This finding gives strong indications for the reversible kinetically controlled interaction of actinides with fracture filling material and demonstrates the strong analytical capabilities of AMS for allowing the long-term in situ studies over a time span of more than a decade.

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REFERENCES

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Multiactinide Analysis with Accelerator Mass Spectrometry for

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