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Measurement of air dose rates over a wide area around the Fukushima Dai-ichi Nuclear Power Plant through a series of car-borne surveys
Journal of Environmental Radioactivity 139 (2015) 266e280
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Journal of Environmental Radioactivity journal homepage: www.elsevier.com/locate/jenvrad
Measurement of air dose rates over a wide area around the Fukushima Dai-ichi Nuclear Power Plant through a series of car-borne surveys Masaki Andoh a, *, Yukio Nakahara a, Shuichi Tsuda a, Tadayoshi Yoshida a, Norihiro Matsuda a, Fumiaki Takahashi a, Satoshi Mikami a, Nobuyuki Kinouchi a, Tetsuro Sato b, Minoru Tanigaki c, Koichi Takamiya c, Nobuhiro Sato c, Ryo Okumura c, Yukio Uchihori d, Kimiaki Saito a a
Japan Atomic Energy Agency, 2-2-2 Uchisaiwai-cho, Chiyoda-ku, Tokyo 100-8577, Japan Hitachi Solutions East Japan, Ltd., NBF Sendai Honcho Bldg., 2-16-10 Honcho, Aoba-ku, Sendai, Miyagi 980-0014, Japan c Kyoto University Research Reactor Institute, 2-1010 Asashiro-nishi, Kumatori, Sennan, Osaka 590-0494, Japan d National Institute of Radiological Sciences, 4-9-1 Anagawa, Inage-ku, Chiba-shi, Chiba 263-8555, Japan b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 30 January 2014 Received in revised form 12 May 2014 Accepted 21 May 2014 Available online 18 June 2014
A series of car-borne surveys using the Kyoto University RAdiation MApping (KURAMA) and KURAMAII survey systems has been conducted over a wide area in eastern Japan since June 2011 to evaluate the distribution of air dose rates around the Fukushima Dai-ichi Nuclear Power Plant and to evaluate the time-dependent trend of decrease in air dose rates. An automated data processing system for the KURAMA-II system was established, which enabled rapid analysis of large amounts of data obtained using about 100 KURAMA-II units. The initial data used for evaluating the migration status of radioactive cesium were obtained in the first survey, followed by other car-borne surveys conducted over more extensive and wider measurement ranges. By comparing the measured air dose rates obtained in each survey (until December 2012), the decreasing trend of air dose rates measured through car-borne surveys was found to be more pronounced than those expected on the basis of the physical decay of radioactive cesium and of the air dose rates measured using NaI (Tl) survey meters in the areas surrounding the roadways. In addition, it was found that the extent of decrease in air dose rates depended on land use, wherein it decreased faster for land used as building sites than for forested areas. © 2014 Elsevier Ltd. All rights reserved.
Keywords: Air dose rate Car-borne survey KURAMA KURAMA-II CsI (Tl) scintillation detector Fukushima Dai-ichi NPP accident
1. Introduction Mobile survey system using vehicles equipped with GPS navigation and gamma-ray spectrometers (i.e., car-borne survey) consists of an instrumentation with crystal packs smaller than those used for airborne survey systems (Sanada et al., 2013). Such a system is useful for evaluating the air dose rate distribution over a wide area for a given time and cost. After the Chernobyl NPP accident, car-borne surveys were conducted in and around the Chernobyl area (Arvela et al., 1990; Sakamoto and Tsutsumi, 1999). The Kyoto University RAdiation MApping System (KURAMA; Tanigaki et al., 2013) is a small car-borne gamma-ray survey system connected to a mobile personal computer (PC), which has * Corresponding author. Tel.: þ81 3 3592 2683. E-mail address: [email protected] (M. Andoh). http://dx.doi.org/10.1016/j.jenvrad.2014.05.014 0265-931X/© 2014 Elsevier Ltd. All rights reserved.
been designed to facilitate car-borne surveys. This system has been succeeded by the KURAMA-II system, which is a simplified (all-in-one) car-borne survey system that uses a built-in computer. These systems are very useful for constructing air dose rate maps over short periods of time without using specially equipped vehicles. Mapping projects have been conducted after the accident in the Fukushima Dai-ichi Nuclear Power Plant (NPP) (Saito, 2014). Many kinds of measurements have been performed in the projects to make detailed maps in order to predict the distribution of radioactive materials in the future. The KURAMA and KURAMA-II carborne survey systems were employed in the mapping projects to construct air dose rate maps over a wide range of eastern Japan. The mapping projects consisted of three series of investigations up until the 2012 fiscal year, and the corresponding car-borne surveys were conducted across five time periods as a part of these mapping projects, which are as follows.
M. Andoh et al. / Journal of Environmental Radioactivity 139 (2015) 266e280
The first mapping project, “Establishment of the Base for Taking Measures for Environmental Impact of Radioactive Substances e Study on Distribution of Radioactive Substances”1 was conducted from June to November 2011. In this project, the first car-borne survey was conducted to obtain the initial data for evaluating the migration status of radioactive materials released by the Fukushima Dai-ichi NPP accident. This survey was conducted by Japan Atomic Energy Agency (JAEA). The second mapping project, “Investigation and Study of the Secondary Distribution of Radioactive Substances due to the Accident at the Fukushima Daiichi Nuclear Power Plant”2 was conducted from November 2011 to March 2012. In this project, the second and third car-borne surveys were conducted to determine the distribution of air dose rates over a wide area of eastern Japan. The second survey was again conducted by JAEA, whereas the third survey was conducted by local governments in 10 prefectures in cooperation with JAEA (during data processing). The third mapping project, “Establishment of Technique for Identifying Long-term Impact of Radioactive Substances due to the Accident at the Fukushima Dai-ichi Nuclear Power Plant”3 was conducted from June 2012 to March 2013. In this project, the fourth and fifth carborne surveys were conducted to investigate in detail the distribution of radioactive cesium in the environment and to contribute to the creation of a model that would delineate the changing trends of air dose rates. In these surveys, JAEA performed measurements on main arterial roads, whereas the local governments performed detailed measurements on the roads including minor ones. The purpose of this study is to evaluate the distribution of air dose rates around the Fukushima Dai-ichi NPP over the region of eastern Japan exhibiting certain contamination levels and to determine the trend of decrease in air dose rates up until December 2012. The details of the measurement systems and methods used for evaluating the air dose rates in this study are described in Section 2. The measurement results are discussed in Section 3. Finally, the results are summarized in Section 4. 2. Measurement systems and methods 2.1. Detector and measurement method The first and second car-borne surveys were conducted (until December 2011) using the KURAMA system developed by Kyoto University. The KURAMA system obtains the ambient dose equivalent rate H*(10). In this paper, the obtained value is represented as the “air dose rate.” The KURAMA system shown in Fig. 1 provides for the simultaneous output of position coordinate data using GPS and air dose rates from the analog output of a NaI (Tl) survey meter (TCS-161, Hitachi Aloka Medical, Mitaka) connected to a mobile PC. An ionization-chamber-type survey meter was used in addition to the NaI (Tl) survey meter mainly inside a 20 km radius from the Fukushima Dai-ichi NPP where measured air dose rates were greater than 30 mSv/h. The KURAMA system was installed in a vehicle with the detection section of the meter set indoors at the top of the rear seat such that the survey meter was arranged to align with the near center of the roadway. The air dose rates were automatically measured using the survey meter, and the output was sent every 5 s to the PC through an interface box (for the first survey, the output was calculated every 10 s) along with the
1 The report is available at http://radioactivity.nsr.go.jp/ja/contents/6000/5235/ view.html (in Japanese). 2 The report is available at http://fukushima.jaea.go.jp/initiatives/cat03/entry02. html (in Japanese). 3 The report is available at http://fukushima.jaea.go.jp/initiatives/cat03/entry05. html (in Japanese).
