Regional comparison of I-129, I-131, and Te-129m in the surface soil at Fukushima

In this study, surface soil samples were collected from 86 locations within Fukushima Prefecture at distances ranging from 13.8 to 123.5 km from the Fukushima Daiichi Nuclear Power Plant (FDNPP), following the nuclear accident on March 11, 2011, and measurements of 129 I, 131 I, and 129m Te were conducted. The ranges of concentration and deposition (decay-corrected to 14:46, March 11, 2011) for 129 I, 131 I, and 129m Te were approximately (0.55–377) × 10 8 atoms g −1 and 1.6–300 mBq m −2 , (1.1–2383) × 10⁶ atoms g −1 and 23–8060 kBq m −2 , and (0.66–1042) × 10⁶ atoms g −1 and 4.4–842 kBq m −2 , respectively. The isotope ratios of 129 I (atoms g −1 ) to 131 I (atoms g −1 ) ranged from 6.4 to 105. The soil sampling locations were roughly classified into five regions, and the relationships between 129 I and 131 I were compared for each region. Factors involved in the deposition of radioactive iodine in soil are very complex, including its release from multiple reactor units at various times and weather conditions, among others. Although it is difficult to identify the specific sources of radioactive iodine release by region, the results of this study suggest that radionuclides were deposited intensively from the same source during the same period, particularly in the northwestern part of the FDNPP. Meanwhile, in the central western region (the Nakadori and Abukuma areas), multiple distinct deposition events for 129 I and 131 I were suggested. In the southern region, the release/deposition pattern of 129m Te was suggested to differ from those in the other four regions. These results are generally consistent with those of previous studies. The mean value of 129 I/ 131 I for the entire area, excluding samples below the background levels, was 22.5 ± 8.4, which is consistent with the ORIGEN2 code and other published data. The mean contribution rate of 129m Te (decay-corrected at 14:46:00 JST on March 11, 2011) to 129 I was approximately 2%. Based on the relationship between 129 , 131 I, and 129m Te deposited in the soil, the proportion released and deposited on land relative to the amount generated within the reactor may have been lower for 129, 131 I than for 129m Te. The results of this study suggest differences in the release, diffusion, and deposition mechanisms from the reactor, based on the element.