ESTIMATION OF EFFECTIVE DOSE FROM EXTERNAL EXPOSURE DUE TO SHORT-LIVED NUCLIDES IN THE PREFECTURES SURROUNDING FUKUSHIMA

ESTIMATION OF EFFECTIVE DOSE FROM EXTERNAL EXPOSURE DUE TO SHORT-LIVED NUCLIDES IN THE... Abstract The Fukushima Daiichi Nuclear Power Plant (FDNPP) accident resulted in a release of radionuclides into the environment. Since the accident, measurements of radiation in the environment such as air dose rate and deposition density of radionuclides have been performed by various organizations and universities. In particular, Japan Atomic Energy Agency (JAEA) has been performing observations of air dose rate using a car-borne survey system continuously over widespread areas. Based on the data measured by JAEA, we estimated effective dose from external exposure in the prefectures surrounding Fukushima. Since car-borne survey started a few months after the accident, the main contribution to measured data comes from 137Cs and 134Cs whose half-lives are relatively long. Using air dose rate of 137Cs and 134Cs and the ratio of deposition density of short-lived nuclides to that of 137Cs and 134Cs, we also estimated contributions to the effective dose from other short-lived nuclides. INTRODUCTION The Fukushima Daiichi Nuclear Power Plant accident caused a release of radionuclides which were deposited onto the ground not only in Fukushima Prefecture but also in nearby prefectures. Since the accident, measurements of radiation in the environment such as air dose rate and deposition density of radionuclides have been performed by various organizations and universities. In particular, Japan Atomic Energy Agency (JAEA) has been repeatedly performing observations of air dose rate using a car-borne survey system over widespread areas. There have been 11 surveys performed as of March 2017(1). In our study, considering some factors such as time spent indoors and outdoors, background radiation dose rate and radiation reduction by houses, and using the air dose rate data measured by JAEA(2), we estimated effective doses from external exposure in the six prefectures adjacent to the Fukushima Prefecture (Iwate, Miyagi, Ibaraki, Tochigi, Gunma and Chiba Prefecture). United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) has also estimated effective doses using measured data of radiation in the environment and considering deposition density of radionuclides(3). There is a similarity between UNSCEAR’s method and ours in terms of using data measured in the environment. However, the data that we have used have been measured repeatedly while the data used by UNSCEAR was obtained by a single measurement. In our study, we estimated effective dose from external exposure based on air dose rate measured by car-borne survey by JAEA, and compared its value to the one estimated by UNSCEAR. MATERIALS AND METHODS In our study, we analyzed measured air dose rate using two models fitted with time-dependent function of air dose rate. Multiplying by conversion factors and subtracting background radiation dose rate, air dose rate can be converted to effective dose rate caused by nuclides that were released by the FDNPP accident. Integrating this, cumulative effective dose from external exposure can be obtained. Models of decrease of air does rate Air dose rate decreases with time due to the radioactive decay and weathering effect. In the case of radioactive decay, air dose rate decrease is determined by the half-life of each nuclide. Weathering effect is a natural phenomenon, in which wind and rainfall promotes the migration of nuclides to another place or into the soil, therefore, this depends on weather conditions or soil characteristics of particular areas. In this study, we use two models to analyze decrease of air dose rate. The first model is described by equation (1). This model assumes that decrease in air dose rate Dair(t) (μSv/h) can be simply expressed by one exponential function: Dair(t)=αexp(−βt)+γ (1) where α (μSv/h) is a factor of an exponential term, β (y−1) is an effective decay constant and γ (μSv/h) is a constant term. The second model is described by equation (2) in which decay of nuclides and weathering are considered: Dair(t)=(D0−DBG)×{pexp(−0.693Tfastt)+(1−p)exp(−0.693Tslowt)}×kexp(−λ134t)+exp(−λ137t)k+1+DBG (2) where D0 (μSv/h) is the initial value of air dose rate and DBG (μSv/h) is the background radiation dose rate. The value of DBG ranges from 0.055 to 0.066 μSv/h, which was measured in the area of interest before the accident(4). According to the previous studies, weathering effect can be expressed empirically as two exponential functions representing a fast reduction component and a slow reduction component as shown in the second term in equation (2), where p is a non-dimensional fraction of fast component, and Tfast (y) and Tslow (y) are the half-lives of fast and slow components(5). The third term in equation (2) corresponds to radioactive decay of 137Cs and 134Cs, where k is a non-dimensional ratio of 134Cs to 137Cs contributing to air dose under the assumption that the initial value of deposition density of each nuclide is the same(6), and λ134 (y−1) and λ137 (y−1) are the decay constants of each nuclide. Conversion to effective dose Considering factors such as time spent indoors Tin (h/d) and time spent outdoors Tout (h/d), background dose rate DBG and radiation reduction factor by houses r (dimensionless), air dose rate Dair(t) can be converted to effective dose rate dE(t)/dt (μSv/y) as shown in equation (3): dE(t)dt=(qDair(t)−DBG)×(rTin+Tout)×d×c (3) where q (dimensionless) is the ratio of air dose rate in living environment to that on streets, d (d/y) is the number of days per year and c (dimensionless) is the conversion factor between air dose rate and effective dose. In car-borne survey, measurement is conducted on streets with detectors set in a car. The measured air dose rate is the one on streets. As a result of traffic, nuclides deposited onto streets are more likely to be removed or penetrate into soil compared to those deposited in living environment. For this reason, air dose rates measured by car-borne survey tend to be smaller than the air dose rate measured in living environment. The latter is ~1.2 times larger than the former as was demonstrated by man-borne measurements simultaneously conducted in the vicinity of the streets where car-borne measurements were done(7). Considering this difference, the coefficient q (=1.2) is multiplied by Dair(t). Air dose rate includes background radiation dose rate. By subtracting background radiation dose rate, only the net contribution from nuclides released by the FDNPP accident is considered. To determine the background dose rate, we used the data measured in prefectural capitals of each prefecture before the accident(4). After the accident, a questionnaire survey on time spent indoors and outdoors of Fukushima City office workers was conducted. According to the results of the survey, indoor workers spent 0.57 h per day outdoors on average (Tin = 23.43 h/d and Tout = 0.57 h/d)(8). Although Fukushima Prefecture was not the subject of this study, we used these values. Radiation is