A new approach for reconstructing the ¹³¹I-spreading due to the 2011 Fukushima nuclear accident by means of measuring ¹²⁹I in airborne particulate matter

To retrieve the diffusion trajectory of the ¹³¹I dispersed in the environment by the nuclear power plant accident in Fukushima in 2011, airborne particulate matter (APM) samples collected in the Tokyo metropolitan area were analyzed for their ¹²⁹I contents by means of accelerator mass spectrometry. In evaluating blank levels of chemicals and filters used for collecting APM, we established the analytical procedure for determining the ¹²⁹I activity of as low as 10⁻⁸ Bq for a small piece of filter samples (about 0.1 cm²). Coupled with ¹³¹I data determined just after the accident, activity ratios of ¹²⁹I/¹³¹I were obtained with a mean value of 2.29 × 10⁻⁸ (±28% of a standard deviation). This value is systematically smaller than a mean value of soil samples by 16–24% and the inventory data by 27%, suggesting that ¹²⁹I was partly lost from APM. As ¹²⁹I can be a proxy of ¹³¹I for APM, it is possible to trace how ¹³¹I in the particulate phase spread in eastern Japan and, furthermore, evaluate the internal radiation exposure due to ¹³¹I by inhalation of ¹³¹I-containing airborne particulates.