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Journal of Environmental Radioactivity 111 (2012) 18e27

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Journal of Environmental Radioactivity

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

Measurement of soil contamination by radionuclides due to the FukushimaDai-ichi Nuclear Power Plant accident and associated estimated cumulativeexternal dose estimation

S. Endo a,*, S. Kimura b, T. Takatsuji c, K. Nanasawa d, T. Imanaka e, K. Shizuma a

aGraduate School of Engineering, Hiroshima University, 1-4-1 Kagamiyama, Higashi-Hiroshima 739-8527, JapanbMedical School, Dokkyo Medical University, 880 Kitakobayashi, Mibu-machi, Shimotsuga-gun, Tochigi 321-0293, JapancGraduate School of Fisheries Science and Environmental Studies, Nagasaki University, 1-14, Bunkyo-machi, Nagasaki 852-8521, Japand The NHK Broadcasting Culture Research Institute, Atago-Mori Tower 16F, 2-5-1, Atago, Minato-ku, Tokyo 105-6216, JapaneResearch Reactor Institute, Kyoto University, Kumatori, Sennangun, Osaka 590-0494, Japan

a r t i c l e i n f o

Article history:Received 14 July 2011Received in revised form4 November 2011Accepted 10 November 2011Available online 1 December 2011

Keywords:Fukushima Dai-ichi nuclear power plantNuclear accidentRadioactivityDose rateCumulative dose

* Corresponding author. Tel.: þ81 82 424 7612; faxE-mail address: endos@hiroshima-u.ac.jp (S. Endo

0265-931X/$ e see front matter � 2011 Elsevier Ltd.doi:10.1016/j.jenvrad.2011.11.006

a b s t r a c t

Soil sampling was carried out at an early stage of the Fukushima Dai-ichi Nuclear Power Plant (FDNPP)accident. Samples were taken from areas around FDNPP, at four locations northwest of FDNPP, at fourschools and in four cities, including Fukushima City. Radioactive contaminants in soil samples wereidentified and measured by using a Ge detector and included 129mTe, 129Te, 131I, 132Te, 132I, 134Cs, 136Cs,137Cs, 140Ba and 140La. The highest soil depositions were measured to the northwest of FDNPP. From thissoil deposition data, variations in dose rates over time and the cumulative external doses at the locationsfor 3 months and 1 y after deposition were estimated. At locations northwest of FDNPP, the external doserate at 3 months after deposition was 4.8e98 mSv/h and the cumulative dose for 1 y was 51 to1.0� 103 mSv; the highest values were at Futaba Yamada. At the four schools, which were used asevacuation shelters, and in the four urban cities, the external dose rate at 3 months after depositionranged from 0.03 to 3.8 mSv/h and the cumulative doses for 1 y ranged from 3 to 40 mSv. The cumulativedose at Fukushima Niihama Park was estimated as the highest in the four cities. The estimated externaldose rates and cumulative doses show that careful countermeasures and remediation will be needed asa result of the accident, and detailed measurements of radionuclide deposition densities in soil will beimportant input data to conduct these activities.

� 2011 Elsevier Ltd. All rights reserved.

1. Introduction

The nuclear accident at the Fukushima Dai-ichi Nuclear PowerPlant (FDNPP) occurred as a consequence of the massive earth-quake and associated tsunami that struck the Tohoku and northKanto regions of Japan on 11 March 2011.

There are six reactor units at FDNPP. Plant units 1, 2 and 3experienced partial core melt several hours after their coolingpumps failed on 11 March 2011. On the afternoon of 12 March, gasin a pressure chamber in unit 1 was vented to avoid damage to thecontainment vessel at about 14:30. After venting, a first hydrogenexplosion occurred at unit 1 at 15:36. On 13 March, a high radiationdose rate was measured at a monitoring post of Onagawa NuclearPower Plant in Miyagi Prefecture. Then, a second hydrogen

: þ81 82 424 2453.).

All rights reserved.

explosion occurred at unit 3 around 11:01 on 14 March. Subse-quently, another explosion was heard from units 1 and 2 atapproximately 06:00 on 15 March. The suppression chamber ofunit 2 appeared to have been damaged as a result of this explosion(Nuclear Emergency Response Headquarters Government of Japan,2011).

The Japanese government established an evacuation zonewithin a 3 km radius from FDNPP, and expanded this zone toa 10 km radius and then to a 20 km radius from FDNPP on 12March2011. In light of this situation, estimates of the radiation doses topeople in the affected areas were needed. However, detailedinformation was lacking on soil deposition densities and on asso-ciated dose rate levels, not only outside of the 20 km exclusion zonebut also within the periphery of the evacuation zone, where someresidents had remained. On 15 March our investigation of doserates and deposition levels was started to provide information insupport of efforts to reduce radiation doses to exposed residents.Initial data suggested that radioactive deposition from the FDNPP

Table 1Conversion factor between deposition density and radiation dose rate at 1 m abovethe ground surface (Beck, 1980).

