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Environmental Measurements Methods 90

137

Atmospheric activity concentration of Sr and Cs after the Fukushima Daiichi Nuclear Accident Zijian Zhang, Kazuhiko Ninomiya, Yoshiaki Yamaguchi, Kazuyuki Kita, Haruo Tsuruta, Yasuhito Igarashi, and Atsushi Shinohara Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b01697 • Publication Date (Web): 06 Aug 2018 Downloaded from http://pubs.acs.org on August 7, 2018

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Environmental Science & Technology

1

Atmospheric activity concentration of 90Sr and 137Cs

2

after the Fukushima Daiichi Nuclear Accident

3 4 Zijian Zhang,a* Kazuhiko Ninomiya,a Yoshiaki Yamaguchi,b Kazuyuki Kita,c Haruo Tsuruta,d 5 Yasuhito Igarashi,e and Atsushi Shinoharaa 6 7

a

Graduate School of Science, Osaka University, 1-1 Machikaneyama, Toyonaka, Osaka 560-0043,

8 Japan 9

b

Radioisotope Research Center, Osaka University, 2-4 Yamadaoka, Suita, Osaka, 565-0871,

10 Japan 11

c

12

d

College of Science, Ibaraki University, 2-1-1 Bunkyo, Mito, Ibaraki, 310-8512, Japan Remote Sensing Technology Center of Japan, 3-17-1Toranomon, Minatoku, Tokyo, 105-0001,

13 Japan 14

e

Meteorological Research Institute, 1-1 Nagamine, Tsukuba, Ibaraki, 305-0052, Japan

15 16 (Z.Z) Phone: (+81)-06-6850-5417; email: [email protected]

1 ACS Paragon Plus Environment


17 Abstract: On 11 March 2011, the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident 18 occurred and a large amount of radionuclides were discharged into the atmosphere. We have 19 operated continuous aerosol samplings at four locations in Japan from the accident until the end of 20 2011. The activities of 90Sr and 137Cs in the aerosol samples were measured using low background 21 liquid scintillation counters and high-purity germanium detectors, respectively. The atmospheric 22

90

Sr and 137Cs concentrations decreased exponentially during 2011. The time variation of the

23

90

Sr/137Cs ratio was obtained, and we found that the ratio rose from 1.2 × 10−3 in March to 1.3 ×

24 10−1 in August of 2011. One reason for the increase in the 90Sr/137Cs ratio could be the change in 25 the primary emission source of activity at the FDNPP, which occurred near June of 2011.


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26 27 Introduction 28

A historically devastating earthquake and tsunami occurred in eastern Japan on 11 March 2011

29 and resulted in one of the most severe nuclear accidents at the Fukushima Daiichi Nuclear Power 30 Plant (FDNPP). A large amount of radioactive fission products as well as activation products were 31 discharged into the environment from the nuclear reactors, contaminating a wide area of 32 northeastern Japan.1 Following the accident, various research groups carried out environmental 33 surveys to estimate the amount of radionuclide discharged and their distribution in the 34 environment. The primary measurement targets in the surveys focused on the measurements of 35 gamma-emitters such as 131I, 134Cs, and 137Cs, and several groups reported on the 95Nb, 110mAg, 36 37

132

Te, and 140Ba-140La concentrations. 2-4

Strontium-90 is a one of the most important radionuclides in severe nuclear accidents generally.

38 It has a large fission yield and a relatively long half-life (28.8 years), similar to 137Cs (30.1 years). 39 Because 90Sr belongs to the same chemical family as Ca, it can easily enter the food chain and 40 ultimately the human body. In the Chernobyl nuclear accident, milk and vegetables5 were 41 contaminated by 90Sr. Once incorporated into the human body, 90Sr can be deposited on bones and 42 teeth for long periods of time.6 Therefore, 90Sr is potentially a hazardous radionuclide, and a rapid 3 ACS Paragon Plus Environment


43 determination of the90Sr concentration and distribution is strongly needed in the aftermath of a 44 nuclear accident. 45

After the FDNPP accident, 90Sr in fallout and aerosol samples,7 soil samples,8-9 and seawater

46 samples was measured.10 Determination of the 90Sr activity in fallout and aerosol samples is 47 important to evaluate the activities in air and their time-dependence. Radionuclide composition in 48 the aerosol samples directly represent characteristics of activity released in the atmosphere, and 49 the activity concentrations in the air are influenced by physical and chemical properties and 50 atmospheric behaviors of radionuclides emitted from the nuclear reactors, primarily in the early 51 phase in 2011. Furthermore, air activity concentration data are important to carry out model 52 simulations to estimate dispersion in the atmosphere and deposition in urban and rural areas, 53 which are necessary to detailed evaluations of the wide-range contamination.11-12 54

However, the 90Sr survey results are fewer and far from those of 137Cs in number. Since 90Sr is a

55 pure beta- emitter, chemical isolation is required to obtain the 90Sr fraction for a 56 beta-measurement with a liquid scintillation counter or a proportional counter. The classical 57 chemical separation methods for 90Sr analysis are time-consuming and generate large quantity of 58 liquid waste.13 This is one reason why 90Sr has not yet been widely surveyed. To solve this 59 problem, our group developed an90Sr isolation method to measure 90Sr activity in aerosol samples 4 ACS Paragon Plus Environment

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60 applied with a solid-phase extraction method.14 Operation time required for the chemical 61 separation excluding acid-pretreatment was about 2.5 hours. By this method, we succeeded to 62 analyze the 90Sr in many aerosol samples. 63

In this paper, we report the 90Sr and 137Cs activity concentrations of aerosol samples and the time

64 variation of those activities since the early phase of the accident. Comparing the 90Sr/137Cs activity 65 ratio with other environmental data and simulation results, we can obtain a better understanding of 66 the radionuclides discharging phenomenon of the FDNPP accident. 67 68 Materials and Methods 69 Aerosol Sampling 70

Our research group has been collecting aerosol samples since March 2011. Sampling sites were

71 installed at four locations: Hitachi city (36.5727, 140.6410) located 87 km south of the FDNPP, 72 Mito city (36.4010,140.4405) located 99 km south-southwest of the FDNPP, Kawasaki city 73 (35.5415,139.73) located 232 km southwest of the FDNPP, and Toyonaka city 74 (34.8220,135.5222) located 580 km southwest of the FDNPP. Map of the sampling locations is 75 shown in SI Fig. S1. At each sampling site, we have collected aerosols on air filters using 76 high-volume samplers. The filter type, sampler type, and flow rate are shown in SI Table S1. Each 5 ACS Paragon Plus Environment


77 sampler had been placed on 13 m (at Hitachi city), 13 m (at Mito city), 12 m (at Kawasaki city) 78 and 1 m (at Toyonaka city) above the ground. Sampling periods were 8 April to 31 December 79 2011 at Hitachi city, 8 April to 25 April 2011 at Mito city, 15 March to 11 May 2011 at Kawasaki 80 city, and 12 March to 30 April 2011 at Toyonaka city.

