137Cs Isotopic Ratio as a New Tracer of Radiocesium

Apr 29, 2014 - Research Center for Radiation Protection, National Institute of Radiological ... (FDNPP) accident in 2011, intensive studies of the dis...
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Cs/137Cs Isotopic Ratio as a New Tracer of Radiocesium Released from the Fukushima Nuclear Accident Jian Zheng,*,† Keiko Tagami,† Wenting Bu,†,‡ Shigeo Uchida,† Yoshito Watanabe,§ Yoshihisa Kubota,§ Shoichi Fuma,§ and Sadao Ihara∥ †

Research Center for Radiation Protection, National Institute of Radiological Sciences, 491 Anagawa, Inage, Chiba 263-8555, Japan State Key Laboratory of Nuclear Physics and Technology, School of Physics, Peking University, Beijing 100871, China § Project for Environmental Dynamics and Radiation Effects, Fukushima Project Headquarters, National Institute of Radiological Sciences, 491 Anagawa, Inage, Chiba 263-8555, Japan ∥ Department of Biology, Ohu University, 31-1 Koriyama, Fukushima 963-8611, Japan ‡

S Supporting Information *

ABSTRACT: Since the Fukushima Daiichi nuclear power plant (FDNPP) accident in 2011, intensive studies of the distribution of released fission products, in particular 134Cs and 137Cs, in the environment have been conducted. However, the release sources, that is, the damaged reactors or the spent fuel pools, have not been identified, which resulted in great variation in the estimated amounts of 137Cs released. Here, we investigated heavily contaminated environmental samples (litter, lichen, and soil) collected from Fukushima forests for the long-lived 135Cs (half-life of 2 × 106 years), which is usually difficult to measure using decay-counting techniques. Using a newly developed triplequadrupole inductively coupled plasma tandem mass spectrometry method, we analyzed the 135Cs/137Cs isotopic ratio of the FDNPP-released radiocesium in environmental samples. We demonstrated that radiocesium was mainly released from the Unit 2 reactor. Considering the fact that the widely used tracer for the released Fukushima accident-sourced radiocesium in the environment, the 134Cs/137Cs activity ratio, will become unavailable in the near future because of the short half-life of 134Cs (2.06 years), the 135Cs/137Cs isotopic ratio can be considered as a new tracer for source identification and long-term estimation of the mobility of released radiocesium in the environment.

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essential for improving the accuracy of estimation of 137Cs release. Accurate determination of radiocesium isotopic composition is important for release source identification. Among the released radiocesium isotopes, 136Cs (t1/2 = 13.2 days) has already decayed out, so that cannot be measured now; intensive determination of 134Cs along with 137Cs has been conducted since the FDNPP accident, and the 134Cs/137Cs activity ratio was immediately used to identify radioactive contamination in the environment.13−19 For example, Komori et al.13 investigated the possibility that the 134Cs/137Cs ratio can be useful as an index for evaluating the contamination from each reactor unit. They reported that the 134Cs/137Cs activity ratio in most samples was ∼1, similar to the ratio of Units 2 and 3; in addition, a lower ratio was found in a sample from the Oshika Peninsula, suggesting that the Oshika Peninsula was contami-

INTRODUCTION

Studies have revealed that large amounts of fission products (FPs), such as 129mTe, 132Te, 131I, 134Cs, 136Cs, and 137Cs, were released into the environment during the early phase of the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident because of venting operations conducted in Units 1−3, hydrogen explosions in the reactor buildings of Units 1 and 3, and the breach of the Unit 2 containment vessel.1−7 There was also hydrogen explosion on the fourth floor of the Unit 4 reactor building, where the largest spent fuel pool (SFP) was located, a little after 06:10 on March 15, 2011 (ref 8). Following this explosion, a fire occurred there. Although the release of FPs, for example, 137Cs, from the SFP in the Unit 4 reactor building was suggested,3,9 it has not been verified. Whether the SFP in the Unit 4 reactor building contributed to the large amounts of FPs released remains an important issue of debate.10,11 This has resulted in great variation in the estimated amounts of 137Cs released; the total amount of 137Cs released has been estimated to range from 15 to 36.6 PBq.3,4,12 Thus, identification of release sources in the FDNPP accident is © 2014 American Chemical Society