Nuclide Half-life (day) Conversion factor (nGy/h)/(kBq/m2)b

Relaxation depth (g/cm2)

0 0.16 1.6129mTe 33.6 0.133 0.116 0.0766129Te 33.6a 0.247 0.217 0.145131I 8.021 1.736 1.503 1.032132Te 3.204 0.913 0.804 0.545132I 3.204a 10.01 8.705 5.779134Cs 754.2 6.897 6.041 3.996136Cs 13.16 9.323 8.134 5.423137Cs 10,958 2.545 2.207 1.463140Ba 2.894 0.133 0.116 0.0766140La 2.894a 0.247 0.217 0.145

a Half-lives of the parent nuclide are used for decay calculation.b Original unit is (mR/h)/(mCi/km2).

S. Endo et al. / Journal of Environmental Radioactivity 111 (2012) 18e27 19

accident was giving rise to a serious situation. In this study, varia-tions over time of dose rates and cumulative doses were estimatedfor several sampling locations around FDNPP from measurementsof radionuclide deposition densities in soil.

2. Material and methods

2.1. Soil sampling

Soil sampling (6.5 cm diameter and at 0e5 cm and 5e10 cmdepths) was carried out during three separate campaigns at thelocations shown in Fig. 1.

i) On 15e16 March 2011, the four locations in blue weresampled in a direction from west to east.

ii) On 20e21 March, the five sampling locations in red wereprimarily south of FDNPP with one to the northwest.

iii) On 28e30 March, the seven sampling locations in yellowwere north, northwest and west of FDNPP.

Thus, 15 sampling points were located 20e40 km away fromFDNPP, two sampling points were 60 km away and one samplingpoint was within 5 km. The soil data included samples from thefour urban cities of Fukushima City (Niihama Park (Pk)), KoriyamaCity (Oshima-Higashi Pk), Minami-Soma City and Iwaki City (ChuoInterchange (IC), Suetsugi Station (St) and Kittoya Mountain (Mt));from Miharu Town; from highly contaminated regions near FutabaTown (Futaba Yamada), Namie Town (Namie Akogi, Namie Tsush-ima) and Iitate Village (Iitate Warabidaira); and from four schools(Miyakoji Junior High School (JHS), Tokiwa JHS, Katsurao JHS andKawauchi Elementary School (ES)) that were being used astemporary residences for evacuees. Uncultivated lands, such asshrubs, school playgrounds and flowerbeds, were selected as

Fig. 1. Sampling locations in an aerial photograph taken from Google Earth (http://earth.goofrom west to east. Red circles: 20e21 March, five sampling locations were south of FDNPP wnorth, northwest and west of FDNPP.

sampling sites, except for the sample at Koriyama City, which wastaken from a sandbox.

2.2. Determination of radioactivity

Soil samples of 40 g fresh weight were placed into acryliccontainers without drying to prevent evaporation of radioiodineisotopes. The radioactive contaminants in the soil samples wereidentified and measured with Ge detectors (GMX-30200-P, Ortec)for 600, 3600 or 20,000 s at Hiroshima University, depending onthe half-life of the radionuclide to be measured. The same soilsamples were independently measured at Nagasaki University tocross-check the measured activity. The obtained data wereconsistent with each other within the limits of the measurement

gle.com/). Blue squares: 15e16 March 2011, four sampling locations were in a directionith one to the northwest. Yellow diamond: 28e30 March, six sampling locations were

Fig. 2. Monitoring data of radiation dose by government for several cities and villages taken from Fukushima Prefecture website: (a) Shirakawa, Aizu-Wakamatsu, Minami-Aizu,Minami-Soma and Iwaki Taira and (b) Fukushima, Tamagawa, Iitate, Iwaki, Tamura and Koriyama.

S. Endo et al. / Journal of Environmental Radioactivity 111 (2012) 18e2720

uncertainty. The detection efficiency of the Ge detectors wasdetermined with less than 5% uncertainty by using nine nuclidemixed activity standard volume sources (MX-033, Japan IsotopeAssociation) containing radionuclides of 109Cd, 57Co, 139Ce, 51Cr,85Sr, 137Cs, 54Mn, 88Y and 60Co.