We performed aerosol sampling for 1 day

81 in weekday and 3 days in weekend until September, then every 3 days until end of 2011 in Hitachi 82 city, for 1 day in weekday and 3 days in Mito city, and for 1 day in Kawasaki city and Toyonaka 83 city. We collected 140 samples in Hitachi city, 16 samples in Mito city, 61 samples in Kawasaki 84 city and 46 samples in Toyonaka city in 2011. 85 86 87

Gamma-measurements All air filters after the sampling were folded into 7 × 5 × 1 cm3 volumes and sealed in

88 polyethylene sheets for gamma-measurements. The aerosol samples were measured using 89 high-purity germanium detectors (GEM40, ORTEC) with durations of more than 80,000 s. 90 Cesium-134 and -137 were quantified via 604.7 keV and 661.7 keV photons, respectively. A 91 standard filter sample, used to determine the efficiency, was made by spotting the standard 92 radioactive solution uniformly on the same type of filter. Radioactive decay of all activities were 93 corrected at 11 March 2011. 6 ACS Paragon Plus Environment

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94 95 Strontium-90 measurements 96

In this paper, we measured the 90Sr-90Y activity via Cherenkov light counting with a liquid

97

scintillation counter (LSC). For an analysis of the 90Sr via LSC, a chemical separation operation

98

is required to obtain the 90Sr counts from the environmental samples. The 90Sr isolation method

99

we used in this study was based on a solid-phase extraction and ion-exchange separation method

100 101

developed by our group.14 We used half or a quarter of the aerosol samples for the chemical separation for the 90Sr

102

analysis. First, the fraction of the aerosol samples was acid pre-treated in aqua regia after adding

103

1 mg of stable Sr as a carrier. Then, the acid solution was passed through a solid-phase

104

extraction reagent, EmporeTM Strontium Rad Disk (3M Inc.) to separate the 90Sr from the other

105

elements. A natural radionuclide 210Pb, which is never eliminated by the reagent, required an

106

additional separation via a cation exchange method. Finally, the 90Sr was eluted from the cation

107

exchange column via a 3.25 M HCl solution to a LSC vial and measured via Cherenkov light

108

using a 1220 QUANTULUSTM Ultra Low Level Liquid Scintillation Spectrometer (Perkin Elmer

109

Inc.).



110

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Since 90Y has a short half-life (64 hours) compared with 90Sr (28.8 years), 90Y reaches secular

111

equilibrium and the same activity with its parents after 2 weeks. The 90Sr activity of the

112

measurement sample can be quantified by continuing to observe the activity of 90Y growing over

113

time. The detection efficiency of the Cherenkov light counting of 90Sr–90Y was 68.7 ± 0.1%, as

114

determined from the 90Sr standard solution. In this measuring system, the counting rate of a

115

blank sample (15 mL of 3.25 M HCl) was 0.028 counts per second and the detection limit of

116

90

117

curves of each measurement sample were obtained from the counting rates of several

118

measurements over 14 days.

119

Sr–90Y with a 500 min counting time was estimated to be 0.004 Bq.

The 90Sr–90Y growth

The chemical yields of Sr were determined by comparing the stable Sr amount before the

120

solid-phase extraction to the amount in the measurement samples. An Agilent/HP 4500 ICP-MS

121

(Agilent Technologies Inc.) was used to determine the stable Sr amount. In the ICP-MS

122

measurement, a standard Sr(NO3)2 solution (1000 mg/L) was used for external calibration. All

123

ICP-MS measurement samples were adjusted using a 2% m/m HNO3 solution. For each

124

measurement, we performed at least six analytical runs and applied the averaged value. The

125

calibration curve between the Sr concentration and the counts from the ICP-MS measurement


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126

had a linear relationship (R2 ≥ 0.9999). The detailed procedures for the separation and

127

measurement are given in a previous paper.14

128



129 Results and Discussion 130

Composite plots of the observed 137Cs and 90Sr activity concentrations in surface air at each

131 sampling location are shown in Figs. 1 and 2, respectively. 132

134

We also had determined activity of

Cs. The 134Cs /137Cs activity ratio was about 1, which was identical with many previous studies.

133 Since the behaviors of 134Cs and 137Cs in the environment are the same, the discussion on 134Cs is 134 omitted. 135 136

The 137Cs and 90Sr activity concentrations decreased with time. The activity concentration of 137

Cs in surface air was an order of magnitude of 1 Bq/m3 in March 2011 and then decreased

137 exponentially to about 10−4 Bq/m3 in December 2011. The activity concentrations of 90Sr in 138 surface air also decreased from an order of magnitude of 10−3 Bq/m3 just after the accident to 139 about 10−5 Bq/m3 in December 2011. In the following sections, we discuss the results of 137Cs 140 first, because there are detailed measurement data and many previous studies related to 137Cs. 141 142 Surface air 137Cs activity concentration 143

A distance dependence of the activity concentration was clearly observed in the first half of 2011.

144 In March, surface air 137Cs activity concentration in Kawasaki reached on the order of 10−1 Bq/m3 145 on 14 and 20 March; conversely, air 137Cs concentration at Toyonaka was less than detection limit. 10 ACS Paragon Plus Environment

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146 In April 2011, the 137Cs concentrations at Hitachi and Mito were similar levels each, whereas the 147

137

Cs concentration at Kawasaki was one or two orders of magnitude lower than that at Hitachi

148 and Mito. The concentration change with distance was caused by the dispersion of the 149 radionuclides released from the FDNPP. In addition, all of our sampling locations were 150 south-west of the FDNPP. Therefore, the HV-samplers predominantly collected aerosol that was 151 transported western direction from the FDNPP. Meteorological simulations are necessary to 152 verify the transportation process; however, this is beyond the scope of the present study. 153

We compared our results for the atmospheric concentration to the data of other research groups.

