Modeling and sensitivity analysis of transport and deposition of radionuclides from the Fukushima Dai-ichi accident

The atmospheric transport and ground deposition of radioactive isotopes 131I and 137Cs during and after the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident (March 2011) are investigated using the Weather Research and Forecasting-Chemistry (WRF-Chem) model. The aim is to assess the skill of WRF in simulating these processes and the sensitivity of the model’s performance to various parameterizations of unresolved physics. The WRF-Chem model is first upgraded by implementing a radioactive decay term into the advection–diffusion solver and adding three parameterizations for dry deposition and two parameterizations for wet deposition. Different microphysics and horizontal turbulent diffusion schemes are then tested for their ability to reproduce observed meteorological conditions. Subsequently, the influence of emission characteristics (including the emission rate, the gas partitioning of 131I and the size distribution of 137Cs) on the simulated transport and deposition is examined. The results show that the model can predict the wind fields and rainfall realistically and that the ground deposition of the radionuclides can also be captured reasonably well. The modeled precipitation is largely influenced by the microphysics schemes, while the influence of the horizontal diffusion schemes on the wind fields is subtle. However, the ground deposition of radionuclides is sensitive to both horizontal diffusion schemes and microphysical schemes. Wet deposition dominated over dry deposition at most of the observation stations, but not at all locations in the simulated domain. To assess the sensitivity of the total daily deposition to all of the model physics and inputs, the averaged absolute value of the difference (AAD) is proposed. Based on AAD, the total deposition is mainly influenced by the emission rate for both 131I and 137Cs; while it is not sensitive to the dry deposition parameterizations since the dry deposition is just a minor fraction of the total deposition. Moreover, for 131I, the deposition is moderately sensitive (AAD between 10 and 40 % between different runs) to the microphysics schemes, the horizontal diffusion schemes, gas-partitioning and wet deposition parameterizations. For 137Cs, the deposition is very sensitive (AAD exceeding 40 % between different runs) to the microphysics schemes and wet deposition parameterizations, but moderately sensitive to the horizontal diffusion schemes and the size distribution.