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Journal of Environmental Radioactivity 139 (2015) 294e299

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Journal of Environmental Radioactivity

journal homepage: www.elsevier .com/locate / jenvrad

Aerial radiation monitoring around the Fukushima Dai-ichi nuclearpower plant using an unmanned helicopter

Yukihisa Sanada*, Tatsuo ToriiHeadquarters of Fukushima Partnership Operations, Japan Atomic Energy Agency, 2-2-2, Uchisaiwai-cho, Chiyoda-ku, Tokyo 100-8577, Japan

a r t i c l e i n f o

Article history:Received 2 December 2013Received in revised form26 June 2014Accepted 26 June 2014Available online 19 July 2014

Keywords:Unmanned helicopterRadiation measurementPlastic scintillatorLaBr3:Ce scintillatorFukushima Dai-ichi nuclear power plantRadioactive cesium

* Corresponding author. Tel.: þ81 3 3592 2351.E-mail address: sanada.yukihisa@jaea.go.jp (Y. San

http://dx.doi.org/10.1016/j.jenvrad.2014.06.0270265-931X/© 2014 Elsevier Ltd. All rights reserved.

a b s t r a c t

The Great East Japan Earthquake on March 11, 2011 generated a series of large tsunami that seriouslydamaged the Fukushima Dai-ichi Nuclear Power Plant (FDNPP), which resulted in the release of radio-active materials into the environment. To provide further details regarding the distribution of air doserate and the distribution of radioactive cesium (134Cs and 137Cs) deposition on the ground within a radiusof approximately 5 km from the nuclear power plant, we carried out measurements using an unmannedhelicopter equipped with a radiation detection system. The distribution of the air dose rate at a height of1 m above the ground and the radioactive cesium deposition on the ground was calculated. Accordingly,the footprint of radioactive plumes that extended from the FDNPP was illustrated.

© 2014 Elsevier Ltd. All rights reserved.

1. Introduction

To measure the distribution of radioactive substances releasedinto the environment over an entire region after the accident at theFukushima Dai-ichi Nuclear Power Plant (FDNPP) in 2011, wedeveloped remote monitoring techniques using a manned heli-copter and applied these techniques to dose rate measurement ofthe whole of Japan (Torii et al., 2012; Sanada et al., 2014a). Mannedhelicopter technology is extremely useful for monitoring remoteradiation because it enables radiation measurements in areas thatpeople cannot easily enter, such as forests and paddy fields. Themap constructed from these data helps to visualize the distributionof the air dose rate and the radioactive cesium deposition on theground over large areas. However, aviation regulations in Japanprohibit flying manned helicopters at altitudes lower than 150 mabove ground level (AGL).

In Japan, unmanned helicopters that fly at a low altitude are notrestricted by the aviation regulations and are typically used foragricultural applications, such as spraying pesticides. For scientificobservations, an unmanned helicopter can be equipped with anautonomous navigation system comprising a computer andcommunication system (Kaneko et al., 2011). An autonomous

ada).

unmanned helicopter (AUH) equippedwith a radiation detector hasbeen developed by the Japan Atomic Energy Agency (JAEA) as aradiation monitor for nuclear emergencies following the Tokai-mura criticality accident that occurred in 1999 (Okuyama et al.,2008).

Radiation can be measured by AUHs quickly over large areas, ascompared to performing measurements on the ground usingportable survey meters. Moreover, because flights can be pro-grammed using an autonomous flight system, confirmation of theeffect of decontamination work and investigation of the migrationof radioactive materials can both be assessed on a single flight(Sanada et al., 2014b). It is also possible to estimate the air dose rateand the radioactive cesium deposition on the ground in evacuationzones near the nuclear power plant, even in mountainous areas,paddy fields and high air dose rate areas that aren't easily accessibleby people. In this paper, we report the results of the AUH moni-toring around the FDNPP.

2. Materials and methods

2.1. AUH

We used an AUH (RMAX G1) manufactured by Yamaha MotorCo., Ltd (Shizuoka, Japan). Although taking off and landing of thisAUH is operated manually, once airborne the AUH functionsautonomously according to a preset program. The operator was

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allowed for a safety check in operated automatically monitoringflights using a computer and a communication system that receivedradio waves (2.4 GHz) from a ground station in a vehicle. Based oninformation about the position of the helicopter obtained by adifferential global positioning system (DGPS), the operator plannedthe flight program, including the flight path, speed and altitude ofthe AUH. The AUH weighs 94 kg and has a maximum payload ca-pacity of 10 kg. Its maximum speed is 72 km/h (about 40 knots). Theflight duration is 1.5 h in a filled-up fuels.

