[M-AG34] Radioisotope migration: New Development of Environmental Dynamics Research on the 1FNPS Accident

[MAG34-P09] Improving the estimation of direct release rates from the Fukushima Daiichi Nuclear Power Plant accident using a higher resolution oceanic dispersion model

*Daisuke Tsumune¹, Takaki Tsubono², Kazuhiro Misumi² (1.University of Tsukuba, 2.Central Research Institute of Electric Power Industry)

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

A series of accidents at the Fukushima Daiichi Nuclear Power Station (F1NPS) following the 11 March 2011 earthquake and tsunami resulted in the release of radioactive substances into the ocean. In particular, the large amount of radioactive caesium released caused damage to the fishing industry, including voluntary restraints on fishing and restrictions on shipments. For environmental impact assessments and the formulation of measures to contain the effects of the accident, oceanic dispersion simulations are useful, based on the estimation of the supply routes and fluxes of radioactive materials. For direct release rate from the F1 NPS site, an estimation method was proposed using the results of nearby monitoring and the estimated seawater exchange rate by numerical simulation. However, the influence of the volume for setting the seawater exchange rate as not considered. Appropriate volumes need to be considered for use in estimating future accidents. In addition, underestimation of coastal directional transport was observed in the reproduction simulations for the F1NPS accident due to lack of resolution. A study using a higher resolution model was carried out to estimate the pathways and fluxes of radioactive material supply to the ocean and to understand the concentration distribution based on the ocean dispersion simulation. The conventional ocean dispersion model ROMS with a horizontal resolution of 1 km was increased to a horizontal resolution of 200 m. The optimal setting of the seawater exchange rate was investigated and the transport process of radiocesium in the coastal direction was improved. It was found that the conventional volume for determining the seawater exchange rate, including the locations of release sources and observation points, is optimal. This validity was confirmed by experimental equations from previous oceanic tracer-release experiments. In the estimation of future release rates, it was found necessary to establish an appropriate volume, e.g. depending on the distance between the locations of release sources and the observation point. In addition, improvements were observed in the transport process in the coastal direction due to the higher resolution, which improved the reproducibility. However, with a horizontal resolution of 200 m, there were problems with repeatability near ports, and it was found that a higher resolution using nesting or other methods would be desirable to deal with smaller-scale release than the F1NPS accident.