reduced indoors because of the shielding effect of houses. In this study, we adopted a value of 0.4 for reduction factor r, which is a value recommended for one and two-story wood-frame house by IAEA(9). In addition, some investigations conducted after the accident in real houses in Japan have shown that radiation reduction factor by houses were ~0.4(7, 10). For conversion factor from air dose rate to effective dose c, we adopted the value of 0.6, where the target is assumed to be adult(11). According to a previous study where it is assumed that radioactive 137Cs and 134Cs are distributed uniformly in the soil over a planar area and at specific depths, the value is also calculated almost 0.6(12). The product of (rTin+Tout)×d×c will be defined as fi2,4 as shown later in Figure 1. Figure 1. View largeDownload slide Estimation flow of cumulative effective dose contributed from each nuclide. Figure 1. View largeDownload slide Estimation flow of cumulative effective dose contributed from each nuclide. Finally, by integrating equation (3), cumulative effective dose (in the period from T1 (y) to T2 (y)) can be estimated as shown in the equation below. E(T1,T2)=∫T1T2dE(t)dtdt (4) Effective dose considering short-lived nuclides Since car-borne survey started three months later after the accident, the contribution of short-lived nuclides such as 131I could be considered negligible and the contribution of 137Cs and 134Cs is dominant in the survey result. Therefore, based on air dose rate of 137Cs and 134Cs and the ratio of deposition density of short half-life nuclides to that of 137Cs and 134Cs, we also estimated contributions to the effective dose not only from 137Cs and 134Cs but also from other short-lived nuclides. The initial value Dair(t) can be obtained by making t = 0 in equation (1) or (2). This value can be considered to be the sum of the contribution of 137Cs, 134Cs and background as shown in equation (5): Dair(0)=D1(0)+D2(0)+DBG (5) where D1(0) (μSv/h) is air dose rate caused by 137Cs in the environment and D2(0) (μSv/h) is that of 134Cs in the environment. The deposition from FDNPP lasted for many days. Moreover, each car-borne survey was conducted within a few weeks. Therefore, it is difficult to define the specific origin of time. However, we estimate the cumulative effective dose for 1 year or 10 years since the accident. Therefore, it is not necessary to focus on and consider daily change of air dose rate precisely. Considering these and to simplify data handling, we defined the origin of time (t = 0) as 12 March 2011 and assumed that all nuclides were deposited to the ground at the same time. As shown in Table 1, index i is defined to indicate each nuclide. Index i = 1, 2 corresponds to 137Cs and 134Cs, respecitvely. By subtracting the background dose rate DBG from Dair(0), value D1(0) + D2(0) can be obtained. By defining deposition density for each nuclide i as Ai (kBq/m2) and conversion factor from deposition density Ai to air dose rate as fi1,2 (μSv/h per kBq/m2) (shown in Table 2), for 137Cs (i = 1) and 134Cs (i = 2), the equation below is obtained. D1(0)+D2(0)=f11,2A1+f21,2A2 (6) Table 1. Deposition ratio and half-life for each nuclide i. i Nuclide θi Tihalf 1 137Cs + 137mBa 1.0 30 y 2 134Cs 1.0 2.1 y 3 136Cs 0.17 13 d 4 131I 12 8.0 d 5 129m + 129Te 1.1 34 d 6 132Te + 132I 8.0 3.2 d 7 110m + 110Ag 0.0028 25 d i Nuclide θi Tihalf 1 137Cs + 137mBa 1.0 30 y 2 134Cs 1.0 2.1 y 3 136Cs 0.17 13 d 4 131I 12 8.0 d 5 129m + 129Te 1.1 34 d 6 132Te + 132I 8.0 3.2 d 7 110m + 110Ag 0.0028 25 d View Large Table 1. Deposition ratio and half-life for each nuclide i. i Nuclide θi Tihalf 1 137Cs + 137mBa 1.0 30 y 2 134Cs 1.0 2.1 y 3 136Cs 0.17 13 d 4 131I 12 8.0 d 5 129m + 129Te 1.1 34 d 6 132Te + 132I 8.0 3.2 d 7 110m + 110Ag 0.0028 25 d i Nuclide θi Tihalf 1 137Cs + 137mBa 1.0 30 y 2 134Cs 1.0 2.1 y 3 136Cs 0.17 13 d 4 131I 12 8.0 d 5 129m + 129Te 1.1 34 d 6 132Te + 132I 8.0 3.2 d 7 110m + 110Ag 0.0028 25 d View Large Table 2. Conversion factors fi1,2 and fi1,3 (10−3 μSv/h per kBq/m2)(14). i Nuclide fi1,2 fi1,3 1 137Cs+137mBa 2.3 — 2 134Cs 6.3 — 3 136Cs — 4.7 4 131I — 0.85 5 129m + 129Te — 0.15 6 132Te + 132I — 5.5 7 110m + 110Ag — 6.1 i Nuclide fi1,2 fi1,3 1 137Cs+137mBa 2.3 — 2 134Cs 6.3 — 3 136Cs — 4.7 4 131I — 0.85 5 129m + 129Te — 0.15 6 132Te + 132I — 5.5 7 110m + 110Ag — 6.1 View Large Table 2. Conversion factors fi1,2 and fi1,3 (10−3 μSv/h per kBq/m2)(14). i Nuclide fi1,2 fi1,3 1 137Cs+137mBa 2.3 — 2 134Cs 6.3 — 3 136Cs — 4.7 4 131I — 0.85 5 129m + 129Te — 0.15 6 132Te + 132I — 5.5 7 110m + 110Ag — 6.1 i Nuclide fi1,2 fi1,3 1 137Cs+137mBa 2.3 — 2 134Cs 6.3 — 3 136Cs — 4.7 4 131I — 0.85 5 129m + 129Te — 0.15 6 132Te + 132I — 5.5 7 110m + 110Ag — 6.1 View Large Assumuing that the initial values of deposition density of 137Cs and 134Cs are the same (A1 = A2)(13), parameter A2 can be eliminated, then the equation below is obtained. A1=D1(0)+D2(0)f11,2+f21,2 (7) Defining the ratio of initial deposition density of each nuclide i to that of 137Cs (i = 1) as θi (dimensionless), the equation below can be obtained. Ai=θiA1 (8) Although this ratio has been studied in many previous studies, in this study, we adopted the same values as UNSCEAR as shown in Table 1(13). Defining the conversion factor from deposition density Ai to effective dose rate dεi/dt as fi1,3 (μSv/h per kBq/m2) (shown in Table 2), effective dose rate contributed from each nuclide i can be expressed as equation (9), where Tihalf is half-life for nuclide i. dεi(t)dt=fi1,3Aiexp(−ln2Tihalft) (9) By multiplying factors defining its product as fi3,4 in equation (10), this can be converted to effective dose that considers time spent indoors and outdoors, and reduction factor as shown in equation (11). fi3,4=(rTin+Tout)×d (10) dEi(t)dt=fi3,4dεi(t)dt (11) Finally, by summing and integrating equation (11), cumulative effective dose including short-lived nuclides can be obtained. Etot(T1,T2)=E(T1,T2)+∑i=37∫T1T2dEi(t)dtdt (12) Figure 1 summarizes the estimation flow of cumulative effective dose contributed from each nuclide explained above. RESULTS AND DISCUSSIONS Time-dependent decrease of air dose rate By fitting the average of measured data of air dose rate with the two models shown above, we analyzed time-dependent decrease of air dose rate in the six prefectures (Iwate, Miyagi, Ibaraki, Tochigi, Gunma Pref and Chiba Prefecture). In the first model, parameters α, β and γ were free. In the second model, D0, p and Tfast were free, although Tslow (=92 y)(5) and the value of DBG of each prefecture were constant. These free parameters were determined by fitting using the two models. However, for Iwate, Gunma and Chiba Prefectures where measured values of air dose rate were relatively small, there were some cases where fitting parameters were not determined appropriately. In order to avoid this, we used only the first model for these three prefectures, and used some fitting parameters determined for the other prefectures. Figure 2 shows the air dose rate in Tochigi Prefecture. The dots show the averaged values of measured data. The solid lines and the dashed lines show the ones determined by fitting