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(137Cs activity ranging from 0.15 to 4.65 MBq/kg) collected from April 2011 to May 2013 in the 20−50 km zone around the FDNPP and found that the release of 137Cs from the Unit 4 SFP due to the explosion and subsequent fire, if any, was negligible. Combined with the 240Pu/239Pu isotopic ratio fingerprint, for the first time, we could conclude that the Unit 2 reactor was the main release source of FPs in the FDNPP accident. The obtained 135Cs/137Cs isotopic ratio is proposed as a new tracer for applications in long-term estimation of environmental behavior of released radionuclides.

nated mainly from the radiocesium released from Unit 1. However, considering the short half-life of 134Cs (2.06 years), the 134Cs/137Cs activity ratio will become unavailable in the future for identification of the FDNPP radioactive contamination. Inspired by the pioneering study of Lee et al. using the 135 Cs/137Cs isotopic ratio as a powerful chronometer tracer in the environment,20 in this study, we discuss the possibility of establishing a new tracer, the 135Cs/137Cs isotopic ratio, for release source identification and consider its application for long-term estimation of the mobility of released radionuclides in the environment. Radiocesium isotopes are FPs with high yields of up to 6.535 and 6.236% for 135Cs and 137Cs, respectively, from the thermal neutron fission of 235U.21 In the fission chains for 135Cs and 137Cs, a shielding of 135Cs occurs due to the neutron capture of its precursor, 135Xe, to form 136 Xe, whereas production of 137Cs is unaffected.22 This process causes a high degree of variance in the 135Cs/137Cs isotopic ratio with source. Thus, this ratio will be characteristic of the reactor operation and shutdown conditions. Delmore et al. measured the 135Cs/137Cs isotopic ratio in the effluent from reactor operations, discussed in detail the multiple paths by which radioactive cesium can reach the effluent, and, thus, demonstrated the use of the cesium isotope ratio as an indicator of nuclear power plant operations.23 Using the ORIGEN code, a Japan Atomic Energy Agency (JAEA) group calculated the 135 Cs and 137Cs inventory in the three damaged reactor cores and the four SFPs in the FDNPP24 and showed that 135 Cs/137Cs isotopic ratios are characteristically different among the damaged reactors and the SFPs (Table 1). Recently,

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EXPERIMENTAL SECTION Litter, Lichen, and Soil Sampling. The collection of organic layer samples from a forest floor was conducted in a deciduous broad-leaf forest at three sampling locations differing in distance and slightly in direction from the nuclear power plant in the Deliberate Evacuation Area (S1, in Katsurao Village, 25 km west northwest of the FDNPP, N37°29′02.8″ E140°45′46.4″; S2, in Namie Town, 26 km northwest of the FDNPP, N37°34′17.1″ E140°47′39.9″; S3, in Iitate Village, 32 km northwest of the FDNPP, N37°36′20.0″ E140°45′17.1″) in May 2011. Another litter sample (S5, in Oguni Village, ∼50 km northwest of the FDNPP, N37°44′36.0″ E140°33′46.0″) was collected in October 2012. For each location, three samples were collected several meters apart for organic layer and soil samples. The organic layer samples were divided into two parts, the upper litter layer (AOL) and the lower fermentation and humus layer (AOF, AOH). A lichen sample (S4, Iitate Village, ∼40 km northwest of the FDNPP, N37°42′12.0″ E140°42′55.0″) was collected in May 2013. A surface soil (0−2 cm) sample (S6, 20 km south of the FDNPP) was collected in April 2011 (ref 26). All environmental samples (S1−S6) were analyzed this time for 135Cs, 137Cs, and the 135 Cs/137Cs isotopic ratio. The upper litter layer samples (S1− S3) and J-Village soil sample (S6) were analyzed for Pu isotopes in our previous work.26 Analytical Procedures for 137Cs and the 135Cs/137Cs Isotopic Ratio. The 137Cs activity was determined using a GE detection system (Seiko EG&G) for 3600 s in most cases. The 137 Cs activity was determined using its peak at 661.6 keV.26 For 135 Cs/137Cs isotopic ratio analysis, a sample (approximately 2− 4 g) was weighed out and put in a sealable Teflon vessel (120 mL). After 20 mL of concentrated HNO3 had been added, the lid of the Teflon vessel was tightened and the vessel was heated on a hot plate (160 °C). During the acid digestion process, 4 mL of H2O2 was added to destroy the organic matter. After the sample had been heated to near dryness, the residue was dissolved in 20 mL of concentrated HNO3. Then the sample solution was filtered through an Advantec filter into a beaker (250 mL). From this filtered sample solution, 0.1 mL was taken out and diluted to 10 mL with 4% HNO3, to be used for the analysis of stable Cs as a yield monitor. The rest of the sample solution was adjusted to the activity of 1.6 M HNO3 by adding Milli-Q water. Then 35 mg of ammonium molybdophosphate (AMP) (Kishida Chemical Co., Ltd.) was added to the sample solution for the adsorption of Cs. The mixture was stirred for 1 h and then passed through a 0.45 μm syringe filter (Sartorius Stedim Biotech, Goettingen, Germany) to retain the Csadsorbed AMP. AMP was dissolved from the filter in 5 mL of 1.5 M NH4OH and was then ready for loading onto an AG MP-1M resin column.

