Abstract
Ten years have elapsed since the accident at the Fukushima Daiichi Nuclear Power Plant in 2011, and the relative contribution of natural radiation is increasing in Fukushima Prefecture due to the reduced dose of artificial radiation. In order to accurately determine the effective dose of exposure to artificial radiation, it is necessary to evaluate the effective dose of natural as well as artificial components. In this study, we measured the gamma-ray pulse-height distribution over the accessible area of Namie Town, Fukushima Prefecture, and evaluated the annual effective dose of external exposure by distinguishing between natural and artificial radionuclides. The estimated median (range) of absorbed dose rates in air from artificial radionuclides as of 1 April 2020, is 133 (67–511) nGy h−1 in the evacuation order cancellation zone, and 1306 (892–2081) nGy h−1 in the difficult-to-return zone. The median annual effective doses of external exposures from natural and artificial radionuclides were found to be 0.19 and 0.40 mSv in the evacuation order cancellation zone, and 0.25 and 3.9 mSv in the difficult-to-return zone. The latest annual effective dose of external exposure discriminated into natural and artificial radionuclides is expected to be utilized for radiation risk communication.
Highlights
In order to comprehensively evaluate the factors that affect the attenuation of radioactivity, the apparent half-life was calculated using the data of the air dose rate that is regularly observed at the monitoring posts widely installed in
The gamma-ray pulse-height distribution was measured over the entire accessible area of Namie Town and was developed using a response matrix to determine the absorbed dose rate in air
The total absorbed dose rate in air obtained in this study is almost in agreement with the 0.041–11 μGy h−1 measured by Shiroma et al [21]
Summary
On 11 March 2011, a magnitude 9.0 earthquake struck the Tohoku region along the eastern coast of Japan. The earthquake caused a tsunami with a height of more than 15 m, and affected the Fukushima Daiichi Nuclear Power Plant (FDNPP). The FDNPP lost power and the cores of Units 1 to 3 became heated and melted. This caused a hydrogen gas explosion [1]. As a result of the FDNPP accident, 132 Te, 131 I, 134 Cs, 137 Cs, and rare gases such as 133 Xe, etc., were released into Fukushima Prefecture and other eastern regions of
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