Influences of Source Term on Long-Range Transport of Radionuclides from the Fukushima Daiichi Nuclear Accident with FLEXPART Model

The lagrangian particle dispersion models require an accurate source term as input to obtain better simulation results. In this paper, two different ¹³⁷Cs source terms of Fukushima accident from Terada and Stohl were utilized. The FLEXPART (FLEXible PARTicle dispersion mode) was used to simulate global atmospheric dispersion and transport of radionuclides released from the Fukushima Daiichi nuclear accident. The atmospheric radiation monitoring data enable us to assess the spatial-temporal distribution of the two radioactive plumes and explore the uncertainty in results due to the different source terms. The results showed that despite the differences in the total emission, emission rate, and height of emission between the source terms of Terada and Stohl, the two source terms were alike in the whole developing trend. Thereby the two radioactive plumes had similar diffusion processes and reached similar regions. Under the influence of mid-latitude westerlies, the two radioactive plumes moved eastward across the Pacific Ocean, the American continent and Continental Europe, and eventually spread over the entire Northern Hemisphere. For short-range stations in the Asia-Pacific and North American regions, the simulated time of first detection of radioactive plume arriving at these stations with the two source terms both agreed well with the observed time. For long-range stations, the simulated time of first detection of the radioactive plume at these stations agreed better with the observed time when using the Stohl source term as input compared to that using the Terada source term as input. Second, using the two different source terms as input respectively, FLEXPART was applied to simulate global atmospheric dispersion of radionuclides. Results indicated that the simulated radioactivity concentrations were more accurate at the short-range stations than at the long-range stations. In addition, the simulated radioactivity concentrations were more accurate with Stohl's source term as input than that with Terada's source term as input. Terada's source term appeared to be lower than observations. Finally, the influence of the number of particles in the global model simulation was evaluated. It was found that differences in the number of particles could affect the statistical indexes of long-range transport of pollutants and the density of the diffusive radioactive plume in its late stage.