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Fig. 1. The KURAMA system.
position coordinate information recorded by GPS. Fig. 2 shows the measurement scheme of the air dose rates conducted using the KURAMA system. The acquired data were transferred to a “gateway server” from the PC via a mobile line. The measurement data transmitted to the gateway server were stored in a “dropbox” using cloud computing. The data stored in the drop box were processed and stored in “data conversion servers” for visualizing using Google Earth (Google Inc.), and the measured values were displayed by superimposing them on the images obtained using Google Earth installed on a PC in order to check the measurement status in nearly real time. The data being sent to the data conversion servers were also processed to correct or delete outliers after daily measurements. The second version of the KURAMA system, KURAMA-II, was also developed by Kyoto University. It was a simplified survey system that can be operated even by users with no technical knowledge of radiation measurements (e.g., to set up the survey meter). As shown in Fig. 3, a thallium-activated cesium iodide (CsI (Tl)) scintillation detector, which has performance similar to the NaI (Tl) scintillation survey meter of the KURAMA system, a data transmission substrate set, and a data processing unit (a built-in computer) were installed in a tool box 35 cm (length) 15 cm (side) 17 cm (height). The crystal size of the CsI (Tl) scintillation detector (C12137, Hamamatsu Photonics, Hamamatsu) was 13 mm 13 mm 20 mm. Optical signals from the scintillator were amplified using a multi-pixel photon counter, and it was possible to measure photons in the range of 30e2000 keV. The
Fig. 2. Data processing scheme used by the KURAMA and KURAMA-II systems.
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Ratio of the air dose rate (outside/inside)
1.6
1.5
1.4
1.3
1.2
1.1
1.0 0
5
10
15
20
25
Air dose rate inside the car (µSv/h) Fig. 3. The KURAMA-II system.
KURAMA-II system measurements were performed by installing the unit behind the headrest on the rear seat of the car similar to that done for the KURAMA system survey meter. Air dose rate data, evaluated every 3 s using the measured pulse height spectrum distribution data and the spectrum dose conversion operator (G(E) function; Tsuda and Tsutsumi, 2012) incorporated in the data processing program, were sent to the gateway server through a 3G line network along with the position coordinate information recorded by GPS. Data processing after transmission to the gateway server was similar to that conducted using the KURAMA system. The statistical error of the air dose rate measurement obtained every 3 s was about 20% of the background level (0.1 mSv/h). The gamma ray detecting characteristics of the KURAMA-II system are described in detail in Tsuda et al. (2014). The KURAMA-II system was used from the third car-borne survey onwards (from March 2012).
Fig. 4. Measured correction factors for the first car-borne survey measurement. The data were obtained in June and July 2011.
tendency to become smaller when the measured air dose rates were relatively low. It was considered that the contribution ratio of the high-energy gamma rays of natural background radiation was relatively high at low air dose rates, and thus, the CF was reduced. In light of the fact that the second car-borne survey was expanded to regions where the air dose rates were lower than those in the first survey, the CF corresponding to low dose rates was reevaluated. In February 2012, the ratios were measured again as before mainly in areas where air dose rates were lower than 1.0 mSv/h. The ratios measured in June and July 2011 and February 2012 are shown in Fig. 5 with the CF determined as follows. The CF for low air dose rates (0.01e1.0 mSv/h) was determined as shown in Eq. (1).
CF ¼ 0:06515lnðDÞþ1:3; whenD ¼ 0:011:0 mSv=h;
(1)
2.2. Correction factor for deriving air dose rates outside of the car where D is the air dose rate inside the car. For air dose rates greater than 1.0 mSv/h and for those smaller than 0.01 mSv/h, the
1.5 Ratio of the air dose rate (outside/inside)
Environmental radiation is usually measured at a height of 1 m from the ground surface; however, the car-borne surveys using the KURAMA or KURAMA-II systems were conducted with the detector installed within the car, and the gamma rays from the surrounding environment were partially shielded by the car body. Therefore, a correction factor (CF) was required to match the air dose rates measured by car-borne surveys with those existing outside the car. For an accurate air dose rate evaluation of the areas adjacent to the roadways based upon data collected from car-borne surveys, a CF should be determined that considers the width of the roadway and the surrounding land usage (Sakamoto and Saito, 2003). In this study, the air dose rate was evaluated at a height of 1 m from near the center of the roadway, and only the shielding effect of the car body was considered. To obtain the CF, air dose rates were measured by a NaI (Tl) survey meter both inside and outside of the car in the same positions at a number of open areas where radioactive cesium was uniformly deposited, and the measurement ratios of air dose rates (outside/inside) were evaluated. The measured ratios are shown in Fig. 4. As a result, the ratio of the air dose rates outside the car (height: 1 m from the ground surface) to those inside the car (rear of the back seat nearby the center of the roadway) was about 1.3, and this value was adopted for the CF in the first carborne survey. It was observed that the air dose rate ratio had a
1.4 CF = 0.06515 ln(D) + 1.3 1.3
1.2
1.1
1.0 -2 10
-1
10
0
10
1
10
2
10
Air dose rate inside the car (µSv/h) Fig. 5. Evaluation of the correction factor for the car-borne survey measurements corresponding to low dose rates.
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corresponding CFs were determined to be constants as shown in Eqs. (2) and (3):
CF ¼ 1:3;
when D > 1:0 mSv=h;
(2)
CF ¼ 1:0;
when D < 0:01 mSv=h:
(3)
CFs that depend on the air dose rates were used for the second carborne survey onwards. The evaluation of CF appropriate for low radiation levels of around 0.1 mSv/h is a future subject of study. However, it should be noted that the CF values for most of the measurements varied from 1.2 to 1.3, and the uncertainty in the CF had little effect on the results of the car-borne surveys. 2.3. Development of the automated data processing system The KURAMA system measurements were manually checked from several viewpoints; air dose rates that significantly differed from those measured before and after (such as data spikes) were excluded by checking the measured time-series data, data measured in tunnels were excluded since data collected there gave no indication of radiation derived from contamination. However, about a hundred units of the KURAMA-II system were used in measuring the air dose rates; therefore, about a million data points were immediately analyzed to rapidly provide local governments with the results. To achieve this, we developed an automated data processing system that automatically performed the correction or deletion of outliers in the measured data. The automated data processing system performed the following processes for the KURAMA-II system measurements: Outlier deletion: B The over-range data of the detector were ascertained and excluded. B Data spikes due to noise (sharply increased air dose rate values) were detected and deleted. B The measurement times were examined, and repeated data values, except for their first occurrences, were deleted. Tunnel data deletion: B The data measured in tunnels were ascertained using GIS data4 and were excluded. Position correction tool: B The data deriving from positions off course were detected using GIS data, the positions of such data within a predetermined range from the roadway were corrected to the nearest roadway, and those beyond the range were excluded as outliers. Mesh display tool B The measured data in the low-air-dose-rate areas included large statistical errors. To reduce these errors, the measured regions were divided into meshes, and the measured data were expressed as representative values of the meshes by averaging the air dose rate values at the measurement points in the mesh. In this study, the mesh area was set to
4 Geographic Information System (GIS) data is distributed by Geospatial Information Authority of Japan, Ministry of Land, Infrastructure, Transport and Tourism.
Fig. 6. Air dose rate distribution map obtained from the first car-borne survey (June 4e13, 2011).
100 m 100 m by considering that the gamma rays originating from a surrounding area of about a 60-m radius where radioactive cesium was deposited on the land surface contributed to approximately 90% of the air dose rate at a height of 1 m.
