Isotopic ratio of radioactive iodine (129I/131I) released from Fukushima Daiichi NPP accident

Abstract

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 I-131 among radionuclides in the initial stage immediately following the release of radionuclides. Since I-131 cannot be detected after several months owing to its short half-life (8 days), the reconstruction by I-129 (half-life: 1.57 x 10(7) yrs) analysis is important. For this reconstruction, it is necessary to know the isotopic ratio of I-129/I-131 of radioactive iodine released from the NPP. In this study, the I-129 concentration was measured in several surface soil samples collected around the Fukushima Daiichi NPP for which the I-131 level had already been determined. The surface deposition amount of I-129 was between 15.6 and 6.06 x 10(3) mBq/m(2) within the region 3.6 to 59.0 km distant from the NPP. I-129 and I-131 data had good linear correlation and the average isotopic ratio was estimated to be I-129/I-131 = 31.6 +/- 8.9 as of March 15, 2011.