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Measurement Data Collection

Main information on human and environmental measurements for reconstructing the thyroid dose to the inhabitants is described as follows.

Table 16.1 Contents of the proceedings of the 1st NIRS symposium on reconstruction of early internal dose in the TEPCO Fukushima Daiichi nuclear power station accident [7]

Human Thyroid Measurements

Through the symposium, it was revealed that human thyroid measurement data of the inhabitants were much fewer in this accident compared to those collected in the Chernobyl accident. The total number of such measurements totals ~1,500 at most; the largest dataset was a screening survey on the thyroid exposure that was conducted for 1,080 children in late March 2011 [8]. The subjects of this screening survey were inhabitants of Kawamata Town, Iwaki City, or Iitate Village where only indoor evacuation was advised because it was outside the restricted zone (20-km radius of FDNPS). The results demonstrated that net readings of the devices used in the screening survey were 0 for 598 of the 1,080 subjects and also suggested that no one had a thyroid dose that exceeded 100 mSv. Kim et al. [9] first introduced the thyroid dose distribution of the 1,080 subjects, showing that the maximum thyroid dose was 43 mSv on the assumption of a chronic intake scenario. However, difficulty was recognized in evaluating the quantitative capability of the nonspectrometric devices being used under elevated levels of radiation background. On the other hand, Tokonami et al. [10] performed thyroid measurements with a NaI(Tl) scintillation spectrometer on 12–16 April 2011 of 62 subjects of Namie Town: 45 evacuees from coastal areas and 17 inhabitants in Tsushima district. Positive detection was found in 39 in the evacuees and 7 in the inhabitants. The maximum thyroid dose among the subjects was 23 mSv in the group less than 20 years old and 33 mSv in the group over 20 years old. Other human thyroid measurements were obtained from responders or workers in charge of emergency operations at FDNPS [11, 12].

Human Whole-Body (WB) Measurements

Although measurement data of WBCs directly give internal doses from Cs only, these data would be available for reconstructing the thyroid dose to the inhabitants if the intake ratio of I to Cs can be determined in some way. NIRS was in charge of the pilot survey as a part of Fukushima Health Management Survey [13]. This survey was performed for 122 inhabitants of pilot survey areas (Namie Town, Iitate Village, and Yamakiya district of Kawamata Town) from June 27 to July 16 in 2011 by means of the thyroid/WB measurements and urinalysis. Table 16.2 summarizes numbers of the subjects with positive detection in the thyroid/WB measurements. Only the WB measurements provided meaningful results, showing that neither 134Cs nor 137Cs was detected for about half the subjects. This result indicates that a median CED of the adult subjects was below 0.035 mSv, which was derived from the detection limit of the WB measurements based on an acute intake scenario via inhalation on 12 March 2011. The Japan Atomic Energy Agency (JAEA) subsequently started WB measurements of the inhabitants on 11 July 2011 at the request of the Fukushima government. The total number of the subjects (mainly children) reached 9,927 at the end of January in 2012. Momose et al. [14] have reported their analysis results of

Table 16.2 Thyroid and whole-body measurement results of subjects of the pilot survey (as of 10 July 2011) [13]

4–7 years

8–12 years

13–17 years

≤18 years

All

Target nuclide

Pos.a No.b

Pos.a No.b

Pos.a No.b

Pos.a No.b

Pos.a No.b

131I 0

9

0

14

0

6

0

80

0

109

134Cs only 2

6

2

42

52

137Cs only 0

0

0

32

32

134Cs and 137Cs 0

0

0

26

26

Detection limits: 38 Bq for 131I, 320 Bq for 134Cs, and 570 Bq for 137Cs aPositive detection

bSubject number for each age group

the CED distributions of two age groups, 13–17 years old and older than 17 years. The median CED values of the two groups were evaluated to be 0.02 and 0.025 mSv, respectively. Here we note that all the individual CEDs were introduced based on the same intake scenario as described above for conservative estimations. Raw data of the WB measurements (e.g., the body content of Cs, measurement date, subject's age and sex, etc.) have not been published so far.

 
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