2.4. Measurement areas and periods In the mapping projects, in addition to car-borne surveys, the air dose rates were measured using NaI (Tl) survey meters (Mikami et al., 2014), and soil sampling analysis was performed (Saito et al., 2014) around the Fukushima Dai-ichi NPP. The air dose rates were measured at a height of 1 m in undisturbed flat regions where major obstacles did not exist in circumference of 10 m, and gamma rays from the radioactive materials deposited on the ground were dominant. The measurement area of the first car-borne survey was set with respect to the areas where air dose rate measurements using NaI (Tl) survey meters or soil sampling analysis were performed. The initial car-borne measurements were conducted along major roadways, mainly within 100 km of the Fukushima Dai-ichi NPP, from June 4 to 13, 2011. The total mileage was about 17,000 km. After the first car-borne survey measurements were made, it was noted from aircraft monitoring results (Nuclear Regulation Authority, 2011) that the deposition range of radioactive cesium extended far beyond 100 km from the Fukushima Dai-ichi NPP and that detailed data were required for areas where decontamination may be performed. As a result,
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Fig. 7. Air dose rate distribution map obtained from the second car-borne survey (December 5e28, 2011).
areas exhibiting levels of 0.2 mSv/h or more, including those surrounding the Fukushima Prefecture, were subjected to the second survey, and the air dose rates over a wide area (11 prefectures) were measured. The measurements were made along the main roadways from December 5 to 28, 2011, and the total mileage was about 40,000 km. Ten KURAMA systems were used in the first and second car-borne surveys. In the third car-borne survey, the measurements were made from March 13 to 30, 2012 by the local governments of 10 prefectures, and the survey included minor local roads. The total mileage was about 70,000 km. In the fourth and fifth car-borne surveys, measurements along the main arterial roadways were made by JAEA, and detailed measurements for roadways including minor ones that were of interest to local governments were conducted in the same way as the third car-borne survey. The measurements were made for 13 prefectures. The fourth survey was conducted from August 20 to October 12, 2012 and the fifth survey from November 5 to December 10, 2012. The total mileage of the roadway measured was about 85,000 km and 65,000 km in the fourth and fifth surveys, respectively.
Fig. 8. Air dose rate distribution map obtained from the third car-borne survey (March 13e30, 2012).
results for within 80 km of the Fukushima Dai-ichi NPP are shown in Appendix. Comparing the various air dose rate maps, a decreasing trend in the air dose rates can be seen. Although measurement areas were extended to the north and southwest of the Fukushima Dai-ichi NPP in the second survey, data were not obtained in northwestern Japan due to snow. The measurement areas in the third survey were unevenly distributed since those were determined by local governments. In the fourth survey, the measurement areas were extended to the west and north of the Fukushima Dai-ichi NPP. It is clear that the density of measurement areas after the third survey was higher than that of previous surveys since the number of measurement devices was increased to about 100 (KURAMA-II). It is recognized that the measured air dose rates in the area between Iwate Prefecture and Miyagi Prefecture were higher than those in the peripheral areas owing to the larger measurement areas involved in the fourth car-borne survey (see the circled area shown in Fig. 9).
3.2. Trend in the air dose rate 3. Results and discussion 3.1. Measurement results Air dose rate maps for the entire area obtained from the measurements of the first to fifth car-borne surveys are shown in Figs. 6e10, respectively, whereas those showing measurement
Several trends can be observed by superimposing the air dose rate data obtained by the car-borne surveys onto topographic maps. Fig. 11 shows the air dose rate data superimposed onto a Google Earth image of the first car-borne survey measured several kilometers northwest of the Fukushima Dai-ichi NPP, in which direction a high-concentration radioactive plume was
M. Andoh et al. / Journal of Environmental Radioactivity 139 (2015) 266e280
Fig. 9. Air dose rate distribution map obtained from the fourth car-borne survey (August 20eOctober 12, 2012). A red circle shows the area between Iwate Prefecture and Miyagi Prefecture where the air dose rates were relatively high. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Fig. 10. Air dose rate distribution map obtained from the fifth car-borne survey (November 5eDecember 10, 2012).
3.3. Decreasing trend of the air dose rate emitted on the afternoon of March 15, 2011 and where radioactive contamination was considered to be deposited according to the JAEA estimation using SPEEDI (Chino et al., 2011). High dose rate data indicated by red continuously appear in the direction of the radioactive plume flow, whereas rapid decreases in the air dose rate are observed with increasing distance along the route where the radioactive plume moved. The air dose rate distribution remarkably features the contamination conditions at the time of the accident, i.e., contamination varied substantially depending on the location. Furthermore, the relationship between the air dose rate distribution and topography could be derived. Fig. 12, a result of the first car-borne survey conducted in the mountainous areas located to the northwest of the Fukushima Dai-ichi NPP, shows that the air dose rate is clearly different in the south and north regions of a mountain ridge, from which it can be inferred that the radioactive plume flow bypassed the north ridge. In addition, the results of the first car-borne survey in the northwest (the mountainous area) of the cities Nihonmatsu and Fukushima (about 70 km west of the Fukushima Dai-ichi NPP) indicate that the air dose rate gradually decreased with increasing altitude from the basin to the mountaintop, as shown in Fig. 13. As mentioned above, the measured data contribute toward the analysis of plume flow or path of plume deposition caused by the accident.
The correlation among the air dose rate measurement results obtained in the same mesh used in the first (June 2011), second (December 2011), and third (March 2012) car-borne surveys is shown in Fig. 14 and that obtained in the same mesh used in the first, fourth (August to October 2012), and fifth (November to December 2012) car-borne surveys is shown in Fig. 15. The air dose rates in the second car-borne survey decreased by 30% in comparison with those in the first survey, as indicated by the slope of the regression line. Likewise, the air dose rates in the third survey decreased by 39%, the fourth by 48%, and the fifth by 56%. The slope of the regression line is easily affected by high dose rate data, and the slope does not always show a decreasing trend in the dose rate precisely. Therefore, the summation of air dose rates (referred to as the “integral value”) was calculated, and the ratios of the integral values (referred to as the “total dose rate ratio”) are shown in Table 1. As observed from the total dose rate ratios, the air dose rates from the second to the fifth car-borne surveys decreased in comparison with those in the first survey by 34%, 41%, 53%, and 58%, respectively. When we compare the total dose rate ratios (shown in Table 1) with the slope of the regression lines (shown in Figs. 14 and 15), the differences in the decrease of the air dose rates was from 3% to 11%. Fig. 16 shows the decreasing tendency of the total dose rate ratios measured by the car-borne surveys and that calculated by the physical decay of radioactive cesium. It was confirmed that the trend of decrease in the air dose rates from the car-borne surveys
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Fig. 11. Three-dimensional image display that overlays the results of the first car-borne survey (1) northwest of the Fukushima Dai-ichi NPP. Background map is the Google Earth map.
was considerably larger than that calculated by the physical decay of radioactive cesium. Land usage was classified on the basis of the national land information provided by National Land Numerical Information, Ministry of Land, Infrastructure, Transport and Tourism. The integral values and total dose rate ratios determined by the car-borne surveys were evaluated according to land usage in the surrounding areas such as forest area, building sites, paddy field, farmlands, and others, as listed in Table 2. It was found that the observed decrease in the air dose rates depended on land usage, wherein the trend of decrease was less pronounced in forest areas and more pronounced in areas classified as building sites. From the decreasing tendency of the air dose rate described above, the migration of radioactive cesium on the road was found to proceed more quickly owing to human activities and vehicle traffic.