2.3. Estimation of exposure

The time variation in air dose rate can be calculated from thedeposition density in the soil. To calculate the rates, a conversionfactor between each radionuclide’s deposition density and itsradiation dose rate at 1 m above the ground surface was used; theconversion factors were taken from Beck (1980) and are shown inTable 1. Variations in the radiation dose rates were derived from thetime of radioactive deposition, which was assumed to be 15 March2011. The estimates also account for diminishing dose due to thephysical half-life of each radionuclide present in the soil. In per-forming the calculations, a dose conversion factor for ground

Fig. 3. Example of g-ray spectrum measured for soil sample. Radionuclides of 1

surface contamination was assumed in this analysis. Actual radio-nuclides are expected to have been distributed from the surface toa depth of 2.5 cm. Therefore, an uncertainty of 30% is provided bythis assumption owing to the difference in dose rates obtained fromthe conversion factors for 0 and 1.6 g/cm2 relaxation depths.

2.4. Monitoring data collected by government authorities

To determine radiation levels around FDNPP, radiation moni-toring was conducted by Japanese government authorities. Moni-toring post (MP) datawere reported on thewebsites of theMinistryof the Education, Culture, Sports, Science and Technology (MEXT,2011), Fukushima Prefecture (Fukushima Prefecture, 2011) andthe Fukushima International Association (Fukushima InternationalAssociation, 2011).

Selected MP data are shown in Fig. 2(a) and (b). The radioactiveemissions from FDNPP were initially detected at about 20:00 on 12March in Minami-Soma City, located about 30 km north of FDNPP,

29mTe, 129Te, 131I, 132Te, 132I, 134Cs, 136Cs, 137Cs, 140Ba and 140La are identified.

S. Endo et al. / Journal of Environmental Radioactivity 111 (2012) 18e27 21

and the radiation level increased intermittently thereafter onseveral occasions (Fig. 2(a)). In Iwaki City high readings were firstobtained at 00:00 on 15 March, and the readings were intermit-tently elevated thereafter. With the exception of these two loca-tions, the radiation level increased at specific times, and theelevated levels coincided with rain or snow falling on 15 March at14:00 in Koriyama City and at about 17:00 in Fukushima City, Aizu-Wakamatsu City and Iitate Village (Fig. 2(b)).

After the initial detection of deposition, government authoritieshave carried out dose monitoring over wide areas by air dosesurveys, in addition to the collection of dust, soil and water formeasurement. Several sets of field survey (FS) data, which arecontinuously updated, are also available from the websites ofMEXT (2011), Fukushima Prefecture (2011) and FukushimaInternational Association (2011). In this paper, our estimation ofdose rates from deposition densities is compared with these MPand FS data.

Table 2Radioactivity concentration, and 131I/137Cs and 134Cs/137Cs ratios in soil samples collected

(a)

Nuclide Concentration (Bq/g)

Futaba Yamada Namie Tsush129mTe 15.2� 0.29 18.9� 0.51129Te 8.73� 0.17 10.9� 0.30131I 2380� 1.77 209� 0.31132Te 1430� 1.29 104� 0.95132I 1330� 4.30 89.7� 1.09134Cs 186� 0.30 124� 0.04136Cs 39.8� 0.28 2.74� 0.04137Cs 193� 0.40 13.8� 0.05140Ba e�e 0.41� 0.14140La 3.50� 0.11 e�e131I/137Cs 12.3� 0.03 15.1� 0.06134Cs/137Cs 0.96� 0.003 9.0� 0.03

(b)

Nuclide Concentration (Bq/g)

Tokiwa JHS Miyakoji J129mTe 0.37� 0.05 1.40� 0.0129Te 0.24� 0.03 0.85� 0.0131I 5.95� 0.03 28.3� 0.0132Te 3.10� 0.04 10.2� 0.0132I 2.47� 0.05 7.66� 0.0134Cs 0.29� 0.00 0.91� 0.0136Cs 0.038� 0.004 0.161� 0.0137Cs 0.34� 0.00 1.03� 0.0140Ba 0.01� 0.02 0.03� 0.0140La 0.0� 0.00 0.05� 0.0131I/137Cs 17.5� 0.09 27.5� 0.2134Cs/137Cs 0.85� 0.000 0.9� 0.0

(c)

Nuclide Concentration (Bq/g)

Miharu Town FukushimaNiihama Pk

Minami-Soma KouOh

129mTe 1.02� 0.07 5.49� 0.05 1.77� 0.12 3.129Te 0.68� 0.04 3.32� 0.03 1.23� 0.07 2.131I 7.58� 0.02 31.9� 0.04 24.8� 0.1 15.132Te 8.23� 0.04 32.5� 0.4 9.59� 0.29 20132I 6.32� 0.05 27.9� 0.5 8.92� 0.31 17134Cs 1.05� 0.01 3.84� 0.03 1.09� 0.004 4.136Cs 0.138� 0.004 0.806� 0.005 0.242� 0.01 0.8137Cs 1.20� 0.01 4.38� 0.003 1.25� 0.004 4.140Ba 0.023� 0.022 0.12� 0.02 0.081� 0.041 0.1140La 0.041� 0.003 e�e e�e131I/137Cs 6.3� 0.06 7.3� 0.01 19.8� 0.10 3134Cs/137Cs 0.88� 0.011 0.9� 0.01 0.87� 0.00 0.

