Chlorine-36 deposition at Tsukuba, Japan, after the Fukushima Daiichi Nuclear Power Plant accident
Dr Kimikazu Sasa1, Mr Yuta Ochiai1, Dt Yuki Tosaki2, Dr Tetsuya Matsunaka3, Mr Tsutomu Takahashi1, Mrs Masumi Matsumura1, Prof Keisuke Sueki1
1University of Tsukuba, Tsukuba, Japan, 2National Institute of Advanced Industrial Science and Technology, Tsukuba, Japan, 3Kanazawa University, Nomi, Japan
Many radionuclides were released to the environment during the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident in Japan in March 2011, including ³⁶Cl (half-life 3.01 × 10⁵ yr), and were transported in the atmosphere and deposited over large areas. However, there have been few studies of the ³⁶Cl release from the FDNPP, and the actual amounts released and deposited have not been accurately quantified. ³⁶Cl is a β-particle emitter (β decay 0.7092 keV, 98.1%; electron capture 1.1422 keV, 1.9%) and is difficult to quantify by radioactivity measurements with gas-flow detectors or liquid-scintillation counters. However, accelerator mass spectrometry (AMS) is suitable for ³⁶Cl measurements, with a detection limit of ~1 × 10E–6 Bq/kg (Sasa et al., 2018). Miyake et al. (2015) analyzed soil samples from around the FDNPP and confirmed that ³⁶Cl/Cl ratios in soils had risen by factors of 2–60 after the accident. However, no reports concerning atmospheric ³⁶Cl were available, and it was not clear how ³⁶Cl was transported or deposited. Matsumura et al. (2018) reported temporal variations of ¹²⁹I in rainwater at Tsukuba from March 2009 to October 2014, with ¹²⁹I concentrations in March 2011 being ~1000 times those of preceding months. It is expected that ³⁶Cl and ¹²⁹I would behave similarly in the environment as both are halogens, and a comparison of depositions of the two should therefore elucidate the dynamics of ³⁶Cl deposition following the FDNPP accident. Concentrations of ³⁶Cl in rainwater samples collected monthly at Tsukuba, Japan, during the period January 2010 to September 2011, were determined by accelerator mass spectrometry at the University of Tsukuba and used to calculate depositions. In March 2011, the monthly deposition of ³⁶Cl was (1.03 ± 0.02) × 10E–4 Bq /m², 18 times higher than the average for March in previous years. Monthly ³⁶Cl deposition was strongly correlated with that of ¹²⁹I (r = 0.7) and ¹³⁷Cs (r = 0.8) measured in the same rainwater samples from April to August 2011. The total amount of ³⁶Cl released by the FDNPP accident was estimated to be (1.4 ± 0.2) × 10⁸ Bq based on the ³⁶Cl/¹²⁹I deposition ratio of March 2011.
Author is engaged in AMS research field at the University of Tsukuba since 2000.