with the two models. Figure 2. View largeDownload slide Air dose rate (Tochigi Pref.). Figure 2. View largeDownload slide Air dose rate (Tochigi Pref.). Conversion of air dose rate to effective dose Based on the time-dependent decrease of air dose rate for each prefecture, we estimated cumulative effective dose. As mentioned above, this includes the contributions not only from 137Cs and 134Cs but also from other short-lived nuclides according to the method shown in Figure 1. Figure 3 shows estimated cumulative effective dose in Tochigi for instance. Figure 3. View largeDownload slide Absolute values of the estimated cumulative effective dose including the contributions not only from 137Cs and 134Cs but also from other short-lived nuclides (Tochigi pref.). Figure 3. View largeDownload slide Absolute values of the estimated cumulative effective dose including the contributions not only from 137Cs and 134Cs but also from other short-lived nuclides (Tochigi pref.). Figure 4 shows the cumulative effective dose in Tochigi normalized by the total value. This figure shows that when t equals 0.01 y corresponding to ~4 days, the contribution from 132Te and 132I was up to ~60% of the total value. Although its contribution is not as large as that of 132Te and 132I, the contribution from 131I is also relatively large (~20%). Although the contribution from these three nuclides gradually decreases as the time passes, the contribution from 137Cs and 134Cs gains significance. When t equals 0.05 y corresponding to ~18 days, the contribution from these short-lived nuclides becomes comparable to that from 134Cs and 137Cs. As the time passes the contribution from 137Cs and 134Cs gets more dominant. When t equals 1 y (1 year after the accident), their contributions in cumulative dose are ~85% and the contribution from 132Te, 132I and 131I are ~15% in total. Figure 4. View largeDownload slide Normalized values of effective dose (Tochigi Pref.) Figure 4. View largeDownload slide Normalized values of effective dose (Tochigi Pref.) Table 3 shows the estimated values of cumulative effective dose for each prefecture. In the column of this study, the values obtained with our method by integrating from T1 = 0 y to T2 = 1 y and 10 y are shown. In the column of UNSCEAR, the values estimated by UNSCEAR are shown. In the report of UNSCEAR(15, 16), the values are estimated at the municipal level for each prefecture. In the case of the value estimated by UNSCEAR for 1 year in Tochigi Prefecture, the minimum is 0.06 mSv in Ashikaga City and the maximum is 1.07 mSv in Nasu Town and Nasushiobara City. Therefore, in Table 3 the values estimated by UNSCEAR are shown as the range from the minimum to the maximum. Table 3. Comparison of estimated results (mSv)(15, 16). Estimated values in this study Estimated values by UNSCEAR Model 1 y 10 y 1 y 10 y Iwate 1 0.24 — 0.14 0.1 Pref. 2 — — ~0.31 ~0.7 Miyagi 1 0.54 0.87 0.05 0.1 Pref. 2 0.54 0.89 ~0.55 ~1.3 Ibaraki 1 0.37 1.2 0.06 0.1 Pref. 2 0.38 1.1 ~0.54 ~1.3 Tochigi 1 0.62 1.7 0.06 0.2 Pref. 2 0.64 1.5 ~1.07 ~2.5 Gunma 1 0.17 — 0.06 0.1 Pref. 2 — — ~0.50 ~1.2 Chiba 1 0.24 — 0.05 0.1 Pref. 2 — — ~0.76 ~1.8 Estimated values in this study Estimated values by UNSCEAR Model 1 y 10 y 1 y 10 y Iwate 1 0.24 — 0.14 0.1 Pref. 2 — — ~0.31 ~0.7 Miyagi 1 0.54 0.87 0.05 0.1 Pref. 2 0.54 0.89 ~0.55 ~1.3 Ibaraki 1 0.37 1.2 0.06 0.1 Pref. 2 0.38 1.1 ~0.54 ~1.3 Tochigi 1 0.62 1.7 0.06 0.2 Pref. 2 0.64 1.5 ~1.07 ~2.5 Gunma 1 0.17 — 0.06 0.1 Pref. 2 — — ~0.50 ~1.2 Chiba 1 0.24 — 0.05 0.1 Pref. 2 — — ~0.76 ~1.8 Table 3. Comparison of estimated results (mSv)(15, 16). Estimated values in this study Estimated values by UNSCEAR Model 1 y 10 y 1 y 10 y Iwate 1 0.24 — 0.14 0.1 Pref. 2 — — ~0.31 ~0.7 Miyagi 1 0.54 0.87 0.05 0.1 Pref. 2 0.54 0.89 ~0.55 ~1.3 Ibaraki 1 0.37 1.2 0.06 0.1 Pref. 2 0.38 1.1 ~0.54 ~1.3 Tochigi 1 0.62 1.7 0.06 0.2 Pref. 2 0.64 1.5 ~1.07 ~2.5 Gunma 1 0.17 — 0.06 0.1 Pref. 2 — — ~0.50 ~1.2 Chiba 1 0.24 — 0.05 0.1 Pref. 2 — — ~0.76 ~1.8 Estimated values in this study Estimated values by UNSCEAR Model 1 y 10 y 1 y 10 y Iwate 1 0.24 — 0.14 0.1 Pref. 2 — — ~0.31 ~0.7 Miyagi 1 0.54 0.87 0.05 0.1 Pref. 2 0.54 0.89 ~0.55 ~1.3 Ibaraki 1 0.37 1.2 0.06 0.1 Pref. 2 0.38 1.1 ~0.54 ~1.3 Tochigi 1 0.62 1.7 0.06 0.2 Pref. 2 0.64 1.5 ~1.07 ~2.5 Gunma 1 0.17 — 0.06 0.1 Pref. 2 — — ~0.50 ~1.2 Chiba 1 0.24 — 0.05 0.1 Pref. 2 — — ~0.76 ~1.8 For Iwate, Gunma and Chiba Prefectures, fitting parameters were not determined appropriately and even though fitting was done appropriately, air dose rate decreases to the original background level before 10 years after the accident. In such cases, we did not estimate the value for 10 years. This table shows that the values estimated using our method are in good agreement with the values estimated by UNSCEAR. Influence of using different combinations of deposition density ratio θi As shown in Figure 1 and equation (8), deposition ratio for each nuclide θi is important parameter in consideration of short-lived nuclides. In Table 1, the values adopted by UNSCEAR are listed, however, there are other values of the measured or estimated ratio θi. Four kinds of the combination of the ratio θi are listed below for example. Ratio1: adopted by the report of UNSCEAR Ratio2: measured in Iitate village in Fkushima prefecture Ratio3: measured in Tokai village in Ibaraki Prefecture Ratio4: value in Tochigi Prefecture estimated using WSPEEDI Table 4 shows each value in detail. By using the values listed in Table 4 and focusing on Tochigi Prefecture, we examined its influence on the estimated results. Table 4. Deposition density ratio θi(13, 17–19). i Nuclide Ratio1 Ratio2 Ratio3 Ratio4 1 137Cs + 137mBa 1 1 1 1 2 134Cs 1 1 1 1 3 136Cs 0.17 0.32 0.12 — 4 131I 12 9.2 8.3 4.6 5 129m + 129Te 1.1 — 0.82 — 6 132Te + 132I 8.0 8.3 6.1 4.6 7 110m + 110Ag 0.0028 — — — i Nuclide Ratio1 Ratio2 Ratio3 Ratio4 1 137Cs + 137mBa 1 1 1 1 2 134Cs 1 1 1 1 3 136Cs 0.17 0.32 0.12 — 4 131I 12 9.2 8.3 4.6 5 129m + 129Te 1.1 — 0.82 — 6 132Te + 132I 8.0 8.3 6.1 4.6 7 110m + 110Ag 0.0028 — — — Table 4. Deposition density ratio θi(13, 17–19). i Nuclide Ratio1 Ratio2 Ratio3 Ratio4 1 137Cs + 137mBa 1 1 1 1 2 134Cs 1 1 1 1 3 136Cs 0.17 0.32 0.12 — 4 131I 12 9.2 8.3 4.6 5 129m + 129Te 1.1 — 0.82 — 6 132Te + 132I 8.0 8.3 6.1 4.6 7 110m + 110Ag 0.0028 — — — i Nuclide Ratio1 Ratio2 Ratio3 Ratio4 1 137Cs + 137mBa 1 1 1 1 2 134Cs 1 1 1 1 3 136Cs 0.17 0.32 0.12 — 4 131I 12 9.2 8.3 4.6 5 129m + 129Te 1.1 — 0.82 — 6 132Te + 132I 8.0 8.3 6.1 4.6 7 110m + 110Ag 0.0028 — — — Table 5 shows the estimated results for 1 year in Tochigi Prefecture with each ratio. The values listed in the row of Ratio1 correspond to the values shown in Figure 3 at 1 year. This table shows that the difference of the total values is within 10%. It is also shown that by using ratio 1 that is adopted by the report of UNSCEAR, it is possible to estimate the dose more conservatively. Table 5. Estimated effective dose in Tochigi Prefecture for 1 year using different ratios (mSv). i Nuclide Ratio1 Ratio2 Ratio3 Ratio4 1 137Cs + 137mBa 0.524 2 134Cs 3 136Cs 0.004 0.008 0.003 — 4 131I 0.032 0.025 0.023 0.013 5 129m + 129Te 0.002 — 0.002 — 6 132Te + 132I 0.057 0.059 0.043 0.033 7 110m + 110Ag 0.001 — — — Total 0.62 0.62 0.60 0.57 i Nuclide Ratio1 Ratio2 Ratio3 Ratio4 1 137Cs + 137mBa 0.524 2 134Cs 3 136Cs 0.004 0.008 0.003 — 4 131I 0.032 0.025 0.023 0.013 5 129m + 129Te 0.002 — 0.002 — 6 132Te + 132I 0.057 0.059 0.043 0.033 7 110m + 110Ag 0.001 — — — Total 0.62 0.62 0.60 0.57 Table 5. Estimated effective dose in Tochigi Prefecture for 1 year using different ratios (mSv). i Nuclide Ratio1 Ratio2 Ratio3 Ratio4 1 137Cs + 137mBa 0.524 2 134Cs 3 136Cs 0.004 0.008 0.003 — 4 131I 0.032 0.025 0.023 0.013 5 129m + 129Te 0.002 — 0.002 — 6 132Te + 132I 0.057 0.059 0.043 0.033 7 110m + 110Ag 0.001 — — — Total 0.62 0.62 0.60 0.57 i Nuclide Ratio1 Ratio2 Ratio3 Ratio4 1 137Cs + 137mBa 0.524 2 134Cs 3 136Cs 0.004 0.008 0.003 — 4 131I 0.032 0.025 0.023 0.013 5 129m + 129Te 0.002 — 0.002 — 6 132Te + 132I 0.057 0.059 0.043 0.033 7 110m + 110Ag 0.001 — — — Total 0.62 0.62 0.60 0.57 Influence of using a different value of reduction factor r In the estimation above, we have used a value of 0.4 for reduction factor r, which is recommended for wooden houses(9). Since not all houses are necessarily wooden, it is necessary to discuss how the estimated values will be varied depending on a different value of reduction factor r. Although the original estimated value is 0.62 mSv for 1 year after the accident in Tochigi Prefecture using the model 1 for fitting of air dose rate (Table 3), it becomes 0.33 mSv if a value of 0.2 recommended for concrete multi-story apartments is used(9). This shows that the estimated values can get approximately twice larger by considering a concrete multi-story apartment than a wooden house. However, statistical data from the 2003 Housing and Land Survey of Japan shows that ~75% of houses are wooden ones in the prefectures surrounding Fukushima(20). Therefore, although the estimated values in this research are conservative, they are not excessively conservative based on actual data of the ratio of the house structure in Japan. CONCLUSION Using the measured data of air dose rate obtained by car-borne survey conducted by JAEA, we estimated cumulative effective dose from external exposure in the six prefectures adjacent to Fukushima Prefecture. However, since car-borne survey started a few months after the accident, the contribution of short-lived nuclides such as 131I could be considered negligible and the contribution of 137Cs and 134Cs is dominant in the survey result. Therefore, based on air dose rates of 137Cs and 134Cs, and the ratio of deposition density of short-lived nuclides to that of 137Cs and 134Cs, we estimated contributions to the cumulative effective dose not only from 137Cs and 134Cs but also from other short-lived nuclides. In the case of Tochigi Prefecture, the cumulative effective doses estimated by using our method are 0.62 mSv for 1 year and 1.7 mSv for 10 years. The ones by UNSCEAR are 0.06–1.07 mSv for 1 year and 0.2–2.5 mSv for 10 years. The cumulative effective doses estimated by using our method are in good agreement with those estimated by UNSCEAR. The contribution from 132Te, 132I and 131I to the cumulative effective dose was estimated ~80% within a short period of time after the accident (when t equals 0.01 y corresponding to ~4 days). However, as the time passes, the contribution from 137Cs and 134Cs becomes more dominant and it reaches ~85% in cumulative effective dose after 1 year. In addition, the contribution from 137Cs and 134Cs gets ~94% when t equals 10 y by estimating with the same method. Therefore, the contribution from short-lived nuclides to the cumulative lifetime effective dose is thought to be negligible (a few percent). The ratio of deposition density of short-lived nuclides is an important parameter in our method. Using four kinds of the combination of the measured or estimated ratio, we examined its influence on the estimated results. The difference of the estimated results was within 10%. In this report, we conducted the estimations by using the averaged value of measured air dose rate for each prefecture. 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Kagaku 84 ( 3 ), 0322 – 0332 ( 2014 ). 18 Imanaka , T. , Endo , S. , Sugai , M. , Ozawa , S. , Shizuma , K. and Yamamoto , M. Early radiation survey of Iitate Village, Which Was Heavily Contaminated by the Fukushima Daiichi Accident, Conducted on 28 and 29 March 2011 . Health Phys. 102 ( 6 ), 680 – 686 ( 2012 ). Google Scholar CrossRef Search ADS PubMed 19 Japan Atomic Energy Agency . Emergency monitoring of environmental radiation and atmospheric radionuclides at Nuclear Science Research Institute, JAEA following the accident of Fukushima Daiichi Nuclear Power Plant. JAEA-Data/Code 2012-010 ( 2012 ). 20 Ministry of Internal Affairs and Communication . 2003 Housing and Land Survey ( 2005 ). Available on https://www.e-stat.go.jp/en/stat-search/files?page=1&toukei=00200522&tstat=000000050002. © The Author(s) 2018. Published by Oxford University Press. All rights reserved. For Permissions, please email: journals.permissions@oup.com This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/about_us/legal/notices) http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Radiation Protection Dosimetry Oxford University Press

ESTIMATION OF EFFECTIVE DOSE FROM EXTERNAL EXPOSURE DUE TO SHORT-LIVED NUCLIDES IN THE PREFECTURES SURROUNDING FUKUSHIMA

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Abstract

Abstract The Fukushima Daiichi Nuclear Power Plant (FDNPP) accident resulted in a release of radionuclides into the environment. Since the accident, measurements of radiation in the environment such as air dose rate and deposition density of radionuclides have been performed by various organizations and universities. In particular, Japan Atomic Energy Agency (JAEA) has been performing observations of air dose rate using a car-borne survey system continuously over widespread areas. Based on the data measured by JAEA, we estimated effective dose from external exposure in the prefectures surrounding Fukushima. Since car-borne survey started a few months after the accident, the main contribution to measured data comes from 137Cs and 134Cs whose half-lives are relatively long. Using air dose rate of 137Cs and 134Cs and the ratio of deposition density of short-lived nuclides to that of 137Cs and 134Cs, we also estimated contributions to the effective dose from other short-lived nuclides. INTRODUCTION The Fukushima Daiichi Nuclear Power Plant accident caused a release of radionuclides which were deposited onto the ground not only in Fukushima Prefecture but also in nearby prefectures. Since the accident, measurements of radiation in the environment such as air dose rate and deposition density of radionuclides have been performed by various organizations and universities. In particular, Japan Atomic Energy Agency (JAEA) has been repeatedly performing observations of air dose rate using a car-borne survey system over widespread areas. There have been 11 surveys performed as of March 2017(1). In our study, considering some factors such as time spent indoors and