2012 年 46 巻 4 号 p. 327-333
In March 2011, there was an accident at the Fukushima Daiichi Nuclear Power Plant (NPP) and a discharge of radionuclides resulting from a powerful earthquake. Considering the impact on human health, the radiation dosimetry is the most important for 131I among radionuclides in the initial stage immediately following the release of radionuclides. Since 131I cannot be detected after several months owing to its short half-life (8 days), the reconstruction by 129I (half-life: 1.57 × 107 yrs) analysis is important. For this reconstruction, it is necessary to know the isotopic ratio of 129I/131I of radioactive iodine released from the NPP. In this study, the 129I concentration was measured in several surface soil samples collected around the Fukushima Daiichi NPP for which the 131I level had already been determined. The surface deposition amount of 129I was between 15.6 and 6.06 × 103 mBq/m2 within the region 3.6 to 59.0 km distant from the NPP. 129I and 131I data had good linear correlation and the average isotopic ratio was estimated to be 129I/131I = 31.6 ± 8.9 as of March 15, 2011.