Internal thyroid doses to Fukushima residents—estimation and issues remaining

Abstract

Enormous quantities of radionuclides were released into the environment following the disastrous accident at the Fukushima Daiichi Nuclear Power Plant (FDNPP) in March 2011. It is of great importance to determine the exposure doses received by the populations living in the radiologically affected areas; however, there has been significant difficulty in estimating the internal thyroid dose received through the intake of short-lived radionuclides (mainly, 131I), because of the lack of early measurements on people. An estimation by the National Institute of Radiological Sciences for 1 April 2012 to 31 March 2013 was thus performed using a combination of the following three sources: thyroid measurement data (131I) for 1080 children examined in the screening campaign, whole-body counter measurement data (134Cs, 137Cs) for 3000 adults, and atmospheric transport dispersion model simulations. In this study, the residents of Futaba town, Iitate village and Iwaki city were shown to have the highest thyroid equivalent dose, and their doses were estimated to be mostly below 30 mSv. However, this result involved a lot of uncertainties and provided only representative values for the residents. The present paper outlines a more recent dose estimation and preliminary analyses of personal behavior data used in the new method.

Development of a tool for calculating early internal doses in the Fukushima Daiichi nuclear power plant accident based on atmospheric dispersion simulation

“…These persons comprised 310 of 1,080 subjects (children) of the thyroid exposure screening campaign conducted by the Nuclear Emergency Response Local Headquarters at the end of March, 2011 and 112 of 174 subjects of the pilot survey by means of a WBC at the NIRS during the period between June 27 and July 28, 2011. The internal doses of the 421 persons (one subject of the screening campaign was not identified) were already evaluated using the direct measurement results in the previous studies [8,14] and were compared with those obtained in this study. However, this comparison is rather tentative because the former doses were evaluated assuming a simple intake scenario, namely acute inhalation on March 15 for the subjects of the screening campaign or March 12 for the subjects of the pilot survey.…”

Estimation of thyroid equivalent doses during evacuation based on body surface contamination levels in the nuclear accident of FDNPS in 2011

“…The average of multiple measurements on head and neck, at most 6, was considered as the representative value of individual, as there were no statistically significant differences in contamination levels among head, hand and clothing in the same individual [6]. Body surface contamination density S i (at plume) (Bq/cm 2 ) of radionuclide-i at the time of exposure of the plume was calculated by a following formula in (2), where T scr (h) is the time from exposure of the plume to measurement of body surface contamination, HL i (h) is a half-life of each radionuclide, and 15 (h) is a half-life of the natural remove.…”

“…As S i (at plume) can be calculated using measurement value S i (T) in equation (1) and equation (2), ED i-thy (at plume) can be estimated if v i is given.…”

“…Recently, Kim et al reported internal thyroid dose estimation at 1-year-old children mostly from inhalation during 15-23 March, 2011. The authors estimated thyroid dose by 1,080 direct thyroid measurements, about 3,000 radio-cesium measurements by WBC and the simulation of behavior pattern by atmospheric dispersion [2]. In order to estimate thyroid doses from WBC data, the authors assumed 131 I/ 137 Cs intake ratio being 3, which was 2-to 3-times lower than those in the air.…”

“…Because of paucity in direct thyroid measurements after the accident [2,3], thyroid equivalent doses were estimated by an atmospheric transfer, dispersion and deposition model (ATDM) using a source term by Terada in UNSCEAR 2013 Report [4]. However, the UNSCEAR's estimates of settlement-averaged thyroid doses were two to five times higher than the corresponding values based on direct measurements.…”