� indicates counting statistics error. e indicates under the detection limit.

3. Results and discussion

3.1. Soil contamination

An example of a g-ray spectrum measured from a soil sample isshown in Fig. 3. Detected radionuclides included 129mTe, 129Te, 131I,132Te, 132I, 134Cs, 136Cs, 137Cs, 140Ba and 140La, and volatile fissionproducts were primarily observed. Moreover, 99mTc and 110mAg alsoappeared likely to be present in the spectrum, but could not bequantified, owing to large Compton tails and overlap with g-raysfrom other radionuclides. Radioactive concentrations in the0e5 cm sample were decay-corrected to 17:00 on 15 March 2011and are summarized in Table 2. The activity concentration ratios of131I and 134Cs with respect to 137Cs are also listed in Table 2. The131I/137Cs ratio at the southerly sites of Iwaki Chuo Interchange (IC)and Suetsugi Station (St) are higher compared with other samples;however, the radioactivity profiles for all samples are generally

from (a) highly contaminated regions, (b) schools and (c) urban cities.

ima Iitate Warabidaira Namie Akogi

39.7� 0.7 76.2� 0.723.7� 0.4 46.3� 0.4123� 0.2 596� 0.6222� 1 414� 2182� 1 361� 228.0� 0.05 49.8� 0.025.97� 0.05 10.1� 0.0831.3� 0.06 56.7� 0.021.76� 0.17 2.33� 0.27

e�e 2.98� 0.063.9� 0.01 10.5� 0.01

0.89� 0.00 0.88� 0.000

HS Kawauchi ES Katsurao JHS

9 0.43� 0.06 1.34� 0.105 0.26� 0.03 1.04� 0.066 14.1� 0.06 60.7� 0.167 7.29� 0.121 36.1� 0.3459 5.27� 0.148 26.6� 0.4301 1.08� 0.012 4.16� 0.02706 0.16� 0.011 0.787� 0.0231 1.26� 0.014 4.91� 0.0323 0.09� 0.05 0.09� 0.100 0.11� 0.01 0.16� 0.017 11.2� 0.13 12.4� 0.091 0.86� 0.01 0.85� 0.008

riyamashima-Higashi Pk

Iwaki Chuo IC Suetsugi St Kittoya Mt

72� 0.17 0.139� 0.145 0.39� 0.07 1.14� 0.0943� 0.11 0.098� 0.085 0.41� 0.04 0.61� 0.0565� 0.09 7.40� 0.076 40.9� 0.07 14.4� 0.05.2� 0.4 0.887� 0.034 4.52� 0.078 9.04� 0.096.0� 0.5 0.863� 0.041 3.44� 0.077 6.82� 0.12401� 0.006 0.051� 0.002 0.626� 0.006 1.09� 0.00950� 0.020 0.008� 0.007 0.114� 0.005 0.20� 0.00859� 0.007 0.074� 0.020 0.726� 0.007 1.29� 0.01146� 0.068 0.022� 0.013 e�e 0.046� 0.039e�e 0.024� 0.003 0.015� 0.003 0.053� 0.005.4� 0.02 100� 27 56.3� 0.55 11.2� 0.1087� 0.002 0.69� 0.188 0.90� 0.01 0.84� 0.01

S. Endo et al. / Journal of Environmental Radioactivity 111 (2012) 18e2722

similar. The radionuclide activity concentrations (Bq/g) were con-verted to deposition densities (kBq/m2) by using the sample massand sampling area size (6.5 cm diameter), as listed in Table 3.

For each sampling location, the radionuclide distributions at thetwo sampling depths were similar to each other. However, for 15samples taken at each depth, 95% of the radionuclides depositedwere in the 0e5 cm sample. Therefore, the radionuclides in the5e10 cm soil samples were omitted from the dose rate analysis.

3.2. Dose rate estimation

The estimated dose rates for soil sampling points were calcu-lated, where the data was split into three groups: (1) highlycontaminated regions, (2) school locations 20e40 km from FDNPPand (3) contaminated cities.

