Chlorine-36 and Iodine-129 Inventories in Difficult-to-return Zone in Fukushima
Prof. Keisuke Sueki1,2, Mr. Yuki Ohta1,2, Mr. Hiroki Yokoyama1,2, Mr. Yuta Ochiai1,2, Mr. Seiji Hosoya1,2, Dr. Maki Honda1,2,3, Dr. Yukihiko Satou1,2,3, Dr. Tetsuya Matsunaka2,4, Mr. Tsutomu Takahashi2, Ms. Masumi Matsumura2, Prof. Kimihiko Sasa1,2
1Graduate School of Pure and Applied Science, University of Tsukuba, Tsukuba, Japan, 2Accelerator Mass Spectrometry Group, Tandem Accelerator Complex, University of Tsukuba, Tsukuba, Japan, 3Japan Atomic Energy Agency (JAEA), Tokai-mura, Japan, 4Low Level Radioactivity Laboratory, Institute of Nature and Environmental Technology, Kanazawa University, Wake, Nomi, Japan
The Fukushima Daiichi Nuclear Power Plant (FDNPP) accident in March 2011 released a large amount of radionuclide. It is necessary to clarify where and how much of chlorine-36 and iodine-129, which have a long half-life and are thought to have long-term effects, are accumulated in the soil. In particular, the investigation of iodine-129 is very important for understanding the extent of the effect of iodine-131 released at the time of the accident. Chlorine-36 has been reported by Miyake et al. [Miyake 2015], iodine-129 has been reported in an attempt to reproduce the amount of iodine-131 inventory [Miyake 2012, Muramatsu 2015]. In this study, we evaluated the accumulation of chlorine-36 and iodine-129 in areas strongly contaminated with radioactive cesium around the nuclear power plant and its northwestern area, which are still designated as difficult-to-return areas. The anarized samples were the soil around the nuclear power plant collected from 2008 to 2009 before the accident and the soil up to 5 cm from the surface layer collected in Okuma-machi, Futaba- machi, and Namie- machi from 2013 to 2016. The dried sample was sieved by 2 mm and then pulverized by a ball mill. It was placed in a U8 container and measured gamma-ray with a Ge detector, part of it was thermally hydrolyzed for iodine-129, and the other part was extracted with dilute nitric acid to extract the inorganic chloride component and measured chlorine-36. Both chlorine-36 and iodine-129 were determined by isotope ratio using the AMS course of the 6MV tandem accelerator at the University of Tsukuba. Stable chlorine was quantified using ion chromatography and stable iodine was quantified using ICP-MS. From the analysis of 31 points, ratios of ³⁶Cl/Cl were obtained from 1.38×10⁻¹³ to 1.00×10⁻¹¹. By evaluating the natural background before the accident as (4.10 ± 0.17) ×10⁻¹³, ³⁶Cl derived from the accident could be evaluated. Based on this, the accumulated amount of inorganic ³⁶Cl was determined, and 0.007-6.10 mBq/m² was obtained. When compared with ¹³⁷Cs, it was found to be as small as 10⁻⁹. However, it has been found that the presence forms of chlorine in soil are considered to be inorganic and organic, and the amounts thereof are almost the same. From the analysis of 44 points of ¹²⁹I in soils, the results of ¹²⁹I specific activity in the range of 1.13-105 mBq/kg were obtained. Since this was 0.38±0.19 mBq/kg before the accident, it could be evaluated that iodine-129 was contaminated by the accident at all points. In comparison with ¹³⁷Cs, the ratio was large at a short distance from the nuclear power plant, but it was constant at 2.3×10⁻⁷ at the distance of 13 km or more to the northwest. It was possible to evaluate the degree of contamination of chlorine-36 and iodine-129 in difficult-to-return area by comparing with the data before the accident. The amount of iodine-131 inventory from the obtained amount of iodine-129, it became possible to estimate the effect of iodine-131 at the time of the accident.
Since 2003, I have been a member of the Accelerator Mass Spectrometry Group at the University of Tsukuba. I mainly investigate the abundance of radionuclides in the environment.