[M-AG44] Dynamics of radionuclides emitted from Fukushima Dai-ichi Nuclear Power Plant in the environment

[MAG44-09] Mass balance and long term behavior of fluxes of ¹³⁷Cs derived from the TEPCO FNPP1 accident among atmosphere, land and ocean

*Michio Aoyama¹ (1.Center for Research in Isotopes and Environmental Dynamic, Faculty of Life and Environmental Sciences, University of Tsukuba)

Keywords:137Cs, flux, river , resuspension, decontamination

The consensus values of total atmospheric release of ¹³⁷Cs from FNPP1 accident is 15-21 PBq, it for atmospheric deposition of ¹³⁷Cs on land is 3-6 PBq, it for atmospheric deposition of ¹³⁷Cs on the North Pacific is 12-15 PBq, it for direct discharge of ¹³⁷Cs to ocean is 3-6 PBq and it for total inventory of ¹³⁷Cs in the North Pacific is 15-18 PBq (Aoyama et al., 2019). Regarding with fluxes from domain to domain for several years after the accident, the amount of ¹³⁷Cs transported by the rivers in the south of Pacific coast of Tohoku region to the ocean might be 40 TBq (Aoyama et al., in revise) which is corresponding to less than 1.3 % of deposited ¹³⁷Cs on land. In terms of annual flux of ¹³⁷Cs, it was estimated to be10–12 TBq year^-1 in 2011-2012 by 14 rivers around FNPP1 including particulate form and dissolved form (Kitamura et al., 2014) and ca. 0.9 TBq year^-1 for particulate form and dissolved form and 0.04 TBq for dissolved form only in Dec. 2016 (Tsumune et al., 2020). In October and November 2019, ¹³⁷Cs activity concentrations in surface water off Tohoku region were reported to be 10-20 Bq m-3 by governmental monitoring after floods by the Typhoon Hagibis in October 2019. Preliminary results of integration of ¹³⁷Cs for coastal region off Miyagi, Fukushima and Ibaraki prefectures, of which size was 200 km x 20 km, showed that flux from land to ocean in this region by floods might be ca. 0.1 TBq per this typhoon event. This might one tenth of annual flux by the rivers. For resuspension, the annual deposition of ¹³⁷Cs at Okuma during the period from 2014 to 2018 means that 4 TBq year-1 to 10 TBq year-1 should be amount of resuspension from land to atmosphere and this amount correspond to 0.1 % to 0.3 % of total deposition of ¹³⁷Cs on land in Japan. The ¹³⁷Cs activity concentration at 56N canal in 2016-2018 correspond to ¹³⁷Cs discharge of 0.73 TBq year-1 to 1.0 TBq year-1 from FNPP1 site to open water. As a result of decontamination works, 134 TBq of ¹³⁷Cs was removed from surface soil until February 2019 which correspond to 4 % of deposited ¹³⁷Cs on land in Japan (Aoyama et al., in revise). The integrated amount of FNPP1 derived ¹³⁷Cs that entered the Sea of Japan, SOJ, until 2017 was 0.27 ± 0.02 PBq, which is 6.4 % of the estimated total amount of FNPP1-derived ¹³⁷Cs in the STMW in the North Pacific. The integrated amount of FNPP1-derived ¹³⁷Cs that returned to the North Pacific Ocean through the Tsugaru Strait from SOJ was 0.11 ± 0.01 PBq, 42 % of the total amount of FNPP1-derived ¹³⁷Cs transported to the SOJ ( Inomata et al., 2020)