著者

Chanis PORNNUMPA Kazuki IWAOKA Naofumi AKATA Masahiro HOSODA Atsuyuki SORIMACHI Shinji TOKONAMI

出版者

日本保健物理学会

雑誌

保健物理 (ISSN:03676110)

巻号頁・発行日

vol.51, no.2, pp.115-121, 2016 (Released:2016-08-09)

参考文献数

27

被引用文献数

4

---

Namie Town, Fukushima Prefecture was instituted as an evacuation zone and a controlled area after the Fukushima Daiichi Nuclear Power Plant accident in April, 2011 which immediately resulted in a significant amount of radionuclides leakage to the environment. In this study, a car-borne survey utilizing a NaI(Tl) scintillation survey meter was carried out along ordinary roads and forest areas in Namie Town from September 29th to October 1st, 2011 to measure the absorbed dose rate by gamma-rays from 137Cs (T1/2: 30.05 y) and 134Cs (T1/2: 2.04 y) for the estimation of the radiation levels over long periods of time. The similar surveys were conducted again on August 22nd-24th, 2014 and September 14th-16th, 2015 using a NaI(Tl) scintillation spectrometer. The ratios of data obtained from urban areas to total surveyed areas in the same measurements were 27%, 34% and 48% in 2011, 2014 and 2015, respectively. The ratios of those measured from forest areas and mountain regions were 73%, 66% and 52% in 2011, 2014 and 2015, respectively. The absorbed dose rates in air obtained from these surveys were used to draw dose rate distribution maps to monitor the variation of the dose rates, and the distribution was found to be heterogeneous. The maximum value, minimum value and geometric mean of the absorbed dose rates in air for 2011 were estimated to be 47.6, 0.15 and 3.7 μGy h-1, respectively. The values for 2014 were estimated to be 5.8, 0.09 and 1.2 μGy h-1, respectively. In 2015, the values were estimated to be 5.6, 0.05 and 0.9 μGy h-1, respectively. The annual effective doses in 2014 and 2015 dropped by 66% and 75% from 2011, respectively. The annual effective doses estimated in 2014 and 2015 were lower than the annual effective dose limit of 20 mSv recommended by the Japanese government for an evacuation directive lift in the prepared area.

著者

Yasutaka Omori Hiroaki Wakamatsu Atsuyuki Sorimachi Tetsuo Ishikawa

出版者

福島医学会

雑誌

FUKUSHIMA JOURNAL OF MEDICAL SCIENCE (ISSN:00162590)

巻号頁・発行日

vol.62, no.1, pp.1-17, 2016 (Released:2016-06-08)

参考文献数

49

被引用文献数

1 14

---

This study was conducted on the Fukushima Medical University (FMU) premises (in Fukushima City, Fukushima Prefecture) about four years after the Fukushima Daiichi Nuclear Power Plant accident. Its objectives were (1) to create a map of the ambient gamma dose rate (air-kerma rate) distribution, (2) to evaluate the air-kerma rate originating from natural radionuclides, and (3) to investigate the effects of snow cover on changes in the air-kerma rate. This man-borne survey revealed that the air-kerma rate varies widely, ranging from 0.038 μGy h-1 to 0.520 μGy h-1, and is higher on grass than on the other investigated surface types, such as soil, asphalt, and bricks. In this area, the mean air-kerma rate from natural radiation was evaluated to be 0.03 ± 0.01 μGy h-1, which is close to 0.04 μGy h-1, which was measured in central Fukushima City by a local authority.Furthermore, snowfall was found to reduce the air-kerma rate by 5%-30%. This reduction was attributed to attenuation of the primary radiation while passing through the snow cover, and the measured contribution of scattered radiation to the air-kerma rate reduction was small. The reduction rate was found to depend on the initial snow depth but to maintain a similar value for a couple of days, after the snow had partially melted and its depth had decreased. Finally, analysis of the daily dose due to external exposure received on the FMU premises revealed that no further health effects due to chronic radiation exposure at this site are to be expected.