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Vertical and horizontal distributions of 137Cs on paved surfaces affected by the Fukushima Dai-ichi Nuclear Power Plant accident
Journal of Environmental Radioactivity 217 (2020) 106213
Contents lists available at ScienceDirect
Journal of Environmental Radioactivity journal homepage: http://www.elsevier.com/locate/jenvrad
Vertical and horizontal distributions of 137Cs on paved surfaces affected by the Fukushima Dai-ichi Nuclear Power Plant accident K. Yoshimura a, *, T. Watanabe b, H. Kurikami b a b
Japan Atomic Energy Agency, 45-169 Sukakeba, Minamisoma, Fukushima, 975-0036, Japan Japan Atomic Energy Agency, 10-2 Fukasaku, Miharu-machi, Fukushima, 963-7707, Japan
A B S T R A C T
Vertical and horizontal distributions are fundamental for sampling and in-situ gamma spectrum measurement strategies. The distributions of 137Cs were investigated for paved surfaces affected by the Fukusima Dai-ichi Nuclear Power Plant accident. Additionally, the effects of the distributions on the measurement uncertainties of in-situ spectrometry were evaluated. Relaxation mass depth, representing the depth profile of 137Cs, was estimated to be less than 0.23 g cm 2. Variation in the relaxation mass depth, of 0.1–0.23 g cm 2, led to a minor error (less than 5%) in the spectral analysis of the137Cs inventory (activity per unit area, kBq m 2). The 137 Cs inventory, within a 20 � 20 m square of 400 cells each measuring 1 m2, showed an uneven distribution with large variation; coefficient of variation ranged from 54 to 136% of geometric average inventory of 424 kBq m 2. Increasing the grid size decreased 137Cs inventory variation among cells, revealing the relationship between instrument field of view and the spatial uncertainty of the results of in-situ gamma spectrometry.
1. Introduction Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident resulted in the deposition of a substantial amount of radionuclides in the terrestrial environment of the North Kanto and South Tohoku regions of Japan (Mikami et al., 2015; Saito et al., 2015; Sanada et al., 2014). Radiocesium, especially 137Cs, is a major source of radiation and con tributes to long-term exposure more than other radionuclides (UNSCEAR, 2013; Yoshimura et al., 2020). Therefore, the distribution and migration of 137Cs are of great concern to the public. Vertical and horizontal distributions of 137Cs within a test area are fundamental for the decision of sampling and in-situ gamma spectrum measurement strategies to investigate 137Cs in terrestrial environments (IAEA, 2019; ICRU, 1994; Khomutinin et al., 2004). The depth distri bution has been typically parameterized as relaxation mass depth (β, g cm 2), representing the mass depth at which radiocesium concentration reduces to 1/e of the concentration at ground level. The β is an essential parameter to analyze gamma spectra, obtained by in-situ measurements using a portable gamma-ray detector. In addition to the depth profile, the horizontal heterogeneity of 137Cs activity is an important informa tion for the interpretation and validation of investigation results, because the heterogeneity affects the uncertainty in measurements relating with sample number, sampling area, and the areas chosen for measurement (the fields of view being monitored for gamma rays) (IAEA, 2019; Khomutinin et al., 2004; Onda et al., 2015; Saito et al.,
2015; Yoshimura et al., 2019). Thus the uncertainties of spectral anal ysis and obtained results (i.e. spatial representativeness) are affected by both the vertical and horizontal distributions, respectively. The vertical and horizontal distributions for various land uses have been properly documented. The vertical distributions of 137Cs and their time dependencies in plain open fields have been continuously and comprehensively investigated, within an 80 km radius of FDNPP, by a national project launched after the accident (JAEA, 2018; Matsuda et al., 2015). The obtained β was applied to the in-situ gamma spectrometry of the project (Mikami et al., 2015). Land uses other than open fields, such as forest and agricultural fields, was also studied for the vertical distri bution (Koarashi et al., 2012; Takahashi et al., 2015; Yamaguchi et al., 2012; Yang et al., 2016). The horizontal heterogeneity of 137Cs distri bution has also been studied for open fields (Onda et al., 2015; Saito et al., 2015), the forest floor (Kato et al., 2018), and agricultural fields (Tanaka et al., 2013). Based on these reports, the measurement uncer tainty for radiocesium levels on ground soil has been statistically eval uated (IAEA, 2019; Khomutinin et al., 2004). The measurement of 137Cs on the paved surfaces which is the major component in residential areas is important for radiation protection. The 137 Cs (activity per unit area, Bq m 2) inventory of paved surfaces had been measured using the portable gamma-ray spectrometer (Andersson et al., 2002; Yoshimura et al., 2017). The vertical and horizontal dis tributions of 137Cs are essential for the spectral analysis and the inter pretation of the obtained results. The 137Cs accumulated at the
* Corresponding author. E-mail address: [email protected] (K. Yoshimura). https://doi.org/10.1016/j.jenvrad.2020.106213 Received 7 October 2019; Received in revised form 14 February 2020; Accepted 16 February 2020 Available online 22 February 2020 0265-931X/© 2020 Elsevier Ltd. All rights reserved.
