Keyword

1 Introduction

The nuclear accident at the Fukushima Daiichi Nuclear Power Plant (FNPP) was triggered by an enormous earthquake and associated tsunami (the Great East Japan Earthquake) on March 11, 2011. A large quantity of radioactive nuclides were released, resulting in the severe contamination of a wide area of the southern Tohoku region to the northern Kanto region. The main depositions occurred on March 15 and March 20–21, 2011 [1, 2].

Since the accident, many studies have examined the effect of radiation on animals, insects and plants [3,4,5,6,7,8]. However, cumulative exposure in animals is quite difficult to estimate because it is challenging to account for habitat and behavioral properties. Estimates of exposure could be improved using a contamination map combined with habitat information. Dose rate and its temporal changes can be estimated for target animals in an area from the contamination map.

The National Nuclear Security Administration (NNSA) performed a rapid survey of radiation and contamination on March 17–19, 2011 using the Aerial Measuring System. These data were used for the initial NNSA March 22, 2011, report and are available for independent analysis [9]. Additionally, the Ministry of Education, Culture, Sports, Science and Technology, Japan (MEXT), conducted a contamination study using a 2-km mesh from June to August 2011 [10]. This study started 3 months after the main deposition on March 15, 2011. The radionuclides with short half-lives already decayed away; however, 134Cs and 137Cs remained over a wide area around FNPP.

Studies of the biological effect of radiation following the FNPP accident were initiated by Tohoku University as a part of a Comprehensive Dose Evaluation Project Concerning Animals Affected by The Fukushima Daiichi Nuclear Power Plant Accident and a Nuclear Energy Science and Technology and Human Resource Development Project by MEXT using Japanese macaques (Macaca fuscata) sampled in the Fukushima area. Macaques, like humans, belong to the order, Primates. Therefore, the biological effect on macaques might provide important information that can be applied to humans. To estimate exposure, the International Commission of Radiological Protection (ICRP) summarized conversion factors for reference animals under internal exposure to uniformly distributed radionuclides and external exposure to contaminated ground with bodies simulated as ellipsoids in ICRP Publication 108 [11]. The factors are available for 75 radionuclides from ICRP Publication 108. However, the Japanese macaque is not one of the ICRP reference animals. Therefore, we estimated the appropriate conversion factors for the species using the Monte Carlo technique.

2 Materials and Methods

2.1 137Cs Contamination Map

NNSA [9] and MEXT data [10] for 137Cs contamination were combined and interpolated by the Universal Kriging method using System for Automated Geoscientific Analyses–Geographic Information System (SAGA–GIS) software [12]. A variogram was generated, which describes the variance of the difference between 137Cs concentrations at two locations fitted by a quartic function, using SAGA-GIS [10]. The reproducibility of the variogram was 95% within 70 km from FNPP. The interpolated values were stored in 1,814 × 1,854 meshes for latitudes from 36.51611 to 38.32911 and longitudes from 139.30506 to 141.15896. The interpolated map is shown in Fig. 14.1. Using this map, 137Cs contamination at the coordinates where macaques were sampled can be calculated.

Fig. 14.1
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Kriging interpolated map for the combined NNSA and MEXT data

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2.2 Ellipsoid Model

In ICRP Publication 108, the reference animals in terrestrial conditions are modeled as various ellipsoids [11], as listed in Table 14.1. Therefore, Japanese macaques were also modeled as ellipsoids with a density of 1 g/cm3 according to ICRP Publication 108 assumption. Japanese macaques were categorized into three groups according to the body size, namely, small (S), middle (M) and large (L), with the weight of less than 5 kg, 5–10 kg and greater than 10 kg, corresponding to the sampled macaques. The ellipsoid dimensions were based on the physical measurements of 52 macaques obtained by Tohoku University and the values are listed in Table 14.2. Two types of postures, the walk and sit postures, were considered and are also listed in Table 14.2.

Table 14.1 Body parameters for the reference animals in ICRP Publication 108 [11] and the calculated effective radius
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Table 14.2 Japanese macaque models for the walk and sit postures based on physical measurements
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2.3 Conversion Factor Calculation

The conversion factors for internal exposure to uniformly distributed radionuclides in an ellipsoid and external exposure to uniformly contaminated ground from 0 to 0.5 cm were calculated using PHITS code [13]. The calculation geometry of air occupying a 10 × 10 × 10 m area and 50-cm-deep ground soil was taken into account. The ellipsoid macaque was set at the center of the ground surface. To minimize calculation time, the four side boundaries were set to a mirror condition. Gamma-rays and β-particles were separately calculated for seven radionuclides, namely, 129Te, 129mTe, 131I, 132Te, 132I, 134Cs and 137Cs. The γ-ray energies and emission rates for the radionuclides were obtained from the National Nuclear Data Center [14]. Beta-particle energy spectra were obtained from the literature [15, 16], and the internal conversion electrons for each radionuclide were obtained from the website of the National Nuclear Data Center [14]. The energy deposition in the ellipsoids was calculated for each γ-ray and each electron source. In addition, to consider the depth profile of radionuclides, the deposition energy in the walk model for body size L was also calculated for sources uniformly distributed in six soil layers, namely, 0.5–1 cm, 1–2 cm, 2–5 cm, 5–10 cm, 10–15 cm and 15–20 cm (Fig. 14.2).