3.4. Comparison of car-borne survey data on roadways with the mapping project data surrounding roadways Air dose rates along the center of the roadway at a height of 1 m were obtained by the car-borne surveys. However, the air dose rates using the NaI (Tl) survey meters in the mapping projects were measured at a height of 1 m in undisturbed flat regions surrounding the roadways. To determine the relationship between the air dose rates along the roadway and those in the surrounding regions, the air dose rates obtained by the fourth and fifth car-borne surveys using the KURAMA-II systems were compared with those measured by the NaI (Tl) survey meter (Mikami et al., 2014) obtained in the same meshes. Fig. 17 shows a clear-cut correlation between the car-borne survey measurements (the fourth and fifth) and the air dose rate measurements using NaI (Tl) survey meters
Fig. 12. Three-dimensional image display that overlays the results of the first car-borne survey (2) in the mountainous region located to the northwest of the Fukushima Dai-ichi NPP. Background map is the Google Earth map.
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Fig. 13. Three-dimensional image display that overlays the results of the first car-borne survey (3) viewed northwest from about 70 km west of the Fukushima Dai-ichi NPP. Background map is the Google Earth map.
made during the same time period. The slope of the regression line shows that the air dose rates of the surrounding regions measured in the two periods of the third mapping project were 1.24 and 1.41 times larger than those of the fourth and fifth car-borne surveys, respectively. It is possible to estimate the air dose rates in the surroundings using the results of car-borne surveys and the values of the slope of the regression lines. In addition, the decreasing tendency of the air dose rates measured by the NaI (Tl) survey meters is also shown in Fig. 16 (shown as the first to the third mapping projects). Air dose rates on the roadways measured by the car-borne surveys showed values lower than those of the surrounding areas, and the ratio between those two measurements gradually increased with increasing time. From this, it is assumed that radioactive cesium on the roadway can be more easily removed, owing to the effect of weather (the so-
called “weathering effect”), traffic, etc., than that on the area surrounding the roadway. It is an important subject to identify which decreasing trend represents the environment for human life in order to precisely predict ambient dose equivalent rates in the future environment.
Fig. 14. Correlation diagram of the air dose rates between the first, second, and third surveys.
Fig. 15. Correlation diagram of the air dose rates between the first, fourth, and fifth surveys.
4. Conclusion A series of car-borne surveys using the KURAMA and KURAMAII systems has been conducted in a wide area of eastern Japan since June 2011 to evaluate the distribution of air dose rates over a wide area around of the Fukushima Dai-ichi NPP and to determine the trend of decrease in air dose rates. An automated data processing system for the air dose rate measurement using the KURAMA-II
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Table 1 Summation of the air dose rates of common meshes (integral values) for two groups of surveys (i.e., the first, second, and third, and the first, fourth, and fifth)a and the ratios of the integral values (total dose rate ratios) of each car-borne survey to the first survey. Car-borne survey period
Integral value (mSv/h) Ratio to the first survey
First
Second
Third
Fourth
Fifth
4.61 27.0 e
3.04 e 0.65
2.72 e 0.59
e 12.8 0.47
e 11.2 0.41
a
The first, second, third, fourth, and fifth surveys were conducted from June 4 to 13, 2011, from December 5 to 28, 2011, from March 13 to 30, 2012, from August 20 to October 12, 2012, and from November 5 to December 10, 2012, respectively.
system was established, which enabled the analysis of large amounts of collected data obtained with the use of about 100 KURAMA-II units. Initial data to study the migration status of the radioactive material was obtained, and its distribution maps were created from the measurement results using a 100 m mesh. These basic data enabled the precise analysis of the spread of deposition conditions of gamma-ray-emitting nuclides. It was found that the decreasing trend of the air dose rates measured by the car-borne surveys (until December 2012) was more pronounced than those calculated by the physical decay of radioactive cesium and that measured using NaI (Tl) survey meters in areas surrounding the roadways. In addition, the decreasing trend in the air dose rate was found to depend on land usage, wherein the trend was more pronounced for land used as building sites than that for forest areas.
Fig. 16. Comparison of the changing trends of air dose rates measured by car-borne surveys and those measured by NaI (Tl) survey meters.
Fig. 17. Comparison of the results of the air dose rate measurements at a height of 1 m measured by NaI (Tl) survey meters and those from car-borne surveys (left: the fourth car-borne survey; right: the fifth car-borne survey).
Acknowledgment Table 2 Integral values and the total dose rate ratios for each type of land use in the surrounding areas obtained from the common meshes among the first, third, and fourth car-borne surveys. Car-borne Integral value (mSv/h) and total dose rate ratioa survey period Total Forest area Building site Paddy field Farmland and others First Third Fourth
7.06 1.96 1.39 4.09 (0.58) 1.20 (0.61) 0.71 (0.51) 3.19 (0.45) 0.97 (0.49) 0.56 (0.40)
2.07 1.21 (0.58) 0.92 (0.44)
1.32 0.80 (0.61) 0.61 (0.46)
a Values in the parentheses are the total dose rate ratios relative to the first survey for each type of land use.
The authors are grateful to Dr. J. Ishida and Dr. S. Matsumoto of Headquarters of Fukushima Partnership Operation, JAEA, for their encouragement regarding this study. In addition, the authors wish to express their gratitude to the Japan Map Center for their contributions in mapping the measured data, to the KURAMA development team of Kyoto University Research Reactor Institute for their support in the car-borne survey measurements, to the Tokyo Electric Power Company and the JAEA staff for their cooperation in the car-borne survey measurements. This study was a part of the distribution-mapping projects financially supported by the Ministry of Education, Culture, Sports, Science, and Technology.
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Appendix
Fig. A.1. Air dose rate distribution map within 80 km of the Fukushima Dai-ichi NPP obtained by the first car-borne survey performed from June 4 to 13, 2011.
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Fig. A.2. Air dose rate distribution map within 80 km of the Fukushima Dai-ichi NPP obtained by the second car-borne survey performed from December 5 to 28, 2011.
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Fig. A.3. Air dose rate distribution map within 80 km of the Fukushima Dai-ichi NPP obtained by the third car-borne survey performed from March 13 to 30, 2012.
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Fig. A.4. Air dose rate distribution map within 80 km of the Fukushima Dai-ichi NPP obtained by the fourth car-borne survey performed from August 20 to October 12, 2012.
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Fig. A.5. Air dose rate distribution map within 80 km of the Fukushima Dai-ichi NPP obtained by the fifth car-borne survey performed from November 5 to December 10, 2012.
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References Arvela, H., Markkanen, M., Lemmela, H., 1990. Mobile survey of environmental gamma radiation and fall-out levels in Finland after the chernobyl accident. Radiat. Prot. Dosim. 32 (3), 177e184. Chino, M., Nakayama, H., Nagai, H., Terada, H., Katata, G., Yamazawa, H., 2011. Preliminary estimation of release amounts of 131I and 137Cs accidentally discharged from the Fukushima Dai-ichi Nuclear Power Plant into the atmosphere. J. Nucl. Sci. Technol. 48 (7), 1129e1134. Mikami, S., Maeyama, T., Hoshide, Y., Sakamoto, R., Sato, S., Okuda, N., Saito, K., 2014. Construction of air dose rate distribution maps around the Fukushima site and discussion on their temporal changes and spatial characteristics until December 2012. J. Environ. Radioact. submitted for publication. Saito, K., 2014. Mapping and modelling of radionuclide distribution on the ground due to the fukushima accident. Radiat. Prot. Dosim., 1e5. 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., 2014. 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. submitted for publication. Sakamoto, R., Saito, K., 2003. Conversion Factors for A Mobile survey method by car in the chernobyl area. Radiat. Prot. Dosim. 106 (2), 165e175.