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Journal of Environmental Radioactivity 217 (2020) 106213
outermost surfaces of the asphalt road was qualitatively determined after the FDNPP accident using autoradiography (Inoue et al., 2017). Maslova et al. (2013) quantitatively evaluated the depth profile of 137Cs in impermeable materials such as asphalt by absorption experiments using artificial radionuclides solutions and demonstrated that most of the 137Cs were retained within the top 0.2 mm after only 28 days. Andersson (2009) reported that 2 years after the Chernobyl Nuclear Power Plant accident, cesium was still within the top 1 mm of asphalt. The International Commission on Radiation Units and Measurement (ICRU, 1994) assumed the β value of 0.1 g cm 2 when 137Cs is in the uppermost few millimeters. However, limited reports are available for the β value of the 137Cs depth profile and the horizontal heterogeneity of 137 Cs activity on the paved surface. To provide fundamental information for the measurement of 137Cs in residential areas, this study evaluated the vertical and horizontal distributions on paved surfaces located around the FDNPP about six years after the accident, the relationships between the distributions and the uncertainties of spectral analysis and spatial representativeness of obtained results were also evaluated.
thickness of 44–59 mm. Averaged density was calculated to be 2.32 g cm 3, with a standard deviation (SD) of �0.05 g cm 3 from the volume and weight of the core sample. The cores were transported to a laboratory, and their sides were coated with resin to avoid crumbling of the edges during surface scraping. Then, the surface of each core was scraped by a grinder (made by ISI Co. Ltd., Japan) to a depth of around 0.5 mm, and the generated dust was collected by vacuum system equipped with a dust trap (Dust collector 182–30 and CYCLONE Shuttle, AS ONE Corp., Japan) (Fig. 2). To confirm a decrease in radiation on the surface due to the scraping, the surface radiation count rate (cpm) was measured by a Geiger-Mueller survey meter (TGS-146, Hitachi-Aloka Medical, Japan) before and after the every scraping. To avoid a contamination on the next layer by a dust residue, the residue was removed by brushing, wiping and was sucked up using a nozzle of the vacuum system before the measurement of the radiation count rate. However, minor contamination possibly led to a slightly deeper distribution more than the actual depth profile. The scraping was repeated four or five times until surface count rates were less than 10% of those of the initial surfaces. Dust samples from the trap were transferred to a 100 ml plastic containers (U-8 type container (56 mm in diameter with 68 mm height), Yamayu Co., Osaka, Japan) for the measurement of their radioactivity. The averaged recovery rate of the dust samples through the scraping and packing in the container was 93.1% (N ¼ 2) calculated as the ratio between the lost weight of core and dust weight in the container. The 137Cs activity was measured using a high purity germanium gamma-ray detector (GSW275L, Canberra, USA) connected with a multichannel analyzer (MCA7600, Seiko EG&G ORTEC, Japan). The setup was calibrated for efficiency using multiple gamma-ray emitting standard sources, including nine nuclides (Japan Radioisotope Associ ation, Japan). To estimate β, 137Cs depth profile was fitted to an exponential function (ICRU, 1994):
2. Methods 2.1. Study sites In this study, we investigated the paved surfaces of five parking areas within the towns of Okuma and Tomioka, about 5–8 km from the FDNPP (Fig. 1). The inventories on permeable ground around the study sites averaged within the area with diameter of 100 m were 3,100 kBq m 2 (St. 1), 1,420 kBq m 2 (St. 2), 1,330 kBq m 2 (St. 3), 1,510 kBq m 2 (St. 4), and 1,590 kBq m 2 (St. 5). The inventory on the paved surface accounted for about 20% of that on nearby permeable ground four years after the accident (Yoshimura et al., 2017). The sites in both towns were within the evacuation zone, and human activities such as traffic and decontamination were limited. All of the investigated surfaces were impermeable asphalt.
(1)
Am(ζ) ¼ Am,0 exp(-ζ/β) 137
where, Am(ζ) and Am,0 are the Cs activity concentrations at a depth ζ (g cm 2) and at the surface, respectively.
2.2. Evaluation of depth distribution To evaluate the depth profile of 137Cs, duplicate core samples were collected at a parking area in Okuma town (St.1) and four parking areas in Tomioka town (Sts. 2, 3, 4, and 5), respectively, using a core cutter on December 5 and 7, 2016. The cores had a diameter of 103 mm and a
2.3. Evaluation of horizontal distribution To evaluate the horizontal heterogeneity of
137
Cs inventory for
Fig. 1. Study sites in Okuma and Tomioka towns. Asphalt core sampling (St. 1–5) and in-situ measurement of 137Cs inventory using a portable gamma ray spec trometer (St. 2) were carried out in residential areas. This map was generated by ArcGIS 10 software. The distribution of 137Cs inventory as the value on November 5, 2011 was derived from the Fourth Airborne Monitoring Survey by Ministry of Education, Culture, Sports, Science and Technology, Japan (2011). 2
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Journal of Environmental Radioactivity 217 (2020) 106213
Fig. 2. Grinder to scrape asphalt core surface and vacuum system to collect the dust generated by the scraping. Grinding part indicated by red box is expanded at right. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
paved surfaces, in-situ gamma-ray spectrum measurements were carried out at the St.2 parking area in Tomioka town from January to February 2017. A space of 20 � 20 m square in the parking area was divided into 400 cells of 1 m2. Gamma-ray spectra were measured using a portable gamma-ray detector (Falcon-5000, CANBERRA, USA) equipped with a cylindrical lead collimator at a center of each cell; detailed descriptions of the collimator can be found in Yoshimura et al. (2017). The gamma rays emitted from the ground were measured at a height of 0.3 m above the ground for 30 min. Efficiency calibrations for the gamma-ray spectra were carried out by In-Situ Object Calibration Software (ISOCS, CAN BERRA, USA). β and density, which are parameters in the calibration using ISOCS, were 0.1 g cm 2 (ICRU, 1994) and 2.32 g cm 3 (this study), respectively. The simulation conducted by ISOCS showed that 89% of the detected gamma rays were derived from the circular area of diam eter, 1 m below the detector under the measurement conditions.