Fig. 14.2
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Calculation geometry for PHITS

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The external exposure conversion factors were derived by weighing each deposition energy for the depth profile of the radionuclide. The factor \( {D}_i^{\mathrm{ex}} \) ((mGy/day)/(Bq/m2)) of the ith radionuclide from depth layer d k is expressed by

$$ {D}_i^{ex}\left({d}_k\right)=\sum \limits_{j=\gamma, \beta}\frac{E_{ij}\left({d}_k\right)}{m}{I}_JS\cdot 3600\cdot 24\cdot 1000, $$
(14.1)

where subscript j indicates emitted γ-rays or β-particles, I j is the emission rate for emitted γ-rays or β-particles, m is the mass of the macaque-simulating ellipsoid, E is the deposition energy in the ellipsoid by emitted γ-rays and β-particles and S is the calculated area expressed in m2. In the case of soil surface contamination, similar to the ICRP Publication 108 assumption, the depth layer d 1 is only 0–0.5 cm. To estimate the factor for the depth profile of radionuclides, the fraction of radionuclides as a function of depth, f(d k), is introduced and summed for seven layers:

$$ {D}_i^{ex}=\sum \limits_{k=1}^7{f}_1\left({d}_k\right)\cdot {D}_i^{ex}\left({d}_k\right) $$
(14.2)

The internal exposure conversion factor \( {D}_i^{\mathrm{in}} \) ((mGy/d)/(Bq/kg)) for each radionuclide i was calculated using each deposition energy as follows:

$$ {D}_i^{in}=\sum \limits_{j=\gamma, \beta}\frac{E_{ij}}{m}{I}_jm\cdot 3600\cdot 24\cdot 1000. $$
(14.3)

2.4 Depth Profile

There is a lack of depth profile data, except for radioactive Cs. The depth profiles of 137Cs radioactivity have been measured at various locations [17,18,19,20]. The relaxation mass depths of 137Cs have been reported as time-dependent values of 1.1–1.5 g/cm2 for the period from December 2012 to December 2013 [17]; 0.9 g/cm2 for soil with a low clay content [18]; 1.56 g/cm2, on average, for paddy soil [19]; and 1.04, 1.38, 1.05 and 1.29 g/cm2 in April 2011, October 2011, December 2011 and March 2012, respectively [20]. The conversion factor for the 137Cs depth-profiled soil was calculated using the relaxation mass depths from 0.5 to 2.7 according to Eq. (14.2). Additionally, the typical depth profiles for our measurement, as shown in Fig. 14.3, were tested.

Fig. 14.3
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Tested depth profiles. Type 1, farmland; Type 2, normal land

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3 Results and Discussion

3.1 Conversion Factor

The calculated conversion factors for the surface layer for seven radionuclides are listed in Table 14.3. The external exposure results for the walk model and the internal exposure results were compared with those in ICRP Publication 108 against the effective radius and are shown in Fig. 14.4. The calculated factor and those of ICRP Publication 108 agree within 20%. Therefore, if we allow for 20% uncertainty, the conversion factor in ICRP Publication 108 for the effective radius can be used.

Table 14.3 Estimated conversion factors for the external exposure rates of the walk and sit model for surface contamination and the internal exposure rate
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Fig. 14.4
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(a) External exposure for the walk model and (b) internal exposure for the Japanese macaque. Values were compared with those in ICRP Publication 108 against the effective radius

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The conversion factor ratio of external exposure for the walk to the sit model is plotted as a function of \( \overline{E} \), which is defined as the sum of mean γ-ray and β-particle energies, in Fig. 14.5. The ratio is consistent with unity within statistical uncertainty, except for the lowest \( \overline{E} \) of 132Te. The ratio for 132Te, which emits low-energy γ-rays and β-particles, is approximately 0.83. This is due to the shielding effect by an individual’s own body. Therefore, except for low-energy β-particles and γ-rays, the conversion factors for both the walk model and sit model are approximately equal.

