Iodine-129 in soils from Northern Ukraine and the retrospective dosimetry of the iodine-131 exposure after the Chernobyl accident

Forty-eight soil profiles down to a depth of 40 cm were taken in Russia and Ukraine in 1995 and 1997, respectively, in order to investigate the feasibility of retrospective dosimetry of the I-131 exposure after the Chernobyl accident via the long-lived I-129. The sampling sites covered areas almost not affected by fallout from the Chernobyl accident such as Moscow/Russia and the Zhitomir district in Ukraine as well as the highly contaminated Korosten and Narodici districts in Ukraine. I-129 was analyzed by radiochemical neutron activation analysis (RNAA) and accelerator mass spectrometry (AMS). I-127 was measured for some profiles by RNAA or ion chromatography (IC). The results for I-127 demonstrated large differences in the capabilities of the soils to store iodine over long time spans. The depth profiles of I-129 and of (CS)-C-137 showed large differences in the migration behavior between the two nuclides but also for each nuclide among the different sampling sites. Though it cannot be quantified how much I-129 and (CS)-C-137 was lost out of the soil columns into deeper depths, the inventories in the columns were taken as proxies for the total inventories. For I-129, these inventories were at least three orders of magnitude higher than a pre-nuclear value of 0.084 +/- 0.017 mBq m(-2) derived from a soil profile taken in 1939 in Lutovinovo/Russia. From the samples from Moscow and Zhitomir, a pre-Chernobyl I-129 inventory of (44 24) mBq m-2 was determined, limiting the feasibility of I-129 retrospective dosimetry to areas where the I-129 inventories exceed 100 mBq m(-2). Higher average I-129 inventories in the Korosten and Narodici districts of 130 and 848 mBq m(-2), respectively, allowed determination of the I-129 fallout: due to the Chernobyl accident. Based on the total I-129 inventories and on literature data for the atomic ratio of I-129/I-131=13.6 +/- 2.8 for the Chernobyl emissions and on aggregated dose coefficients for I-131, the thyroid exposure due to I-131 after the Chernobyl accident was estimated for the inhabitants of four villages in the Korosten and of three villages in the Narodici districts. The limitations and uncertainties of the I-129 retrospective dosimetry are discussed. (c) 2004 Elsevier B.V. All rights reserved.