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Results and Discussion

Estimated Effective Doses

18.3.1.1 Effective Dose in the First Year After the Contamination Event

To assess doses, the set of values for time spent outdoors, tl,out,j, and initial value of the surface activity density of 137Cs, ACs137 (0), was selected based on the statistical characteristics using the global sensitivity analysis code GSALab [19], which was developed by the Japan Atomic Energy Agency (JAEA). The calculations of doses were performed by 10,000 sets of sample values. Relative errors of these calculations were less than 0.05.

Table 18.6 lists the 50th and 95th percentiles of the effective doses in the first year after the contamination, which were obtained from the probabilistic assessment. The following discussions are based on the 95th percentile.

The effective doses received by the population groups of Namie Town and Iitate Village in the first year after the contamination were estimated to be in the 10–50 mSv dose band. Namie Town had two evacuation scenarios, nos. 7 and 13. In evacuation scenario 7, the inhabitants were rapidly evacuated on March 16, 2011. On the other hand, the evacuation of Namie Town according to scenario 13 was implemented 7 days after evacuation scenario 7. The difference in the annual effective doses

Table 18.6 Effective doses in the first year after the contaminationa (mSv)

Evacuation

scenario no.

Pensioner

Indoor

worker

Outdoor worker

WHOb

Tomioka

Town

1

50th–95th percentile Groundshine (%)

1.3–5.4

91

1.3–5.0

90

1.8–8.1

94

Cloudshine (%)

1

1

0

Inhalation (%)

8

9

6

Okuma Town

2

50th–95th percentile

0.74–3.3

0.71–3.0

1.0–4.8

Groundshine (%)

89

88

92

Cloudshine (%)

1

1

1

Inhalation (%)

10

11

7

Futaba Town

3, 4

50th–95th percentile

0.45–1.2

0.43–1.2

0.54–1.5

Groundshine (%)

65

64

71

Cloudshine (%)

2

2

2

Inhalation (%)

33

34

27

Hirono Town

10

50th–95th percentile

0.55–0.81

0.53–0.75

0.72–1.1

Groundshine (%)

69

68

76

Cloudshine (%)

2

2

2

Inhalation (%)

29

30

22

Naraha Town

5

50th–95th percentile

0.72–2.6

0.69–2.3

0.98–4.0

1–10

Groundshine (%)

87

86

91

Cloudshine (%)

1

1

1

Inhalation (%)

12

13

8

6

50th–95th percentile

0.34–0.53

0.33–0.50

0.44–0.68

Groundshine (%)

63

61

71

Cloudshine (%)

3

3

2

Inhalation (%)

34

36

27

Namie Town

7

50th–95th percentile

4.3–18

4.1–17

5.7–21

10–50

Groundshine (%)

61

60

69

Cloudshine (%)

3

3

2

Inhalation (%)

36

37

29

13

50th–95th percentile

6.2–39

6.0–37

8.4–52

Groundshine (%)

79

78

84

Cloudshine (%)

1

1

1

Inhalation (%)

20

21

15

Minami Soma City

9

50th–95th percentile Groundshine (%)

2.5–6.1

94

2.4–5.7

93

3.5–9.3

96

1–10

Cloudshine (%)

0

1

0

Inhalation (%)

6

6

4

17

50th–95th percentile

1.9–9.9

1.8–9.2

2.7–15

Groundshine (%)

93

93

95

Cloudshine (%)

1

0

0

Inhalation (%)

6

7

5

Table 18.6 (continued)

Evacuation

scenario no.

Pensioner

Indoor

worker

Outdoor worker

WHOb

Iitate Village

15

50th–95th percentile

6.7–16

6.5–14

9.3–22

10–50

Groundshine (%)

90

89

92

Cloudshine (%)

1

1

1

Inhalation (%)

9

10

7

16

50th–95th percentile

7.3–17

6.9–16

9.9–24

Groundshine (%)

90

90

93

Cloudshine (%)

1

1

1

Inhalation (%)

9

9

6

Tamura City

8

50th–95th percentile

1.2–4.5

1.2–4.1

1.7–6.8

Groundshine (%)

92

92

95

Cloudshine (%)

1

1

0

Inhalation (%)

7

7

5

Kawamata

Town

18

50th–95th percentile Groundshine (%)

2.5–6.8

94

2.3–6.3

93

3.5–9.5

95

Cloudshine (%)

0

1

0

Inhalation (%)

6

6

5

Kawachi

11

50th–95th percentile

1.4–5.5

1.3–5.0

1.9–8.3

Village

Groundshine (%)

91

90

94

Cloudshine (%)

1

1

0

Inhalation (%)

8

9

6

Katsurao

Village

12

50th–95th percentile Groundshine (%)

2.2–7.5

94

2.0–6.5

93

3.1–11

96

1–10

Cloudshine (%)

