Ensemble Simulation of Atmospheric Dispersion of Radionuclides During the Fukushima Nuclear Accident

Enormous amounts of radionuclides were emitted into the atmosphere by a nuclear accident at the Fukushima Daiichi nuclear power plant (FDNPP) after the earthquake and tsunami on 11 March 2011. The radionuclides were dispersed and deposited mainly over eastern Japan and the North Pacific. A lot of numerical simulations of the radionuclide dispersion and deposition have been attempted repeatedly since the nuclear accident. However, none of them are able to perfectly simulate the distribution of dose rates over eastern Japan observed after the accident. This is partly due to the error of the wind vectors and precipitations used in the numerical simulations, but unfortunately those deterministic simulations cannot deal with the probability distribution of the simulation results and errors. Therefore, we performed ensemble simulations of the atmospheric radionuclides using the ensemble Kalman filter (EnKF) data assimilation system coupled with the Japan Meteorological Agency (JMA) non-hydrostatic model (NHM). The ensemble simulations provided us the multiple results of the radionuclide dispersion and deposition, and consequently probabilistic information about the dispersion and deposition was acquired successfully.