[M-AG34] Radioisotope migration: New development for radioisotope migration study related to 1F NPP accident

[MAG34-P09] Oceanic dispersion simulation of ¹³⁷Cs derived from the Fukushima Daiichi Nuclear Power Plant accident

*Daisuke Tsumune¹, Tsubono Takaki¹, Kazuhiro Misumi¹, Michio Aoyama² (1.Central Research Institute of Electric Power Industry, 2.Center for Research in Isotopes and Environmental Dynamics, University of Tsukuba)

Keywords:Fukushima Daiichi Nuclear Power Plant accident, Direct release, Caesium-137, Regional ocean model

In order to elucidate the actual state of marine contamination from the Fukushima Daiichi Nuclear Power Station (F1NPS) accident, a reproduction calculation was conducted using an oceanic dispersion model, primarily for ¹³⁷Cs. The Regional Ocean Model System (ROMS) with a horizontal resolution of 1 km was driven by realistic driving forces from meteorological reanalysis data. Data assimilation (nudging) was also employed using results from the ocean reanalysis data (JCOMP2) to reproduce the effects of the Kuroshio and meso-scale eddies. By using realistic driving forces, it was possible to reproduce the large temporal changes in ¹³⁷Cs concentrations in the early stages of the F1NPS accident. The oceanic dispersion model was particularly useful for estimating the source term of direct releases from the F1NPS site. The released ¹³⁷Cs diluted significantly in a north-south direction due to the influence of coastal currents. In addition, the ¹³⁷Cs activity concentration distribution was complicated by the strong influence of meso-scale eddies. The temporal variation of the ¹³⁷Cs activity concentration distribution was large, although the annual mean distribution had smaller spatial variability. The reproducibility was verified by comparing the annual mean calculated values with the annual mean observed values. The influence of atmospheric deposition was predominant for ¹³⁷Cs activity concentration in the ocean immediately after the accident, and the influence of direct release was predominant for about a year from March 26, 2011. After that, the direct release effect continued to dominate in the vicinity of the F1NPS site, and the reproduced annual mean calculation results agreed with the annual mean observations. Predictive simulations of APLS treated water have also been conducted using this verified model. However, the slightly remote coastal area was found to be affected by recirculation of ¹³⁷Cs deposited on the North Pacific scale. In order to understand the effects of recirculation, it is necessary to understand the distribution of atmospheric deposition on the North Pacific scale, although this is still unresolved due to the paucity of observational data. A combined approach of an atmospheric transport model and an oceanic dispersion model is needed. In order to estimate the impact of river discharge, it is necessary to understand the complex process of settlement, resuspension, and re-leaching of ¹³⁷Cs supplied in particle form from rivers into the ocean. Quantitative discussion of the effects of atmospheric deposition and river discharge on the current underestimation.