0

1

0

Inhalation (%)

6

6

4

14

50th–95th percentile

3.0–7.2

2.8–6.7

4.1–11

Groundshine (%)

90

89

93

Cloudshine (%)

1

1

1

Inhalation (%)

9

10

6

Fukushima City

–c

50th–95th percentile Groundshine (%)

2.2–7.5

94

2.1–6.9

94

3.1–11

95

Cloudshine (%)

0

0

0

Inhalation (%)

6

6

5

Koriyama

City

–c

50th–95th percentile Groundshine (%)

1.2–5.5

92

1.1–5.4

91

1.7–8.6

94

Cloudshine (%)

1

1

1

Inhalation (%)

7

8

5

Iwaki City

–c

50th–95th percentile

0.55–1.3

0.53–1.2

0.72–1.9

1–10

Groundshine (%)

78

77

84

Cloudshine (%)

2

2

1

Inhalation (%)

20

21

15

aContributions of exposure pathways were calculated using the arithmetic mean of the distributions of each pathway

bIt is noted that the estimated values by WHO include contributions of internal exposures from

ingestion of radionuclides in food and water

cThe calculations of doses were performed with the assumption that the inhabitants had lived continuously in these cities during the first year after the contamination occurred

Table 18.7 Effective lifetime doses (60 years) (mSv)

between the rapid evacuation (scenario 7) and the deliberate evacuation (scenario 13) is almost double for each population group. This result indicates that the doses received by the population living in the highly contaminated area were significantly influenced by the delayed evacuation at the early phase after the contamination.

However, there was no significant difference among the evacuation scenarios for inhabitants living in Iitate Village. The entire population of Iitate Village was evacuated 2–3 months after the accident onset. Thus, most of the inhabitants had already been exposed to radiation before they were evacuated to Fukushima City. In our estimations, about 80 % of the effective doses received by the inhabitants living in Iitate Village throughout the first year were delivered before the evacuation was implemented.

In addition, the effective doses received by outdoor workers had the potential to be above 10 mSv for 1 year after the contamination in Minami Soma City, Katsurao Village, and Fukushima City. The effective doses received by the inhabitants evacuated according to scenarios 1–5, 8–12, 14, and 18 and to the inhabitants living in Koriyama City and Iwaki City were assessed to be in the 1–10 mSv dose band. The contributions to the annual effective dose from the doses received in the final evacuation facilities in the municipalities ranged from 60 % to 75 % for each scenario.

The effective doses reported by WHO [2] are shown in Table 18.6. The effective doses received by inhabitants living in Iitate Village and Namie Town in the first year after the accident are estimated to be in the 10–50 mSv dose band. At other locations considered in Fukushima Prefecture, the effective doses are estimated to be in the 1–10 mSv dose band. The range of the assessed values in this chapter corresponds approximately to that of the results reported by WHO [2]. In addition, NIRS [3] reported the external doses received by the evacuees during the 4 months after the accident. The results reported by NIRS cannot be compared directly with the assessed values in this chapter because the period subject to assessment is different. The results of this chapter, however, are consistent with the results reported by NIRS [3].

18.3.1.2 Effective Lifetime Doses

The lifetime doses received by the inhabitants of Fukushima City, Koriyama City, and Iwaki City are listed in Table 18.7. The values of the 95th percentile of the effective doses to the three population groups are 16–34, 13–26, and 2.8–5.8 mSv in Fukushima City, Koriyama City and Iwaki City, respectively. For each city, 20–30 % of the lifetime effective dose was delivered during the first year.

Contributions of Different Exposure Pathways

For evacuation scenarios 10, and the continuously living scenario of Iwaki City, the contributions of doses through inhalation range from 15 % to 30 %. Because the doses for these evacuation scenarios were calculated under the condition that radionuclides were deposited on dry property, the deposition velocities are less than those for average scenarios with a wet property. Thus, the dose contributions through inhalation are larger than those in average scenarios.

For evacuation scenarios 3 and 4, the inhabitants of Futaba Town were evacuated to Saitama Prefecture on March 19, 2011. The contamination level in Saitama Prefecture is considerably lower than that in Fukushima Prefecture. Therefore, the prolonged doses from groundshine after the evacuation to Saitama Prefecture are small. Consequently, the dose contributions through inhalation are larger than those in the other average scenarios.

The inhabitants of Namie Town were evacuated according to evacuation scenarios 7 and 13. These inhabitants received doses through internal exposure before evacuation from the highly contaminated area. Therefore, the doses through this pathway are larger than those through external exposure to groundshine in Nihonmatsu City after evacuation.

Contributions of the doses from groundshine and inhalation to the annual effective dose are 85–95 % and 5–15 %, respectively. The contributions from cloudshine are much less than those from groundshine and inhalation. For several evacuation scenarios, the contribution of inhalation is larger than that already mentioned.

 
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