Reassessment of early ¹³¹I inhalation doses by the Fukushima nuclear accident based on atmospheric ¹³⁷Cs and ¹³¹I/¹³⁷Cs observation data and multi-ensemble of atmospheric transport and deposition models

The Fukushima Dai-ichi Nuclear Power Plant accidents following the March 11, 2011 Tohoku earthquake, and subsequent tsunami released radioactive materials into the atmosphere and caused significant public health concerns, particularly thyroid cancers in children. However, the lack of measurement data for atmospheric concentrations of ¹³¹I has caused persistent and widespread uncertainty. This study estimated the maximum potential thyroid doses of inhaled ¹³¹I in the early post-accident phase between March 12 and 23, 2011 by using the hourly measured data of the ¹³⁷Cs concentrations at 101 suspended particulate matter (SPM) monitoring sites, a new multi-model ensemble (MME) method of simulating ¹³⁷Cs concentrations using two Atmospheric Transport and Deposition Models (ATDMs), the ¹³¹I/¹³⁷Cs ratio obtained from measurement data analysis, and the internal exposure model. Based on the measurements, the maximum potential thyroid doses were estimated at 3.1–160 mSv at 5 sites in the Fukushima-Hamadori area for 1-year-old children assumed to remain outdoors, whereas they were less than 4.3 mSv at the other sites in the base case of the ¹³¹I/¹³⁷Cs ratio. The spatial distribution of the maximum potential of early inhalation doses was estimated by using the MME and measurements. The inhalation thyroid doses in the evacuation scenarios were compared to the estimates reported by previous studies. The results of the present study were almost congruent with the outcomes of previous investigations except for thyroid doses contributed by highly contaminated plumes on March 12 and 15. The sensitivity analysis for the ¹³¹I/¹³⁷Cs ratio indicated that these plumes carried the potential to significantly increase the thyroid doses of residents.