Sakamoto, R., Tsutsumi, M., 1999. Development of carborne gamma-ray survey system. J. Atom. Energy Soc. Jpn. 41 (3), 116e123 (in Japanese). Sanada, Y., Kondo, A., Sugita, T., Nishizawa, Y., Yuuki, Y., Ikeda, K., Shoji, Y., Torii, T., 2013. Radiation monitoring using an unmanned helicopter in the evacuation zone around the Fukushima Daiichi nuclear power plant. Explor. Geophys. 45, 3e7. Tanigaki, M., Okumura, R., Takamiya, K., Sato, N., Yoshino, H., Yamana, H., 2013. Development of a car-borne g-ray survey system, KURAMA. Nucl. Instrum. Methods Phys. Res. A 726, 162e166. Tsuda, S., Tsutsumi, M., 2012. Calculation and verification of the spectrum e dose conversion operator of various CsI(Tl) scintillation counters for gamma-ray. Hoken Butsuri 47 (4), 260e265 (in Japanese). Tsuda, S., Yoshida, T., Tsutsumi, M., Saito, K., 2014. Characteristics and verification of a car-borne survey system for dose rates in air: KURAMA-II. J. Environ. Radioact. submitted for publication.
Web references Google Inc. Google Earth. http://www.google.com/earth/index.html. Nuclear Regulation Authority, 2011. Results of the Third Airborne Monitoring Survey by MEXT. http://radioactivity.nsr.go.jp/en/contents/5000/4182/24/ 1304797_0708e.pdf (Press release).
Contents lists available at ScienceDirect
Journal of Environmental Radioactivity journal homepage: www.elsevier.com/locate/jenvrad
Measurement of air dose rates over a wide area around the Fukushima Dai-ichi Nuclear Power Plant through a series of car-borne surveys Masaki Andoh a, *, Yukio Nakahara a, Shuichi Tsuda a, Tadayoshi Yoshida a, Norihiro Matsuda a, Fumiaki Takahashi a, Satoshi Mikami a, Nobuyuki Kinouchi a, Tetsuro Sato b, Minoru Tanigaki c, Koichi Takamiya c, Nobuhiro Sato c, Ryo Okumura c, Yukio Uchihori d, Kimiaki Saito a a
Japan Atomic Energy Agency, 2-2-2 Uchisaiwai-cho, Chiyoda-ku, Tokyo 100-8577, Japan Hitachi Solutions East Japan, Ltd., NBF Sendai Honcho Bldg., 2-16-10 Honcho, Aoba-ku, Sendai, Miyagi 980-0014, Japan c Kyoto University Research Reactor Institute, 2-1010 Asashiro-nishi, Kumatori, Sennan, Osaka 590-0494, Japan d National Institute of Radiological Sciences, 4-9-1 Anagawa, Inage-ku, Chiba-shi, Chiba 263-8555, Japan b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 30 January 2014 Received in revised form 12 May 2014 Accepted 21 May 2014 Available online 18 June 2014
A series of car-borne surveys using the Kyoto University RAdiation MApping (KURAMA) and KURAMAII survey systems has been conducted over a wide area in eastern Japan since June 2011 to evaluate the distribution of air dose rates around the Fukushima Dai-ichi Nuclear Power Plant and to evaluate the time-dependent trend of decrease in air dose rates. An automated data processing system for the KURAMA-II system was established, which enabled rapid analysis of large amounts of data obtained using about 100 KURAMA-II units. The initial data used for evaluating the migration status of radioactive cesium were obtained in the first survey, followed by other car-borne surveys conducted over more extensive and wider measurement ranges. By comparing the measured air dose rates obtained in each survey (until December 2012), the decreasing trend of air dose rates measured through car-borne surveys was found to be more pronounced than those expected on the basis of the physical decay of radioactive cesium and of the air dose rates measured using NaI (Tl) survey meters in the areas surrounding the roadways. In addition, it was found that the extent of decrease in air dose rates depended on land use, wherein it decreased faster for land used as building sites than for forested areas. © 2014 Elsevier Ltd. All rights reserved.
Keywords: Air dose rate Car-borne survey KURAMA KURAMA-II CsI (Tl) scintillation detector Fukushima Dai-ichi NPP accident
1. Introduction Mobile survey system using vehicles equipped with GPS navigation and gamma-ray spectrometers (i.e., car-borne survey) consists of an instrumentation with crystal packs smaller than those used for airborne survey systems (Sanada et al., 2013). Such a system is useful for evaluating the air dose rate distribution over a wide area for a given time and cost. After the Chernobyl NPP accident, car-borne surveys were conducted in and around the Chernobyl area (Arvela et al., 1990; Sakamoto and Tsutsumi, 1999). The Kyoto University RAdiation MApping System (KURAMA; Tanigaki et al., 2013) is a small car-borne gamma-ray survey system connected to a mobile personal computer (PC), which has * Corresponding author. Tel.: þ81 3 3592 2683. E-mail address: [email protected] (M. Andoh). http://dx.doi.org/10.1016/j.jenvrad.2014.05.014 0265-931X/© 2014 Elsevier Ltd. All rights reserved.
been designed to facilitate car-borne surveys. This system has been succeeded by the KURAMA-II system, which is a simplified (all-in-one) car-borne survey system that uses a built-in computer. These systems are very useful for constructing air dose rate maps over short periods of time without using specially equipped vehicles. Mapping projects have been conducted after the accident in the Fukushima Dai-ichi Nuclear Power Plant (NPP) (Saito, 2014). Many kinds of measurements have been performed in the projects to make detailed maps in order to predict the distribution of radioactive materials in the future. The KURAMA and KURAMA-II carborne survey systems were employed in the mapping projects to construct air dose rate maps over a wide range of eastern Japan. The mapping projects consisted of three series of investigations up until the 2012 fiscal year, and the corresponding car-borne surveys were conducted across five time periods as a part of these mapping projects, which are as follows.
M. Andoh et al. / Journal of Environmental Radioactivity 139 (2015) 266e280
The first mapping project, “Establishment of the Base for Taking Measures for Environmental Impact of Radioactive Substances e Study on Distribution of Radioactive Substances”1 was conducted from June to November 2011. In this project, the first car-borne survey was conducted to obtain the initial data for evaluating the migration status of radioactive materials released by the Fukushima Dai-ichi NPP accident. This survey was conducted by Japan Atomic Energy Agency (JAEA). The second mapping project, “Investigation and Study of the Secondary Distribution of Radioactive Substances due to the Accident at the Fukushima Daiichi Nuclear Power Plant”2 was conducted from November 2011 to March 2012. In this project, the second and third car-borne surveys were conducted to determine the distribution of air dose rates over a wide area of eastern Japan. The second survey was again conducted by JAEA, whereas the third survey was conducted by local governments in 10 prefectures in cooperation with JAEA (during data processing). The third mapping project, “Establishment of Technique for Identifying Long-term Impact of Radioactive Substances due to the Accident at the Fukushima Dai-ichi Nuclear Power Plant”3 was conducted from June 2012 to March 2013. In this project, the fourth and fifth carborne surveys were conducted to investigate in detail the distribution of radioactive cesium in the environment and to contribute to the creation of a model that would delineate the changing trends of air dose rates. In these surveys, JAEA performed measurements on main arterial roads, whereas the local governments performed detailed measurements on the roads including minor ones. The purpose of this study is to evaluate the distribution of air dose rates around the Fukushima Dai-ichi NPP over the region of eastern Japan exhibiting certain contamination levels and to determine the trend of decrease in air dose rates up until December 2012. The details of the measurement systems and methods used for evaluating the air dose rates in this study are described in Section 2. The measurement results are discussed in Section 3. Finally, the results are summarized in Section 4. 2. Measurement systems and methods 2.1. Detector and measurement method The first and second car-borne surveys were conducted (until December 2011) using the KURAMA system developed by Kyoto University. The KURAMA system obtains the ambient dose equivalent rate H*(10). In this paper, the obtained value is represented as the “air dose rate.” The KURAMA system shown in Fig. 1 provides for the simultaneous output of position coordinate data using GPS and air dose rates from the analog output of a NaI (Tl) survey meter (TCS-161, Hitachi Aloka Medical, Mitaka) connected to a mobile PC. An ionization-chamber-type survey meter was used in addition to the NaI (Tl) survey meter mainly inside a 20 km radius from the Fukushima Dai-ichi NPP where measured air dose rates were greater than 30 mSv/h. The KURAMA system was installed in a vehicle with the detection section of the meter set indoors at the top of the rear seat such that the survey meter was arranged to align with the near center of the roadway. The air dose rates were automatically measured using the survey meter, and the output was sent every 5 s to the PC through an interface box (for the first survey, the output was calculated every 10 s) along with the
1 The report is available at http://radioactivity.nsr.go.jp/ja/contents/6000/5235/ view.html (in Japanese). 2 The report is available at http://fukushima.jaea.go.jp/initiatives/cat03/entry02. html (in Japanese). 3 The report is available at http://fukushima.jaea.go.jp/initiatives/cat03/entry05. html (in Japanese).