Fig. 14.5
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Conversion factor ratio of the walk-to-sit model as a function of \( \overline{E} \)

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3.2 Conversion Factor for Depth-Profiled Radionuclides

The conversion factors for the seven depth-profiled radionuclides with a relaxation depth from 0.6 to 2.6 and measured depth profiles of 134Cs and 137Cs in Iitate Village, Fukushima Prefecture, in comparison with the 0–0.5 cm uniform factor are shown in Fig. 14.6a, b, respectively. The relaxation depths of 137Cs radioactivity from previous studies [17,18,19,20] are shown in light red and ranged from 0.9 to 1.5 g/cm2. A soil self-shielding factor introduced as a ratio of the conversion factor for the depth profiles to that for the uniformly distributed factor in surface soil characterized by the relaxation depth is summarized in Table 14.4. The soil self-shielding factors for 134Cs and 137Cs were 0.74 and 0.71, respectively. The shielding factor for Type 1 for the slowest measured depth profiles for farmland in Fig. 14.6b was less than that for a relaxation depth of 2.6 g/cm2. However, the shielding factor for Type 2 for the typical depth profile for undisturbed land is consistent with those for the relaxation depth of 0.9–1.5 g/cm2. The variation within the range of 0.9–1.5 g/cm2 corresponds to the variation in the conversion factor for 134Cs or 137Cs, which was less than 8%.

Fig. 14.6
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(a) Calculated conversion factors of walk model for seven radionuclides of relaxation depth and (b) typical measured depth profiles

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Table 14.4 Soil self-shielding factor for seven radionuclides
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3.3 Example of External Exposure Estimation

To consider cumulative exposure to soil contamination, temporal changes in the air dose from radionuclides deposited on the ground are good references. The air dose rate from seven radionuclides from radioactivity concentrations was estimated around FNPP in 2011 [9, 10]. The air dose rate changes were estimated for several locations. For example, the estimated dose rate change in Iitate Village assuming only the physical half-life [2] is shown in Fig. 14.7. Integrating over time, the cumulative air dose can be estimated. The cumulative air dose normalized by the dose at 200 years after deposition and the radionuclide contribution as a function of time are shown in Fig. 14.8a, b, respectively. The sum of the 134Cs and 137Cs contributions to the cumulative air dose at 1 year after deposition or later exceeds 90%. In the sampled macaques, exposure is expected to be the same as the contributions to the air dose. Accordingly, the external exposure for some samples after 1 year following the FNPP accident can be estimated by only radioactive 134Cs and 137Cs, neglecting the 10% contribution of external exposure to other radionuclides.

Fig. 14.7
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Estimated dose rate change in Iitate Village assuming only the physical half-life

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Fig. 14.8
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(a) Cumulative dose in Japanese macaques against time since deposition normalized by the dose at 200 years; (b) radionuclide contribution as a function of time

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Using the 134Cs and 137Cs conversion factors for the walk model, the external, the internal and the total exposures were calculated for the sampled macaques at Namie Town, Fukushima Prefecture, in 2013. Only 137Cs concentration and 134Cs concentration, which is the same as 137Cs concentration in March 2011, were used in this estimation. The ground concentration of 137Cs (interpolated values in Fig. 14.1) and the 137Cs concentration in macaque bodies measured at Tohoku University [21] were used in this calculation. To account for the effect of the depth profile, the soil self-shielding factor of the depth profile for a relaxation depth of 1.3 g/cm2 was assumed to be 0.74 for 134Cs and 0.71 for 137Cs. The macaques were caught in 2013 or later; therefore, only exposure to 134Cs and 137Cs was taken into account. Additionally, the ratio of the accumulated concentration of 134Cs to 137Cs was assumed to be unity. The estimated total exposures ranged from 0.26 to 1.6 Gy, as shown in Table 14.5. Two sampled macaques showed the body weight less than 2kg. There is a possibility that these 2 macaques were born after the FNPP accident. The average exposure was 0.64 Gy in averaged over the 11 sampled macaques except for the 2 macaques.

Table 14.5 Summary of results for a sample of Japanese macaques in Namie Town. \( \dot{D} \) and D show the exposure rates and cumulative exposure. t = 0 indicates the values at the time of deposition
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4 Conclusions

The cumulative external and internal exposures attributed to the FNPP accident were estimated for Japanese macaques around FNPP. Conversion factors for Japanese macaques, modeled as ellipsoids, for external exposure from contaminated ground and internal exposure uniformly distributed in the body were calculated. The estimated conversion factors for seven radionuclides, namely, 129Te, 129mTe, 131I, 132Te, 132I, 134Cs and 137Cs, agreed within 20% with those in ICRP Publication 108 using an effective radius.

The conversion factors for depth-profiled 134Cs and 137Cs characterized by the relaxation depth were also estimated. The conversion factors are expressed by the factor for a uniform distribution at the 0–0.5 cm surface multiplied by a soil self-shielding factor of 0.74 for 134Cs and 0.71 for 137Cs and with a relaxation depth of 1.3 g/cm2.

The external, the internal and the total exposures for 13 sampled macaques were estimated using the calculated conversion factors. The estimated cumulative exposures for the periods from the accident occurred to the sampling date, ranged from 0.26 to 1.6 Gy. The average exposure was 0.64 Gy in averaged over the 11 sampled macaques except for the 2 macaques which might be born after the FNPP accident.