154 Haba et al. collected aerosol samples in Wako 220 km from the FDNPP and measured the activity 155 of radionuclides such as 137Cs.15 They observed specific activity concentration increases on 15, 156 20-21, and 29–31 March 2011. These increases of the surface air 137Cs coincided with the 157 temporal change at Kawasaki, in which both sites are located in the Kanto area, Japan. The events 158 with increasing activity concentrations observed in March 2011 have been discussed in the several 159 papers based on the actual measurement data and on atmospheric model simulations.16-17 All 160 increases have been explained by a radioactive plume that was continuously emitted from the 161 FDNPP and was transported via wind. We confirmed two peaks on 6 April and 17 April at 162 Toyonaka. These peaks were consistent with previous study, that had collected aerosol samples at 11 ACS Paragon Plus Environment


163 Fukuoka city, 1000 km west of the FDNPP18. However, studies on the time variation of the 164 atmospheric activity concentration primarily covered the period until March 2011. We could not 165 find a simulation study that explains causes of the temporal variations of the FDNPP-derived 166 radionuclides in the air of terrestrial environment in Japan from April to the end of 2011. 167

Radioactive cesium emitted into the atmosphere mainly in March 2011 was subsequently

168 deposited on land. Removal half-life of 137Cs in air from the atmosphere was calculated to be 169 10.0–13.9 days, estimated from measurement data of 137Cs/133Xe .19 We estimated activity 170 concentration that would remain in the surface air of Hitachi city during 2011 using these removal 171 time, and compared with the monthly 137Cs activity in Hitachi city (Fig. 3). The median value of 172 the activity concentration in April, 0.051 Bq/m3, was used as the starting value. As a result, if 173 there were no additional releases and treated the 137Cs emission as a single pulse in March 2011, 174 the activity concentration in Hitachi should have been less than 10−4 Bq/m3 after July 2011. 175 However, in the actual measurements, the activity concentration remained high at a maximum of 176 10−3 Bq/m3, even in August. In addition, the activity concentration after October was stable at 177 approximately 10−4 Bq/m3.Therefore, it is highly likely that there were additional supplies of 178 radionuclides from the FDNPP or from the contaminated environment into the atmosphere.


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179


Resuspension of radionuclide from the surroundings of the sampling location was considered

180 first. Hirose et al. suggested that the resuspension of 137Cs deposited on the land did not have a 181 significant effect on the activity concentration in the atmosphere.20 This is because the average 182 deposited amount of activity per month was higher than the estimated resuspension amount. He 183 concluded that there has been little influence of the resuspended fraction on the atmospheric 184 activity concentration since July 2011. In addition, from the resuspension factor K, we can 185 estimate the contribution of 137Cs resuspended form the environment. The resuspension factor is 186 defined as: 187 K=

/ /

188 189

The deposited amount of 137Cs to the land at Hitachi city was calculated from the reported data

190 of land surface contamination of places near our sampling locations by the Nuclear Regulation 191 Authority Japan. 21 The data were 9.2 kBq/m2 (36.6158, 140.6688), 13 kBq/m2 192 (36.5767,140.6466), 9.8 kBq/m2 (36.6393, 140.5729) as of June 14, 201, and we calculated the 193 deposition amount of our sampling point as the arithmetic mean of these data, and it was 10.7 194 kBq/m2. Kaneyasu et al. reported the resuspension factor of Tsukuba city (92 km south-west from 13 ACS Paragon Plus Environment


195 Hitachi city) in end of May 2011, and it was 1.5 – 4.2 ×10-9 m-1. 22 The estimated activity 196 concentration in May 2011 was calculated from the resuspension factor and the estimated 197 deposition amount in Hitachi city, and it was 4.5×10-5 Bq/m3. However, the average of the actual 198 concentration in May of Hitachi city was 3.0×10-3, and the minimum was in 10-4 order. In other 199 words, the resuspension factor in Hitachi city was higher than the value in Tsukuba city. Since 200 both sampling location are only 90 km apart, weather conditions would not very different. Thus, it 201 is highly likely that Hitachi, which is close to the FDNPP, has been influenced by inflows from 202 highly contaminated areas around the FDNPP. 203

Therefore, the atmospheric activity in Hitachi in the latter half of 2011 had not originated from

204 the deposition of activity released in the early stage of the accident, and it not caused by 205 resuspension from the surrounding environment. It would have resulted from the influence of 206 additional emissions from the FDNPP. TEPCO had measured activities up on the reactor 207 buildings and estimated the activity release rate (Bq/h) from the FDNPP during 2011.The activity 208 release rate has become 10−4 times smaller, from 8 × 1011 Bq/h on 5 April to 6 × 107 Bq/h on 6 209 December, 2011. 23 The maximum 137Cs activity concentration of the aerosol sample collected in 210 Hitachi was 1 Bq/m3 in April and was 10−4 Bq/m3 in December 2011. This is consistent with the 211 obtained time deviation from April to December in 2011. In other words, the decrease in the 14 ACS Paragon Plus Environment

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212 atmospheric activity concentration at Hitachi in 2011 showed a similar trend to the decrease in the 213 amount of additional releases from the FDNPP.



214 215

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Surface air 90Sr activity concentration The 90Sr activities were detected from the aerosol samples collected from each sampling

216 location, even from the farthest location, Toyonaka. Because there are few reports on the 217 atmospheric activity concentration of 90Sr, the validity of the measurement data will be described. 218 First, we evaluated the contribution of 90Sr due to the environmental background. Subsequently, 219 we compared this with the 90Sr values obtained in other previous studies. 220

In Japan, 90Sr activity concentration of surface air derived from the global fallout should be

221 less than 1.0 x10-6 Bq/m3 before the FDNPP accident. The global 90Sr activity concentration in 222 surface air was 10−6 Bq/m3 in the Northern Hemisphere in 1983.24 The 90Sr activity concentration 223 in surface air in Japan increased to 7.9 x10-5 Bq/m3 in April to June 1981 after the last nuclear 224 testing operated by China, then decreased to 4.3x10-6 Bq/m3 in 1983.25 After the Chernobyl 225 accident in 1986, the surface air concentration of 90Sr temporary increased to 3.2 x10-5 Bq/m3 in 226 May 1986 26, however, the concentration dropped to the level before the accident. We could not 227 find a 90Sr activity concentration data in Japan after 2000, but we can estimate the concentration 228 form global fallout trend in Japan. Monthly deposition amount of 90Sr was 100-500 229 mBq/m2/month in 1983, and it decreased to 0.15-19 mBq/m2/month during 2005-2011.27

We

230 can say that the activity concentration of 90Sr originated from global fallout would be less than 1.0 16 ACS Paragon Plus Environment

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231 ×10-6 Bq/m3. In contrast, the surface air 90Sr activity concentrations measured in this study 232 exceeded 10−5 Bq/m3; these values are higher than the value estimated from the global fallout. In 233 addition, we obtained two data below the detection limit. This indicates that we could not detect 234 the 90Sr from the global fallout using the sampling method and the measuring apparatus. We 235 conclude that the influence of the global fallout was negligible in this study and that the measured 236 237

90

Sr directly released from the FDNPP reactors.