2.4. Elevation of surface in parking area To investigate the effect of ground level on the heterogeneity of 137Cs inventory, the elevation of the surface at St.2 parking area was measured using a 3D laser scanner (Focus3D S120, FARO, USA). The elevation was scanned from four points outside the 20 � 20 m square space. Data processing operations such as unifying coordinates, data cleaning, and the creation of digital surface models, was carried out by the Point Cloud Software Package (3D GEOKOSMOS, Japan). 2.5. Verification of spectral analysis This study evaluated the uncertainty of the spectral analysis to calculate 137Cs inventory using variable β. For the verification, a gamma-ray spectrum was obtained at St. 1 by the portable gamma ray detector without collimation. The gamma-rays emitted from the ground were detected at 1 m above ground level for 30 min.
Fig. 3. Depth profile of 3
137
Cs in asphalt cores.
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Journal of Environmental Radioactivity 217 (2020) 106213
3. Results and discussion
In the case of permeable ground, β shows large variation between sites and changes with time which is due to many factors (JAEA, 2018; Koarashi et al., 2012; Matsuda et al., 2015; Takahashi et al., 2015, 2018). On the other hand, the β of an impermeable paved surface is inferred to be independent of location and elapsed time after the acci dent because all the βs obtained in this study were sufficiently small, leading to minor impacts on the spectral analysis. Therefore, the β of paved surfaces were suggested to be more constant, resulting in smaller uncertainty of spectra analysis to calculate 137Cs inventory than those of other land uses.
3.1. Vertical distribution Vertical distributions of 137Cs measured for the asphalt cores are shown in Fig. 3. It can be observed that the activity of 137Cs decreases with an increase in depth. Minor differences were found between the duplicate samples, this shows that this research is reproducible. More than 90% of 137Cs was distributed in the layer of mass depth 0.45 g cm 2, which is comparable to 1.9 mm. The depth profile mostly decreased exponentially, but the profile curves of some sites (Sts. 1, 2 and 4) showed brake off at the top layer. This low activity at the top layer could be as a result of wear by anthropogenic activities such as traffic or weathering of surface, possibly. The average β value obtained by fitting equation was 0.16 g cm 2 with a SD of �0.04 g cm 2 while the maximum value was 0.23 g cm 2. Although this study evaluated β, it was only an apparent value due to the roughness of asphalt surfaces. For example, the surface conditions of core at St.2 through the scraping process are shown in Fig. 4. Surface depressions were not removed by the first and second scrapings, due to the uneven surface, but were removed by the third and fourth stages. This partial removal of the actual surface at each scraping stage meant that the third and fourth scrapings included some actual surface layer, resulting in the overestimation of the β in this study. At the least, however, the β on the impermeable paved surface can be estimated to be less than 0.23 g cm 2 which is the maximum value observed in this study. To evaluate the effect of the uncertainty of β on the spectral analysis of 137Cs inventory, the gamma-ray spectrum obtained at St. 1 from 1 m above the ground was analyzed with various β values ranging from 0.05 to 0.3 g cm 2. The calculated 137Cs inventories were increased to about 10% with an increase in β from 0.05 to 0.3 g cm 2. There was a 5% difference between the results calculated using β of 0.1 and 0.23 g cm 2, the former is the value assumed by ICRU (1994) and the latter was obtained from this study. This suggests that the effect of the uncertainty of β on the spectral analysis of 137Cs inventory is limited within the range of β less than 0.23 g cm 2.
3.2. Horizontal distribution Distribution of 137Cs inventory measured for 400 cells of size 1 m2 at the St. 2 parking area showed uneven distribution (Fig. 5). Large
Fig. 5. Distribution map of 137Cs inventory measured 400 grids in the 20 m square at St.2. Circles indicate the location of crack.
Fig. 4. Surfaces before and after every scraping. Pictures were taken for the core at St. 2. 4
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variations in 137Cs activity within the several-meter-scale have also been reported for other terrestrial environments (Kato et al., 2018; Onda et al., 2015; Tanaka et al., 2013). The reported variations are inferred to be a result of the corrugation of ground surfaces and hydrological redistribution, also the structure of canopy, in the case of the forest floor. In the case of paved surface in this study, there were cracks of various sizes with a width from less than 1 mm to several millimeters and a part of the crack accumulated soil in their gap. Hydrological redistribution of 137 Cs on the surface was expected to result in both the increase in ac tivity due to the accumulation of 137Cs in the cracks and decrease in activity due to the shielding effect associating to the isolation of 137Cs into inside the gap. However, the cells showing obvious high and low activities distributed independently from the crack location, although some cells containing cracks showed high activity. The hydrological redistribution can be also affected by slope on the surface. The elevation map (Fig. 6) shows the slope of the parking area, marking the decrease in elevation from the upper left area toward the lower right area. However, the 137Cs distribution was also independent of the elevation. These re sults suggest that the distribution pattern was not formed solely by the hydrological processes after the accident, but the uneven distribution was probably decided at the time of initial deposition. The dust on paved surfaces showed high 137Cs activity (Murakami et al., 2017; Yamashita et al., 2015) and a large affinity to radiocesium (Andersson, 2009). Some puddles were found in the study area after precipitation, meaning that there were some small depressions, which was difficult to be detected by the 3D laser scanner. Therefore, heterogeneous accumulation of the dust on the surface, especially on the small depression and roadside, could be a factor deciding the patch distribution of 137Cs inventory. Spatial variability of 137Cs inventories within the area are important information to decide sampling and measurement strategies and to consider the spatial representativeness of the results. The variation has been statistically evaluated and was usually described by a log-normal distribution (IAEA, 2019; Khomutinin et al., 2004). The histogram of log-137Cs inventory obtained in this study is shown in Fig. 7. The aver aged log-137Cs inventory (kBq m 2) was 2.6 with an SD of �0.13, cor responding to the coefficient of variation ranged from 54 to 136% of the geometric mean 137Cs-inventory of 424 kBq m 2, under the measure ment conditions of this study with a spatial resolution of almost 1 m2. For the in-situ spectrometry, the instruments’ fields of view contributed largely to the uncertainty in the measurements (Malins et al., 2015; Yoshimura et al., 2019). To evaluate the variation in the 137 Cs inventory and the fields of view, the 137Cs inventories were
Fig. 7. Distribution of log
137
Cs inventory observed at a parking area of St. 2.
