Modeling and sensitivity analysis for atmospheric transport and ground deposition from the Fukushima Daiichi nuclear accident

Large amounts of radioactive materials such as I-131 and Cs-137 were released into the atmosphere following the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident, which occurred in March 2011. To understand the atmospheric fate and transport of the radionuclides, and to highlight the most important parameters that influence ground deposition, we used the Weather Research and Forecasting/Chemistry (WRF/Chem) model version 3.5 to simulate these processes. First, we implemented the process of radioactive decay into the advection-diffusion equation solver of WRF/Chem, and coupled it to dry and wet deposition models available in WRF/CHEM. In total, we test and compare 3 dry deposition and 2 wet deposition parameterization methods. Moreover, to assess the influence of the model setup parameters on the deposition, different microphysics and horizontal diffusion schemes were compared. Furthermore, we have also taken the uncertainty in the imposed emission characteristics into account, including the emission rates (estimated by JAEA and TEPCO respectively), the gas/aerosol partitioning of I-131, and the size distribution of Cs-137. To assess the model performance, we compared the simulated meteorological fields and the total depositions to the observed data. The results show that the simulated rates have the largest sensitivity to the emission rates for both I-131 and Cs-137. Furthermore, for Cs-137, the microphysics scheme is also a relatively important parameter to which accumulated deposition are sensitive. Overall, based on the simulated deposition rates and accumulated deposition amounts, simulation for Cs-137 has better accuracy than for I-131.->