2.2. Radiation measurement system

We employed three LaBr3:Ce scintillation detectors (38.1 mm indiameter� 38.1mm in length), packagedwith supporting electronicsto create the 6.5 kg Aerial Radiation Measurement System (ARMS),which was installed under the helicopter. The ARMS was electricallyinsulated by the AUH to reduce the occurrence of mutual in-terferences. Noise derived from the AUH was not detected in ARMSdata. The count rate of the three detectors, and the spectral and po-sition data of the DGPS were acquired and stored in the ARMS everysecond. The pulse signal of the LaBr3:Ce scintillation detector from0 keV to 3000 keV is preserved in each of the 1024 channels of themulti channel analyzer (MCA) in the ARMS. Any signal greater than3000 keV was only counted without information on gamma energy.Any signal less than 50 keV was removed by the pulse heightdiscriminator. The energy resolution (fullwidthathalfmaximumoverthepeakposition) for the662keVphotopeakwas2.8%. This resolutionis sufficient to discriminate between the photopeaks of 605 keV(134Cs) and 662 keV (137Cs). The gamma spectra of the three detectorswere stored individually and summed after each flight. Detectorsensitivity dependency on temperature was corrected with a tem-perature compensation circuit. The output from the ARMSwas downlinked to the ground station using awireless local area network; thusthe count rate data were displayed in real time on a map.

Samples of the gamma spectra measured in the evacuation zonearound the FDNPP are shown in Fig. 1. These spectra, representing aperiod of 100 s, were obtained hovering at 100mAGL and above thesea in December 2012. Both measurement areas were in Futaba-machi, which is 5 km north of the FDNPP. The spectrum repre-sented by the black line was obtained in the area in where the airdose rate at 1mAGLwas 10mSv/h. Another spectrum representedbya gray line was obtained above the sea 200 m from the shore.

2.3. Data acquisition

Most of the flights were carried out at a speed of 8 m/s and 80 mAGL. We selected this flight altitude as a standard altitude (Hsd) in

Fig. 1. Gamma spectra of the ARMS (three LaBr3:Ce detectors) above the ground andabove the sea in Futaba-machi, 5 km from the FDNPP. Measurement time was 100 s.Measurement date was 22 December 2012.

consideration of the safety and reliability of the data. A measure-ment at 80 m AGL means that the ARMS measured the averagegamma rays emitted from the ground within a circle 80 m in radiusbelow the helicopter in the case of an infinite plate source (IAEA,2003). Furthermore, considering the 80 m AGL, we employed aflight line spacing of 80 m.

2.4. Calibration parameter

To convert the total count rate measured in the air into the airdose rate at 1 m AGL, we measured the air dose rate at a height of1m using a NaI surveymeter (TGS-172B, Hitachi AlokaMedical, Ltd,Tokyo Japan) at 30 locations in a test area within the evacuationzones near the FDNPP. This survey meter was calibrated based on aJapanese standard (JIS, 2014). The test area was selected for its flattopography and little variation in air dose-rate results. At the testarea, the helicopter hovered at 10 m or 20 m AGL intervals from10 m to 100 m AGL, and we measured the total count rate at eachheight. Fig. 2 shows the relationship between total count ratesmeasured as a function of AGL altitudes. An effective attenuationfactor (AF) was selected from an exponential regression of eachbackground-corrected count rate and height. Furthermore, the airdose-rate conversion factor (CDsd) was calculated as the ratio of thetotal count rate of at 80 m AGL to the air dose rate at 1 m AGL in thetest area.

A background spectrum was collected at 80 m above the sea(gray line in Fig. 1). The background spectrum represents radio-logical contributions from (1) inherent contaminants within theLaBr3:Ce crystal, (2) any sources in/on the AUH and associated at-tachments, and (3) cosmic radiation. The inherent-contaminationof the LaBr3:Ce crystal is due to the presence of natural radionu-clides of the detector material itself, such as 138La, and contami-nation by 227Ac, a chemical homolog of lanthanum (Nicolini et al.,2007). The influence of cosmic rays varies with the altitude of theflight. However, because variation of count rate due to cosmic raysis smaller than the count rate of the radioactive cesium around theFDNPP, the influence of cosmic rays on the calibration parameter isconsidered as negligible.

2.5. Data analysis

2.5.1. Air dose rateAlthough the gamma-ray energy spectrum can be collectedwith

the ARMS, we used the gross count rate for estimation of the airdose rate, which includes the contribution from natural radiation.

Fig. 2. Example of the relationship between the count rate of the ARMS and AGL flightaltitude. AF is the inclination of the exponential fitting curve. Air dose rate of this point(in Namie-machi, 8 km from the FDNPP) was 0.6 mSv/h. The error was twice thestandard deviation of the measurement result.