averaged within different cell sizes (2 m2, 4 m2, 5 m2, and 10 m2). Then, coefficients of variation were evaluated for each cell (Fig. 8). The co efficient of variation decreased with cell size to reach less than 15% for cell sizes over 4 m2. This study investigated on a relatively flat surface as shown in Fig. 6. On the other hand, the spatial representativeness of the results should increase on hollowed and more uneven surfaces; also, the surface roughness influences the uncertainty of radiation measurement in terrain over larger scale due to shielding of radiation emitted from depression surfaces by little hills (slightly raised site). Therefore, further studies and investigations under different conditions will help in un derstanding the uncertainty of measurement on paved surfaces. 4. Conclusions This study measured depth profiles of 137Cs for asphalt cores ob tained from five parking areas affected by the FDNPP accident. At the least, β was estimated to be less than 0.23 g cm 2. The variation in β, ranging from 0.1 to 0.23 g cm 2 led to a minor error (less than 5%) in the result of the spectral analysis of 137Cs inventory. This study also evalu ated the horizontal heterogeneity of the 137Cs inventory in 20 � 20 m
Fig. 6. Elevation map in the 20 m square at St. 2. Circles indicate the location of crack, and the white area indicates the area at which elevation data could not be obtained.
Fig. 8. Coefficient of variation as a function of grid size. 5
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Journal of Environmental Radioactivity 217 (2020) 106213
square with a spatial resolution of 1 m2-cell; an uneven distribution with large variation (coefficient of variation ranged from 54 to 136%) was observed. The relationship between the variation in 137Cs inventory and the instruments’ fields of view was also evaluated. These information obtained in this study should be useful to validate the uncertainty of the investigation of radiocesium on paved surface.
Mikami, S., et al., 2015. Spatial distributions of radionuclides deposited onto ground soil around the Fukushima Dai-ichi Nuclear Power Plant and their temporal change until December 2012. J. Environ. Radioact. 139, 320–343. Ministry of Education, Culture, Sports, Science and Technology, Japan, 2011. Results of the Fourth Airborne Monitoring Survey by MEXT. http://radioactivity.nsr.go.jp/en /contents/4000/3179/24/1270_1216.pdf. (Accessed 3 October 2019). Murakami, M., Saha, M., Iwasaki, Y., Yamashita, R., Koibuchi, Y., Tsukada, H., Takada, H., Sueki, K., Yasutaka, T., 2017. Source analysis of radiocesium in river waters using road dust tracers. Chemosphere 187, 212–220. Onda, Y., Kato, H., Hoshi, M., Takahashi, Y., Nguyen, M.L., 2015. Soil sampling and analytical strategies for mapping fallout in nuclear emergencies based on the Fukushima Dai-ichi Nuclear Power Plant accident. J. Environ. Radioact. 139, 300–307. Saito, K., Tanihata, I., Fujiwara, M., Saito, T., Shimoura, S., Otsuka, T., Onda, Y., Hoshi, M., Ikeuchi, Y., Takahashi, F., Kinouchi, N., Saegusa, J., Seki, A., Takemiya, H., Shibata, T., 2015. Detailed deposition density maps constructed by large-scale soil sampling for gamma-ray emitting radioactive nuclides from the Fukushima Dai-ichi Nuclear Power Plant accident. J. Environ. Radioact. 139, 308–319. Sanada, Y., Sugita, T., Nishizawa, Y., Kondo, A., Torii, T., 2014. The aerial radiation monitoring in Japan after the Fukushima Daiichi nuclear power plant accident. Prog. Nucl. Sci. Tech. 4, 76–80. Takahashi, J., Onda, Y., Hihara, D., Tamura, K., 2018. Six-year monitoring of the vertical distribution of radiocesium in three forest soils after the Fukushima Dai-ichi Nuclear Power Plant accident. J. Environ. Radioact. 192, 172–180. Takahashi, J., Tamura, K., Suda, T., Matsumura, R., Onda, Y., 2015. Vertical distribution and temporal changes of 137Cs in soil profiles under various land uses after the Fukushima Dai-ichi Nuclear Power Plant accident. J. Environ. Radioact. 139, 351–361. Tanaka, K., Iwatani, H., Takahashi, Y., Sakaguchi, A., Yoshimura, K., Onda, Y., 2013. Investigation of spatial distribution of radiocesium in a paddy field as a potential sink. PloS One 8 (11), e80794. UNSCEAR, 2013. Sources, Effects and Risks of Ionizing Radiation. Report to the General Assembly with Scientific Annexes, vol. I. Scientific Annex A. Yamaguchi, N., Eguchi, S., Fujiwara, H., Hayashi, K., Tsukada, H., 2012. Radiocesium and radioiodine in soil particles agitated by agricultural practices: field observation after the Fukushima nuclear accident. Sci. Total Environ. 425, 128–134. Yamashita, R., Murakami, M., Iwasaki, Y., Shibayama, N., Sueki, K., Saha, M., Mouri, G., Lamxay, S., Haechong, O., Koibuchi, Y., Takada, H., 2015. Temporal variation and source analysis of radiocesium in an urban river after the 2011 nuclear accident in Fukushima, Japan. J. Water Environ. Technol. 13, 179–194. Yang, B., Onda, Y., Wakiyama, Y., Yoshimura, K., Sekimoto, H., Ha, Y., 2016. Temporal changes of radiocesium in irrigated paddy fields and its accumulation in rice plants in Fukushima. Environ. Pollut. 208 (Pt B), 562–570. Yoshimura, K., Fujiwara, K., Nakama, S., 2019. Applicability of autonomous unmanned helicopter survey of air dose rate in suburban area. Radiat. Protect. Dosim. https:// doi.org/10.1093/rpd/ncz116. Yoshimura, K., Saegusa, J., Sanada, Y., 2020. Initial decrease in the ambient dose equivalent rate after the Fukushima accident and its difference from Chernobyl. Sci. Rep. (in press). Yoshimura, K., Saito, K., Fujiwara, K., 2017. Distribution of 137Cs on components in urban area four years after the Fukushima Dai-ichi Nuclear Power Plant accident. J. Environ. Radioact. 178–179, 48–54.
Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. References Andersson, K.G., 2009. Migration of radionuclides on outdoor surfaces. Chapter 5. In: Andersson, K.G. (Ed.), Airborne Radioactive Contamination in Inhabited Areas, Book Series Radioactivity in the Environment, vol.15. Elsevier, pp. 107–146. Andersson, K.G., Roed, J., Fogh, C.L., 2002. Weathering of radiocaesium contamination on urban streets, walls and roofs. J. Environ. Radioact. 62, 49–60. IAEA, 2019. Guidelines on Soil and Vegetation Sampling for Radiological Monitoring. Vienna, Austria, IAEA Technical Reports Series No. 486. ICRU, 1994. Gamma-ray spectrometry in the environment. ICRU (Int. Comm. Radiat. Units Meas.) Rep. 53. Inoue, K., Arai, M., Fujisawa, M., Saito, K., Fukushi, M., 2017. Detailed distribution map of absorbed dose rate in air in tokatsu area of chiba prefecture, Japan, constructed by car-borne survey 4 Years after the fukushima daiichi nuclear power plant accident. PloS One 12 (1), e0171100. JAEA, 2018. Report for the Project Entrusted from the Nuclear Regulation Authority. htt ps://radioactivity.nsr.go.jp/ja/list/574/list-1.html. (Accessed 3 October 2019) (in Japanese). Kato, H., Onda, Y., Wakahara, T., Kawamori, A., 2018. Spatial pattern of atmospherically deposited radiocesium on the forest floor in the early phase of the Fukushima Daiichi Nuclear Power Plant accident. Sci. Total Environ. 615, 187–196. Khomutinin, Y.V., Kashparov, V.A., Zhebrovska, K.I., 2004. Sampling Optimization when Radioecological Monitoring. UIAR. Ukraine Institute for Agricultural Radiology, Kiev, Ukraine, ISBN 966-646-034-3, p. 133. Koarashi, J., Atarashi-Andoh, M., Matsunaga, T., Sato, T., Nagao, S., Nagai, H., 2012. Factors affecting vertical distribution of Fukushima accident-derived radiocesium in soil under different land-use conditions. Sci. Total Environ. 431, 392–401. Malins, A., Okumura, M., Machida, M., Takemiya, H., Saito, K., 2015. Fields of view for environmental radioactivity. In: Proceedings of the International Symposium on Radiological Issues for Fukushima’s Revitalized Future, pp. 28–34. Maslova, K., Stepina, I., Konoplev, A., Popov, V., Gusarov, A., Pankratov, F., Lee, S.D., Il’icheva, N., 2013. Fate and transport of radiocesium, radiostrontium and radiocobalt on urban building materials. J. Environ. Radioact. 125, 74–80. Matsuda, N., Mikami, S., Shimoura, S., Takahashi, J., Nakano, M., Shimada, K., Uno, K., Hagiwara, S., Saito, K., 2015. Depth profiles of radioactive cesium in soil using a scraper plate over a wide area surrounding the Fukushima Dai-ichi Nuclear Power Plant. Japan. J. Environ. Radioact. 139, 427–434.
6
Contents lists available at ScienceDirect
Journal of Environmental Radioactivity journal homepage: http://www.elsevier.com/locate/jenvrad
Vertical and horizontal distributions of 137Cs on paved surfaces affected by the Fukushima Dai-ichi Nuclear Power Plant accident K. Yoshimura a, *, T. Watanabe b, H. Kurikami b a b
Japan Atomic Energy Agency, 45-169 Sukakeba, Minamisoma, Fukushima, 975-0036, Japan Japan Atomic Energy Agency, 10-2 Fukasaku, Miharu-machi, Fukushima, 963-7707, Japan
A B S T R A C T
Vertical and horizontal distributions are fundamental for sampling and in-situ gamma spectrum measurement strategies. The distributions of 137Cs were investigated for paved surfaces affected by the Fukusima Dai-ichi Nuclear Power Plant accident. Additionally, the effects of the distributions on the measurement uncertainties of in-situ spectrometry were evaluated. Relaxation mass depth, representing the depth profile of 137Cs, was estimated to be less than 0.23 g cm 2. Variation in the relaxation mass depth, of 0.1–0.23 g cm 2, led to a minor error (less than 5%) in the spectral analysis of the137Cs inventory (activity per unit area, kBq m 2). The 137 Cs inventory, within a 20 � 20 m square of 400 cells each measuring 1 m2, showed an uneven distribution with large variation; coefficient of variation ranged from 54 to 136% of geometric average inventory of 424 kBq m 2. Increasing the grid size decreased 137Cs inventory variation among cells, revealing the relationship between instrument field of view and the spatial uncertainty of the results of in-situ gamma spectrometry.
