A New Approach For Reconstructing The I-131-Spreading Due To The 2011 Fukushima Nuclear Accident By Means Of Measuring I-129 In Airborne Particulate Matter

To retrieve the diffusion trajectory of the I-131 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 I-129 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 I-129 activity of as low as 10(-8) Bq for a small piece of filter samples (about 0.1 cm(2)). Coupled with I-131 data determined just after the accident, activity ratios of I-129/I-131 were obtained with a mean value of 2.29 x 10(-9) (+/- 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 I-129 was partly lost from APM. As I-129 can be a proxy of I-131 for APM, it is possible to trace how I-131 in the particulate phase spread in eastern Japan and, furthermore, evaluate the internal radiation exposure due to I-131 by inhalation of I-131-containing airborne particulates.