Determination of 129I and 127I concentration in soil samples from the chernobyl 30-km zone by AMS and ICP-MS

Abstract

A large amount of radioiodine isotopes (mainly 131I, t1/2 = 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 (t1/2 = 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 dif- ferent 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.