Analysis of 129I in the soils of Fukushima Prefecture: preliminary reconstruction of 131I deposition related to the accident at Fukushima Daiichi Nuclear Power Plant (FDNPP)

Iodine-131 is one of the most critical radionuclides to be monitored after release from reactor accidents due to the tendency for this nuclide to accumulate in the human thyroid gland. However, there are not enough data related to the reactor accident in Fukushima, Japan to provide regional information on the deposition of this short-lived nuclide (half-life = 8.02 d). In this study we have focused on the long-lived iodine isotope, 129I (half-life of 1.57 × 107 y), and analyzed it by accelerator mass spectrometry (AMS) for surface soil samples collected at various locations in Fukushima Prefecture. In order to obtain information on the 131I/129I ratio released from the accident, we have determined 129I concentrations in 82 soil samples in which 131I concentrations were previously determined. There was a strong correlation (R2 = 0.84) between the two nuclides, suggesting that the 131I levels in soil samples following the accident can be estimated through the analysis of 129I. We have also examined the possible influence from 129mTe on 129I, and found no significant effect. In order to construct a deposition map of 131I, we determined the 129I concentrations (Bq/kg) in 388 soil samples collected from different locations in Fukushima Prefecture and the deposition densities (Bq/m2) of 131I were reconstructed from the results.