Strontium-89, which was also discharged from the reactors, coexists with the 90Sr in the

238 measurement samples and can be detected by the LSC. However, the 89Sr had decayed to an 239 undetectable level when we started the 90Sr analysis. The 89Sr/90Sr activity ratio of the nuclear 240 fuels in the FDNPP was calculated to be 0.43 by the ORIGEN 2 code.28 We began our 241 measurements in 2015 (approximately 1500 days after the accident), and the 89Sr/90Sr activity 242 ratio was estimated to be 1.6 × 10−10 when we analyzed the samples. Therefore, there was no 243 serious interference by 89Sr in the data. 244

The obtained 90Sr concentration in Kawasaki in March was consistent with reported values

245 measured by Igarashi et al.7. The activity concentrations of 90Sr in Tsukuba city on 15 and 20 246 March where the main radionuclide deposition event in Kanto occurred, and it was 1.04–1.50 × 247 10−3 Bq/m3.7 Even though Kawasaki is far from the FDNPP compared to Tsukuba and it is 17 ACS Paragon Plus Environment


248 expected that the activity concentration there is lower than that in Tsukuba, in the early stages of 249 the accident, the activity concentration fluctuated greatly depending on the collection period and 250 the sampling point; therefore, this variation would be reasonable. 251 252 253

90

Sr/137Cs activity ratio

The 90Sr/137Cs activity ratios varied over a range of 10−4 -10−1 during 2011 (see Fig. 4).

254 Especially in the early phase, the 90Sr/137Cs ratios were relatively small and were on the order of 255 10−4–10−3. This result is consistent with expectations because the activity discharged during this 256 period was primarily deposited on the ground, that is, the 90Sr activity deposited to the ground 257 during the FDNPP accident. The 90Sr/137Cs ratio of the radionuclides that existed in the nuclear 258 reactors at the shutdown (inventory) was 0.68–0.75.27 If the release behavior of both nuclides was 259 the same, the activity ratio observed in the environment should also be close to these ratios. 260 However, because cesium compounds have generally showed lower boiling point than those of 261 strontium compounds (e.g. CsOH with 1263 K , CsI with 1550 K, SrO with 3473 K), radiocesium 262 was more easily released during the initial stage of the heating of the reactor fuel. This difference 263 has been confirmed in past simulation experiments.28-29 Therefore, during the FDNPP accident, 264 strontium was not released as predominantly as cesium due to their chemical properties. 18 ACS Paragon Plus Environment

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The atmospheric radioactive concentrations of 90Sr were measured by Igarashi et al.7 The

265 266

90

Sr/137Cs ratios they found were 4.3 × 10−5–2.1 × 10−4 in March, which are similar to but little bit

267 lower than our data for the same period. Strontium-90 activity concentrations in soil samples have 268 been measured in several studies. Sahoo et al. reported 16 results of 90Sr in soil collected from the 269 exclusion zone of the FDNPP.8 The 90Sr/137Cs activity ratios were 1.1 × 10−4 – 4.3 × 10−3.8 Mishra 270 et al. reported four results of 90Sr in soil near the FDNPP in November 2011.30 The 90Sr/137Cs 271 activity ratios were 1.4 × 10−4 – 2.2 × 10−2. This range of 90Sr/137Cs ratios covers our data from 272 aerosol samples. As discussed, the 90Sr/137Cs ratios of the soil primary reflect the main fraction of 273 the discharged activity in the early phase of the FDNPP accident. The 90Sr/137Cs activity ratio of 274 the sea-water was measured by Castrillejo et al.31 and Casacubetra et al.33, and the 90Sr/137Cs 275 ratios were found to be 0.28 ± 0.02 and 0.0265 ± 0.0006, respectively. The 90Sr/137Cs activity 276 ratios in sea-water near the FDNPP were much higher than those in the aerosol and soil samples. 277 This difference may result from the release mechanisms of the radionuclides. Povinec et al. 278 reported that surface sea-water near the Units 1 and 2 had different 90Sr/137Cs ratios.10 The activity 279 ratios varied at Unit 1 from 0.01 to a maximum of 0.95 and at the Unit 2 from 0.006 to a 280 maximum of 64.5. This suggests that the activity ratio of seawater near the FDNPP was affected



281 by the direct release (or leakage) of cooling water or ground water from the reactors, which had 282 high 90Sr/137Cs ratios. 283

We have samples that were collected at different locations on the same date, and we obtained

284 similar 90Sr/137Cs ratios for these samples. The aerosol samples collected in Toyonaka city at the 285 farthest sampling location had a similar 90Sr/137Cs ratio (2.1 ×10−3) than those at the nearest 286 sampling location, the Hitachi samples, in the middle of April 2011. The radioactive materials 287 came from the same plume; therefore, the 90Sr/137Cs ratio did not change after being released from 288 the nuclear reactors. These results indicate that 90Sr had the same transportation behavior through 289 the atmosphere as did 137Cs. That means that 90Sr attached to aerosols, similar to 137Cs, and was 290 not transported in gaseous form as for 131I. Miyamoto et al. reported that 140Ba, which belongs to 291 the same chemical family as 90Sr, had the same particle size distribution as 137Cs when observed 292 with a cascade impactor during the FDNPP accident 33 and that the carrier of 90Sr can be estimated 293 to be sulfate aerosols similar to that of 137Cs.34 We can conclude that 90Sr and 137Cs are 294 transported through the atmosphere in aerosol form and that the 90Sr/137Cs ratio of the radioactive 295 plume remained constant even after its long-range transport. 296


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297 Time variation of 90Sr/137Cs 298

In the present study, we found a clear time variation in the 90Sr/137Cs activity ratio during 2011.

299 The ratio was 8.1 × 10−4 in Kawasaki on 15 March. In April 2011, the ratio was almost on the 300 order of 10−3; however, a few samples had ratios greater than 10−2. Conversely, on a larger time 301 scale, a large ratio of over 10−1 was obtained in Hitachi in July 2011.Therefore, the 90Sr/137Cs 302 activity ratio rose by nearly two orders of magnitude during 2011. 303

Our measurement results are the first example of observing a relatively high 90Sr/137Cs ratio over

304 10-2 in the environmental aerosol samples after the FDNPP accident. As shown in below, there are 305 some results of fallout samples and soil samples that had high 90Sr/137Cs ratios; however, the 306 effect of resuspension of global fallout 90Sr and 137Cs are limited. Hirose summarized the monthly 307

90

Sr/137Cs activity ratio of fallout samples collected in Japan after the FDNPP accident.35 In March

308 2011, the activity ratio was on the order of approximately 10−4 at all points; however, the activity 309 ratio varied after April and there was a month when the activity ratio reached a maximum on the 310 order of 10−2. The Ministry of Education, Culture, Sports, Science and Technology (MEXT) 311 conducted a soil survey in June 2011 and measured the 90Sr activity in soil samples.36 The survey 312 results indicate that the 90Sr/137Cs ratios were 1.6 × 10−4 to 5.8 × 10−2 (average: 2.6 × 10−3), which 313 was largely variable. MEXT conducted an additional survey for the soil samples collected from 21 ACS Paragon Plus Environment


