External Dose Estimation
There are two main pathways in external exposure: exposure from a radioactive plume released from nuclear facilities into the atmosphere in the early stages after an accident, and exposure from radionuclides deposited in the surrounding environment, such as land and buildings, after the plume has passed through. A few approaches are generally used to estimate the external radiation doses. A direct and reliable method is personal monitoring with a dosimeter. Personal dosimeters, such as a glass badge and an electric dosimeter, can be used for this purpose. Such direct monitoring is most common for radiation workers in a radiation-controlled area. However, if such direct personal monitoring is not possible, the dose should be estimated using models with some assumptions and environmental monitoring data such as air radiation dose rates.
In the early stages when the radioactive plume was a major source, it was almost impossible to monitor the personal doses of residents directly, and there was no such report in the case of the Fukushima accident. It was speculated that some radiation workers with a personal dosimeter worked with the refugees and were exposed to the radioactive plume. However, personal doses for those radiation workers have not been reported anywhere. In the future, personal dosimeters should be provided to certain public facilities, such as schools and city halls, in specific areas near nuclear facilities such as the emergency planning zone (EPZ), and used for monitoring actual personal radiation doses.
If direct monitoring or measurement of a personal dose is difficult, it may be possible to estimate it using monitoring data (e.g., air radiation dose rates) and analyses of personal behavior, with relatively better accuracy than the estimation of internal doses. The Fukushima Prefecture carried out an estimation of personal doses on a large scale, with the support of the National Institute of Radiological Sciences: this was done as part of the Fukushima Health Management Survey, and 400,000 personal doses were assessed over 4 months, from March to July 2011. This assessment is retrospective; therefore, it is sometimes referred as “dose reconstruction.” Detailed data of this assessment are provided at the prefecture's Internet site  and are also commented on by Inaba (see Chap. 4).
In the later stages when the deposited radiocesium became the dominant radiation source, some trials began to include the direct monitoring of personal radiation doses, which was handled by local government, groups, and individuals using dosimeters. For example, Fukushima City provided glass badges to 16,223 citizens for 3 months and reported that the cumulative dose of the period was less than
0.2 mSv . In contrast with this large-scale governmental monitoring, some small groups also tried to clarify the actual personal radiation doses locally. Yoshida et al. monitored the personal dose equivalent in local towns in Miyagi Prefecture by using optically stimulated luminescent dosimeters (see Chap. 19). This study provides not only information about the actual radiation doses of the residents, but also scientific explanations of the discrepancy between the personal doses derived from the air dose rates and those measured directly with a personal dosimeter. In Chap. 17, Kinashi et al. report the personal radiation doses of the staff of Kyoto University, who were dispatched to Fukushima to screen the radio-contamination of refugees in the evacuation shelters. This report shows a typical personal radiation dose of a person who stayed in a specific shelter.
Some researchers also estimated personal doses from the environmental monitoring data (e.g., air dose rates), retrospectively and prospectively. Some of these have been reported in this book as well as in the Symposium. Takahara et al. provides an excellent research work on the probabilistic dose assessment of the residents living in contaminated areas (see Chap. 18). Yamamoto et al. measured the air dose rate and estimated the annual dose after the accident in Tochigi Prefecture . Similar assessments had been carried out by Endo et al. for Miyagi Prefecture and by Amano et al. for Chiba Prefecture [4, 5].