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We selected the total count rates of this background spectrum (grayline in Fig. 1) as the background count rate (BG). The air dose rate at1 m AGL (D1m) was calculated from the airborne count rate bysubtracting BG from the gross count rate (GC). The AGL altitude wasevaluated to subtract the altitude of the digital elevation model(DEM) data (GSI, 2011) of the 10 m mesh from the GPS altitude(altitude above sea level). The difference of the real flight altitude(H) and Hsd was corrected using AF. The D1m is given by Eq. (1).

D1m ¼ ðGC� BGÞ � CDsd � exp½ � AF� ðH � HsdÞ� (1)

To evaluate the radioactive cesium deposition on the ground, weused the method of calculating radioactive cesium deposition onthe ground from the air dose rate of the radioactive cesium. Here,the air dose rate of the radioactive cesium was calculated by sub-tracting the air dose rate by the cosmic ray and natural radionu-clides in AUH and ARMS from the measured air dose rate. Therelationship between the radioactive cesium deposition on theground and the air dose rate of the radioactive cesiumwas obtainedusing an in-situ Ge detector.

Fig. 3a shows a comparison between the ground measurementresults of 2500 points by the NaI survey meter at 1 m AGL and theARMS measurements. These data all agreed well within the sloperange of 0.5e2, indicating that the two systems can accuratelymeasure the air dose rate distribution on the ground within anuncertainty factor of 2. However, at certain points, the air dose ratemeasured by the ARMS was higher or lower than that obtained bythe ground measurement. This could be because such points arelocations where the air dose rate is heterogeneous (for example1.0e10 mSv/h within 1 km2). The measurement results obtained bythe ARMS may not match the ground measurement data at every

Fig. 3. Comparison of AUH and measurement. Top left (a) shows results from the NaI surveybetween the two lines (y ¼ 0.5x, y ¼ 2.0x) shown in the figure. The figures on the bottomsurement (d).

point, because the data obtained by the unmanned helicopter arean average value representing the area below the helicopter.Moreover, the environmental effects pointed out by IAEA (2003)(e.g. shielding condition from source to detector, soil moisture, Rnprogeny in air and topographic effects) may also be responsible forthe slight data mismatch. Evaluation of these effects will benecessary in the future.

The minimum detectable air dose rate (MDD) is 0.07 mSv/h at80mAGL. TheMDDwas calculated from the BG count rate obtainedabove the sea, and the MDDs were converted into the air dose rateat 1 m AGL using Eq (1). To create a map of the air dose rate dis-tribution, the air dose rate data obtained were interpolated usingthe Kriging method (IAEA, 2003).

2.5.2. Radioactive cesium deposition on the groundThe radioactive cesium deposition on the ground (Rd) was

calculated from the air dose rate excluding the radiation fromnatural radionuclides in the ground. This is possible because thosefrom cesium are much lower than the gamma-ray energies fromradioactive cesium from the naturally occurring K, U, and Th at1471 keV, 1765 keV, and 2615 keV, respectively. Due to the inherentcontamination within the LaBr3:Ce crystal, it is difficult to correctlyestimate the terrestrial component of natural radiation because theinternal 138La emission (1436 keV) closely coincides with the nat-ural 40K emission (1461 keV). To overcome this challenge, thecontribution of the natural radionuclides to the air dose rate wasmeasured at 104 locations around the FDNPP using an in situ Gedetector (Falcon 5000, CANBERRA Industries Inc., Meriden, CT), andthe average air dose rate from natural radionuclides in the groundof 31.3 nSv/h, was estimated to be the background air dose rate(Dnat). The ratio of 134Cs/137Cs (B) was determined by the ratio of

meter. Top right (b) shows results of the in situ Ge measurement. Almost all plots areshow the distributions of the ratios for the NaI survey meter (c) and in-situ Ge mea-

Fig. 4. Flight path of the AUH. There are gaps in the flight path because of a powertransmission line.

Fig. 5. (a) Distribution map of the air dose-rate at 1 m AGL as measured by the AUH. The mmap of the air dose-rate at 1 m AGL as measured by the manned helicopter. The mesh of thflight path.

Y. Sanada, T. Torii / Journal of Environmental Radioactivity 139 (2015) 294e299 297

photopeak counts of 661 keV (137Cs) and 796 keV (134Cs) by usingthe gamma-ray energy spectrum. The B on the measurement datewas similar to the value of the physical decay of the radioactivecesium from the calibration date (the ratio of 134Cs/137Cs was 0.917on Aug. 13, 2011 (MEXT, 2011)). The radioactive cesium depositionon the ground was calculated using the conversion factor(CF ¼ [kBq/m2][mSv/h]�1) obtained from the air dose rate. The CFwas given by the relaxation mass per unit (b) of 1.0 g cm�2 (ICRU,1994). The b was used for consistency with the depth profilemeasurements of the FDNPP (Kato et al., 2012). The Rd is shown inEq. (2).

