A large amount of radioiodine isotopes (mainly ¹³¹I, t_1/2 = 8 days) was released from the accident at Chernobyl Nuclear Power Plant (CNPP) in April-May 1986. An increase in childhood-thyroid cancer in the contaminated areas in Belarus, Russia and the Ukraine was demonstrated to be caused by radioiodine released at the time of the accident. However, there is a lack of quantitative data on the ¹³¹I levels in the local environment (e.g. air, plant, soil). At this point, a long-lived iodine isotope, ¹²⁹I (t_1/2 = 15.7 million years), also released with a certain ratio to ¹³¹I from CNPP, could be used for estimating the ¹³¹I levels in the environment. In this paper we present analytical results of the ¹²⁹I concentrations and ¹²⁹I/¹²⁷I atom ratios in soil samples collected from the CNPP exclusion zone (30-km zone), with the aim of assessing current contamination levels and distribution patterns. For the analysis of the iodine fraction in the investigated soil samples, the pyrohydrolysis method was utilized for separation of ¹²⁷I and ¹²⁹I nuclides, and subsequently their concentration was determined using inductively coupled plasma mass spectrometry (ICP-MS) and accelerator mass spectrometry (AMS), respectively. The concentration of ¹²⁹I and the ¹²⁹I/¹²⁷I atom ratio in the surface soil samples in the 30 km-zone of CNPP ranged from 4.6 to 170 mBq/kg, and from 1.4 × 10^-6 to 13 × 10^-6, respectively. These values are significantly higher than those from global ¹²⁹I fallout, indicating that most of the measured ¹²⁹I was due to the deposition of the accident. Stable iodine concentrations in this area were found to be very low (below 1 ppm) for most of the samples, suggesting the environmental iodine levels in this area to be potentially low. The ¹²⁹I/¹³⁷Cs activity ratio in surface and sub-surface soils was not so constant, i.e., in the range (7.3-20.2) × 10^-7. This might be due to the different behavior of deposition and/or migration of these nuclides in soil. These results suggest the obtained data of ¹²⁹I to be useful for the reconstruction of the ¹³¹I deposition in the contaminated areas.

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