Estimation of internal exposure of the thyroid to 131I on the basis of 134Cs accumulated in the body among evacuees of the Fukushima Daiichi Nuclear Power Station accident

• The thyroid dose was estimated using the results of whole body counting examinations.
• 131I/134Cs ratio was estimated using the results obtained by direct measurements.
• 131I/134Cs ratio changed significantly depending on the thyroid uptake factor.
• The maximum thyroid equivalent dose of 18 mSv was obtained for one child.

Namie Town was heavily contaminated by the Fukushima Daiichi Nuclear Power Station accident. The thyroid equivalent dose for residents who lived in Namie was estimated using results of whole body counting examinations which were carried out by the Japan Atomic Energy Agency a few months after the nuclear accident. Photon peaks of 131I and 134Cs were previously measured by the authors using a NaI(Tl) scintillation spectrometer and that information was used to estimate the 131I/134Cs activity ratio of total intake in the present study. The maximum values of 131I/134Cs activity ratio corresponding to thyroid uptake factors of 0.3, 0.1 and 0.03 were evaluated to be 0.9, 2.6 and 8.7, respectively. The maximum value of the 131I/134Cs activity ratio was used to obtain the most conservative thyroid equivalent dose estimation. The maximum internal exposure of the thyroid to 131I on the basis of 134Cs accumulated in the body measured by the whole body counter was estimated to be 18 mSv. This value was much smaller than 50 mSv that the International Atomic Energy Agency recommends as the dose at which exposed persons should take stable iodine tablets.