267
Fig. 1. The KURAMA system.
position coordinate information recorded by GPS. Fig. 2 shows the measurement scheme of the air dose rates conducted using the KURAMA system. The acquired data were transferred to a “gateway server” from the PC via a mobile line. The measurement data transmitted to the gateway server were stored in a “dropbox” using cloud computing. The data stored in the drop box were processed and stored in “data conversion servers” for visualizing using Google Earth (Google Inc.), and the measured values were displayed by superimposing them on the images obtained using Google Earth installed on a PC in order to check the measurement status in nearly real time. The data being sent to the data conversion servers were also processed to correct or delete outliers after daily measurements. The second version of the KURAMA system, KURAMA-II, was also developed by Kyoto University. It was a simplified survey system that can be operated even by users with no technical knowledge of radiation measurements (e.g., to set up the survey meter). As shown in Fig. 3, a thallium-activated cesium iodide (CsI (Tl)) scintillation detector, which has performance similar to the NaI (Tl) scintillation survey meter of the KURAMA system, a data transmission substrate set, and a data processing unit (a built-in computer) were installed in a tool box 35 cm (length) 15 cm (side) 17 cm (height). The crystal size of the CsI (Tl) scintillation detector (C12137, Hamamatsu Photonics, Hamamatsu) was 13 mm 13 mm 20 mm. Optical signals from the scintillator were amplified using a multi-pixel photon counter, and it was possible to measure photons in the range of 30e2000 keV. The
Fig. 2. Data processing scheme used by the KURAMA and KURAMA-II systems.
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Ratio of the air dose rate (outside/inside)
1.6
1.5
1.4
1.3
1.2
1.1
1.0 0
5
10
15
20
25
Air dose rate inside the car (µSv/h) Fig. 3. The KURAMA-II system.
KURAMA-II system measurements were performed by installing the unit behind the headrest on the rear seat of the car similar to that done for the KURAMA system survey meter. Air dose rate data, evaluated every 3 s using the measured pulse height spectrum distribution data and the spectrum dose conversion operator (G(E) function; Tsuda and Tsutsumi, 2012) incorporated in the data processing program, were sent to the gateway server through a 3G line network along with the position coordinate information recorded by GPS. Data processing after transmission to the gateway server was similar to that conducted using the KURAMA system. The statistical error of the air dose rate measurement obtained every 3 s was about 20% of the background level (0.1 mSv/h). The gamma ray detecting characteristics of the KURAMA-II system are described in detail in Tsuda et al. (2014). The KURAMA-II system was used from the third car-borne survey onwards (from March 2012).
Fig. 4. Measured correction factors for the first car-borne survey measurement. The data were obtained in June and July 2011.
tendency to become smaller when the measured air dose rates were relatively low. It was considered that the contribution ratio of the high-energy gamma rays of natural background radiation was relatively high at low air dose rates, and thus, the CF was reduced. In light of the fact that the second car-borne survey was expanded to regions where the air dose rates were lower than those in the first survey, the CF corresponding to low dose rates was reevaluated. In February 2012, the ratios were measured again as before mainly in areas where air dose rates were lower than 1.0 mSv/h. The ratios measured in June and July 2011 and February 2012 are shown in Fig. 5 with the CF determined as follows. The CF for low air dose rates (0.01e1.0 mSv/h) was determined as shown in Eq. (1).
CF ¼ 0:06515lnðDÞþ1:3; whenD ¼ 0:011:0 mSv=h;
(1)
2.2. Correction factor for deriving air dose rates outside of the car where D is the air dose rate inside the car. For air dose rates greater than 1.0 mSv/h and for those smaller than 0.01 mSv/h, the
1.5 Ratio of the air dose rate (outside/inside)
Environmental radiation is usually measured at a height of 1 m from the ground surface; however, the car-borne surveys using the KURAMA or KURAMA-II systems were conducted with the detector installed within the car, and the gamma rays from the surrounding environment were partially shielded by the car body. Therefore, a correction factor (CF) was required to match the air dose rates measured by car-borne surveys with those existing outside the car. For an accurate air dose rate evaluation of the areas adjacent to the roadways based upon data collected from car-borne surveys, a CF should be determined that considers the width of the roadway and the surrounding land usage (Sakamoto and Saito, 2003). In this study, the air dose rate was evaluated at a height of 1 m from near the center of the roadway, and only the shielding effect of the car body was considered. To obtain the CF, air dose rates were measured by a NaI (Tl) survey meter both inside and outside of the car in the same positions at a number of open areas where radioactive cesium was uniformly deposited, and the measurement ratios of air dose rates (outside/inside) were evaluated. The measured ratios are shown in Fig. 4. As a result, the ratio of the air dose rates outside the car (height: 1 m from the ground surface) to those inside the car (rear of the back seat nearby the center of the roadway) was about 1.3, and this value was adopted for the CF in the first carborne survey. It was observed that the air dose rate ratio had a
1.4 CF = 0.06515 ln(D) + 1.3 1.3
1.2
1.1
1.0 -2 10
-1
10
0
10
1
10
2
10
Air dose rate inside the car (µSv/h) Fig. 5. Evaluation of the correction factor for the car-borne survey measurements corresponding to low dose rates.
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corresponding CFs were determined to be constants as shown in Eqs. (2) and (3):
CF ¼ 1:3;
when D > 1:0 mSv=h;
(2)
CF ¼ 1:0;
when D < 0:01 mSv=h:
(3)
CFs that depend on the air dose rates were used for the second carborne survey onwards. The evaluation of CF appropriate for low radiation levels of around 0.1 mSv/h is a future subject of study. However, it should be noted that the CF values for most of the measurements varied from 1.2 to 1.3, and the uncertainty in the CF had little effect on the results of the car-borne surveys. 2.3. Development of the automated data processing system The KURAMA system measurements were manually checked from several viewpoints; air dose rates that significantly differed from those measured before and after (such as data spikes) were excluded by checking the measured time-series data, data measured in tunnels were excluded since data collected there gave no indication of radiation derived from contamination. However, about a hundred units of the KURAMA-II system were used in measuring the air dose rates; therefore, about a million data points were immediately analyzed to rapidly provide local governments with the results. To achieve this, we developed an automated data processing system that automatically performed the correction or deletion of outliers in the measured data. The automated data processing system performed the following processes for the KURAMA-II system measurements: Outlier deletion: B The over-range data of the detector were ascertained and excluded. B Data spikes due to noise (sharply increased air dose rate values) were detected and deleted. B The measurement times were examined, and repeated data values, except for their first occurrences, were deleted. Tunnel data deletion: B The data measured in tunnels were ascertained using GIS data4 and were excluded. Position correction tool: B The data deriving from positions off course were detected using GIS data, the positions of such data within a predetermined range from the roadway were corrected to the nearest roadway, and those beyond the range were excluded as outliers. Mesh display tool B The measured data in the low-air-dose-rate areas included large statistical errors. To reduce these errors, the measured regions were divided into meshes, and the measured data were expressed as representative values of the meshes by averaging the air dose rate values at the measurement points in the mesh. In this study, the mesh area was set to
4 Geographic Information System (GIS) data is distributed by Geospatial Information Authority of Japan, Ministry of Land, Infrastructure, Transport and Tourism.
