Environmental Radiation Monitoring and External Dose Estimation in Aomori Prefecture after the Fukushima Daiichi Nuclear Power Plant Accident

Abstract

Many nuclear facilities are located within Aomori Prefecture, Japan. However, no detailed dose rate distribution map of Aomori Prefecture, including its mountain regions has been reported since the Fukushima Daiichi Nuclear Power Plant accident. A car-borne survey which used a 3-in × 3-in NaI(Tl) scintillation spectrometer was carried out throughout the prefecture for the purposes of making a dose distribution map and estimating the annual external dose. The average absorbed dose rate in air and the annual effective dose were found to be 22 ± 5 nGy h-1 and 0.20 ± 0.08 mSv, respectively. These average values for all of Aomori Prefecture were respectively 44% and 59% of the nationwide average values. The average values with standard deviations of activity concentrations in soil of 40 K, 238 U and 232 Th were 234 ± 148, 15 ± 6, 12 ± 6 Bq kg-1 , respectively. The average values of contributions of 40 K, 238 U and 232 Th to absorbed dose rates in air were 39%, 29% and 32%, respectively. The contributions of 134 Cs and 137 Cs to the absorbed dose rates in air were judged to be negligible. KEY WORDS: absorbed dose rate in air, annual effective dose, car-borne survey, Aomori Prefecture, activity concentration, potassium-40, uranium-238, thorium-232, dose rate distribution map. Original Paper IINTRODUCTION Large amounts of articial radionuclides were released by the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident on 11 March 2011. 1) The evaluations of contamination by the radionuclides, 2) distribution of ambient dose rate, 3, 4) external dose 5, 6) and internal dose 7~10) have been carried out by staff members of the national and local governments and researchers of universities and institutes. Moreover, airborne surveys for making distribution maps of ambient dose rate (ambient dose equivalent rate) and contaminations by 134 Cs and 137 Cs in the whole area of Japan were also carried out by the Japanese government. 11) These results are available on the website of the Nuclear Regulation Authority. 12) The minimum level of the distribution maps of ambient dose equivalent rate which were reported by the government was shown as 0.1 Sv h-1. A fact that is often neglected by the general public is that people were exposed to natural radiation sources such as radon, cosmic-rays and terrestrial gamma-rays before the FDNPP accident, 13) and that such exposure continues on a daily basis. In order to estimate the external dose for Japanese, a nationwide survey of the ambient dose rate (exposure rate) by natural radiation was carried out by researchers of the National Institute of Radiological Sciences (NIRS), 14) and the distribution map of ambient dose rate (converted to the absorbed dose rate in air) was drawn. 15) Portable type NaI(Tl) scintillation survey meters which were calibrated by the same ionization survey meter were used for this nationwide survey, and almost all the measurements were made on the grounds of schools located in each prefecture. 14) According to the results by this nationwide survey, 15, 16) the arithmetic mean of absorbed dose rate in air by terrestrial gamma-rays was reported as 50 nGy h-1 , and the annual effective dose was 0.33 mSv. Two nuclear power plants (NPPs) and Japan's rst commercial nuclear fuel cycling facilities are located in Aomori Prefecture (Fig. 1).