Table 1. Model Calculation Results of Cs and Pu Isotopic Compositions of Nuclear Fuels and Estimated Amounts of 137 Cs Released and Inventory of 135Cs 135

Cs/137Cs isotopic ratio ORIGEN calculationa Core 1 Core 2 Core 3 SEP-1 SEP-2 SEP-3 SEP-4

0.396 0.341 0.350 0.514 0.435 0.377 0.415

240

Pu/239Pu isotopic ratio

0.344 0.320 0.356 0.394 0.442 0.468 0.417

135 Cs released (PBq)

135 Cs inventory (kg)

total of 15

core total of 77.5 24.7 26.8 26.0 25.9 60.0 45.6 113.0

0.59 14.0 0.71 − − − −

a Data for the Cs isotopic compositions and 135Cs inventory in nuclear fuels in the reactor cores of Units 1−3 and in SFPs reported by Nishihara et al.24 Data for the amounts of 137Cs released into the environment estimated by METI using model simulation.4

Ohno and Muramatsu reported the 134 Cs/ 137 Cs and Cs/137Cs isotopic ratios in rainwater samples collected in Fukushima, Tsukuba, Chiba, and Tokyo.25 They found that the measured 135Cs/137Cs isotopic ratios in rainwater samples are different from the values of global fallout and the Chernobyl accident, indicating that this isotopic ratio can be used as a radiocesium tracer in the environment. Here, by applying a newly developed inductively coupled plasma tandem mass spectrometry (ICP-MS/MS) technique, we successfully measured 135Cs/137Cs isotopic ratios in heavily contaminated environmental samples (litter, lichen, and soil) 135

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Table 2. Analytical Results of 137Cs Activities, 135Cs/137Cs Isotopic Ratios, and 240Pu/239Pu Isotopic Ratios in Environmental Samples