314 Souma city, which had the highest 90Sr/137Cs ratios (5.8 × 10−2) in the first survey 37. In the 315 additional survey results, MEXT could not obtain the high 90Sr/137Cs ratios but obtained the ratio 316 about 2.0 × 10−3 from the soil samples in Soma city; therefore, they concluded that the deposition 317 of 90Sr on the soil was more dispersive than that of 137Cs even in the same sampling spot. 318

Because the 90Sr/137Cs activity ratio of the radioactive plumes did not change during dispersion,

319 the increase in the 90Sr/137Cs ratio of the aerosol samples may have been affected by changes in 320 the ratio of the emission source itself. However, the increase in the 90Sr/137Cs activity ratio cannot 321 be explained by the chemical features of cesium and strontium compounds. First, the fission 322 product 90Sr is known to be a non-volatile nuclide in a severe nuclear accident phase, unlike 323

137

Cs.38 Therefore, 90Sr would not be more prominently discharged compared to 137Cs from the

324 nuclear reactors of the FDNPP under the condition of the nuclear reactors being relatively cool. 325 We need to consider the influences from emission sources that had 90Sr/137Cs ratios that were 326 relatively higher than that of the main emission source. 327

Resuspension processes could also explain the change in the 90Sr/137Cs ratio. In the 1990s and

328 2000s, 90Sr in surface air is controlled by resuspension of global fallout 90Sr deposited on land 329 surface. The 90Sr/137Cs activity ratio of global fallout was in range of 0.17-1.7 in Japan.41 330 However, the activity ratio of land near the FDNPP have changed to 10-4 to 10-3 due to the 22 ACS Paragon Plus Environment

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331 deposition of radionuclide from the FDNPP 36-37. If 90Sr was remarkably released from the land to 332 the atmosphere compared to 137Cs, the 90Sr/137Cs activity ratio in the atmosphere would rise. 333 However, such features of the deposited 90Sr have not been reported. According to long-term 334 observations in Munich, Germany, after the Chernobyl nuclear accident, the 90Sr/137Cs ratio in the 335 atmosphere was constant.40 Considering the non-volatility of 90Sr and its compounds, it is unlikely 336 that 90Sr deposited on the soil would volatilize more than 137Cs. Therefore, there was likely no 337 specific injection of 90Sr into the atmosphere due to resuspension, and resuspension from the land 338 with 90Sr/137Cs ratio of 10-3 order would not contribute to the rise of the ratio in the surface air 339 observed in this study. However, resuspension of 137Cs from forest ecosystems have observed 41, 340 and alkali-earth elements from the vegetation and the biosphere are still large sources of 341 uncertainty. 342 343 Influences of decommission operations 344

Additional release from the FDNPP may have caused the time variation of 90Sr/137Cs activity

345 ratio. There is previous study observed the additional release event of radionuclide even in 2013.42 346 Since there are three nuclear reactors destroyed by the accident, the contribution of additional 347 release from each nuclear reactor would influence the change in the activity in the atmosphere. 23 ACS Paragon Plus Environment


348 One reason for the 90Sr/137Cs rise in the second half of 2011 could be a change of main emission 349 source through the decommissioning process in the FDNPP. 350

TEPCO reported the analysis result of the 90Sr activity concentration in the air at the upper part

351 of the reactor building for each reactor in December 2011.43 The measurement results of each 352 sample are shown in Table 1. The 90Sr/137Cs activity ratio was the lowest at 1.3 × 10−4 in Unit 3 353 and the highest at 3.5 × 10−2 in Unit 1. Therefore, the activity ratio was different for each reactor 354 building; the order was Unit 3 < Unit 2 < Unit 1. The International Research Institute for Nuclear 355 Decommissioning (IRID) and the Japan Atomic Energy Agency (JAEA) collected debris from the 356 nuclear reactor buildings of the individual nuclear reactors and quantified various radionuclides.49 357 The main component of the debris was concrete, surface coating films, heat insulating materials, 358 and deck plates. The highest 90Sr/137Cs activity ratio of 4.2 × 10−2 was observed in the surface 359 coating film from Unit 1. The lowest 90Sr/137Cs activity ratio of 3.0 × 10−4 was observed in a 360 concrete sample from Unit 3. There were no samples with 90Sr/137Cs activity ratio of 10−2 or less 361 in the samples from Units 2 and 3. Overall, the 90Sr/137Cs activity ratio of the Unit 1 samples 362 tended to be higher than that of t the other reactors. The result is the same trend as that of the 363 activity ratio of the air in the reactor buildings.


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364


The amount of activity discharged from each reactor of the FDNPP to the atmosphere was

365 estimated by the Modular Accident Analysis Program (MAAP code), which simulates light water 366 reactor responses in severe accident sequences.50 The main emission source of activity was Unit 2, 367 which accounted for 90% of all the discharged 137Cs. The 90Sr/137Cs ratio of the radioactive plume 368 was directly correlated with the 90Sr/137Cs ratio of the Kawasaki’s sample, i.e., 1.2 × 10−3; 369 therefore, the radioactive plumes released from Units 2 and 3 have lower 90Sr/137Cs ratios on the 370 order of 10−4 - 10−3. 371

Decommissioning operations in the FDNPP influenced the additional releases of radionuclides.

372 Focusing on Unit 2, which was the largest emission source of the activity, a filtering operation 373 had started to reduce additional releases from the Unit 2 nuclear reactor building on 11 June 374 2011.51 Two filter drain units were installed in the Unit 2 waste disposal building and drained the 375 air through an airlock in the Unit 2 nuclear reactor building. Due to this operation, the activity 376 concentration of 137Cs was changed from 1.6 × 105 Bq/m3 on June 4 to 9.7 × 103 Bq/m3 on June 377 18, which was 1/10 or less.52 The filter drain operation likely prevented additional activity 378 escaping from the Unit 2; therefore, the main emission source of activity in the atmosphere 379 changed at this time. Until 18 June 2011, radioactive plume with low 90Sr/137Cs activity ratio had 380 been released from the Unit 2. After the decommissioning operations like the filter drain 25 ACS Paragon Plus Environment


381 operations, the additional release from the Unit 2 was suppressed. Although the activity 382 concentration in the surrounding area have been decreased, however, the plume derived from Unit 383 1 with high activity ratio would become remarkable. 384 385 Supporting information 386 Latitude and longitude of each sampling point, sampling start period, types of air filters, types of 387 high volume samplers, and flow rates of each sampler (Table S1). Map of the sampling locations 388 of aerosol samples in the present study (Figure S1). 389 390 Corresponding author 391 *Email: [email protected] 392 ORCID 393 Zijian Zhang : 0000-0002-7673-8162 394 395 Acknowledgments 396 The study was supported by Japan Society for the Promotion of Science (JSPS) KAKENHI Grant 397 Number JP16K45678 and a Grant-in-Aid for Scientific Research (KAKENHI) on Innovative 26 ACS Paragon Plus Environment

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398 Areas under the A04-08 research teams (grant number 24110009). We express our sincere thanks 399 to the former Kawasaki Municipal Research Institute for Environmental Protection for using the 400 filters. We would like to thank Dr. Tomioka for appropriately teaching to the corresponding author 401 how to use ICP-MS.