1. Introduction Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident resulted in the deposition of a substantial amount of radionuclides in the terrestrial environment of the North Kanto and South Tohoku regions of Japan (Mikami et al., 2015; Saito et al., 2015; Sanada et al., 2014). Radiocesium, especially 137Cs, is a major source of radiation and con tributes to long-term exposure more than other radionuclides (UNSCEAR, 2013; Yoshimura et al., 2020). Therefore, the distribution and migration of 137Cs are of great concern to the public. Vertical and horizontal distributions of 137Cs within a test area are fundamental for the decision of sampling and in-situ gamma spectrum measurement strategies to investigate 137Cs in terrestrial environments (IAEA, 2019; ICRU, 1994; Khomutinin et al., 2004). The depth distri bution has been typically parameterized as relaxation mass depth (β, g cm 2), representing the mass depth at which radiocesium concentration reduces to 1/e of the concentration at ground level. The β is an essential parameter to analyze gamma spectra, obtained by in-situ measurements using a portable gamma-ray detector. In addition to the depth profile, the horizontal heterogeneity of 137Cs activity is an important informa tion for the interpretation and validation of investigation results, because the heterogeneity affects the uncertainty in measurements relating with sample number, sampling area, and the areas chosen for measurement (the fields of view being monitored for gamma rays) (IAEA, 2019; Khomutinin et al., 2004; Onda et al., 2015; Saito et al.,
2015; Yoshimura et al., 2019). Thus the uncertainties of spectral anal ysis and obtained results (i.e. spatial representativeness) are affected by both the vertical and horizontal distributions, respectively. The vertical and horizontal distributions for various land uses have been properly documented. The vertical distributions of 137Cs and their time dependencies in plain open fields have been continuously and comprehensively investigated, within an 80 km radius of FDNPP, by a national project launched after the accident (JAEA, 2018; Matsuda et al., 2015). The obtained β was applied to the in-situ gamma spectrometry of the project (Mikami et al., 2015). Land uses other than open fields, such as forest and agricultural fields, was also studied for the vertical distri bution (Koarashi et al., 2012; Takahashi et al., 2015; Yamaguchi et al., 2012; Yang et al., 2016). The horizontal heterogeneity of 137Cs distri bution has also been studied for open fields (Onda et al., 2015; Saito et al., 2015), the forest floor (Kato et al., 2018), and agricultural fields (Tanaka et al., 2013). Based on these reports, the measurement uncer tainty for radiocesium levels on ground soil has been statistically eval uated (IAEA, 2019; Khomutinin et al., 2004). The measurement of 137Cs on the paved surfaces which is the major component in residential areas is important for radiation protection. The 137 Cs (activity per unit area, Bq m 2) inventory of paved surfaces had been measured using the portable gamma-ray spectrometer (Andersson et al., 2002; Yoshimura et al., 2017). The vertical and horizontal dis tributions of 137Cs are essential for the spectral analysis and the inter pretation of the obtained results. The 137Cs accumulated at the
* Corresponding author. E-mail address: [email protected] (K. Yoshimura). https://doi.org/10.1016/j.jenvrad.2020.106213 Received 7 October 2019; Received in revised form 14 February 2020; Accepted 16 February 2020 Available online 22 February 2020 0265-931X/© 2020 Elsevier Ltd. All rights reserved.
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Journal of Environmental Radioactivity 217 (2020) 106213
outermost surfaces of the asphalt road was qualitatively determined after the FDNPP accident using autoradiography (Inoue et al., 2017). Maslova et al. (2013) quantitatively evaluated the depth profile of 137Cs in impermeable materials such as asphalt by absorption experiments using artificial radionuclides solutions and demonstrated that most of the 137Cs were retained within the top 0.2 mm after only 28 days. Andersson (2009) reported that 2 years after the Chernobyl Nuclear Power Plant accident, cesium was still within the top 1 mm of asphalt. The International Commission on Radiation Units and Measurement (ICRU, 1994) assumed the β value of 0.1 g cm 2 when 137Cs is in the uppermost few millimeters. However, limited reports are available for the β value of the 137Cs depth profile and the horizontal heterogeneity of 137 Cs activity on the paved surface. To provide fundamental information for the measurement of 137Cs in residential areas, this study evaluated the vertical and horizontal distributions on paved surfaces located around the FDNPP about six years after the accident, the relationships between the distributions and the uncertainties of spectral analysis and spatial representativeness of obtained results were also evaluated.
thickness of 44–59 mm. Averaged density was calculated to be 2.32 g cm 3, with a standard deviation (SD) of �0.05 g cm 3 from the volume and weight of the core sample. The cores were transported to a laboratory, and their sides were coated with resin to avoid crumbling of the edges during surface scraping. Then, the surface of each core was scraped by a grinder (made by ISI Co. Ltd., Japan) to a depth of around 0.5 mm, and the generated dust was collected by vacuum system equipped with a dust trap (Dust collector 182–30 and CYCLONE Shuttle, AS ONE Corp., Japan) (Fig. 2). To confirm a decrease in radiation on the surface due to the scraping, the surface radiation count rate (cpm) was measured by a Geiger-Mueller survey meter (TGS-146, Hitachi-Aloka Medical, Japan) before and after the every scraping. To avoid a contamination on the next layer by a dust residue, the residue was removed by brushing, wiping and was sucked up using a nozzle of the vacuum system before the measurement of the radiation count rate. However, minor contamination possibly led to a slightly deeper distribution more than the actual depth profile. The scraping was repeated four or five times until surface count rates were less than 10% of those of the initial surfaces. Dust samples from the trap were transferred to a 100 ml plastic containers (U-8 type container (56 mm in diameter with 68 mm height), Yamayu Co., Osaka, Japan) for the measurement of their radioactivity. The averaged recovery rate of the dust samples through the scraping and packing in the container was 93.1% (N ¼ 2) calculated as the ratio between the lost weight of core and dust weight in the container. The 137Cs activity was measured using a high purity germanium gamma-ray detector (GSW275L, Canberra, USA) connected with a multichannel analyzer (MCA7600, Seiko EG&G ORTEC, Japan). The setup was calibrated for efficiency using multiple gamma-ray emitting standard sources, including nine nuclides (Japan Radioisotope Associ ation, Japan). To estimate β, 137Cs depth profile was fitted to an exponential function (ICRU, 1994):
2. Methods 2.1. Study sites In this study, we investigated the paved surfaces of five parking areas within the towns of Okuma and Tomioka, about 5–8 km from the FDNPP (Fig. 1). The inventories on permeable ground around the study sites averaged within the area with diameter of 100 m were 3,100 kBq m 2 (St. 1), 1,420 kBq m 2 (St. 2), 1,330 kBq m 2 (St. 3), 1,510 kBq m 2 (St. 4), and 1,590 kBq m 2 (St. 5). The inventory on the paved surface accounted for about 20% of that on nearby permeable ground four years after the accident (Yoshimura et al., 2017). The sites in both towns were within the evacuation zone, and human activities such as traffic and decontamination were limited. All of the investigated surfaces were impermeable asphalt.