(1) Dose rate variation over time for highly contaminated regions

The most highly contaminated area was located to the north-west of FDNPP. In this area we sampled soil from four sites: FutabaYamada, Namie Akogi, Iitate Warabidaira and Namie Tsushima. Theestimated time variations for dose rates at these sites are shown inFig. 4(a)e(d). For comparison, the FS data collected by the

Table 3Radioactivity deposition density for (a) highly contaminated regions, (b) schools and (c)

(a)

Nuclide Deposition density (kBq/m2)

Futaba Yamada Namie Tsushim129mTe 8.00� 102� 15 6.99� 102� 19129Te 4.60� 102� 9 4.05� 102� 11131I 1.25� 105� 93 7.74� 102� 12132Te 7.53� 104� 68 3.87� 102� 35132I 6.98� 104� 226 3.32� 102� 41134Cs 9.78� 103� 16 4.61� 102� 1.4136Cs 2.10� 102� 15 1.02� 102� 1.4137Cs 1.01� 104� 21 5.10� 102� 1.7140Ba e�e 9.19� 101� 11140La 184� 6 e�e

(b)

Nuclide Deposition density (kBq/m2)

Tokiwa JHS Miyakoji JHS129mTe 3.29� 101� 4.4 1.11� 102� 7129Te 2.18� 101� 2.4 6.71� 101� 4131I 5.33� 102� 2.3 2.24� 103� 4132Te 2.77� 102� 3.5 8.05� 102� 6132I 2.21� 102� 4.7 6.07� 102� 8134Cs 2.58� 101� 0.1 7.18� 101� 0136Cs 3.4� 0.3 1.28� 101� 0137Cs 3.03� 101� 0.4 8.19� 101� 0140Ba 1.0� 1.7 2.8� 2140La 1.6� 0.2 4.2� 0

(c)

Nuclide Deposition density (kBq/m2)

Miharu FukushimaNiihama Pk

Minami-Soma KouriyamOhshima

129mTe 71.3� 4.6 4.85� 102� 4 5.86� 101� 0.4 9.96� 10129Te 47.4� 2.5 2.93� 102� 2 4.06� 101� 2.3 6.51� 10131I 528� 1.4 2.82� 103� 4 8.20� 102� 4 4.18� 10132Te 573� 3 2.87� 103� 37 3.18� 102� 9 5.39� 10132I 440� 4 2.47� 103� 41 2.95� 102� 10 4.53� 10134Cs 73.3� 0.4 3.39� 102� 0.2 3.62� 101� 0.1 1.07� 10136Cs 9.6� 0.3 7.13� 101� 0.5 8.0� 0.4 2.27� 10137Cs 83.4� 0.5 3.88� 102� 0.3 4.12� 101� 0.1 1.23� 10140Ba 1.6� 1.5 1.09� 101� 2.2 2.7� 1.4 3140La 2.9� 0.2 e�e e�e

� indicates counting statistics error. e indicates under the detection limit.

government authorities (MEXT, 2011; Fukushima Prefecture, 2011;Fukushima International Association, 2011) near our samplingpoints (within a few kilometers) are also shown in the figures. Theestimated dose rate variations are consistent with the FS data,except for Futaba Yamada, which is located 4.4 km northwest ofFDNPP. When the time variation for Futaba Yamada is comparedwith Futaba Yamada FS data, which is located 1 km away from oursampling point, the estimated rates are 3-fold higher at FutabaYamada. However, the estimated dose rates for Namie Akogi, NamieTsushima and Iitate Warabidaira are in good agreement with thecorresponding FS data. These comparisons indicate that themethod used to estimate air dose rate variations over time givesreasonably accurate predictions. The estimated dose rates for thefour locations immediately after deposition and 3 months afterdeposition are listed in Table 4. High dose rates are still expected toexist in each of these locations.

(2) Dose rate variation over time for schools

The examined JHSs and ES have been used as shelters forevacuated residents. Initial estimates of the dose rates at theseschools are highly relevant for calculating the radiation exposure ofthese evacuees. Estimates of future dose rates are also relevant if

urban cities.

a Iitate Warabidaira Namie Akogi

2.34� 103� 39 2.97� 103� 281.40� 103� 22 1.80� 103� 177.23� 103� 13 2.32� 104� 221.31� 104� 60 1.61� 104� 721.07� 104� 87 1.40� 104� 931.65� 103� 3 1.94� 103� 13.51� 102� 3 3.95� 102� 31.84� 103� 3 2.21� 103� 11.03� 102� 10 9.1� 101� 10e�e 1.16� 102� 2

Kawauchi ES Katsurao JHS

1.90� 101� 2.6 7.16� 101� 5.5.2 1.14� 101� 1.4 5.54� 101� 3.4

6.14� 102� 2.8 3.24� 103� 93.18� 102� 5.3 1.93� 103� 182.30� 102� 6.5 1.42� 103� 23