402 403 References 404 405 [1] Tanaka, S. Accident at the Fukushima Dai-ichi Nuclear Power Stations of TEPCO —Outline 406 & lessons learned— Proc. Jpn. Acad. Ser. B Phys. Biol. Sci. 2012, 88, 471-484, DOI: 407 10.2183/pjab.88.471 408 409 [2] Kanai, Y. Monitoring of aerosols in Tsukuba after Fukushima Nuclear Power Plant incident in 410 2011 J. Environ. Radioact. 2012, 111, 33-37, DOI: 10.1016/j.jenvrad.2011.10.011. 411 412 [3] A. de Vismes Ott,; Gurriaran, R.; Cagnat, X.; Masson, O. Fission product activity ratios 413 measured at trace level over France during the Fukushima accident J. Environ. Radioact. 2013, 414 125, 6-16,DOI: 10.1016/j.jenvrad.2013.02.014. 415 416 [4] Tagami, K.; Uchida, S; Ishii, N.; Zheng, J. Estimation of Te-132 distribution in Fukushima 417 Prefecture at the early stage of the Fukushima Daiichi Nuclear Power Plant reactor failures 418 Environ. Sci. Technol. 2013, 47, 5007-5012, DOI: 10.1021/es304730b. 28 ACS Paragon Plus Environment

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419 420 [5] Južnič, K.; Fedina, Š. Distribution of 89Sr and 90Sr in Slovenia, Yugoslavia, after the 421 Chernobyl accident J. Environ. Radioact. 1987, 5, 159-163, DOI: 422 10.1016/0265-931X(87)90030-0 423 424 [6] Stamoulis, K. C.; Assimakopoulos, P. A.; Ioannides, K. G.; Johnson, E.; Soucacos, P. N. 425 Strontium-90 concentration measurements in human bones and teeth in Greece Sci. Total Environ. 426 1999, 229, 165-182, DOI: 10.1016/S0048-9697(99)00052-2 427 428 [7] Igarashi, Y.; Kajino, M.; Zaizen, Y.; Adachi, K.; Mikami, M. Atmospheric activity over 429 Tsukuba, Japan: a summary of three years of observations after the FDNPP accident. Prog. Earth 430 Planet. Sci. 2015, 2, 1-19, DOI: 10.1186/s40645-015-0066-1 431 432 [8] Sahoo, S. K.; Kavasi, N.; Sorimachi, A.; Arae, H.; Tokonami, S.; Mietelski, J. W.; Yoshida, 433 S.S. Strontium-90 activity concentration in soil samples from the exclusion zone of the 434 Fukushima daiichi nuclear power plant Sci. Rep., 2016, 6, 1-10, DOI: 10.1038/srep23925 435 29 ACS Paragon Plus Environment


436 [9] Kavasi, N.; Sahoo, S. K.; Sorimachi, A.; Tokonami, S.; Aono, T.; Yoshida, S. Measurement of 437

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468 [17] Nakajima, T.; Misawa, S.; Morino, Y.; Tsuruta, H.; Goto, D.; Uchida, J.; Takemura, T.; 469 Ohara, T.; Oura, Y.; Ebihara, M.; Satoh, M. Model depiction of the atmospheric flows of 470 radioactive cesium emitted from the Fukushima Daiichi Nuclear Power Station accident Prog. 471 Earth Planet. Sci. 2017, 4, 1-18, DOI: 10.1186/s40645-017-0117-x 472 473 [18] Momoshima, N.; Sugihara, S.; Ichikawa; R.; Yokoyama, H. Atmospheric radionuclides 474 transported to Fukuoka, Japan remote from the Fukushima Dai-ichi nuclear power complex 475 following the nuclear accident J. Environ. Radioact. 2012, 111, 28-32, DOI: 476 10.1016/j.jenvrad.2011.09.001 477 478 [19] Kristiansen, N. I.; Stohl, A.; Wotawa, G. Atmospheric removal times of the aerosol-bound 479 radionuclides 137Cs and 131I measured after the Fukushima Dai-ichi nuclear accident a constraint 480 for air quality and climate models Atmos. Chem. Phys. 2012, 12, 10759-10769, DOI: 481 10.5194/acp-12-10759-2012 482


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483 [20] Hirose, K. 2011 Fukushima Dai-ichi nuclear power plant accident: summary of regional 484 radioactive deposition monitoring results J. Environ. Radioact. 2012, 111, 13-17, DOI: 485 10.1016/j.jenvrad.2011.09.003 486 487 [21] Nuclear Regulation Authority, Japan, 2017. Monitoring information of environmental 488 radioactivity level, Extension site of distribution map of radiation dose, etc., Results of the first 489 investigation, Soil concentration map of Cesium-137 as of June 14, 2011 (in Japanese) 490 https://ramap.jmc.or.jp/map/eng (Accessed June 23, 2018) 491 492 [22] Kaneyasu, N.; Ohashi, H.; Suzuki, F.; Okuda, T.; Ikemori, F.; Akata, N.; Kogure, T. Weak 493 size dependence of resuspended radiocesium adsorbed on soil particles collected after the 494 Fukushima nuclear accident J. Environ. Radioact. 2017, 172, 122-129, DOI: 495 10.1016/j.jenvrad.2017.03.001 496 497 [23] Tokyo Electric Power Company Holdings, Incorporated (TEPCO). Completion of the path 498 toward the convergence of the accident (in Japanese),



499 http://www.tepco.co.jp/nu/fukushima-np/outline/pdf/f12np-gaiyou_2.pdf. (Accessed July 23, 500 2018) 501 502 [24] United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR). 2000, 503 UNSCEAR 2000 Report to the General Assembly, with Scientific Annexes. 504 505 [25] Hirose, K.; Sugimura, Y.; Katsuragi, Y. 90Sr and 239+240Pu in the Surface Air in Japan: Their 506 Concentrations and Size Distributions Pap. Meteor. Geophys. 1986, 37, 255-269, DOI: 507 10.2467/mripapers.37.255 508 509 [26] Hirose, K.; Sukeyoshi, T.; Michio, A. Wet deposition of radionuclides derived from the 510 Chernobyl accident J. Atoms. Chem. 1993, 17, 61-71, DOI: 10.1007/BF00699114 511 512 [27] Nishihara, K.; Iwamoto, H.; Suyama, K. Estimation of Fuel Compositions in 513 Fukushima-Daiichi Nuclear Power Plant 2012, JAEA-Data/Code 2012-018, DOI: 514 10.11484/JAEA-Data-Code-2012-018 515 34 ACS Paragon Plus Environment

