Evaluation of the parameters of size distribution of emitted aerosols for simulation of radionuclides atmospheric transport following Fukushima accident using FLEXPART model

In this study, the FLEXPART atmospheric transport model was used to simulate the atmospheric transport of Cs-137 following the Fukushima NPP accident in March 2011, using the source term estimation described in [1]. The simulation results were compared with airborne concentration measurements conducted in Japan and globally. It was shown that FLEXPART results as compared to Fukushima are highly sensitive to assumptions regarding size distribution of the radioactivity in the source. Based on our study, 'conventional' assumptions regarding the mean aerodynamic diameter (MAD) of particles, which are typically reported in the range of 0.2 to 0.7 μm in various studies, yielded reasonable agreement between simulated and observed concentrations within Japan. However, at greater distances from the source (e.g., across Eurasia), our results showed that the calculated concentrations were significantly overestimated. As discussed in our study, the size distribution of particles in the plume evolves over time. Therefore, measurements of AMAD conducted in Europe [2], which report a similar range of AMAD values, may not be representative enough to determine the initial AMAD at the source. To address this, we curve-fitted the observed size distribution of emitted particles measured in Japan shortly after the accident, as presented in [3]. This approach resulted in the following estimates for the size distribution: AMAD ≈ 2.5 μm and geometric standard deviation (GSD) ≈ 1.8. Simulations conducted with these updated size distribution parameters showed significant improvement in the agreement between calculated and observed airborne concentrations, as compared to using conventional assumptions.

References

• 1. Terada H, Nagai H., Tsuduki K., et al. (2020) Refinement of source term and atmospheric dispersion simulations of radionuclides during the Fukushima Daiichi Nuclear Power Station accident, Journal of Environmental Radioactivity, Volume 213,106104, https://doi.org/10.1016/j.jenvrad.2019.106104
• 2. Masson O., Ringer W., Malá H., et al. (2013) Size Distributions of Airborne Radionuclides from the Fukushima Nuclear Accident at Several Places in Europe. Environmental Science & Technology 47 (19), 10995-11003 DOI: 10.1021/es401973c
• 3. Miyamoto Y., Yasuda K., Magara M., (2014) Size distribution of radioactive particles collected at Tokai, Japan 6 days after the nuclear accident, Journal of Environmental Radioactivity (132) 1-7, https://doi.org/10.1016/j.jenvrad.2014.01.010