Temporal Change in Atmospheric Radiocesium during the First Seven Years after the Fukushima Dai-ich Nuclear Power Plant Accident

After the Fukushima Daiichi Nuclear Power Plant accident, the Nuclear Regulation Authority monitored the atmospheric radiocesium concentration as a national project to assess its temporal changes from August 2011 till November 2017. The concentration ranged from 10–1 to 100 Bq m–3 during the first two years but from 10–5 to 10–1 Bq m–3 approximately seven years following the accident. Additionally, the resuspension factor (RF) fell between 10–7 and 10–6 m–1 at the beginning of the third year but eventually declined to values between 10–11 and 10–7 m–1. To investigate the effect of anthropogenic activities on the temporal change in this parameter, we categorized the monitoring data into those from within and without the entry-restricted zone. The latter exhibited a higher rate of decrease in the RF, which agrees with the previously reported data on the time dependence of the air dose rate and suggests that anthropogenic activities promote environmental remediation and reduce atmospheric radiocesium. Finally, as the rate observed in this study exceeds that reported for the Chernobyl Nuclear Power Plant accident over the corresponding period, it warrants ongoing assessment based on current monitoring data.


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The severe accident that broke out at the Fukushima Daiichi Nuclear Power Plant (FDNPP) in 32 March 2011 released a large amount of radiocesium into the atmosphere. Radiocesium is 33 considered to be an important radioactive substance deposited on the ground after the accident in 34 terms of radiation exposure due to its high level of release and long half-life (2.06 y for 134  inhalation of resuspended atmospheric aerosol, which is crucial for the retrospective assessment 39 and prediction of the radiation risk. Therefore, the radiocesium activity and temporal changes in its 40 atmospheric content need to be evaluated and understood to estimate the effects of inhalation 41 exposure. 42

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5 (NRA) project (NRA, 2020). These monitoring sites were located both outside and inside of entry-86 restricted zone (ERZ), where all activities other than emergency monitoring were restricted due to 87 a high level of contamination. Since anthropogenic activities such as traffic, decontamination and 88 cultivation differed significantly between the outside and inside of ERZ, the data set provided an 89 assessment of the anthropogenic influence on the temporal trend of RF. 90 To assess the atmospheric radiocesium levels and the characteristics of the RF temporal decrease 91 for longer periods, we analyzed the atmospheric aerosol monitoring data obtained at 11 sites in the 92 first seven years after the FDNPP accident. Additionally, anthropogenic impact on the time 93 dependence of RF and differences in the time dependence of the Chernobyl case has been studied. 94

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Sample collection and analysis method 97 Figure 1 shows the location of monitoring sites. After the FDNPP accident, the area within a 98 radius of 20 km from the accident site, which is the possible place of a radiation dose exceeded 20 99 mSv y −1 , was designated as the evacuation order zone (EOZ) in which entry was restricted. The 100 atmospheric aerosol was collected at five locations (I-1, I-2, I-3, I-4, and I-5) within the zone from 101

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6 The entry in the DRZ was restricted as well as EOZ, but the entry in the other two zones was 104 allowed. The monitoring sites from May 2013 to November 2017 were changed to two locations 105 (I-6 and I-7) inside the ERZ and four locations (O-1, O-2, O-3, and O-4) outside the ERZ, 106 respectively. Hence, the monitoring sites were classified into two groups; the first was called 107 "inside ERZ," including seven sites located in both EOZ and DRZ, and the second was called 108 "outside ERZ," including the four sites in the other zones. Table 1   Furthermore, after the first five years of the accident, the RF values outside ERZ were further

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11 where anthropogenic activities were inferred. Besides, the yearly average values in the first 4-5, 192 5-6, and 6-7 years after the accident were 2.1 × 10 −9 , 9.8 × 10 −10 , and 7.1 × 10 −10 m −1 , respectively, 193 suggesting that anthropogenic activities possibly reduce the RF by half during these three years. 194

RF time dependence 195
The RF showed a slow decay after two years of the FDNPP accident based on the findings 196 discussed in the previous section (Fig. 3). To accurately determine the decreasing rate of RF, Eq. 197 (1) was used for the data obtained only in the first 2-5 years due to uncertainties arising from the 198 poor quality of data collected during the initial period and the anthropogenic effect observed five 199 years after the accident. Furthermore, the variation in RF showed log-normal distribution, the Eq. 200 (1) was fitted to the geometric mean value of the RF every 0.5 years. Finally, the obtained models 201 for the sites inside and outside ERZ could be expressed using Eqs. The changes in the RF over time inside and outside ERZ during that period (2-5 y) can be 208 estimated, as shown in Fig. 4. In particular, we found that the effective half-life outside ERZ (0.22 209 y) was shorter than that inside the zone (0.29 y). Moreover, based on recent studies, the air dose 210 rate outside ERZ decreased faster than inside ERZ, whereas the reduction was considered to be 211  fate of radiocesium between Fukushima and Chernobyl as described below.

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was faster in Fukushima than in the area affected by the CNPP accident due to higher annual 226 precipitation. This probably led to a faster decrease in radiocesium activity at the surface ground 227 in Fukushima, resulting in a faster decrease in the activity of resuspended matter. Furthermore, 228 observed in the present study may reflect the increasing reduction rate of RF as atmospheric aerosol 239 derives from a wide area. 240 M A N U S C R I P T 14 rate of RF was higher than that obtained from the CNPP case. These results suggest that the long-243 term inhalation exposure in Fukushima is lower than estimated by the models reported after the 244 CNPP accident if the initial deposition amounts are the same. Furthermore, the present study could 245 not evaluate the RF during the first two years after the FDNPP accident, which could significantly 246 affect the initial internal exposure (World Health Organization, 2013), with a significant temporal 247 decrease in atmospheric radiocesium concentrations for medium-long periods after the accident. 248 The present data are expected to be useful, particularly in the future for estimating additional 249 exposure to radiocesium. This study evaluates the change in atmospheric radiocesium content and RF value over time in 254 the first seven years after the FDNPP accident. Our observations revealed that in the first two years, 255 the atmospheric radiocesium concentrations ranged from 10 −1 -10 0 Bq m −3 , while they decreased 256 to 10 −5 -1 0 −1 Bq m −3 in the next five years. Moreover, the RF values ranged from 6.7 × 10 −11 -2.8 257 × 10 −8 m −1 and 1.7 × 10 −10 -2.0 × 10 −7 m −1 inside and outside ERZ, respectively, 2-5 years after the 258 accident. It is also worth noting that the effective half-life outside ERZ was shorter than that within 259 the zone, suggesting that the anthropogenic activities significantly facilitate the atmospheric