After the AG MP-1M resin column had been preconditioned with 10 mL of 1.5 M NH4OH, the sample solution was loaded on the column. An additional 3 mL of 1.5 M NH4OH was used for the elution of Cs. In the presence of an alkaline solution, Mo in the sample solution was dissolved in the hexavalent state as MoO42− molybdate ions, which will be retained on the AG MP-1M column, while Ba as positively charged Ba2+ ions will be eluted together with Cs. The effluent during sample loading and the eluent were collected together in a 30 mL Teflon vessel. Then the sample solution was evaporated to dryness and dissolved in 5 mL of 0.15 M NH4OH, ready for loading onto an AG 50 WX8 resin column. The 2 mL AG 50 WX8 resin column was preconditioned with 10 mL of 1.5 M HCl, 10 mL of H2O, and 10 mL of 0.15 M NH4OH. After the sample had been loaded, the column was washed with 10 mL of 0.15 M NH4OH and 10 mL of H2O. Cs was eluted from the AG 50 WX8 resin by adding 30 mL of 1.5 M HCl, and Ba was retained on the resin, thus yielding the required separation between Cs and Ba. The eluent was evaporated to near dryness and dissolved in 4 mL of 4% HNO3, in preparation for ICP-MS analysis. With our chromatographic separation system, we achieved high decontamination factors of 1.8 × 104 for Ba and 4.1 × 105 for Mo. We used a triplequadrupole ICP-MS/MS system (Agilent 8800) that features an additional quadrupole mass filter, situated in front of the octopole reaction cell and quadrupole mass filters.27 N2O gas was introduced into the octopole reaction cell to eliminate polyatomic and isobaric interference, such as MoAr+, 135Ba+, and 137Ba+, for the measurement of 135Cs and 137Cs.28 As shown in Figure S1 of the Supporting Information, the Ba signal intensity was suppressed by 4 orders of magnitude. In addition, the triple-quadrupole ICP-MS/MS system uses the tandem mass spectrometer configuration with two quadrupole mass filters (Q1 and Q2), which significantly improved the abundance sensitivity. In MS/MS mode, both Q1 and Q2 are operated as unit mass filters, so the overall abundance sensitivity of the instrument is the product of the Q1 abundance sensitivity times the Q2 abundance sensitivity. With two high-frequency and hyperbolic quadrupoles, each operating with an abundance sensitivity of 10−7, the combined abundance sensitivity of ICP-MS/MS is theoretically 10−14, providing sufficient abundance sensitivity for 135Cs isotope measurement to eliminate the interference of stable 133Cs in the environmental samples.29 The analytical method employed for 135 Cs and 137Cs measurement was validated by the analysis of the IAEA-375 reference material (soil collected from the 30 km zone of Chernobyl). The 135Cs/137Cs isotopic ratio we found is 0.517 ± 0.045, which is in good agreement with the value of 0.50 ± 0.05 reported by Taylor et al.,30 indicating the applicability of our analytical method for environmental samples. The analysis of environmental samples for radiocesium isotopes was conducted at the Tokyo Analytical Division, Agilent Technologies International Japan, Ltd.

sample S1 (litter, Katsurao) S2 (litter, Namie) S3 (litter, Iitate) S4 (lichen, Iitate) S5 (litter, Oguni) S6 (soil, JVillage)

135 Cs/137Cs isotopic ratio

0.333 ± 0.007

137

Cs activity (Bq/g)

148 ± 1a

240 Pu/239Pu isotopic ratio

not determined

0.340 ± 0.001

a

1416 ± 4

0.323 ± 0.017a

0.341 ± 0.002 0.342 ± 0.020

4649 ± 9a 659 ± 3

0.330 ± 0.032a not determined

0.343 ± 0.009

119 ± 1

not determined

0.375 ± 0.024

11.5 ± 0.5

0.303 ± 0.030a

a

Data on 137Cs activities and 240Pu/239Pu isotopic ratios of litter samples reported by Zheng et al.26

