Assessment of Internal Exposure Doses in Fukushima by a Whole Body Counter Within One Month after the Nuclear Power Plant Accident

TLDR

Early internal radiation dose data in Fukushima after the March 2011 accident are scarce because of organizational challenges, high background radiation, and contaminated measuring devices. A horizontal bed‑type whole‑body counter with two NaI(Tl) detectors measured 173 residents and visitors in Fukushima from March 11 to April 10, 2011, converting their internal radioactivity to estimated intake under an acute inhalation scenario to calculate committed effective and thyroid doses. More than 30 % of the 173 subjects had detectable (¹³¹I, ¹³⁴Cs, and ¹³⁷Cs), with detection rates up to 50 % higher during March 12–18, and the peak committed effective dose was 1 mSv and thyroid dose 20 mSv, indicating that early post‑accident intake is unlikely to cause health effects.

Abstract

Information on early internal radiation doses in Fukushima after the nuclear power plant accident on March 11, 2011, is quite limited due to initial organizational difficulties, high background radiation and contamination of radiation measuring devices. In Nagasaki, approximately 1,200 km away from Fukushima, the internal radioactivity in evacuees and short-term visitors to Fukushima has been measured by a whole body counter (WBC) since March 15, 2011. A horizontal bed-type scanning WBC equipped with two NaI(Tl) scintillation detectors was used for 173 people who stayed in the Fukushima prefecture between March 11 and April 10, 2011. The average length of stay was 4.8 days. The internal radioactivity was converted to an estimated amount of intake according to the scenario of acute inhalation, and then the committed effective dose and the thyroid dose were evaluated. (131)I, (134)Cs and (137)Cs were detected in more than 30% of examined individuals. In subjects who stayed in Fukushima from March 12 to March 18, the detection rate was approximately 50% higher for each radionuclide and 44% higher for all three nuclides. The maximum committed effective dose and thyroid equivalent dose were 1 mSv and 20 mSv, respectively. Although the number of subjects and settlements in the study are limited, the results suggest that the internal radiation exposure in Fukushima due to the intake of radioactive materials shortly after the accident will probably not result in any deterministic or stochastic health effects.