outdoors, background radiation dose rate and radiation reduction by houses, and using the air dose rate data measured by JAEA(2), we estimated effective doses from external exposure in the six prefectures adjacent to the Fukushima Prefecture (Iwate, Miyagi, Ibaraki, Tochigi, Gunma and Chiba Prefecture). United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) has also estimated effective doses using measured data of radiation in the environment and considering deposition density of radionuclides(3). There is a similarity between UNSCEAR’s method and ours in terms of using data measured in the environment. However, the data that we have used have been measured repeatedly while the data used by UNSCEAR was obtained by a single measurement. In our study, we estimated effective dose from external exposure based on air dose rate measured by car-borne survey by JAEA, and compared its value to the one estimated by UNSCEAR. MATERIALS AND METHODS In our study, we analyzed measured air dose rate using two models fitted with time-dependent function of air dose rate. Multiplying by conversion factors and subtracting background radiation dose rate, air dose rate can be converted to effective dose rate caused by nuclides that were released by the FDNPP accident. Integrating this, cumulative effective dose from external exposure can be obtained. Models of decrease of air does rate Air dose rate decreases with time due to the radioactive decay and weathering effect. In the case of radioactive decay, air dose rate decrease is determined by the half-life of each nuclide. Weathering effect is a natural phenomenon, in which wind and rainfall promotes the migration of nuclides to another place or into the soil, therefore, this depends on weather conditions or soil characteristics of particular areas. In this study, we use two models to analyze decrease of air dose rate. The first model is described by equation (1). This model assumes that decrease in air dose rate Dair(t) (μSv/h) can be simply expressed by one exponential function: Dair(t)=αexp(−βt)+γ (1) where α (μSv/h) is a factor of an exponential term, β (y−1) is an effective decay constant and γ (μSv/h) is a constant term. The second model is described by equation (2) in which decay of nuclides and weathering are considered: Dair(t)=(D0−DBG)×{pexp(−0.693Tfastt)+(1−p)exp(−0.693Tslowt)}×kexp(−λ134t)+exp(−λ137t)k+1+DBG (2) where D0 (μSv/h) is the initial value of air dose rate and DBG (μSv/h) is the background radiation dose rate. The value of DBG ranges from 0.055 to 0.066 μSv/h, which was measured in the area of interest before the accident(4). According to the previous studies, weathering effect can be expressed empirically as two exponential functions representing a fast reduction component and a slow reduction component as shown in the second term in equation (2), where p is a non-dimensional fraction of fast component, and Tfast (y) and Tslow (y) are the half-lives of fast and slow components(5). The third term in equation (2) corresponds to radioactive decay of 137Cs and 134Cs, where k is a non-dimensional ratio of 134Cs to 137Cs contributing to air dose under the assumption that the initial value of deposition density of each nuclide is the same(6), and λ134 (y−1) and λ137 (y−1) are the decay constants of each nuclide. Conversion to effective dose Considering factors such as time spent indoors Tin (h/d) and time spent outdoors Tout (h/d), background dose rate DBG and radiation reduction factor by houses r (dimensionless), air dose rate Dair(t) can be converted to effective dose rate dE(t)/dt (μSv/y) as shown in equation (3): dE(t)dt=(qDair(t)−DBG)×(rTin+Tout)×d×c (3) where q (dimensionless) is the ratio of air dose rate in living environment to that on streets, d (d/y) is the number of days per year and c (dimensionless) is the conversion factor between air dose rate and effective dose. In car-borne survey, measurement is conducted on streets with detectors set in a car. The measured air dose rate is the one on streets. As a result of traffic, nuclides deposited onto streets are more likely to be removed or penetrate into soil compared to those deposited in living environment. For this reason, air dose rates measured by car-borne survey tend to be smaller than the air dose rate measured in living environment. The latter is ~1.2 times larger than the former as was demonstrated by man-borne measurements simultaneously conducted in the vicinity of the streets where car-borne measurements were done(7). Considering this difference, the coefficient q (=1.2) is multiplied by Dair(t). Air dose rate includes background radiation dose rate. By subtracting background radiation dose rate, only the net contribution from nuclides released by the FDNPP accident is considered. To determine the background dose rate, we used the data measured in prefectural capitals of each prefecture before the accident(4). After the accident, a questionnaire survey on time spent indoors and outdoors of Fukushima City office workers was conducted. According to the results of the survey, indoor workers spent 0.57 h per day outdoors on average (Tin = 23.43 h/d and Tout = 0.57 h/d)(8). Although Fukushima Prefecture was not the subject of this study, we used these values. Radiation is reduced indoors because of the shielding effect of houses. In this study, we adopted a value of 0.4 for reduction factor r, which is a value recommended for one and two-story wood-frame house by IAEA(9). In addition, some investigations conducted after the accident in real houses in Japan have shown that radiation reduction factor by houses were ~0.4(7, 10). For conversion factor from air dose rate to effective dose c, we adopted the value of 0.6, where the target is assumed to be adult(11). According to a previous study where it is assumed that radioactive 137Cs and 134Cs are distributed uniformly in the soil over a planar area and at specific depths, the value is also calculated almost 0.6(12). The product of (rTin+Tout)×d×c will be defined as fi2,4 as shown later in Figure 1. Figure 1. View largeDownload slide Estimation flow of cumulative effective dose contributed from each nuclide. Figure 1. View largeDownload slide Estimation flow of cumulative effective dose contributed from each nuclide. Finally, by integrating equation (3), cumulative effective dose (in the period from T1 (y) to T2 (y)) can be estimated as shown in the equation below. E(T1,T2)=∫T1T2dE(t)dtdt (4) Effective dose considering short-lived nuclides Since car-borne survey started three months later after the accident, the contribution of short-lived nuclides such as 131I could be considered negligible and the contribution of 137Cs and 134Cs is dominant in the survey result. Therefore, based on air dose rate of 137Cs and 134Cs and the ratio of deposition density of short half-life nuclides to that of 137Cs and 134Cs, we also estimated contributions to the effective dose not only from 137Cs and 134Cs but also from other short-lived nuclides. The initial value Dair(t) can be obtained by making t = 0 in equation (1) or (2). This value can be considered to be the sum of the contribution of 137Cs, 