Rd ¼ ðD1m � DnatÞ ��B� CF134 þ CF137

Bþ 1

�(2)

where CF134 and CF137 are the conversion factors for 134Cs and 137Cs,respectively.

A comparison between the ground measurement results ob-tained by the in-situ Ge detector at 128 points that were 1 m AGLand the analyzed results measured by the ARMS are shown inFig. 3c. These data agree well within the range of 0.5e2, indicatingthat the ARMS can accurately measure the radioactive cesiumdeposition on the ground. As shown by the preceding measure-ment and the analysis, we can detect a minimum detectable ac-tivity (MDA) of 4 kBq/m2 of radioactive cesium at 80 m AGL.

3. Results and discussion

3.1. Air dose-rate distribution

Prior to this study, aerial monitoring using manned helicoptershad only been used to measure data at a distance of 3 km or greater

esh of the map was 5 m. Data were decay-corrected to March 20, 2013. (b) Distributione map is 250 m. Data were decay-corrected to March 20, 2013. Black dots indicate the

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from the FDNPP due to the aviation regulations in Japan. However,these regulations do not pertain to low altitude unmanned heli-copters, so we decided to measure the air dose rate within a 5 kmdistance from FDNPP using the AUH. The entire survey wascompleted over a period of 52 d from January 28, 2013 to March 20,2013 using a line spacing of 80mAGL. Fig. 4 shows the flight path ofthe AUH.

Fig. 5a shows the distribution map of the air dose rate measuredby the AUH. These data are decay-corrected to March 20, 2013 bythe physical half-lives of radionuclides. Four deposition patterns inwhich the air dose rate is particularly high extend from the FDNPP,as clearly seen in the figure. This indicates that there were fourunique releases of radioactive material during the accidents. Highair dose rate areas consist of four zones around the FDNPP. Thesouthern area had the highest air dose rate. The western areaseemed to be divided into two sections, and the high air dose-ratearea in the west appeared from a location several hundred metersaway from the boundary of the FDNPP site as shown in Fig. 5a. Theair dose rate in the northwestern area was comparatively low.

The distributionmap produced from aerial monitoring using themanned helicopter is shown in Fig. 5b. These data were collectedfrom October 31, 2012 to November 16, 2012 (MEXT, 2013) and aredecay-corrected to March 20, 2013 by the physical half life. Becausethe spacing of the flight line with a manned helicopter (1.8 km) waswider than that of the unmanned helicopter, the position resolutionof the map was relatively rough. Because flight conditions (altitude,speed and detector) are different, both maps do not completelyagree, but the tendency of the distribution maps was similar.

Prior to this study, the air dose-rate distribution around theFDNPP had not been measured by either airborne monitoring orground measurement. Our results show the first detailed air dose-

Fig. 6. Distribution map of radioactive cesium deposition on the soil. Data were decay-corrected to March 20, 2013.

rate distribution around the FDNPP obtained after the accident,providing important information for analysis of the accident.

3.2. Distribution of radioactive cesium deposition on the ground

The distribution of the radioactive cesium deposition on theground was obtained along with the air dose rate. The radioactivecesium deposition on the ground was evaluated using the air doserate as previously mentioned, and the results are shown in Fig. 6.The area with the greatest concentration of radioactive cesiumwasin the southern and western parts up to 3 km from the FDNPP site.We were able to obtain the distribution of the radioactive cesiumdeposition on the ground by this AUH monitoring.

4. Conclusion

To clarify the distribution of radioactive cesium deposition, weperformed measurements over a wide area around the FDNPP us-ing the ARMS. This technique combines airborne monitoring andground monitoring, and is useful in clarifying the detailed distri-bution. This technique visually expressed the dispersion patterns ofthe radioactive material around the FDNPP and clearly showed thedeposition from the radioactive plumes emitted by the accident.Because duplicate measurements are easily performed using AUHmonitoring, this method is effective for investigating the migrationof radioactive materials in the environment. In subsequent exper-iments, we will perform observational and analytical research onthe migration of the radioactive cesium around the FDNPP.

Acknowledgment

We thank Mr. Y. Yuuki, Mr. M. Matsui, Mr. Y. Nishizawa, Mr. M.Ishida, and Mr. Y. Urabe for their cooperation in taking measure-ments and processing data. We are also grateful to Mr. J. Ishida andthe collaborators at the Fukushima Environmental Safety Center,JAEA, for their encouragement of this study.

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