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516 [28] Rest, J. An improved model for fission product behavior in nuclear fuel under normal and 517 accident conditions J. Nucl. Mater. 1984, 120, 195-212, DOI: 10.1016/0022-3115(84)90057-6 518 519 [29] Cubicciotti, D. Thermodynamics of vaporization of fission products and materials under 520 severe reactor accident conditions Pure Appl. Chem. 1985, 57, 1-13, DOI: 521 10.1016/0022-3115(85)90290-9 522 523 [30] Mishra, S.; Sahoo, S. K.; Arae, H.; Watanabe, Y.; Mietelski, J. W. Activity Ratio of Caesium, 524 Strontium and Uranium with Site Specific Distribution Coefficients in Contaminated Soil near 525 Vicinity of Fukushima Daiichi Nuclear Power Plant J. Chromatogr. Sep. Tech. 2014, 5, 1-6. DOI: 526 10.4172/2157-7064.1000250 527 528 [31] Castrillejo, M.; Casacuberta, N.; Breier, C. F.; Pike, S. M.; Masqué, P.; Buesseler K. O. 529 Reassessment of 90Sr, 137Cs, and 134Cs in the Coast off Japan Derived from the Fukushima 530 Dai-ichi Nuclear Accident Environ. Sci. Technol. 2016, 50, 173-180, DOI: 531 10.1021/acs.est.5b03903 532 35 ACS Paragon Plus Environment


533 [32] Casacuberta, N.; Masque´, P.; Garcia-Orellana, J.; Garcia-Tenorio, R.; Buesseler, K. O. 90Sr 534 and 89Sr in seawater off Japan as a consequence of the Fukushima Dai-ichi nucelar accident 535 Biogeosciences 2013, 10, 3649-3659, DOI: 10.5194/bg-10-3649-2013 536 537 [33] Miyamoto, Y.; Yasuda, K.; Magara, M. Size distribution of radioactive particles collected at 538 Tokai, Japan 6 days after the nuclear accident J. Environ. Radioact. 2014, 132, 1-7, DOI: 539 10.1016/j.jenvrad.2014.01.010 540 541 [34] Kaneyasu, N.; Ohashi, H.; Suzuki, F.; Okuda, T.; Ikemori, F. Sulfate aerosol as a potential 542 transport medium of radiocesium from the Fukushima nuclear accident Environ. Sci. Technol. 543 2012, 46, 5720-5726, DOI: 10.1021/es204667h. 544 545 [35] Hirose, K. 90Sr Deposition Observed in Central and Northeast Honshu Island after the 546 Fukushima Dai-Ichi Nuclear Power Plant Accident Int. J. Earth. Environ. Sci. 2017, 2, 134. 547 548 [36] Ministry of Education, Culture, Sports, Science and Technology (MEXT). About the results 549 of nuclide analysis Plutonium and Strontium by MEXT (in Japanese), 36 ACS Paragon Plus Environment

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550 http://radioactivity.nsr.go.jp/ja/contents/6000/5048/24/5600_110930_rev130701.pdf (Accessed 551 June 23, 2018) 552 553 [37] Ministry of Education, Culture, Sports, Science and Technology (MEXT). About the analysis 554 results of gamma-ray emitting nuclides by MEXT, (2) analysis results of strontium-89, 90 555 (secondary distribution situation survey) (in Japanese) 556 http://activity.nsr.go.jp/ja/contents/7000/6213/24/6213_20120912_rev20130701.pdf. (Accessed 557 March 12, 2018) 558 559 [38] Lewis, J.; Corse, J.; Thompson, T.; Kaye, H.; Iglesias, C.; Elder, P.; Dickson, R.; Liu, Z. Low 560 volatile fission-product release and fuel volatilization during severe reactor accident conditions J. 561 Nucl. Mater 1998, 252, 235-256, DOI: 10.1016/S0022-3115(97)00292-4 562 563 [39] Igarashi, Y.; Aoyama, M.; Hirose, K.; Miyao, T.; Nemoto, K.; Tomita, M.; Fujikawa, T. 564 Resuspension: decadal monitoring time series of the anthropogenic radioactivity deposition in 565 Japan J. Radiat. Res. 2003, 44, 319-328, DOI: doi.org/10.1269/jrr.44.319 566 37 ACS Paragon Plus Environment


567 [40] Rosnr, G.; Winkler, R. Long-term variation (1986-1998) of post-Chernobyl 90Sr, 137Cs, 238Pu 568 and 239, 240Pu concentrations in air, depositions to ground, resuspension factors and resuspension 569 rates in south Germany Sci. Total Environ. 1997, 273, 11-25, DOI: 570 10.1016/S0048-9697(00)00716-6 571 572 [41] Kinase, T.; Kita, K.; Igarashi, Y.; Adachi, K.; Ninomiya, K.; Shinohara, A.; Okochi H.; 573 Ogata, H.; Ishizuka, M.; Toyoda, S.; Yamada, K.; Yoshida, N.; Zaizen, Y.; Mikami, M.; Demizu, 574 H.; Onda, Y. The seasonal variations of atmospheric 134,137 Cs activity and possible host particles 575 for their resuspension in the contaminated areas of Tsushima and Yamakiya, Fukushima, Japan 576 PEPS, 2018, 5, 12, 1-17, DOI: 10.1186/s40645-018-0171-z 577 578 [42] Steinhauser, G.; Niisoe, T.; Harada, K. H.; Shozugawa, K.; Schneider, S.; Synal, A.; Walther, 579 C.; Christl, M.; Nanba, K.; Ishikawa, H.; Koizumi, A. Post-Accident Sporadic Releases of 580 Airborne Radionuclides from the Fukushima Daiichi Nuclear Power Plant Site Environ. Sci. 581 Technol. 2015, 49, 14028-14035, DOI: 10.1021/acs.est.5b03155.,vbt 582