Fig. 6. Air dose rate distribution map obtained from the first car-borne survey (June 4e13, 2011).
100 m 100 m by considering that the gamma rays originating from a surrounding area of about a 60-m radius where radioactive cesium was deposited on the land surface contributed to approximately 90% of the air dose rate at a height of 1 m.
2.4. Measurement areas and periods In the mapping projects, in addition to car-borne surveys, the air dose rates were measured using NaI (Tl) survey meters (Mikami et al., 2014), and soil sampling analysis was performed (Saito et al., 2014) around the Fukushima Dai-ichi NPP. The air dose rates were measured at a height of 1 m in undisturbed flat regions where major obstacles did not exist in circumference of 10 m, and gamma rays from the radioactive materials deposited on the ground were dominant. The measurement area of the first car-borne survey was set with respect to the areas where air dose rate measurements using NaI (Tl) survey meters or soil sampling analysis were performed. The initial car-borne measurements were conducted along major roadways, mainly within 100 km of the Fukushima Dai-ichi NPP, from June 4 to 13, 2011. The total mileage was about 17,000 km. After the first car-borne survey measurements were made, it was noted from aircraft monitoring results (Nuclear Regulation Authority, 2011) that the deposition range of radioactive cesium extended far beyond 100 km from the Fukushima Dai-ichi NPP and that detailed data were required for areas where decontamination may be performed. As a result,
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Fig. 7. Air dose rate distribution map obtained from the second car-borne survey (December 5e28, 2011).
areas exhibiting levels of 0.2 mSv/h or more, including those surrounding the Fukushima Prefecture, were subjected to the second survey, and the air dose rates over a wide area (11 prefectures) were measured. The measurements were made along the main roadways from December 5 to 28, 2011, and the total mileage was about 40,000 km. Ten KURAMA systems were used in the first and second car-borne surveys. In the third car-borne survey, the measurements were made from March 13 to 30, 2012 by the local governments of 10 prefectures, and the survey included minor local roads. The total mileage was about 70,000 km. In the fourth and fifth car-borne surveys, measurements along the main arterial roadways were made by JAEA, and detailed measurements for roadways including minor ones that were of interest to local governments were conducted in the same way as the third car-borne survey. The measurements were made for 13 prefectures. The fourth survey was conducted from August 20 to October 12, 2012 and the fifth survey from November 5 to December 10, 2012. The total mileage of the roadway measured was about 85,000 km and 65,000 km in the fourth and fifth surveys, respectively.
Fig. 8. Air dose rate distribution map obtained from the third car-borne survey (March 13e30, 2012).
results for within 80 km of the Fukushima Dai-ichi NPP are shown in Appendix. Comparing the various air dose rate maps, a decreasing trend in the air dose rates can be seen. Although measurement areas were extended to the north and southwest of the Fukushima Dai-ichi NPP in the second survey, data were not obtained in northwestern Japan due to snow. The measurement areas in the third survey were unevenly distributed since those were determined by local governments. In the fourth survey, the measurement areas were extended to the west and north of the Fukushima Dai-ichi NPP. It is clear that the density of measurement areas after the third survey was higher than that of previous surveys since the number of measurement devices was increased to about 100 (KURAMA-II). It is recognized that the measured air dose rates in the area between Iwate Prefecture and Miyagi Prefecture were higher than those in the peripheral areas owing to the larger measurement areas involved in the fourth car-borne survey (see the circled area shown in Fig. 9).
3.2. Trend in the air dose rate 3. Results and discussion 3.1. Measurement results Air dose rate maps for the entire area obtained from the measurements of the first to fifth car-borne surveys are shown in Figs. 6e10, respectively, whereas those showing measurement
Several trends can be observed by superimposing the air dose rate data obtained by the car-borne surveys onto topographic maps. Fig. 11 shows the air dose rate data superimposed onto a Google Earth image of the first car-borne survey measured several kilometers northwest of the Fukushima Dai-ichi NPP, in which direction a high-concentration radioactive plume was
M. Andoh et al. / Journal of Environmental Radioactivity 139 (2015) 266e280
Fig. 9. Air dose rate distribution map obtained from the fourth car-borne survey (August 20eOctober 12, 2012). A red circle shows the area between Iwate Prefecture and Miyagi Prefecture where the air dose rates were relatively high. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
271
Fig. 10. Air dose rate distribution map obtained from the fifth car-borne survey (November 5eDecember 10, 2012).
3.3. Decreasing trend of the air dose rate emitted on the afternoon of March 15, 2011 and where radioactive contamination was considered to be deposited according to the JAEA estimation using SPEEDI (Chino et al., 2011). High dose rate data indicated by red continuously appear in the direction of the radioactive plume flow, whereas rapid decreases in the air dose rate are observed with increasing distance along the route where the radioactive plume moved. The air dose rate distribution remarkably features the contamination conditions at the time of the accident, i.e., contamination varied substantially depending on the location. Furthermore, the relationship between the air dose rate distribution and topography could be derived. Fig. 12, a result of the first car-borne survey conducted in the mountainous areas located to the northwest of the Fukushima Dai-ichi NPP, shows that the air dose rate is clearly different in the south and north regions of a mountain ridge, from which it can be inferred that the radioactive plume flow bypassed the north ridge. In addition, the results of the first car-borne survey in the northwest (the mountainous area) of the cities Nihonmatsu and Fukushima (about 70 km west of the Fukushima Dai-ichi NPP) indicate that the air dose rate gradually decreased with increasing altitude from the basin to the mountaintop, as shown in Fig. 13. As mentioned above, the measured data contribute toward the analysis of plume flow or path of plume deposition caused by the accident.
