Determination of I-129/I-131 in surface soils related to the Fukushima Daiichi Nuclear Power Plant accident
Dr Kimikazu Sasa1, Mrs Masumi Matsumura1, Dr Tetsuya Matsunaka2, Mr Tsutomu Takahashi1, Dr Yukihiko Satou3, Dr Norikazu Kinoshita4, Prof Hiroyuki Matsuzaki5, Prof Keisuke Sueki1
1University of Tsukuba, Tsukuba, Japan, 2Kanazawa University, Nomi, Japan, 3Japan Atomic Energy Agency, Tokai, Japan, 4Shimizu Corporation, Koto-ku, Japan, 5The University of Tokyo, Bunkyo-ku, Japan
Total amounts of radionuclides discharged into the atmosphere were estimated to be 8.1 GBq for ¹²⁹I (half-life 1.57 × 10⁷ y) (Hou et al., 2013) and 120 – 200 PBq for ¹³¹I (half-life 8.02 d) (Chino et al., 2011) released from the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident. Although the accident-derived ¹³¹I in soil extinguished in a few months, the long-lived ¹²⁹I can be used as a tracer to retrospectively infer the level of ¹³¹I. The ¹²⁹I/¹²⁷I ratio for the prepared AgI sample including iodine carrier with an ¹²⁹I/¹²⁷I ratio of 1.8 × 10E–13 was measured using AMS at the MALT, The University of Tokyo. The ¹²⁹I/¹²⁷I ratios and ¹²⁹I concentrations in surface soils were calculated using the ¹²⁷I concentration obtained from an inductively coupled plasma-mass spectrometry and the ¹²⁹I/¹²⁷I ratio obtained from the AMS, and by subtracting the ¹²⁷I and ¹²⁹I contents of the background during the sample preparation. Pre-accident levels of ¹²⁹I concentration were estimated to be (2.7 ± 1.4) × 10⁸ atoms/g at Fukushima. We determinated the ¹²⁹I/¹³¹I atomic ratio in surface soils of Fukushima area for reconstruction of the ¹³¹I deposition using the long-lived ¹²⁹I, which has same behavioral response with ¹³¹I in the environment. For this purpose, we investigated the concentrations of ¹²⁹I among 5-cm-long surface soils in and around Fukushima area, which were measured for ¹³¹I immediately after the accident by gamma-ray spectrometry (Kinoshita et al., 2011). Average ratio of ¹²⁹I/¹³¹I was estimated to be 26.0 ± 5.7 as at March 11, 2011. The correlation coefficient (R²) is 0.98 between concentrations of ¹²⁹I and ¹³¹I in surface soils at Fukushima. We also found the deference of ¹²⁹I/¹³¹I ratios depending on the locations. It is thought to be due to reflect differences related to emission sources from the FDNPP. Previous studies has reported the ¹²⁹I/¹³¹I ratios of 31.6 ± 8.9 (Miyake et al., 2012), 26.1 ± 5.8 (Miyake et al., 2015), and 21 (Muramatsu et al., 2015) in surface soils. The calculations for ¹²⁹I/¹³¹I ratios by the ORIGEN2 code resulted 31.4 for the Unit 1 reactor, 21.9 for the Unit 2 reactor, and 20.8 for the Unit 3 reactor (Nishihara et al., 2012). Our result of 26.0 ± 5.7 for ¹²⁹I/¹³¹I ratio was consistent with previous studies and the estimates by the ORIGEN2 code.
Author is engaged in AMS research field at the University of Tsukuba since 2000.