Initial 131I physicochemical composition ratios during the Fukushima Daiichi nuclear power station accident and impact of 131I re-emission process from surface deposition on air concentrations: three-dimensional atmospheric dispersion simulation approach

This study validated previously reported initial physicochemical composition ratios of ¹³¹I plumes released during the Fukushima Daiichi Nuclear Power Station (FDNPS) accident using three-dimensional atmospheric dispersion simulations performed with a chemical transport model (CTM). Using these reported ratios, we simulated air concentrations of organic gas, inorganic gas, and particulate forms of ¹³¹I and compared the relative abundance of gaseous and particulate forms with observational data collected in Ibaraki prefecture, a neighboring region of Fukushima prefecture. Simulations were generally consistent with observations, demonstrating significantly improved agreement with observations for releases occurring around midnight on March 15. Refining the source term based on these initial composition ratios is beneficial when utilizing a previously proposed ¹³¹I source term for the FDNPS accident. Subsequently, we investigated the influence of ¹³¹I re-emission processes on air concentrations. Simulations using the CTM revealed that air concentrations at the observation site from March 17 to 19 were largely influenced by re-emission from surface deposition. The re-emission parameter used in our simulations suggests that direct volatilization from surface deposition is a key mechanism in the ¹³¹I re-emission process during the FDNPS accident. Previous source term estimates of ¹³¹I following the FDNPS accident did not account for re-emission, which limited the effective use of observational data. The findings of this study are expected to improve the integration of observational data into future assessments.