The correlation among the air dose rate measurement results obtained in the same mesh used in the first (June 2011), second (December 2011), and third (March 2012) car-borne surveys is shown in Fig. 14 and that obtained in the same mesh used in the first, fourth (August to October 2012), and fifth (November to December 2012) car-borne surveys is shown in Fig. 15. The air dose rates in the second car-borne survey decreased by 30% in comparison with those in the first survey, as indicated by the slope of the regression line. Likewise, the air dose rates in the third survey decreased by 39%, the fourth by 48%, and the fifth by 56%. The slope of the regression line is easily affected by high dose rate data, and the slope does not always show a decreasing trend in the dose rate precisely. Therefore, the summation of air dose rates (referred to as the “integral value”) was calculated, and the ratios of the integral values (referred to as the “total dose rate ratio”) are shown in Table 1. As observed from the total dose rate ratios, the air dose rates from the second to the fifth car-borne surveys decreased in comparison with those in the first survey by 34%, 41%, 53%, and 58%, respectively. When we compare the total dose rate ratios (shown in Table 1) with the slope of the regression lines (shown in Figs. 14 and 15), the differences in the decrease of the air dose rates was from 3% to 11%. Fig. 16 shows the decreasing tendency of the total dose rate ratios measured by the car-borne surveys and that calculated by the physical decay of radioactive cesium. It was confirmed that the trend of decrease in the air dose rates from the car-borne surveys
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Fig. 11. Three-dimensional image display that overlays the results of the first car-borne survey (1) northwest of the Fukushima Dai-ichi NPP. Background map is the Google Earth map.
was considerably larger than that calculated by the physical decay of radioactive cesium. Land usage was classified on the basis of the national land information provided by National Land Numerical Information, Ministry of Land, Infrastructure, Transport and Tourism. The integral values and total dose rate ratios determined by the car-borne surveys were evaluated according to land usage in the surrounding areas such as forest area, building sites, paddy field, farmlands, and others, as listed in Table 2. It was found that the observed decrease in the air dose rates depended on land usage, wherein the trend of decrease was less pronounced in forest areas and more pronounced in areas classified as building sites. From the decreasing tendency of the air dose rate described above, the migration of radioactive cesium on the road was found to proceed more quickly owing to human activities and vehicle traffic.
3.4. Comparison of car-borne survey data on roadways with the mapping project data surrounding roadways Air dose rates along the center of the roadway at a height of 1 m were obtained by the car-borne surveys. However, the air dose rates using the NaI (Tl) survey meters in the mapping projects were measured at a height of 1 m in undisturbed flat regions surrounding the roadways. To determine the relationship between the air dose rates along the roadway and those in the surrounding regions, the air dose rates obtained by the fourth and fifth car-borne surveys using the KURAMA-II systems were compared with those measured by the NaI (Tl) survey meter (Mikami et al., 2014) obtained in the same meshes. Fig. 17 shows a clear-cut correlation between the car-borne survey measurements (the fourth and fifth) and the air dose rate measurements using NaI (Tl) survey meters
Fig. 12. Three-dimensional image display that overlays the results of the first car-borne survey (2) in the mountainous region located to the northwest of the Fukushima Dai-ichi NPP. Background map is the Google Earth map.
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Fig. 13. Three-dimensional image display that overlays the results of the first car-borne survey (3) viewed northwest from about 70 km west of the Fukushima Dai-ichi NPP. Background map is the Google Earth map.
made during the same time period. The slope of the regression line shows that the air dose rates of the surrounding regions measured in the two periods of the third mapping project were 1.24 and 1.41 times larger than those of the fourth and fifth car-borne surveys, respectively. It is possible to estimate the air dose rates in the surroundings using the results of car-borne surveys and the values of the slope of the regression lines. In addition, the decreasing tendency of the air dose rates measured by the NaI (Tl) survey meters is also shown in Fig. 16 (shown as the first to the third mapping projects). Air dose rates on the roadways measured by the car-borne surveys showed values lower than those of the surrounding areas, and the ratio between those two measurements gradually increased with increasing time. From this, it is assumed that radioactive cesium on the roadway can be more easily removed, owing to the effect of weather (the so-
called “weathering effect”), traffic, etc., than that on the area surrounding the roadway. It is an important subject to identify which decreasing trend represents the environment for human life in order to precisely predict ambient dose equivalent rates in the future environment.
Fig. 14. Correlation diagram of the air dose rates between the first, second, and third surveys.
Fig. 15. Correlation diagram of the air dose rates between the first, fourth, and fifth surveys.
4. Conclusion A series of car-borne surveys using the KURAMA and KURAMAII systems has been conducted in a wide area of eastern Japan since June 2011 to evaluate the distribution of air dose rates over a wide area around of the Fukushima Dai-ichi NPP and to determine the trend of decrease in air dose rates. An automated data processing system for the air dose rate measurement using the KURAMA-II
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Table 1 Summation of the air dose rates of common meshes (integral values) for two groups of surveys (i.e., the first, second, and third, and the first, fourth, and fifth)a and the ratios of the integral values (total dose rate ratios) of each car-borne survey to the first survey. Car-borne survey period
Integral value (mSv/h) Ratio to the first survey
First
Second
Third
Fourth
Fifth
4.61 27.0 e
3.04 e 0.65
2.72 e 0.59
e 12.8 0.47
e 11.2 0.41
a
The first, second, third, fourth, and fifth surveys were conducted from June 4 to 13, 2011, from December 5 to 28, 2011, from March 13 to 30, 2012, from August 20 to October 12, 2012, and from November 5 to December 10, 2012, respectively.
system was established, which enabled the analysis of large amounts of collected data obtained with the use of about 100 KURAMA-II units. Initial data to study the migration status of the radioactive material was obtained, and its distribution maps were created from the measurement results using a 100 m mesh. These basic data enabled the precise analysis of the spread of deposition conditions of gamma-ray-emitting nuclides. It was found that the decreasing trend of the air dose rates measured by the car-borne surveys (until December 2012) was more pronounced than those calculated by the physical decay of radioactive cesium and that measured using NaI (Tl) survey meters in areas surrounding the roadways. In addition, the decreasing trend in the air dose rate was found to depend on land usage, wherein the trend was more pronounced for land used as building sites than that for forest areas.
Fig. 16. Comparison of the changing trends of air dose rates measured by car-borne surveys and those measured by NaI (Tl) survey meters.
Fig. 17. Comparison of the results of the air dose rate measurements at a height of 1 m measured by NaI (Tl) survey meters and those from car-borne surveys (left: the fourth car-borne survey; right: the fifth car-borne survey).
Acknowledgment Table 2 Integral values and the total dose rate ratios for each type of land use in the surrounding areas obtained from the common meshes among the first, third, and fourth car-borne surveys. Car-borne Integral value (mSv/h) and total dose rate ratioa survey period Total Forest area Building site Paddy field Farmland and others First Third Fourth
7.06 1.96 1.39 4.09 (0.58) 1.20 (0.61) 0.71 (0.51) 3.19 (0.45) 0.97 (0.49) 0.56 (0.40)
2.07 1.21 (0.58) 0.92 (0.44)
1.32 0.80 (0.61) 0.61 (0.46)
a Values in the parentheses are the total dose rate ratios relative to the first survey for each type of land use.
The authors are grateful to Dr. J. Ishida and Dr. S. Matsumoto of Headquarters of Fukushima Partnership Operation, JAEA, for their encouragement regarding this study. In addition, the authors wish to express their gratitude to the Japan Map Center for their contributions in mapping the measured data, to the KURAMA development team of Kyoto University Research Reactor Institute for their support in the car-borne survey measurements, to the Tokyo Electric Power Company and the JAEA staff for their cooperation in the car-borne survey measurements. This study was a part of the distribution-mapping projects financially supported by the Ministry of Education, Culture, Sports, Science, and Technology.
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Appendix
Fig. A.1. Air dose rate distribution map within 80 km of the Fukushima Dai-ichi NPP obtained by the first car-borne survey performed from June 4 to 13, 2011.
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Fig. A.2. Air dose rate distribution map within 80 km of the Fukushima Dai-ichi NPP obtained by the second car-borne survey performed from December 5 to 28, 2011.
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Fig. A.3. Air dose rate distribution map within 80 km of the Fukushima Dai-ichi NPP obtained by the third car-borne survey performed from March 13 to 30, 2012.
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Fig. A.4. Air dose rate distribution map within 80 km of the Fukushima Dai-ichi NPP obtained by the fourth car-borne survey performed from August 20 to October 12, 2012.
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Fig. A.5. Air dose rate distribution map within 80 km of the Fukushima Dai-ichi NPP obtained by the fifth car-borne survey performed from November 5 to December 10, 2012.
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