(S1−S3 and S5) and the lichen sample (S4) collected in the 20−50 km zone in the northwest direction from the plant site and a soil sample (S6) collected in J-Village south of the plant site (Figure 1) were analyzed in this study; the 240Pu/239Pu isotopic ratios in litter samples S2 and S3 and soil sample S6 were reported previously.26 We found that 135Cs/137Cs isotopic ratios in the litter and lichen samples collected northwest from the FDNPP site had very similar values, ranging from 0.333 to 0.343 (reference to March, 11, 2011). No significant variation in the 135Cs/137Cs isotopic ratio could be observed, although the activities of 137Cs were extremely different, ranging from 0.12 to 4.65 MBq/kg, in these litter and lichen samples, indicating that radioactive Cs isotopes deposited on the surface of litter and lichen were mostly released from the same source in the FDNPP. Similarly, the 240Pu/239Pu isotopic ratio showed constant values (0.323−0.330) in the investigated forest litter samples. In the surface soil sample from J-Village, 20 km south of the plant, a slightly high 135Cs/137Cs isotopic ratio of 0.375 ± 0.024 was observed. To investigate if there was significant release of FPs from the nuclear fuels in the SFPs, in particular the SFP in the Unit 4 reactor building where the hydrogen explosion and fire had occurred, we compared 135 Cs/137 Cs isotopic ratios in Fukushima environmental samples with those in the damaged reactors (Units 1−3) and the SFPs (Units 1−4). As shown in Figure 2, the observed 135Cs/137Cs isotopic ratios (0.333− 0.343) in environmental samples taken from areas northwest of the FDNPP site were distinctly different from those in the SFPs (0.377−0.514), but coincident with 135Cs/137Cs isotopic ratios in the cores of Units 2 and 3 (0.341 and 0.350, respectively) (Table 2). This result indicates that the possible release of FPs from the SFPs was negligible, if any occurred. The damaged reactors were the sources of the radioactive releases. Moreover, the 135Cs/137Cs isotopic ratio of Core 1 (0.396) is much higher than the ratios observed in these samples, suggesting that Core 2 and/or Core 3 were the major release sources, and Core 1 made a relatively small contribution to the total amount of FPs released. A higher 135Cs/137Cs isotopic ratio of 0.375 ± 0.024 was found in J-Village soil; two possibilities could be considered for this observation. (1) A relatively significant deposition of radiocesium released from the Unit 1 reactor occurred in the area south of the FDNPP site, and (2) this slightly high 135 Cs/137Cs isotopic ratio was due to the mixing of global fallout radiocesium with the FDNPP-sourced radiocesium because we have found that ∼10% 239+240Pu had its origin in

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RESULTS AND DISCUSSION Identification of the Source of Radiocesium Released by the FDNPP Accident. Table 2 summarizes the analytical results of the 135Cs/137Cs isotopic ratio, the 240Pu/239Pu isotopic ratio in Fukushima environmental samples, and nuclear fuel isotopic compositions of radioactive Cs and Pu isotopes in the damaged reactors, and the SFPs in the FDNPP obtained by ORIGEN code calculation conducted by JAEA24 are listed in Table 1. The 135Cs/137Cs isotopic ratios of forest litter samples 5435

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Figure 1. Map showing sampling locations of Fukushima environmental samples (litter, lichen, and surface soil) with information about activities and 135Cs/137Cs isotopic ratios.

137

Cs

Figure 2. Comparison of 135Cs/137Cs isotopic ratios observed in litter and lichen samples and those in nuclear fuels in the damaged reactors (Cores 1−3) and in the spent fuel pools (SFPs).

Figure 3. Comparison of Cs and Pu isotopic compositions among litter (S2 and S3) and surface soil (S6) samples and nuclear fuels in the damaged reactors and in the SFPs.

global fallout in this sample.26 Further investigation of the distribution of the 135Cs/137Cs isotopic ratio in the area south of the FDNPP site is needed in the future. Figure 3 compares the isotopic compositions of Cs (135Cs/137Cs) and Pu (240Pu/239Pu) isotopes in litter (S2 and S3) and soil (S6) samples, and nuclear fuels in the damaged reactor cores and in the SFPs. Again, the isotopic compositions of the SFPs were completely different from those observed in the heavily contaminated forest litter samples (levels of 137Cs ranging from 0.12 to 4.65 MBq/kg), eliminating the possibility of significant release of FPs from the SFP sources. It was interesting to note that there is a striking similarity between the Cs and Pu isotopic compositions observed in the forest litter samples and those in the Unit 2 reactor core (Core 2). The coincidence of this isotopic composition strongly indicated that the major source of FPs release during the FDNPP accident was damaged reactor Unit 2. Compared to the venting operations and hydrogen explosions, the breach of the Unit 2 containment vessel resulted in the largest radioactive release. As shown in Table 1, the Ministry of Economy, Trade and Industry (METI) estimated the released amounts of 137Cs from Units 1−3 were 0.59, 14.0, and 0.71 PBq, respectively,4 using a