(1)
Am(ζ) ¼ Am,0 exp(-ζ/β) 137
where, Am(ζ) and Am,0 are the Cs activity concentrations at a depth ζ (g cm 2) and at the surface, respectively.
2.2. Evaluation of depth distribution To evaluate the depth profile of 137Cs, duplicate core samples were collected at a parking area in Okuma town (St.1) and four parking areas in Tomioka town (Sts. 2, 3, 4, and 5), respectively, using a core cutter on December 5 and 7, 2016. The cores had a diameter of 103 mm and a
2.3. Evaluation of horizontal distribution To evaluate the horizontal heterogeneity of
137
Cs inventory for
Fig. 1. Study sites in Okuma and Tomioka towns. Asphalt core sampling (St. 1–5) and in-situ measurement of 137Cs inventory using a portable gamma ray spec trometer (St. 2) were carried out in residential areas. This map was generated by ArcGIS 10 software. The distribution of 137Cs inventory as the value on November 5, 2011 was derived from the Fourth Airborne Monitoring Survey by Ministry of Education, Culture, Sports, Science and Technology, Japan (2011). 2
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Fig. 2. Grinder to scrape asphalt core surface and vacuum system to collect the dust generated by the scraping. Grinding part indicated by red box is expanded at right. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
paved surfaces, in-situ gamma-ray spectrum measurements were carried out at the St.2 parking area in Tomioka town from January to February 2017. A space of 20 � 20 m square in the parking area was divided into 400 cells of 1 m2. Gamma-ray spectra were measured using a portable gamma-ray detector (Falcon-5000, CANBERRA, USA) equipped with a cylindrical lead collimator at a center of each cell; detailed descriptions of the collimator can be found in Yoshimura et al. (2017). The gamma rays emitted from the ground were measured at a height of 0.3 m above the ground for 30 min. Efficiency calibrations for the gamma-ray spectra were carried out by In-Situ Object Calibration Software (ISOCS, CAN BERRA, USA). β and density, which are parameters in the calibration using ISOCS, were 0.1 g cm 2 (ICRU, 1994) and 2.32 g cm 3 (this study), respectively. The simulation conducted by ISOCS showed that 89% of the detected gamma rays were derived from the circular area of diam eter, 1 m below the detector under the measurement conditions.
2.4. Elevation of surface in parking area To investigate the effect of ground level on the heterogeneity of 137Cs inventory, the elevation of the surface at St.2 parking area was measured using a 3D laser scanner (Focus3D S120, FARO, USA). The elevation was scanned from four points outside the 20 � 20 m square space. Data processing operations such as unifying coordinates, data cleaning, and the creation of digital surface models, was carried out by the Point Cloud Software Package (3D GEOKOSMOS, Japan). 2.5. Verification of spectral analysis This study evaluated the uncertainty of the spectral analysis to calculate 137Cs inventory using variable β. For the verification, a gamma-ray spectrum was obtained at St. 1 by the portable gamma ray detector without collimation. The gamma-rays emitted from the ground were detected at 1 m above ground level for 30 min.
Fig. 3. Depth profile of 3
137
Cs in asphalt cores.
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3. Results and discussion
In the case of permeable ground, β shows large variation between sites and changes with time which is due to many factors (JAEA, 2018; Koarashi et al., 2012; Matsuda et al., 2015; Takahashi et al., 2015, 2018). On the other hand, the β of an impermeable paved surface is inferred to be independent of location and elapsed time after the acci dent because all the βs obtained in this study were sufficiently small, leading to minor impacts on the spectral analysis. Therefore, the β of paved surfaces were suggested to be more constant, resulting in smaller uncertainty of spectra analysis to calculate 137Cs inventory than those of other land uses.
