Proposed Methods

Figure 16.3 displays proposed methods for reconstructing the thyroid dose to the inhabitants. The methods are classified into direct and indirect approaches: the former uses human data and the latter uses nonhuman data (including atmospheric dispersion simulations). The top priority in the dose reconstruction is placed on dose estimations from thyroid measurements. However, as mentioned earlier, such data are insufficient in terms of number and area. Thus, the direct approach utilizes available WB measurement data, which requires determining an appropriate intake ratio of I to Cs. Realistic scenarios of the intake are essential in the direct approach.

The dose estimation from the WB measurements is illustrated as follows. Here suppose the use of a CED value (134Cs and 137Cs) of the adult as the WB measurement data (currently available). Figure 16.4 displays a calculation flow from the CED of the adult to the thyroid doses to the children on the assumption that they both inhale the nuclides (131I and Cs) with average ventilation rates for each age group under the same environment. The intake amounts of 134Cs and 137Cs giving a unit dose (1 mSv in the CED) are calculated as 90,000 Bq for each; here the existing ratio of 134Cs/137Cs is set as equal. When the intake ratio of 131I/137Cs is 1, the thyroid dose to the adult is 30 mSv, taking into account the gaseous/particulate ratio of 131I and thyroid doses from nuclides other than 131I. The thyroid doses to a 1-year-old child and a 10-year-old child are also calculated as 60 and 50 mSv, respectively. Note that these thyroid doses increase in proportion to the intake ratio of 131I/137Cs. For instance, when using a CED value of 0.025 mSv (from the average CED for the adult inhabitants in Fukushima) and an intake ratio of 10 (from the average existing ratio in air at the JAEA site), the thyroid dose to the 1-year-old child is calculated as 15 mSv (= 0.025 × 60 × 10).

In the direct approach, internal doses received via inhalation or ingestion are separately estimated using materials on the figure. Dose estimation results from the indirect approach need to be compared with those from the direct approach for validation.

Fig. 16.3 Proposed methods for reconstruction of thyroid dose to inhabitants

Fig. 16.4 Calculation flow of thyroid doses to different age groups from committed effective dose (CED) value of adult

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