model simulation; that is, the release from Unit 2 accounted for more than 93% of the total releases from the three damaged reactors. Our analysis is consistent with the METI model simulation, and for the first time, we clearly identified the major release source as the release from the Unit 2 reactor based on the Cs and Pu isotopic composition fingerprint. Therefore, the estimation of a release of 36.6 PBq of 137Cs by Stohl et al.,3 with the assumption that the explosion and fire in the Unit 4 reactor building caused significant release of FPs from nuclear fuels in the Unit 4 SFP, was undoubtedly an overestimation. Using a 135Cs/137Cs isotopic ratio of 0.341 observed in the S3 litter sample that was most heavily contaminated with a 137Cs activity of 4.65 MBq/kg and a total amount of 137Cs release of 15 PBq estimated by the METI,4 we could calculate the total amount of 135Cs release to be 6.74 × 10−5 PBq or 1.58 kg. Then, on the basis of the inventory of 135Cs in the cores of Units 1−3 (Table 1) estimated by the JAEA group,24 we estimated that the percentage of 135Cs core inventory released was 2.0%, which is in good agreement with the estimated 137Cs percentage of release (∼2%) by the IAEA.31 The agreement of the release percentages between 135Cs and 137Cs, again, 5436

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Ltd.) for assistance with the 135Cs and 137Cs analysis using ICPMS/MS. Dr. Kenji Nishihara (JAEA) is thanked for constructive discussion about the isotopic compositions of Cs in reactor cores and SFPs. We thank Dr. N. Ishii and Dr. Y. Uchihori for providing Oguni litter sample and J-Village soil samples. We thank the editor and four reviewers for their constructive comments that significantly improved our manuscript.

provides evidence of only negligible release of FPs from the SFPs, if any. Potential Application of the 135Cs/137Cs Isotopic Ratio as a New Tracer for Long-Term Migration of Radiocesium in the Environment. Among the released FPs, 137Cs is the most important radionuclide for radiation dose estimation because of the large amount released (∼15 PBq) and the relatively long half-life (30.2 years). However, many important issues with respect to its atmospheric transport, deposition processes, and distributions in terrestrial and marine environments remain to be investigated. It has been estimated that ∼80% of the atmospherically released 137Cs was deposited in the western North Pacific Ocean, in addition to 3.6 PBq of 137 Cs directly discharged into the ocean due to the discharge of radioactive waste waters.32 Thus, we estimate that ∼7.01 × 10−5 PBq (1.64 kg) of 135Cs has been released into the ocean since the FDNPP accident. Furthermore, the continuous input of 137 Cs into the ocean due to river runoff of the 137Cs deposited in heavily contaminated Fukushima forest soil can be expected. Recent studies have revealed the start of the transport of the Fukushima accident-sourced 137Cs into the ocean interior, and a possible pathway of Fukushima accident-derived radionuclides in the North Pacific Ocean was proposed.33,34 On the basis of this proposed pathway, it is predicted that in 30 years the Fukushima accident-derived 137Cs will come back to the ocean surface in the western North Pacific Ocean off the Fukushima coast through its transport by the Kuroshio current. Thus, to understand the environmental behavior and the fate of Fukushima accident-sourced radionuclides in the environment, a powerful Cs tracer is strongly required, because the currently widely used 134Cs/137Cs activity ratio tracer will become unavailable in several years because of the rapid decay of 134 Cs activity in the environment. The 135Cs/137Cs isotopic ratio of the Fukushima accident-sourced radioactive Cs was characterized by a value of 0.341, which is different from those of global fallout Cs (2.7 ± 0.5, reference to 2009)35 and the Chernobyl accident (0.50 ± 0.05, reference to 2006).30 In addition, 135Cs has a half-life of 2 × 106 years; therefore, we are confident that the 135Cs/137Cs isotopic ratio can be considered as a new powerful tracer for long-term source identification and environmental behavior studies.