134Cs and background as shown in equation (5): Dair(0)=D1(0)+D2(0)+DBG (5) where D1(0) (μSv/h) is air dose rate caused by 137Cs in the environment and D2(0) (μSv/h) is that of 134Cs in the environment. The deposition from FDNPP lasted for many days. Moreover, each car-borne survey was conducted within a few weeks. Therefore, it is difficult to define the specific origin of time. However, we estimate the cumulative effective dose for 1 year or 10 years since the accident. Therefore, it is not necessary to focus on and consider daily change of air dose rate precisely. Considering these and to simplify data handling, we defined the origin of time (t = 0) as 12 March 2011 and assumed that all nuclides were deposited to the ground at the same time. As shown in Table 1, index i is defined to indicate each nuclide. Index i = 1, 2 corresponds to 137Cs and 134Cs, respecitvely. By subtracting the background dose rate DBG from Dair(0), value D1(0) + D2(0) can be obtained. By defining deposition density for each nuclide i as Ai (kBq/m2) and conversion factor from deposition density Ai to air dose rate as fi1,2 (μSv/h per kBq/m2) (shown in Table 2), for 137Cs (i = 1) and 134Cs (i = 2), the equation below is obtained. D1(0)+D2(0)=f11,2A1+f21,2A2 (6) Table 1. Deposition ratio and half-life for each nuclide i. i Nuclide θi Tihalf 1 137Cs + 137mBa 1.0 30 y 2 134Cs 1.0 2.1 y 3 136Cs 0.17 13 d 4 131I 12 8.0 d 5 129m + 129Te 1.1 34 d 6 132Te + 132I 8.0 3.2 d 7 110m + 110Ag 0.0028 25 d i Nuclide θi Tihalf 1 137Cs + 137mBa 1.0 30 y 2 134Cs 1.0 2.1 y 3 136Cs 0.17 13 d 4 131I 12 8.0 d 5 129m + 129Te 1.1 34 d 6 132Te + 132I 8.0 3.2 d 7 110m + 110Ag 0.0028 25 d View Large Table 1. Deposition ratio and half-life for each nuclide i. i Nuclide θi Tihalf 1 137Cs + 137mBa 1.0 30 y 2 134Cs 1.0 2.1 y 3 136Cs 0.17 13 d 4 131I 12 8.0 d 5 129m + 129Te 1.1 34 d 6 132Te + 132I 8.0 3.2 d 7 110m + 110Ag 0.0028 25 d i Nuclide θi Tihalf 1 137Cs + 137mBa 1.0 30 y 2 134Cs 1.0 2.1 y 3 136Cs 0.17 13 d 4 131I 12 8.0 d 5 129m + 129Te 1.1 34 d 6 132Te + 132I 8.0 3.2 d 7 110m + 110Ag 0.0028 25 d View Large Table 2. Conversion factors fi1,2 and fi1,3 (10−3 μSv/h per kBq/m2)(14). i Nuclide fi1,2 fi1,3 1 137Cs+137mBa 2.3 — 2 134Cs 6.3 — 3 136Cs — 4.7 4 131I — 0.85 5 129m + 129Te — 0.15 6 132Te + 132I — 5.5 7 110m + 110Ag — 6.1 i Nuclide fi1,2 fi1,3 1 137Cs+137mBa 2.3 — 2 134Cs 6.3 — 3 136Cs — 4.7 4 131I — 0.85 5 129m + 129Te — 0.15 6 132Te + 132I — 5.5 7 110m + 110Ag — 6.1 View Large Table 2. Conversion factors fi1,2 and fi1,3 (10−3 μSv/h per kBq/m2)(14). i Nuclide fi1,2 fi1,3 1 137Cs+137mBa 2.3 — 2 134Cs 6.3 — 3 136Cs — 4.7 4 131I — 0.85 5 129m + 129Te — 0.15 6 132Te + 132I — 5.5 7 110m + 110Ag — 6.1 i Nuclide fi1,2 fi1,3 1 137Cs+137mBa 2.3 — 2 134Cs 6.3 — 3 136Cs — 4.7 4 131I — 0.85 5 129m + 129Te — 0.15 6 132Te + 132I — 5.5 7 110m + 110Ag — 6.1 View Large Assumuing that the initial values of deposition density of 137Cs and 134Cs are the same (A1 = A2)(13), parameter A2 can be eliminated, then the equation below is obtained. A1=D1(0)+D2(0)f11,2+f21,2 (7) Defining the ratio of initial deposition density of each nuclide i to that of 137Cs (i = 1) as θi (dimensionless), the equation below can be obtained. Ai=θiA1 (8) Although this ratio has been studied in many previous studies, in this study, we adopted the same values as UNSCEAR as shown in Table 1(13). Defining the conversion factor from deposition density Ai to effective dose rate dεi/dt as fi1,3 (μSv/h per kBq/m2) (shown in Table 2), effective dose rate contributed from each nuclide i can be expressed as equation (9), where Tihalf is half-life for nuclide i. dεi(t)dt=fi1,3Aiexp(−ln2Tihalft) (9) By multiplying factors defining its product as fi3,4 in equation (10), this can be converted to effective dose that considers time spent indoors and outdoors, and reduction factor as shown in equation (11). fi3,4=(rTin+Tout)×d (10) dEi(t)dt=fi3,4dεi(t)dt (11) Finally, by summing and integrating equation (11), cumulative effective dose including short-lived nuclides can be obtained. Etot(T1,T2)=E(T1,T2)+∑i=37∫T1T2dEi(t)dtdt (12) Figure 1 summarizes the estimation flow of cumulative effective dose contributed from each nuclide explained above. RESULTS AND DISCUSSIONS Time-dependent decrease of air dose rate By fitting the average of measured data of air dose rate with the two models shown above, we analyzed time-dependent decrease of air dose rate in the six prefectures (Iwate, Miyagi, Ibaraki, Tochigi, Gunma Pref and Chiba Prefecture). In the first model, parameters α, β and γ were free. In the second model, D0, p and Tfast were free, although Tslow (=92 y)(5) and the value of DBG of each prefecture were constant. These free parameters were determined by fitting using the two models. However, for Iwate, Gunma and Chiba Prefectures where measured values of air dose rate were relatively small, there were some cases where fitting parameters were not determined appropriately. In order to avoid this, we used only the first model for these three prefectures, and used some fitting parameters determined for the other prefectures. Figure 2 shows the air dose rate in Tochigi Prefecture. The dots show the averaged values of measured data. The solid lines and the dashed lines show the ones determined by fitting with the two models. Figure 2. View largeDownload slide Air dose rate (Tochigi Pref.). Figure 2. View largeDownload slide Air dose rate (Tochigi Pref.). Conversion of air dose rate to effective dose Based on the time-dependent decrease of air dose rate for each prefecture, we estimated cumulative effective dose. As mentioned above, this includes the contributions not only from 137Cs and 134Cs but also from other short-lived nuclides according to the method shown in Figure 1. Figure 3 shows estimated cumulative effective dose in Tochigi for instance. Figure 3. View largeDownload slide Absolute values of the estimated cumulative effective dose including the contributions not only from 137Cs and 134Cs but also from other short-lived nuclides (Tochigi pref.). Figure 3. View largeDownload slide Absolute values of the estimated cumulative effective dose including the contributions not only from 137Cs and 134Cs but also from other short-lived nuclides (Tochigi pref.). Figure 4 shows the cumulative effective dose in Tochigi normalized by the total value. This figure shows that when t equals 0.01 y corresponding to ~4 days, the contribution from 132Te and 132I was up to ~60% of the total value. Although its contribution is not as large as that of 132Te and 132I, the contribution from 131I is also relatively large (~20%). Although the contribution from these three nuclides gradually decreases as the time passes, the contribution from 137Cs and 134Cs gains significance. When t equals 0.05 y corresponding to ~18 days, the contribution from these short-lived nuclides becomes comparable to that from 134Cs and 137Cs. As the time passes the contribution from 137Cs and 134Cs gets more dominant. When t equals 1 y (1 year after the accident), their contributions in cumulative dose are ~85% and the contribution from 132Te, 132I and 131I are ~15% in total. Figure 4. View largeDownload