[M-AG34] [EJ] Dynamics of radionuclides emitted from Fukuchima Dai-ichi Nuclear Power Plant in the environment

[MAG34-08] Radioactivity in soil from near the Fukushima Dai-ichi Nuclear Power Plant at five years after the accident

*Haruka Minowa¹, Kazuyuki Kita², Atsushi Shinohara³, Kencho Kawatsu⁴, Kazuhiko Ninomiya³, Yuki Inai³, Tsutomu Ohtsuki⁵, Yasushi Kino⁶, Kazuma Koarai⁶, Takashi Saito⁷, Yukihiko Satou⁸, Keisuke Sueki⁹, Koichi Takamiya⁵, Yukio Takeuchi¹⁰, Taeko Doi¹⁰, Masaki Uesugi¹¹, Satoru Endo¹², Shingo Okumura¹³, Takahiro Ono¹⁴, Seika Onozaki¹⁴, Naoya Katsumi¹⁵, Akimitsu Kanda³, Nguyen Tat Thanh ¹², Kenya Kubo¹⁶, Shuntaro Kinno¹⁵, Anna Suzuki³, Masatoshi Suzuki⁶, Kenji Suzuki⁴, Masaomi Takahashi³, Sota Takenaka¹¹, Zi Jian Zhang³, Izumi Nakai¹⁴, Shunsuke Nakamura¹², Akihiro Nambu³, Yudai Nishiyama⁵, Junpei Nishiyama⁶, Daisuke Fukuda¹³, Kengo Fujii¹³, Nobufumi Fujita³, Naoki Miyazawa¹¹, Tomo Muranoi⁶, Yuichi Moriguchi¹⁷, Akiyo Yatagai¹⁸, Kouhei Yamamori¹¹, Akihiko Yokoyama¹¹, Go Yoshida³, Takashi Yoshimura³ (1.The Jikei University School of Medicine, 2.Ibaraki University, 3.Osaka University, 4.Fukushima University, 5.Kyoto University, 6.Tohoku University, 7.Shokei Gakuin University, 8.Japan Atomic Energy Agency, 9.Tsukuba University, 10.National Institute for Environmental Studies, 11.Kanazawa University, 12.Hiroshima University, 13.Meiji University, 14.Tokyo University of Science, 15.Waseda University, 16.International Christian University, 17.Tokyo University, 18.Hirosaki University)

Keywords:Fukushima Dai-ichi Nuclear Power Plant, radiocesium, Cs-134, Cs-137, soil deposition density

[Introduction] A large-scale soil sampling project for radionuclides from the Fukushima Daiichi Nuclear Power Plant accident was conducted in June 2011 by a research group on the Japan Geoscience Union and the Japan Society of Nuclear and Radiochemical Sciences. New research project followed by was implemented with the objective to know the transition process of radioactivity and the current contamination in 2016 when five years passed since the accident.
Between June and September 2016, 176 researcher of total were work to measure the air dose rate and collect soil samples at 105 locations near the Fukushima Dai-ichi nuclear power plant, mainly on the difficult-to-return zone, for 9 days in total. The outline of the project will be reported in another presentation. In this report, we will present the results of the inventory of ¹³⁴Cs and ¹³⁷Cs in the soil, the degree of destribution, the ratio of ¹³⁴Cs/¹³⁷Cs concentration, and the correlation with the air dose rate.
[Sample and Measurement] Surface soil of 5 samples per one location was collected from 36 public facilities in the difficult-to-return area of Futaba-machi or Okuma-machi in Fukushima Prefecture in June and July 2016. The sample from depths of 5 cm was divided into two parts of 0 - 2.5 cm depth and 2.5 - 5 cm depth, filled in a U-8 container after drying. Radiocesium was quantified using gamma ray spectrometry with Ge detectors.
[Results and Discussion] The results are shown in Fig.1 The maximum ¹³⁷Cs concentration was 68400 kBq/m² in inventory and 1180 kBq/kg(dry) in specific activity. The inventory clearly correlated with the air dose rate. There was large difference in soil distribution degree (the ratio of radioactivity of 2.5 - 5.0 cm and 0 - 2.5 cm in depth) for each sample, and the values were almost less than 1. Moreover, it was found that there are points where the concentration ratio of ¹³⁴Cs /¹³⁷Cs clearly has a low value of 0.87 - 0.93 in the north-north-west direction from the nuclear power plant.