Determination of 129I and 127I concentrations in Chernobyl contaminated soil samples

Abstract

A large amount of radioiodine isotopes (mainly ¹³¹I, T_1/2=8days) were released from the accident at Chernobyl Nuclear Power Plant (CNPP) in April-May 1986. An increase in childhood-thyroid cancer in the contaminated area was assumed to be the cause of radioiodine released at the time of the accident. However, there is a lack of relevant data on the ¹³¹I levels in the local environment (e.g. air, plant, soil), and therefore, it is difficult to assess radiation doses received by the patients from this nuclide following the accident. However, a long-lived iodine isotope, ¹²⁹I (T_1/2=15.7 million years), which was also released with a certain ratio to ¹³¹I from CNPP, could be used for estimating the ¹³¹I levels in the environment. In this presentation, we will discuss analytical results of the ¹²⁹I concentrations and ¹²⁹I/¹²⁷I atom ratio in soil samples collected from CNPP exclusion zone (30-km zone) with the aim of assessing current contamination levels and distribution patterns. For the analysis of iodine fraction in soil samples, pyrohydrolysis method was utilized for the separation of ¹²⁷I and ¹²⁹I nuclides and subsequently determined using inductively coupled plasma mass spectrometry (ICP-MS) and accelerator mass spectrometry (AMS), respectively. The concentration of ¹²⁹I and ¹²⁹I/¹²⁷I atom ratio in surface soil samples in the 30km-zone of CNPP ranged from 4.6 to 170mBq/kg and from 1.4 x10^-6 to 13 x10^-6, respectively. These values are significantly higher than the global fallout values of ¹²⁹I, indicating the most of ¹²⁹I was attributed to the fallout of the accident. Stable iodine concentrations in this area were found to be very low (below 1ppm) for most of the samples, suggesting the environmental iodine levels in this area are 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 x 10^-7. This might be due to the different behavior of deposition and/or migration of these nuclides. The results suggest that the obtained data of ¹²⁹I should be useful reconstructing the ¹³¹I deposition in the contaminated areas.