.6 4.70� 101� 0.5 2.22� 102� 1

.5 7.2� 0.5 4.20� 101� 1.2

.7 5.49� 101� 0.6 2.62� 102� 1.7

.6 4.1� 2.3 4.7� 5.6

.3 4.8� 0.4 8.4� 0.7

a-Higashi Pk

Iwaki Chuo IC Suetsugi St Kittoya Mt

1� 4.7 4.29� 4.5 1.33� 101� 2.2 4.62� 101� 3.61� 2.9 3.02� 2.6 1.40� 101� 1.5 2.48� 101� 1.92� 2.3 2.29� 102� 2.4 1.39� 103� 2.5 5.83� 102� 2.02� 10 2,75� 101� 1.1 1.53� 102� 2.7 3.67� 102� 3.92� 14 2,67� 101� 1.3 1.17� 102� 2.6 2.77� 102� 5.02� 0.2 1.57� 0.07 2.13� 101� 0.2 4.43� 101� 0.41� 0.5 0.25� 0.21 3.9� 0.2 8.2� 0.31� 0.2 2.27� 0.61 2.46� 101� 0.3 5.24� 101� 0.4.9� 1.8 0.69� 0.39 e�e 1.9� 1.6e�e 0.75� 0.08 0.5� 0.1 2.1� 0.2

Fig. 4. Estimated variation of dose rates over time for (a) Futaba Yamada, (b) Namie Akogi, (c) Iitate Warabidaira and (d) Namie Tsushima. The plots are of FS data found at nearbylocations, (a) Futaba Yamada ( ); (b) Namie Akogi ( ) and Namie Akogi Teshichiro ( ); (c) Iitate Wrabidaira ( ) (d) Namie Tsushima ( ) and Namie Shimo-Tsushima ( ),respectively (MEXT, 2011; Fukushima Prefecture, 2011; Fukushima International Association, 2011).

S. Endo et al. / Journal of Environmental Radioactivity 111 (2012) 18e27 23

the schools return to their intended use and children are presentduring a normal school day. Changes in estimated dose rates overtime for the four sampled schools are shown in Fig. 5(a)e(d).

These figures also show each school’s monitoring data and FSdata taken at nearby locations (MEXT, 2011; Fukushima Prefecture,2011; Fukushima International Association, 2011). The estimateddose rate variations match all the FS datawell. Estimated dose ratesfor the schools immediately after deposition and 3 months afterdeposition are listed in Table 5. Our estimates suggest that doserates at three of the schools will be lower than 1 mSv/h after 3months. However, a higher rate is found at Katsurao JHS, which islocated in the path of the northwesterly deposition plume.

(3) Dose rate variation over time for cities

There are several cities near FDNPP with relatively high pop-ulations, and thus there was considerable concern about the

Table 4Dose rates estimated from radionuclide deposition density immediately after and 90 d aftnear our sampling points of Futaba Yamada, Namie Akogi, Iitate Nagadoro and Namie Ts

Location Distance fromFDNPP (km)

Air dose rate (mSv/h)

Estimated from depo

At deposition

Futaba Yamada 4.4 1000Namie Akogi 26 220Iitate Warabidaira 29 150Namie Tsushima 29 55

potential radiation exposure to the residents of these cities. Theestimated dose rate changes for Fukushima Niihama Pk, KoriyamaOshima-Higashi Pk, Miharu, Minami-Soma, Iwaki Chuo IC andSuetsugi St are shown in Fig. 6(a)e(f). Estimated dose rates for thecities immediately after deposition and 3 months after depositionare listed in Table 6.

For Fukushima Niihama Pk, our estimated air dose rates areabout 2-fold higher than those from MP data at Fukushima cityoffice. This disparity is most likely caused by the difference inlocation between the soil sampling point and the MP point ofFukushima city office, reported by the government (MEXT, 2011;Fukushima Prefecture, 2011; Fukushima International Association,2011). The decreasing pattern was similar, but the absolute valuesare different with a factor of 2.2. The factor of 2.2 corresponds toa ratio between the dose rates of 3.9 mSv/h at Niihama Pk from FSdata and 1.8 mSv/h from MP data at the Fukushima city officemonitoring post on 22 April. The estimated dose rates for Koriyama

er deposition for highly contaminated regions. FS data collected byMEXT at locationsushima about 90 d after deposition (MEXT, 2011).

sition density FS data at 90 d after deposition

At 90 d after deposition

98 e

20 18.217 15.34.8 7.4

Fig. 5. Dose rate variations for soil sampled at four schools: (a) Miyakoji JHS, (b) Tokiwa JHS; (c) Katsurao JHS and (d) Kawauchi ES. The plots are of FS data found at nearby location,(a) Miyakoji ( ) and Miyakoji JHS ( ); (b) Tokiwa Yamane ( ) and Tokiwa JHS ( ); (c) Katsurao Uenogawa ( ) and Katsurao JHS ( ); (d) Kami-Kawauchi ( ) and Kawauchi ES ( ) byMEXT, respectively (MEXT, 2011; Fukushima Prefecture, 2011; Fukushima International Association, 2011).

