[M-AG41] Dynamics of radionuclides emitted from Fukuchima Dai-ichi Nuclear Power Plant in the environment

[MAG41-P10] Estimating impacts of direct release and riverine discharge on oceanic ¹³⁷Cs derived from the Fukushima Dai-ichi Nuclear Power Plant accident by an regional ocean model

★Invited Papers

*Daisuke Tsumune¹, Tsubono Takaki¹, Kazuhiro Misumi¹, Yutaka Tateda¹, Yuichi Onda², Michio Aoyama³ (1.Central Research Institute of Electric Power Industry, 2.Center for Research in Isotopes and Environmental Dynamics, University of Tsukuba, 3.Institute of Environmental Radioactivity, Fukushima University)

Keywords:Fukushima Dai-ichi Nuclear Power Plant accident, Cs-137, Regional Ocean Model, Direct release, River input

A series of accidents at the Fukushima Dai-ichi Nuclear Power Plant (1F NPP) following the Great East Japan Earthquake and tsunami of 11 March 2011 resulted in the release of radioactive materials to the ocean. Measured ¹³⁷Cs activities in the coastal zone adjacent to the 1F NPP are still higher than the one before the accident because sources to the ocean are still existing. The Regional Ocean Model System (ROMS) was employed for regional-scale simulation of ¹³⁷Cs activity in the ocean offshore of Fukushima, the sources of radioactivity being direct release, atmospheric deposition, the inflow of ¹³⁷Cs deposited into the ocean by atmospheric deposition outside the domain of the model, and river discharges. Direct release of ¹³⁷Cs was estimated for 7 years after the accident by comparing simulated results and measured activities adjacent to the accident site. In addition, riverine discharge rates ¹³⁷Cs were also estimated by multiplication between river flow simulation rate and measured ¹³⁷Cs activities. Simulated atmospheric deposition to the ocean was employed by atmospheric transport model. Inflow of ¹³⁷Cs from boundary sections was set by the results of the North Pacific scale ocean model. Sensitivity experiments were carried out to investigate the contributions of each source to measured ¹³⁷Cs activities in the ocean. We focused on the term from 2013 to 2016 because there were few data in the river before 2012, and also focused on dissolved ¹³⁷Cs because most of ¹³⁷Cs is dissolved form in the ocean.

Simulated ¹³⁷Cs activity attributable to direct release were in good agreement with measured data in the coast zone adjacent to the 1F NPP, because the effect of direct release was dominant from 2013 to 2016. On the other hands, simulated results attributable to inflow from boundary sections were slightly underestimated to the measured data offshore area. This suggests that recirculation of subducted ¹³⁷Cs to the surface layer was underestimated in the North Pacific model. Apparent half-life of direct released and river discharged ¹³⁷Cs activity were estimated to be about 1 year and 2 years, respectively. And apparent half-life of inflow of ¹³⁷Cs activity was much longer due to time scale of dilution process in the North Pacific. Apparent half-life of each source should be similar to the measured one attributable to each source. Apparent half-life of measured ¹³⁷Cs activity adjacent to the 1F NPP was about 1 year, on the other hand, the ones increased with increasing distance from the 1F NPP. Apparent half-life of measured data was about 2 years in front of the Uda river mouth where is far from the 1F NPP. Although ¹³⁷Cs activity in this area was mainly affected by the river input, simulated ¹³⁷Cs activities with river input were one fifth of observations. There is a brackish lagoon, Matsukawa-ura in front of Uda river mouth. The observed ¹³⁷Cs activities in the Matsukawa-ura were 3-5 times larger than the one in the Uda river. This suggests the removal process from particle ¹³⁷Cs to dissolved form in the brackish lagoon may be important.