slide Normalized values of effective dose (Tochigi Pref.) Figure 4. View largeDownload slide Normalized values of effective dose (Tochigi Pref.) Table 3 shows the estimated values of cumulative effective dose for each prefecture. In the column of this study, the values obtained with our method by integrating from T1 = 0 y to T2 = 1 y and 10 y are shown. In the column of UNSCEAR, the values estimated by UNSCEAR are shown. In the report of UNSCEAR(15, 16), the values are estimated at the municipal level for each prefecture. In the case of the value estimated by UNSCEAR for 1 year in Tochigi Prefecture, the minimum is 0.06 mSv in Ashikaga City and the maximum is 1.07 mSv in Nasu Town and Nasushiobara City. Therefore, in Table 3 the values estimated by UNSCEAR are shown as the range from the minimum to the maximum. Table 3. Comparison of estimated results (mSv)(15, 16). Estimated values in this study Estimated values by UNSCEAR Model 1 y 10 y 1 y 10 y Iwate 1 0.24 — 0.14 0.1 Pref. 2 — — ~0.31 ~0.7 Miyagi 1 0.54 0.87 0.05 0.1 Pref. 2 0.54 0.89 ~0.55 ~1.3 Ibaraki 1 0.37 1.2 0.06 0.1 Pref. 2 0.38 1.1 ~0.54 ~1.3 Tochigi 1 0.62 1.7 0.06 0.2 Pref. 2 0.64 1.5 ~1.07 ~2.5 Gunma 1 0.17 — 0.06 0.1 Pref. 2 — — ~0.50 ~1.2 Chiba 1 0.24 — 0.05 0.1 Pref. 2 — — ~0.76 ~1.8 Estimated values in this study Estimated values by UNSCEAR Model 1 y 10 y 1 y 10 y Iwate 1 0.24 — 0.14 0.1 Pref. 2 — — ~0.31 ~0.7 Miyagi 1 0.54 0.87 0.05 0.1 Pref. 2 0.54 0.89 ~0.55 ~1.3 Ibaraki 1 0.37 1.2 0.06 0.1 Pref. 2 0.38 1.1 ~0.54 ~1.3 Tochigi 1 0.62 1.7 0.06 0.2 Pref. 2 0.64 1.5 ~1.07 ~2.5 Gunma 1 0.17 — 0.06 0.1 Pref. 2 — — ~0.50 ~1.2 Chiba 1 0.24 — 0.05 0.1 Pref. 2 — — ~0.76 ~1.8 Table 3. Comparison of estimated results (mSv)(15, 16). Estimated values in this study Estimated values by UNSCEAR Model 1 y 10 y 1 y 10 y Iwate 1 0.24 — 0.14 0.1 Pref. 2 — — ~0.31 ~0.7 Miyagi 1 0.54 0.87 0.05 0.1 Pref. 2 0.54 0.89 ~0.55 ~1.3 Ibaraki 1 0.37 1.2 0.06 0.1 Pref. 2 0.38 1.1 ~0.54 ~1.3 Tochigi 1 0.62 1.7 0.06 0.2 Pref. 2 0.64 1.5 ~1.07 ~2.5 Gunma 1 0.17 — 0.06 0.1 Pref. 2 — — ~0.50 ~1.2 Chiba 1 0.24 — 0.05 0.1 Pref. 2 — — ~0.76 ~1.8 Estimated values in this study Estimated values by UNSCEAR Model 1 y 10 y 1 y 10 y Iwate 1 0.24 — 0.14 0.1 Pref. 2 — — ~0.31 ~0.7 Miyagi 1 0.54 0.87 0.05 0.1 Pref. 2 0.54 0.89 ~0.55 ~1.3 Ibaraki 1 0.37 1.2 0.06 0.1 Pref. 2 0.38 1.1 ~0.54 ~1.3 Tochigi 1 0.62 1.7 0.06 0.2 Pref. 2 0.64 1.5 ~1.07 ~2.5 Gunma 1 0.17 — 0.06 0.1 Pref. 2 — — ~0.50 ~1.2 Chiba 1 0.24 — 0.05 0.1 Pref. 2 — — ~0.76 ~1.8 For Iwate, Gunma and Chiba Prefectures, fitting parameters were not determined appropriately and even though fitting was done appropriately, air dose rate decreases to the original background level before 10 years after the accident. In such cases, we did not estimate the value for 10 years. This table shows that the values estimated using our method are in good agreement with the values estimated by UNSCEAR. Influence of using different combinations of deposition density ratio θi As shown in Figure 1 and equation (8), deposition ratio for each nuclide θi is important parameter in consideration of short-lived nuclides. In Table 1, the values adopted by UNSCEAR are listed, however, there are other values of the measured or estimated ratio θi. Four kinds of the combination of the ratio θi are listed below for example. Ratio1: adopted by the report of UNSCEAR Ratio2: measured in Iitate village in Fkushima prefecture Ratio3: measured in Tokai village in Ibaraki Prefecture Ratio4: value in Tochigi Prefecture estimated using WSPEEDI Table 4 shows each value in detail. By using the values listed in Table 4 and focusing on Tochigi Prefecture, we examined its influence on the estimated results. Table 4. Deposition density ratio θi(13, 17–19). i Nuclide Ratio1 Ratio2 Ratio3 Ratio4 1 137Cs + 137mBa 1 1 1 1 2 134Cs 1 1 1 1 3 136Cs 0.17 0.32 0.12 — 4 131I 12 9.2 8.3 4.6 5 129m + 129Te 1.1 — 0.82 — 6 132Te + 132I 8.0 8.3 6.1 4.6 7 110m + 110Ag 0.0028 — — — i Nuclide Ratio1 Ratio2 Ratio3 Ratio4 1 137Cs + 137mBa 1 1 1 1 2 134Cs 1 1 1 1 3 136Cs 0.17 0.32 0.12 — 4 131I 12 9.2 8.3 4.6 5 129m + 129Te 1.1 — 0.82 — 6 132Te + 132I 8.0 8.3 6.1 4.6 7 110m + 110Ag 0.0028 — — — Table 4. Deposition density ratio θi(13, 17–19). i Nuclide Ratio1 Ratio2 Ratio3 Ratio4 1 137Cs + 137mBa 1 1 1 1 2 134Cs 1 1 1 1 3 136Cs 0.17 0.32 0.12 — 4 131I 12 9.2 8.3 4.6 5 129m + 129Te 1.1 — 0.82 — 6 132Te + 132I 8.0 8.3 6.1 4.6 7 110m + 110Ag 0.0028 — — — i Nuclide Ratio1 Ratio2 Ratio3 Ratio4 1 137Cs + 137mBa 1 1 1 1 2 134Cs 1 1 1 1 3 136Cs 0.17 0.32 0.12 — 4 131I 12 9.2 8.3 4.6 5 129m + 129Te 1.1 — 0.82 — 6 132Te + 132I 8.0 8.3 6.1 4.6 7 110m + 110Ag 0.0028 — — — Table 5 shows the estimated results for 1 year in Tochigi Prefecture with each ratio. The values listed in the row of Ratio1 correspond to the values shown in Figure 3 at 1 year. This table shows that the difference of the total values is within 10%. It is also shown that by using ratio 1 that is adopted by the report of UNSCEAR, it is possible to estimate the dose more conservatively. Table 5. Estimated effective dose in Tochigi Prefecture for 1 year using different ratios (mSv). i Nuclide Ratio1 Ratio2 Ratio3 Ratio4 1 137Cs + 137mBa 0.524 2 134Cs 3 136Cs 0.004 0.008 0.003 — 4 131I 0.032 0.025 0.023 0.013 5 129m + 129Te 0.002 — 0.002 — 6 132Te + 132I 0.057 0.059 0.043 0.033 7 110m + 110Ag 0.001 — — — Total 0.62 0.62 0.60 0.57 i Nuclide Ratio1 Ratio2 Ratio3 Ratio4 1 137Cs + 137mBa 0.524 2 134Cs 3 136Cs 0.004 0.008 0.003 — 4 131I 0.032 0.025 0.023 0.013 5 129m + 129Te 0.002 — 0.002 — 6 132Te + 132I 0.057 0.059 0.043 0.033 7 110m + 110Ag 0.001 — — — Total 0.62 0.62 0.60 0.57 Table 5. Estimated effective dose in Tochigi Prefecture for 1 year using different ratios (mSv). i Nuclide Ratio1 Ratio2 Ratio3 Ratio4 1 137Cs + 137mBa 0.524 2 134Cs 3 136Cs 0.004 0.008 0.003 — 4 131I 0.032 0.025 0.023 0.013 5 129m + 129Te 0.002 — 0.002 — 6 132Te + 132I 0.057 0.059 0.043 0.033 7 110m + 110Ag 0.001 — — — Total 0.62 0.62 0.60 0.57 i Nuclide Ratio1 Ratio2 Ratio3 Ratio4 1 137Cs + 137mBa 0.524 2 134Cs 3 136Cs 0.004 0.008 0.003 — 4 131I 0.032 0.025 0.023 0.013 5 129m + 129Te 0.002 — 0.002 — 6 132Te + 132I 0.057 0.059 0.043 0.033 7 110m + 110Ag 0.001 — — — Total 0.62 0.62 0.60 0.57 Influence of using a different value of reduction factor r In the estimation above, we have used a value of 0.4 for reduction factor r, which is recommended for wooden houses(9). Since not all houses are necessarily wooden, it is necessary to discuss how the estimated values will be varied depending on a different value of reduction factor r. Although the original estimated value is 0.62 mSv for 1 year after the accident in Tochigi Prefecture using the model 1 for fitting of air dose rate (Table 3), it becomes 0.33 mSv if a value of 0.2 recommended for concrete multi-story apartments is used(9). This shows that the estimated values can get approximately twice larger by considering a concrete multi-story apartment than a wooden house. However, statistical data from the 2003 Housing and Land Survey of Japan shows that ~75% of houses are wooden ones in the prefectures surrounding Fukushima(20). Therefore, although the