S. Endo et al. / Journal of Environmental Radioactivity 111 (2012) 18e2724

Oshima-Higashi Pk (Fig. 6(b)) are lower than those from the MPdata of Koriyama joint government building, which is about 1.7 kmwest from the soil sampling location. The reason for this differenceis not clear. One possibility relates to our sampling point; the soilsample was taken from a sandbox in Ohshima-Higashi Pk. Sincerain water is easily drained in sand, the deposited radionuclidesmay have migrated to a depth lower than 5 cm.

For Miharu (Fig. 6(c)), the estimated dose rate change repro-duces theMP data. In contrast, for Minami-Soma, Iwaki Chuo IC andSuetsugi St, the MP data of Iwaki Taira and Iwaki Chuodai indicatethat several depositions occurred at different times, and thus theestimated dose rate changes do not match the MP data. However,the general trends are consistent with MP data. The spikes in thedose rate in the MP data may be due to radioactive noble gases(mainly 133Xe), that are not deposited on the ground. Soil was

Table 5Dose rates immediately after and 90 d after deposition for soil sampled at schools. FS daTokiwa Yamane, Katsurao Kaminogawa and Kami-Kawauchi about 90 days after deposit

Location Distance from FDNPP (km) Air dose rate (mS

Estimated from d

At deposition

Miyakoji JHS 22 12Tokiwa JHS 35 3.7Katsurao JHS 25 24Kawauchi ES 23 4.2

sampled at Minami-Soma City about 300 h after 17:00 on 15 March2011. Taking into account this sampling time, the estimated doserate from the soil contamination is consistent with the MP data.Similarly at Suetsugi St, the MP data increased about 130 h after17:00 on 15 March 2011, which was exactly when we sampled thissite. As shown in Fig. 6(e) and (f), the time variation of dose ratesobtained by the soil contamination for Suetsugi St and Iwaki ChuoIC is different from the MP data, possibly due to further depositionoccurring after the soil sampling at these sites.

3.3. Cumulative dose

Cumulative doses were estimated based on the dose rate valuescalculated above. The cumulative doses for 3 months (D3m) and 1 y(D1y) after the deposition are obtained by integrating the changes in

ta collected by MEXT at locations near our sampling points of Miyakoji Furumichi,ion (MEXT, 2011).

v/h)

eposition density FS data at 90 d after deposition

At 90 d after deposition

0.8 1.00.28 0.42.3 2.30.49 0.5

Fig. 6. Dose rate variation over time for: (a) Fukushima Niihama Pk, (b) Koriyama Oshima-Higashi Pk, (c) Miharu, (d) Minami-Soma, (e) Iwaki Chuo IC and (f) Suetsugi St. The plotsare of FS data found at nearby locations, (a) Fukushima city office MP ( ); (b) Koriyama prefectural office MP ( ) and Oshima ES ( ); (c) Miharu Town ( ); (d) Minami-Soma MP ( );(e) Iwaki Taira MP ( ) and Iwaki Chuodai MP ( ); (f) Iwaki Taira MP ( ) and Iwaki Hisanohama ( ) by MEXT, respectively (MEXT, 2011; Fukushima Prefecture, 2011; FukushimaInternational Association, 2011).

S. Endo et al. / Journal of Environmental Radioactivity 111 (2012) 18e27 25

dose rate over the respective time period and are summarized inTable 7. The D3m and D1y values at the locations greater than 20 kmfrom FDNPP, namely, Namie Akogi, Iitate Warabidaira and NamieTsushima, are 15e57 mSv and 51e210 mSv, respectively. Thesevalues exceed the maximum value of the 1e20 mSv/y referencelevel based on the recommendations by ICRP (2007, 2009) fora post-emergency situation. From our measurements, the indoor

radiation dose rate was reduced by a factor of about 0.5e0.7 fromthe outdoor value with the shielding of the building, depending onits structural materials.

For the schools where soils were sampled, the dose rates areestimated to be 0.8e2.3 mSv/h at 3 months after the deposition.Cumulative doses for 3 months and 1 y after the deposition are1.9e6.7 mSv and 3.0e25.0 mSv, respectively. The cumulative dose

Table 6Dose rates immediately after and 90 d after deposition for four cities and Miharu Twon. FS data collected by MEXT at locations near our sampling points of FukushimaUniversity ES, Koriyama Ohshima-Higashi Pk, Miharu Town office, Minami-Soma town hall, Iwaki joint government building and Iwaki Hisanohama about 90 days afterdeposition (MEXT, 2011).

