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Description
I-131 is a radionuclide that, in the event of a nuclear accident, contributes the most to the radiation dose to members of the public. Since such exposure occurs within the first few days following a radioactive release, large-scale determination of the effective dose is virtually impossible.
Current monitoring of I-131 activity concentrations in the Czech Republic relies on HPGe gamma spectrometry measurements of samples from only ten high-volume air sampling stations. Most of the area remains therefore uncovered.
Aim of the presented work is to build a system capable of determining I-131 activity concentration as well as thyroid equivalent dose retrospectively via I-129 in soil samples analyses. The condition of the procedure viability is to be capable to distinguish between I-129 of pre-accident and accident origin and the knowledge of I-131 to I-129 deposition density ratio during the accident (Michel et al. 2005). While I-131 to I-129 deposition density ratio in particular geographical area can be accessed from mathematical simulation retrospectively, reliable determination of I-129 of pre-accident origin has to be conducted in advance due to huge differences in I-129 activities caused by Chornobyl accident. Specific areas of interest within the Czech Republic were selected for the monitoring of current I-129 activity concentrations in soil. These included regions poorly covered by the existing monitoring system, areas heavily affected by Chornobyl fallout, densely populated zones, and border regions in close proximity to nuclear power plants (NPPs).
For needs of determination of I-129 activity concentration in soil samples a radiochemical procedure was utilized and optimized for best optimal yields of iodine separation from solid environmental media, involving especially various types of soils and sediments. For these materials, 5–10% TMAH solutions typically provide near-quantitative yields for total iodine separation. This approach was combined with subsequent liquid-liquid extraction to chloroform in the acidic pH range, followed up with re-extraction and co-precipitation as silver iodide. The oxidation states during the procedure were adjusted accordingly with K2S2O8, Na2SO3 and NaNO2 agents. Samples were analysed with MILEA AMS beamline at ÚJF Řež for 129I content, total iodine concentration was determined in parallel by ICP-MS. The analysis of 129I was performed utilizing I-/I2+ analytical setup, with the primary ions being extracted from AgI(+Nb) target matrices in Ti cathodes, used for suppression of molecular isobars, and the high energy beamline efficiency corrected to the ETH Zurich isotope reference material C.2-39.
The retrospective calculation of the thyroid equivalent dose is based on the methodology established by Pietrzak-Flis et al. (2003).
This conference contribution presents a comprehensive methodology proposal for the retrospective estimation of I-131 thyroid equivalent doses, based on accelerator mass spectrometry (AMS) analyses of I-129.
This research is co-funded with state support from the Czech Technology Agency under the SARA project (SQ01010334), Environment for Life 2 Programme. The infrastructure of the AMS laboratory built under the RAMSES project was used in the research realization.