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583 [43] Tokyo Electric Power Company Holdings, Incorporated (TEPCO). Results of Sr analysis of 584 radioactive materials in air at the upper part of reactor buildings of the Fukushima Daiichi Nuclear 585 Power Plant (in Japanese), 586 http://www.tepco.co.jp/nu/fukushima-np/images/handouts_111202_01-j.pdf (Accessed July 23, 587 2018) 588 589 [44] Tokyo Electric Power Company Holdings, Incorporated (TEPCO). Results of radionuclide 590 analysis of airborne radioactive substance in the upper part of Unit 1 reactor building of the 591 FDNPP (in Japanese), 592 http://www.tepco.co.jp/nu/fukushima-np/images/handouts_111004_02-j.pdf (Accessed July 23, 593 2018) 594 595 [45] Tokyo Electric Power Company Holdings, Incorporated (TEPCO). Results of radionuclide 596 analysis of airborne radioactive substance in the upper part of Unit 1 reactor building of the 597 FDNPP (in Japanese), 598 http://www.tepco.co.jp/nu/fukushima-np/images/handouts_111013_03-j.pdf (Accessed July 23, 599 2018) 39 ACS Paragon Plus Environment


600 601 [46] Tokyo Electric Power Company Holdings, Incorporated (TEPCO). Results of radionuclide 602 analysis of airborne radioactive substance in the upper part of Unit 2 reactor building of the 603 FDNPP (in Japanese), 604 http://www.tepco.co.jp/nu/fukushima-np/images/handouts_111014_01-j.pdf (Accessed July 23, 605 2018) 606 607 [47] Tokyo Electric Power Company Holdings, Incorporated (TEPCO). Results of radionuclide 608 analysis of airborne radioactive substance in the upper part of Unit 3 reactor building of the 609 FDNPP (in Japanese), 610 http://www.tepco.co.jp/nu/fukushima-np/images/handouts_111013_02-j.pdf (Accessed July 23, 611 2018) 612 613 [48] Tokyo Electric Power Company Holdings, Incorporated (TEPCO). Results of radionuclide 614 analysis of airborne radioactive substance in the upper part of Unit 3 reactor building of the 615 FDNPP (in Japanese),


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616 http://www.tepco.co.jp/nu/fukushima-np/images/handouts_111013_04-j.pdf (Accessed July 23, 617 2018) 618 619 [49] International Research Institute for Nuclear Decommissioning (IRID) and Japan Atomic 620 Energy Agency (JAEA). Analysis of debris collected in Fukushima Daiichi Nuclear Power 621 Station (in Japanese),

http://irid.or.jp/wp-content/uploads/2016/04/20160428_1.pdf (Accessed

622 July 23, 2018) 623 624 [50] Nuclear and Industrial Safety Agency (NISA). For some error of radioactive material 625 emissions data (in Japanese). 626 http://dl.ndl.go.jp/view/download/digidepo_6017196_po_20111020001.pdf?contentNo=3&altern 627 ativeNo=. (Accessed July 23, 2018) 628 629 [51] Nuclear and Industrial Safety Agency (NISA). Tokyo Electric Power Company Fukushima 630 Daiichi Nuclear Power Plant Unit. 2. Implementation measures for reducing the concentration of 631 radioactive materials in the reactor building (in Japanese),



632 http://dl.ndl.go.jp/view/download/digidepo_6017503_po_20110619001-6.pdf. (Accessed July 633 23, 2018) 634 635 [52] Tokyo Electric Power Company Holdings, Incorporated (TEPCO). Measures to improve the 636 environment by local exhaust ventilator at Fukushima Daiichi Nuclear Power Station Unit 2 (in 637 Japanese), http://www.tepco.co.jp/cc/press/betu11_j/images/110608d.pdf. (Accessed July 23, 638 2018) 639


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640 Figures 641 Fig. 1.

642 643



644 Fig. 2.

645


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646 Fig. 3.

647 648



649 Fig. 4.

650 651


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652 Table 1 Activity of the radionuclides

and the

90

Sr/137Cs activity ratio in aerosol samples

653 collected from each reactor building reported by TEPCO.43-48 The 90Sr activity was measured by 654 Japan Chemical Analysis Center and was summarized in one document published by TEPCO. 655 The

137

Cs activity was reported in various documents published by TEPCO. The

137

Cs data was

656 assigned to the 90Sr data via sampling date and sampling location in each document. 657 Reactor Sampling location

Unit 1

Sampling date

Upper part of 2011/10/3

137

Cs activity

concentration

90

Sr activity

concentration

90

Sr/137Cs

activity ratio

(Bq/m3)

(Bq/m3)

290

3

0.01

5.3

0.04

Unit 1 reactor building (West side of the nuclear reactor) Unit 1

Upper part of 2011/10/12 140 Unit 1 reactor 47

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building (On the equipment hatch) Unit 2

Upper part of 2011/10/13 170

0.41

0.002

2.5

0.0003

Unit 2 reactor building (Center west of the blow-out panel) Unit 3

Upper part of 2011/10/11 7300 Unit 3 reactor building (West side of the nuclear reactor(lateral 48


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direction)) Unit 3

Upper part of 2011/10/12 110

0.66

Unit 3 reactor building (the opening of the equipment hatch) 658


0.006


659 660 Figure captions 661 662 Figure 1. 137Cs activity concentration in the surface air at (a) Hitachi city, (b) Mito city, (c) 663 Kawasaki city, (d) Toyonaka city during 2011. The collection period in (a) Hitachi was 8 April to 664 31 December 2011, in (b) Mito 8 was April to 25 April 2011, in (c) Kawasaki was 15 March to 11 665 May 2011, and (d) in Toyonaka was 12 March to 30 April 2011. The filled plot shows the 137Cs 666 activity concentration of the samples we used for 90Sr analysis. All activity concentration values 667 are plotted in the middle of the sampling collection period. 668 669 Figure 2. 90Sr activity concentration in the surface air at (a) Hitachi city, (b) Mito city, (c) 670 Kawasaki city, (d) Toyonaka city during 2011. The collection period in (a) Hitachi was 8 April to 671 31 December 2011, in (b) Mito 8 was April to 25 April 2011, in (c) Kawasaki was 15 March to 11 672 May 2011, and (d) in Toyonaka was 12 March to 30 April 2011. The sample with relatively high 673

137

Cs activity had been analyzed. All activity concentration values are plotted in the middle of the

674 sampling collection period. 675 50 ACS Paragon Plus Environment

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676 677 Figure 3. Changes in the monthly activity concentration in Hitachi in 2011. The orange bar is the 678 median of the activity concentration of 137Cs. The lines indicate the changes in the activity 679 concentration estimated from the removal time starting from the median in April. All box pots are 680 plotted in the middle of the month. 681 682 Figure 4. 90Sr/137Cs activity ratio in the surface air from March to December in 2011. The plot 683 shows the data from Hitachi city (red stars), Mito city (blue triangles), Kawasaki city (green 684 circles), and Toyonaka city (black squares). The collection period in Hitachi was 8 April to 31 685 December 2011, in Mito 8 was April to 25 April 2011, in Kawasaki was 15 March to 11 May 686 2011, and in Toyonaka was 12 March to 30 April 2011. All activity ratios are plotted in the 687 middle of the sampling collection period.



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