Method

Scope

In the early phase of the accident, inhabitants were evacuated to prevent and reduce radiation exposure. The National Institute of Radiological Sciences (NIRS) suggested 18 evacuation scenarios according to the Fukushima health management survey [3]. These scenarios are listed in Table 18.1. Figure 18.1 shows the municipalities related to the evacuation scenarios and area classification of Fukushima Prefecture. Most people within the 20 km from the nuclear power plant were rapidly evacuated within a few days after the accident (evacuation scenario no. 1–12). However, some areas including Namie Town, Katsurao Village, Iitate Village, Minami Soma City, and Kawamata Town were later designated as “deliberate evacuation areas” based on environmental monitoring data (evacuation scenario no. 13–18).

Doses were assessed for the inhabitants evacuated, as well as for the inhabitants who continued to live in Fukushima City, Koriyama City, and Iwaki City after the contamination occurred. The doses were assessed for the population living in an urban environment, such as Fukushima City and Koriyama City, whereas the rural environment prevails in some municipalities in Fukushima Prefecture. Further assessments will be needed taking into account both urban and rural environments.

The dosimetric endpoints of the study are the effective doses received by adults in the first year after the contamination and over the inhabitants' lifetimes.^[1] The total effective doses were calculated as the summation of those received by inhabitants in the municipalities listed in each evacuation scenario. The present study assumed that other protective actions such as sheltering and stable iodine uptake were not

Table 18.1 Evacuation scenarios for the population living in the evacuation area or the deliberate evacuation area based on the Fukushima health management survey [3]

aThe dose assessment was performed with the assumption that the inhabitants stayed in the same municipality after movement to the final evacuation facility

implemented. Radiation exposure occurs through several pathways. The present study assessed the doses from external exposure to radionuclides deposited on the ground (hereafter referred to as groundshine) and to radionuclides in the radioactive cloud (hereafter referred to as cloudshine) as well as the doses caused by internal exposure through inhalation of radionuclides in the radioactive cloud.

Fig. 18.1 Municipalities related to the 18 evacuation scenarios and area classification of Fukushima Prefecture. The numbers shown in this map represent municipalities listed in Table 18.5

The doses from inhalation of noble gases and radioactive materials resuspended from the ground surface were not included in the assessments. This assumption was adopted according to a World Health Organization (WHO) report [2], which mentions such inhalations are not expected to provide a significant contribution to radiation exposure. Also, the doses from cloudshine caused by noble gases cannot be considered in the present study. In addition, internal radiation doses from ingestion pathways were not included. The measurements of the doses resulting from the ingestion of contaminated food and water are being performed using a whole-body counter. The doses acquired from the ingestion pathway should be assessed with considerations about the results of measurements in the future.

• [1] The integrated period is 60 years for adults