estimated values in this research are conservative, they are not excessively conservative based on actual data of the ratio of the house structure in Japan. CONCLUSION Using the measured data of air dose rate obtained by car-borne survey conducted by JAEA, we estimated cumulative effective dose from external exposure in the six prefectures adjacent to Fukushima Prefecture. However, since car-borne survey started a few months after the accident, the contribution of short-lived nuclides such as 131I could be considered negligible and the contribution of 137Cs and 134Cs is dominant in the survey result. Therefore, based on air dose rates of 137Cs and 134Cs, and the ratio of deposition density of short-lived nuclides to that of 137Cs and 134Cs, we estimated contributions to the cumulative effective dose not only from 137Cs and 134Cs but also from other short-lived nuclides. In the case of Tochigi Prefecture, the cumulative effective doses estimated by using our method are 0.62 mSv for 1 year and 1.7 mSv for 10 years. The ones by UNSCEAR are 0.06–1.07 mSv for 1 year and 0.2–2.5 mSv for 10 years. The cumulative effective doses estimated by using our method are in good agreement with those estimated by UNSCEAR. The contribution from 132Te, 132I and 131I to the cumulative effective dose was estimated ~80% within a short period of time after the accident (when t equals 0.01 y corresponding to ~4 days). However, as the time passes, the contribution from 137Cs and 134Cs becomes more dominant and it reaches ~85% in cumulative effective dose after 1 year. In addition, the contribution from 137Cs and 134Cs gets ~94% when t equals 10 y by estimating with the same method. Therefore, the contribution from short-lived nuclides to the cumulative lifetime effective dose is thought to be negligible (a few percent). The ratio of deposition density of short-lived nuclides is an important parameter in our method. Using four kinds of the combination of the measured or estimated ratio, we examined its influence on the estimated results. The difference of the estimated results was within 10%. In this report, we conducted the estimations by using the averaged value of measured air dose rate for each prefecture. However, measured air dose rate has certain distribution and the parameters used to convert air dose rate to effective dose rates also have uncertainties. In our next step, we will try to conduct the estimations considering such distributions and estimate uncertainties of the estimated results. FUNDING This study was conducted as part of the comprehensive study on the dose estimation of residents in the Fukushima Daiichi Nuclear Power Plant accident funded by the Ministry of the Environment, Japan (FY. 2014–2016). REFERENCES 1 Japan Atomic Energy Agency . Report written in Japanese. 102–119 ( 2016 ). Available on http://radioactivity.nsr.go.jp/ja/list/564/list-1.html 2 Japan Atomic Energy Agency . Database for Radioactive Substance Monitoring Data. Available on http://emdb.jaea.go.jp/emdb/en/ 3 United Nations Scientific Committee on the Effects of Atomic Radiation . Sources, effects and risks of ionizing radiation, United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) 2013 Report. Volume I ( 2013 ). 4 Japan Chemical Analysis Center . Environmental Radioactivity and Radiation in Japan. Available on http://www.kankyo-hoshano.go.jp/study_menu.html 5 Gale , H. J. , Humphreys , D. L. O. and Fisher , E. M. R. Weathering of Caesium-137 of soil . Nature 4916 , 257 – 261 ( 1964 ). Google Scholar CrossRef Search ADS 6 Japan Atomic Energy Agency . Report written in Japanese. 102–119 ( 2016 ). Available on http://radioactivity.nsr.go.jp/ja/contents/12000/11995/33/part2.pdf. 7 Japan Atomic Energy Agency . Report written in Japanese ( 2014 ). Available on https://fukushima.jaea.go.jp/initiatives/cat03/entry06.html. 8 Takahara , S. , Iijima , M. , Shimada , K. , Kimura , M. and Homma , T. Probabilistic assessment of doses to the public living in areas contaminated by the Fukushima Daiichi Nuclear Power Plant Accident. Radiation Monitoring and Dose Estimation of the Fukushima Nuclear Accident, 197–214 ( 2014 ). 9 International Atomic Energy Agency . Generic procedures for assessment and response during a radiological emergency. IAEA-TECDOC-1162 ( 2000 ). 10 Yoshida-Ohuchi , H. , Hosoda , M. , Kanagami , T. , Uegaki , M. and Tashima , H. Reduction factors for wooden houses due to external γ-radiation based on in situ measurements after the Fukushima nuclear accident . Sci. Rep. 4 , 7541 ( 2014 ). Google Scholar CrossRef Search ADS PubMed 11 International Commission on Radiological Protection . Conversion coefficients for use in Radiological Protection against External Radiation. ICRP Publication 74 ( 1996 ). 12 Satoh , D. , Furuta , T. , Takahashi , F. , Endo , A. , Lee , C. and Bolch , W. E. Age-dependent dose conversion coefficients for external exposure to radioactive cesium in soil . J. Nucl. Sci. Technol. 53 , 69 – 81 ( 2016 ). Google Scholar CrossRef Search ADS 13 United Nations Scientific Committee on the Effects of Atomic Radiation . Methodology for the assessment of dose from external exposure and inhalation of radioactive material. UNSCEAR 2013 Report Attachment C-12 ( 2013 ). 14 Saito , K. and Petoussi-Henss , N. Ambient dose equivalent conversion coefficients for radionuclides exponentially distributed in the ground . J. Nucl. Sci. Technol. 51 , 1274 – 1287 ( 2014 ). Google Scholar CrossRef Search ADS 15 United Nations Scientific Committee on the Effects of Atomic Radiation . Effective doses in Japan for the first year. UNSCEAR 2013 Report Attachment C-14 ( 2013 ). 16 United Nations Scientific Committee on the Effects of Atomic Radiation . Total effective doses in Japan for 10-year and lifetime exposure. UNSCEAR 2013 Report Attachment C-19 ( 2013 ). 17 Imanaka , T. External dose assessment for inhabitants in iitate village until evacuation after the Fukushima-1 NPP Accident . Kagaku 84 ( 3 ), 0322 – 0332 ( 2014 ). 18 Imanaka , T. , Endo , S. , Sugai , M. , Ozawa , S. , Shizuma , K. and Yamamoto , M. Early radiation survey of Iitate Village, Which Was Heavily Contaminated by the Fukushima Daiichi Accident, Conducted on 28 and 29 March 2011 . Health Phys. 102 ( 6 ), 680 – 686 ( 2012 ). Google Scholar CrossRef Search ADS PubMed 19 Japan Atomic Energy Agency . Emergency monitoring of environmental radiation and atmospheric radionuclides at Nuclear Science Research Institute, JAEA following the accident of Fukushima Daiichi Nuclear Power Plant. JAEA-Data/Code 2012-010 ( 2012 ). 20 Ministry of Internal Affairs and Communication . 2003 Housing and Land Survey ( 2005 ). Available on https://www.e-stat.go.jp/en/stat-search/files?page=1&toukei=00200522&tstat=000000050002. © The Author(s) 2018. Published by Oxford University Press. All rights reserved. For Permissions, please email: journals.permissions@oup.com This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/about_us/legal/notices)

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Radiation Protection DosimetryOxford University Press

Published: May 11, 2018

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