Location Distance from FDNPP (km) Air dose rate (mSv/h)

Estimated from deposition density FS data at 90 d after deposition

At deposition At 90 d after deposition

Fukushima Niihama Pk 62 39 3.8 2.3Koriyama Ohshima-Higashi Pk 60 7.0 1.0 1.2a

Miharu 48 6.6 0.74 0.5Minami-Soma 26 5.1 0.39 0.5Iwaki Chuo IC 44 0.71 0.03 0.3Suetsugi St 29 4.0 0.27 0.2

a Dose rate at 50 cm

S. Endo et al. / Journal of Environmental Radioactivity 111 (2012) 18e2726

for 1 y at Katsurao JHS was estimated as the highest value of25 mSv, which exceeds 20 mSv/y. Although the doses at the otherschools do not exceed 20 mSv, an exposure limit of 1 mSv/y shouldbe applied to children under normal circumstance (UNSCEAR,2008; WHO, 2006). Therefore, careful surveillance is needed evenif the cumulative dose for 1 y does not exceed 20 mSv.

The cumulative dose for Fukushima Niihama Pk for 1 y is esti-mated to be high at 40 mSv, also exceeding the emergency expo-sure reference value. As mentioned above, the dose rates estimatedfor Koriyama Oshima-Higashi Pk from soil deposition data wereabout 1/3 of those estimated from MP data; therefore, the esti-mated cumulative dose at this location remains an open question.The cumulative doses at Koriyama Oshima-Higashi Pk are 2.3 mSvand 10 mSv for 3 months and 1 y, respectively. However, there isa factor of 3 discrepancy between estimated dose rate and Kor-iyama prefectural office MP data, and if the former is tripled, thecumulative dose exceeds 20 mSv/y. Cumulative doses in MiharuTown, Iwaki City and Minami-Soma City are 0.3e7.7 mSv for 1 y.These values do not exceed the emergency exposure referencelevel. Clearly, the need for further surveillance of external doserates is different from location to location.

These estimates are based on the assumption that people stayoutside of houses or buildings for a year. Since people will spenda certain percentage of time indoors, the effective values areexpected to be found by multiplying the estimations to account forthe shielding factor provided by houses or buildings. Thus, MEXTuses a effective shielding factor of 0.6, averaged between spending8 h in awooden housewith a 0.4 shielding factor and 16 h outdoors

Table 7Cumulative dose for 3 months (D3m) and 1 year (D1y) without effective shielding andwith effective shielding factor of 0.6.

Location Distance fromFDNPP (km)

D3m (mSv) D1y (mSv)

Withoutshielding

Withoutshielding

Withshielding

Futaba Yamada 4.4 290 1000 600Namie Akogi 26 57 210 126Iitate Warabidaira 29 42 170 102Namie Tsushima 29 15 51 31Miyakoji JHS 22 2.9 8.7 5.2Tokiwa JHS 35 0.9 3.0 1.8Katsurao JHS 25 6.7 25 15Kawauchi ES 23 1.3 5.0 3Fukushima Niihama Pk 62 10 40 24Koriyama

Ohshima-Higashi Pk60 2.3 10 6

Miharu 48 1.9 7.7 4.6Minami-Soma 26 1.3 4.2 3.5Iwaki Chuo IC 44 0.17 0.31 0.19Suetsugi St 29 1.1 2.8 1.7Kittoya Mt 31 1.3 4.9 2.9

with a factor of 1.0 (0.4�16/24þ1.0� 8/24¼ 0.6) (MEXT, 2011). Ifwe assume the effective shielding factor of 0.6 is used for thecumulative dose for 1 y, the values are summarized in Table 7.However, the highly contaminated areas and Fukushima City areestimated to have still elevated external doses, even if the effectiveshielding factor is included. Therefore, remediation will almostcertainly be needed for residents to return to these cities.

4. Conclusion

High radioactive deposition in soil taken from locations nearFDNPP was detected soon after deposition occurred. The calculateddose rates from detected radionuclides in soil samples weregenerally consistent with monitoring data collected by MEXT. Onthe basis of a limited sampling campaign, the external dose rate at3 months after the deposition and the cumulative dose for 1 y tothe northwest of FDNPP were estimated to be 4.8e98 mSv/h and 51to 1�103 mSv, respectively. The highest values, 98 mSv/h and1�103 mSv, were found at Futaba Yamada 4.4 km away fromFDNPP. For the four schools and four cities, where are at least22 km away from FDNPP, the external dose rate at 3 months was0.03e3.8 mSv/h and the cumulative dose for 1 y was 3e40 mSv. Thehighest exposure was at Fukushima Niihama Park where thecumulative dose was estimated to be 40 mSv for 1 y. Carefulremediation strategies must be developed to deal with theseextremely high external dose rates. Finally, we note that a contin-uous and intensive survey of radionuclide deposition densitieswill be needed for the area surrounding FDNPP.

Acknowledgment

We thank Mr. M. Ikeza of Japan Broadcasting Corporation for hisassistance. The measurements with the Ge detector were carriedout at the Radiation Laboratory of the Graduate School of Engi-neering, Hiroshima University.

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