On 11 March 2011, a 9.0 magnitude earthquake, which occurred at Northern Japan, and subsequent tsunami caused serious damage to the Fukushima Daiichi Nuclear Power Station (FDNPS). People living within a radius of 30 km were evacuated from their homes. Residents of Namie Town stayed for several days at a location 30 km northwest of the FDNPS. However, as a highly concentrated radioactive plume passed over this location, the evacuees are very much worried about their radiation exposure. On the other hand, there are several nuclear facilities in Aomori Prefecture. Such circumstances may produce concerns among people regarding an increased risk of cancer or other radiation-induced disease. In this study, focusing on the citizens of Namie Town and Aomori Prefecture (Hirosaki, Aomori and Hachinohe Cities), their understanding of basic knowledge on radiation was investigated through anonymous questionnaires. The present study has revealed that people recognized that they are exposed to natural radiation of more than 1 mSv, Despite this fact, however, people in Namie Town believe even radiation of 1 mSv will cause them some biological effects. Although basic general knowledge on radiation should be provided, a reliable relationship between the general public and experts also needs to be established.
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.
Ambient dose rates are continuously monitored in Ningyo-toge Environmental Engineering Center, Japan Atomic Energy Agency. The present study discussed the variations in ambient dose rates, observed from April 2014 to March 2015, due to snowfall as well as rainfall. It is much snowy as one of climatic features in this area. Rain or snow was sampled for a certain period in the day of interest (17 cases in total), and then the concentration of radon progeny was measured. With the measured data, the variation in ambient dose rate was calculated considering the accumulation of the radon progeny on the ground. As a whole, this calculation was found to reasonably reproduce the time trends of observed dose rates, except for four cases. Based on the backward trajectory analysis, it was explained that the discrepancy in two cases out of the four was induced by changes of radon progeny concentration in precipitation around sampling period. It was suggested that the other two cases were caused by the run-off of rain from the ground surface.
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