3.1. Vertical distribution Vertical distributions of 137Cs measured for the asphalt cores are shown in Fig. 3. It can be observed that the activity of 137Cs decreases with an increase in depth. Minor differences were found between the duplicate samples, this shows that this research is reproducible. More than 90% of 137Cs was distributed in the layer of mass depth 0.45 g cm 2, which is comparable to 1.9 mm. The depth profile mostly decreased exponentially, but the profile curves of some sites (Sts. 1, 2 and 4) showed brake off at the top layer. This low activity at the top layer could be as a result of wear by anthropogenic activities such as traffic or weathering of surface, possibly. The average β value obtained by fitting equation was 0.16 g cm 2 with a SD of �0.04 g cm 2 while the maximum value was 0.23 g cm 2. Although this study evaluated β, it was only an apparent value due to the roughness of asphalt surfaces. For example, the surface conditions of core at St.2 through the scraping process are shown in Fig. 4. Surface depressions were not removed by the first and second scrapings, due to the uneven surface, but were removed by the third and fourth stages. This partial removal of the actual surface at each scraping stage meant that the third and fourth scrapings included some actual surface layer, resulting in the overestimation of the β in this study. At the least, however, the β on the impermeable paved surface can be estimated to be less than 0.23 g cm 2 which is the maximum value observed in this study. To evaluate the effect of the uncertainty of β on the spectral analysis of 137Cs inventory, the gamma-ray spectrum obtained at St. 1 from 1 m above the ground was analyzed with various β values ranging from 0.05 to 0.3 g cm 2. The calculated 137Cs inventories were increased to about 10% with an increase in β from 0.05 to 0.3 g cm 2. There was a 5% difference between the results calculated using β of 0.1 and 0.23 g cm 2, the former is the value assumed by ICRU (1994) and the latter was obtained from this study. This suggests that the effect of the uncertainty of β on the spectral analysis of 137Cs inventory is limited within the range of β less than 0.23 g cm 2.
3.2. Horizontal distribution Distribution of 137Cs inventory measured for 400 cells of size 1 m2 at the St. 2 parking area showed uneven distribution (Fig. 5). Large
Fig. 5. Distribution map of 137Cs inventory measured 400 grids in the 20 m square at St.2. Circles indicate the location of crack.
Fig. 4. Surfaces before and after every scraping. Pictures were taken for the core at St. 2. 4
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variations in 137Cs activity within the several-meter-scale have also been reported for other terrestrial environments (Kato et al., 2018; Onda et al., 2015; Tanaka et al., 2013). The reported variations are inferred to be a result of the corrugation of ground surfaces and hydrological redistribution, also the structure of canopy, in the case of the forest floor. In the case of paved surface in this study, there were cracks of various sizes with a width from less than 1 mm to several millimeters and a part of the crack accumulated soil in their gap. Hydrological redistribution of 137 Cs on the surface was expected to result in both the increase in ac tivity due to the accumulation of 137Cs in the cracks and decrease in activity due to the shielding effect associating to the isolation of 137Cs into inside the gap. However, the cells showing obvious high and low activities distributed independently from the crack location, although some cells containing cracks showed high activity. The hydrological redistribution can be also affected by slope on the surface. The elevation map (Fig. 6) shows the slope of the parking area, marking the decrease in elevation from the upper left area toward the lower right area. However, the 137Cs distribution was also independent of the elevation. These re sults suggest that the distribution pattern was not formed solely by the hydrological processes after the accident, but the uneven distribution was probably decided at the time of initial deposition. The dust on paved surfaces showed high 137Cs activity (Murakami et al., 2017; Yamashita et al., 2015) and a large affinity to radiocesium (Andersson, 2009). Some puddles were found in the study area after precipitation, meaning that there were some small depressions, which was difficult to be detected by the 3D laser scanner. Therefore, heterogeneous accumulation of the dust on the surface, especially on the small depression and roadside, could be a factor deciding the patch distribution of 137Cs inventory. Spatial variability of 137Cs inventories within the area are important information to decide sampling and measurement strategies and to consider the spatial representativeness of the results. The variation has been statistically evaluated and was usually described by a log-normal distribution (IAEA, 2019; Khomutinin et al., 2004). The histogram of log-137Cs inventory obtained in this study is shown in Fig. 7. The aver aged log-137Cs inventory (kBq m 2) was 2.6 with an SD of �0.13, cor responding to the coefficient of variation ranged from 54 to 136% of the geometric mean 137Cs-inventory of 424 kBq m 2, under the measure ment conditions of this study with a spatial resolution of almost 1 m2. For the in-situ spectrometry, the instruments’ fields of view contributed largely to the uncertainty in the measurements (Malins et al., 2015; Yoshimura et al., 2019). To evaluate the variation in the 137 Cs inventory and the fields of view, the 137Cs inventories were
Fig. 7. Distribution of log
137
Cs inventory observed at a parking area of St. 2.
averaged within different cell sizes (2 m2, 4 m2, 5 m2, and 10 m2). Then, coefficients of variation were evaluated for each cell (Fig. 8). The co efficient of variation decreased with cell size to reach less than 15% for cell sizes over 4 m2. This study investigated on a relatively flat surface as shown in Fig. 6. On the other hand, the spatial representativeness of the results should increase on hollowed and more uneven surfaces; also, the surface roughness influences the uncertainty of radiation measurement in terrain over larger scale due to shielding of radiation emitted from depression surfaces by little hills (slightly raised site). Therefore, further studies and investigations under different conditions will help in un derstanding the uncertainty of measurement on paved surfaces. 4. Conclusions This study measured depth profiles of 137Cs for asphalt cores ob tained from five parking areas affected by the FDNPP accident. At the least, β was estimated to be less than 0.23 g cm 2. The variation in β, ranging from 0.1 to 0.23 g cm 2 led to a minor error (less than 5%) in the result of the spectral analysis of 137Cs inventory. This study also evalu ated the horizontal heterogeneity of the 137Cs inventory in 20 � 20 m
Fig. 6. Elevation map in the 20 m square at St. 2. Circles indicate the location of crack, and the white area indicates the area at which elevation data could not be obtained.
Fig. 8. Coefficient of variation as a function of grid size. 5
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square with a spatial resolution of 1 m2-cell; an uneven distribution with large variation (coefficient of variation ranged from 54 to 136%) was observed. The relationship between the variation in 137Cs inventory and the instruments’ fields of view was also evaluated. These information obtained in this study should be useful to validate the uncertainty of the investigation of radiocesium on paved surface.
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