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(1) Katata, G.; Terada, H.; Nagai, H.; Chino, M. Numerical reconstruction of high dose rate zones due to the Fukushima Daiichi Nuclear Power Plant accident. J. Environ. Radioact. 2012, 111, 2− 12. (2) Kinoshita, N.; Sueki, K.; Sasa, K.; Kitagawa, J. I.; Ikarashi, S.; Nishimura, T.; Wong, Y. S.; Satou, Y.; Handa, K.; Takahashi, T.; Sato, M.; Yamagata, T. Assessment of individual radionuclide distributions from the Fukushima nuclear accident covering central-east Japan. Proc. Natl. Acad. Sci. U.S.A. 2011, 108, 19526−19529. (3) Stohl, A.; Seibert, P.; Wotawa, G.; Arnold, D.; Burkhart, J. F.; Eckhardt, S.; Tapia, C.; Vargas, A.; Yasunari, T. J. Xenon-133 and caesium-137 releases into the atmosphere from the Fukushima Daiichi nuclear power plant: Determination of the source term, atmospheric dispersion, and deposition. Atmos. Chem. Phys. 2012, 12, 2313−2343. (4) Ministry of Economy, Trade and Industry. Data on the amount of released radioactive materials, 2011 (http://www.meti.go.jp/press/ 2011/10/20111020001/20111020001.pdf). (5) Tagami, K.; Uchida, S.; Ishii, N.; Zheng, J. Estimation of Te-132 distribution in Fukushima Prefecture at the early stage of the Fukushima daiichi nuclear power plant reactor failures. Environ. Sci. Technol. 2013, 47, 5007−5012. (6) Steinhauser, G.; Brandl, A.; Johnson, T. E. Comparison of the Chernobyl and Fukushima nuclear accidents: A review of the environmental impacts. Sci. Total Environ. 2014, 470−471, 800−817. (7) Kaneyasu, N.; Ohashi, H.; Suzuki, F.; Okuda, T.; Ikemori, F. Sulfate aerosol as a potential transport medium of radiocesium from the Fukushima nuclear accident. Environ. Sci. Technol. 2012, 46, 5720− 5726. (8) National diet of Japan Fukushima nuclear accident independent investigation commission. Official report, 2012 (http://warp.da.ndl.go. jp/info:ndljp/pid/3856371/naiic.go.jp/en/report). (9) Kirchner, G. K.; Bossew, P.; De Cort, M. Radioactivity from Fukushima Dai-ichi in air over Europe; part 2: What can it tell us about the accident? J. Environ. Radioact. 2012, 114, 35−40. (10) Merz, S.; Steinhauser, G.; Hamada, N. Anthropogenic radionuclides in Japanese food: Environmental and legal implications. Environ. Sci. Technol. 2013, 47, 1248−1256. (11) Schwantes, J. M.; Orton, C. R.; Clark, R. A. Analysis of a nuclear accident: Fission and activation product releases from the Fukushima Daiichi nuclear facility as remote indicators of source identification, extent of release, and state of damaged spent nuclear pool. Environ. Sci. Technol. 2012, 46, 8621−8627. (12) Chino, M.; Nakayama, H.; Nagai, H.; Terada, H.; Katata, G.; Yamazawa, H. Preliminary estimation of release amounts of 131I and 137 Cs accidentally discharged from the Fukushima Daiichi nuclear power plant into the atmosphere. J. Nucl. Sci. Technol. 2011, 48, 1129− 1134. (13) Komori, M.; Shozugawa, K.; Nogawa, N.; Matsuo, M. Evaluation of radioactive contamination caused by each plant of Fukushima Daiichi nuclear power station using 134Cs/137Cs activity ratio as an index. Bunseki Kagaku 2013, 62, 475−483. (14) Tagami, K.; Uchida, S.; Uchihori, Y.; Ishii, N.; Kitamura, H.; Shirakawa, Y. Specific activity and activity ratios of radionuclides in soil collected about 20 km from the Fukushima Daiichi Nuclear Power Plant: Radionuclide release to the south and southwest. Sci. Total Environ. 2011, 409, 4885−4888.

ASSOCIATED CONTENT

S Supporting Information *

Effect of N2O gas flow rate on the signal intensity of 133Cs and 137 Ba using Agilent 8800 triple-quadrupole ICP-MS/MS (Figure S1). This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*Telephone: 81-043-206-4634. Fax: 81-043-255-0721. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was supported by the Sumitomo Environmental Foundation and partially by the Agency for Natural Resources and Energy, the Ministry of Economy, Trade and Industry (METI), Japan. We thank Mr. Yasuyuki Shikamori and Ms. Kazumi Nakano (Agilent Technologies International Japan, 5437

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dx.doi.org/10.1021/es500403h | Environ. Sci. Technol. 2014, 48, 5433−5438