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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, (129)I (half-life of 1.57 × 10(7) 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 (131)I/(129)I ratio released from the accident, we have determined (129)I concentrations in 82 soil samples in which (131)I concentrations were previously determined. There was a strong correlation (R(2) = 0.84) between the two nuclides, suggesting that the (131)I levels in soil samples following the accident can be estimated through the analysis of (129)I. We have also examined the possible influence from (129m)Te on (129)I, and found no significant effect. In order to construct a deposition map of (131)I, we determined the (129)I concentrations (Bq/kg) in 388 soil samples collected from different locations in Fukushima Prefecture and the deposition densities (Bq/m(2)) of (131)I were reconstructed from the results.

In order to understand the behavior of radionuclides released from the Fukushima Daiichi nuclear power plant, the depth distributions of radiocesium and radioiodine were investigated in a wheat field, a rice paddy, an orchard, and a cedar forest in Koriyama, Fukushima Prefecture. Our results demonstrate that, following the nuclear power plant disaster, more than 90% of the radionuclides were distributed in the upper 6 cm of the soil column in the wheat field and within 4 cm of the surface in the rice paddy, orchard, and cedar forest. According to the measurement of radionuclides in the three adjacent agricultural fields, the variation of deposition densities in the wheat field was smaller than that of the orchard and rice paddy, suggesting that the low permeability of the orchard and paddy soils may cause horizontal migration of radionuclides during the initial deposition. This result indicates that the deposition densities in the wheat field should be appropriate for estimating the amount of fallout in the area. The deposition densities of 134 Cs, 137 Cs, and 131 I in this area were estimated to be 512 ± 76 (SD, n = 5), 522 ± 80 (SD, n = 5), and 608 ± 79 (SD, n = 5) kBq/m 2 (decay corrected to April 1, 2011), respectively. A comparison of the deposition density between the wheat field and the cedar forest suggests that more than half of the radionuclides are distributed in the tree canopies of the evergreen forestland.

Atmospheric fallout samples collected from Tokyo between 1963 and 2003 were analyzed using accelerator mass spectrometry (AMS) in order to determine (129)I/(127)I ratios and to examine the deposition rate of (129)I and its secular variation in Tokyo. The (129)I/(127)I ratios in the atmosphere during 1963-1977 ranged from 1 × 10(-8) to 2 × 10(-8). This is roughly 4 orders of magnitude higher than pre-atomic levels, possibly due to atmospheric nuclear weapons tests. The calculated monthly atmospheric deposition rates of (129)I differed from those produced by nuclear fallout of (90)Sr and (137)Cs, indicating that the variations in (129)I deposition are not influenced exclusively by either nuclear bomb testing or by the Chernobyl accident. After 1978, high (129)I depositions (up to 0.13 mBq/m(2)/month) were observed. The (129)I depositions started to increase markedly at the latter half of the 1970s. The secular variation of the estimated annual (129)I deposition in Tokyo showed a close relationship between the annual atmospheric discharge of (129)I from the Tokai Reprocessing plant. Therefore, the atmospheric fallout collected from Tokyo after the late 1970s is influenced primary by the (129)I discharge from the Tokai Reprocessing plant.

Atmospheric (129)I deposition was studied in different locations of Japan (Akita, Tsukuba, Tokyo, and Ishigaki Island) with samples collected between 1963 and 2005 in order to understand the distribution and sources of this nuclide and provide a reference deposition level prior to the Fukushima accident. Over this time period, the deposition pattern of (129)I in Tsukuba and Tokyo (on the Pacific side) differed from that of Akita (on the Japan Sea side). The primary source of deposition in Tsukuba and Tokyo is related to the (129)I discharge from domestic reprocessing in Tokai-mura. In contrast, the time-series pattern of deposition in Akita seems to have been influenced by (129)I discharges from reprocessing facilities in Europe and the transport of this radionuclide by westerly winds to coastlines of the Japan Sea. The (129)I deposition in Ishigaki (one of the southernmost islands in Japan) is influenced primarily by oceanic air masses (easterly winds), and deposition was 1 order of magnitude lower than that observed in Tsukuba and Tokyo. Cumulative (129)I deposition in Tokyo before the Fukushima accident was estimated at 13 mBq/m(2). The results of this study on deposition contribute to understanding the deposition levels of (129)I prior to the accident.