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VII Dose Assessment

NIRS's Project for the Reconstruction of Early Internal Dose to Inhabitants in Fukushima After the Nuclear Disaster

Osamu Kurihara, Eunjoo Kim, Kumiko Fukutsu, Masaki Matsumoto, Soheigh Suh, Keiichi Akahane, and Kazuo Sakai

Abstract In 2012, National Institute of Radiological Sciences (NIRS) launched a project on the reconstruction of the early internal dose, namely the thyroid dose to inhabitants in Fukushima and adjacent prefectures from the intake of short-lived nuclides, mainly 131I after the nuclear disaster. Limited human measurements make it difficult to estimate individual doses, and thus it is necessary to collect all information available and explore possible methods for dose reconstruction. As the first action of the project, a symposium was organized by NIRS on 10–11 July 2012. This chapter provides human or environmental measurement data collected and describes the current status of the project.

Keywords Dose reconstruction • Early internal dose • Fukushima • Inhabitants

• Nuclear disaster • Thyroid dose


The Tokyo Electric Power Company (TEPCO) Fukushima Daiichi Nuclear Power Station (FDNPS) accident triggered by the Tohoku Region Pacific Coast Earthquake on 11 March 2011 resulted in reactor core meltdown with several hydrogen explosion events, following an enormous release of radioactive materials into the environment [1]. Airborne surveys after this nuclear disaster have shown the spatial distribution of ambient dose rates and the Cs ground deposition on a regional scale, revealing that heavily contaminated areas were generated mainly in a northwesterly direction from FDNPS [2]. It is thus of great importance to understand the magnitude of individual doses to populations in affected areas from the aspect of foreseeing health effects attributed to radiation exposure in the accident. Preliminary dose estimations for inhabitants in Fukushima Prefecture and the whole of Japan have been reported by the World Health Organization (WHO) [3].

The Fukushima government has also initiated dose estimations of the inhabitants inside the prefecture and results obtained are described as follows. External dose estimations of 228,512 inhabitants have been completed as of the end of October 2012, demonstrating that 99.6 % of these are 5 mSv in effective dose for the first 4 months after the accident [4]. These estimations have been performed using a system for calculating individual external doses based on time-series maps of ambient dose rates in Fukushima Prefecture and personal behavior records. Human measurements of the inhabitants with whole-body counters (WBCs) have been performed for estimating their internal doses. It has been reported that 26 of 90,050 subjects exceeded 1 mSv in the committed effective dose (CED) [5]. However, it should be noted that this result is for internal doses only from 134Cs and 137Cs. The early internal dose, which is expected to be received by the intake of short-lived nuclides (e.g., 131I, 132I, 132Te) during the first few months after the accident, still remains unknown because of limited information available. Concern is thus raised concerning thyroid exposure to small children from the intake of radioiodine, as with situations found after Chernobyl accident [6].

In 2012, the National Institute of Radiological Sciences (NIRS) launched a project on the reconstruction of the early internal dose to the inhabitants in Fukushima and adjacent prefectures. As the first action of this project, the 1st NIRS symposium on “Reconstruction of early internal dose in the TEPCO Fukushima nuclear power station accident” was held on 10–11 July 2012. The main aims of this symposium were twofold: (1) to collect human and environmental measurement data available for estimating the early internal dose to the inhabitants and (2) to discuss methods for reconstructing the thyroid equivalent dose (hereinafter, thyroid dose) among invited experts from Japan and overseas. The proceedings of the symposium, including 17 peer-reviewed 17 papers and the summary of the discussion, have been published by NIRS [7]. Table 16.1 lists authors and titles of the papers included in the proceedings. The goals of the project this year are set as (1) to propose best methods for the internal dose estimation and (2) to estimate representative doses to the inhabitants of each area in Fukushima prefecture. This chapter briefly describes the current status of the project.

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