Keywords: tailings, objects of uranium legacy site, natural radionuclides, contamination, soils, aerosols, groundwater and surface water, radioecological monitoring

The article summarizes the results of long-term (2005-2021) site characterization and radioecological monitoring at the former U-production “Prydniprovsk Chemical Plant” (PChP site) and adjacent territories during period of preparedness and justification of the large scale of remediation project activities, which have been recently started at the site. Data analysis have been determined the activity levels of the wide spectra of naturally occurring radionuclides (in particular ²³⁸U, ²²⁶Ra, ²³⁰Th, ²¹⁰Pb, ²¹⁰Po, ²²²R) identified in soils, groundwaters and surface waters, air, atmospheric fallout and some biota at the contaminated site. The results present the dynamics and time-series in contamination of the varies environment elements, which play significant role in exposure of workers at the site and public living at the surrounding areas. The radiological surveys carried out at the site show the areas with elevated dose rate (≥0.5 μSv·h^-1) at the more than 30% of the entire legacy site territory. The activity ²³⁸U in the local soils identified with high range of variability and level of contamination exceeding its local background (30-50 Bq·kg^-1) from 10 to 1000 times. The ambient activity concentrations of ²³⁸U and ²²⁶Ra in aerosols in some weather condition may exceed the background level at the surrounding areas (0.1 μBq·m^-2·s^-1, 11.5 μBq·m³) in 100-200 times (in particular locally at the areas were earthworks had place. In case of no actions the typical air contaminations were observed at the legacy site in 1.5-2.0 times higher of the levels outside of the U-legacy site. The monthly averaged activity ²²²Rn in the air of open space are 4-5 times higher of the local background contamination level for the dwellings (median 200±68 Bq∙m^-3), however in some locations at the site (former U-production facilities) the ambient activity concentration of ²²²Rn can exceed its background levels by up to 50 and even 100 times. The data analysis proves that main ways of exposure of workers are external gamma radiation, the inhalation exposure routes from aerosol pollution and the spread of OA 222Rn in the air can be significant only for workers work at the legacy site and do not pose a threat to the city‘s population. The time-series of radionuclides in the groundwaters waters and some surface rivers at the site are given as well. Radiological assessment showed that aquatic pathways cannot form any significant contributions to the radiation exposure dose of the public (at least in nearest future). The analysis of site characterization studies and obtained monitoring were used for justification of the remediation strategy legacy site. It was shown the main sources of radiological hazard at the site are facilities where concentrated residues of uranium-containing materials are concentrated (in the former ore processing buildings, ore storage facilities and tailings of the U-processing residues). These objects are requiring the application of the priority remediation measures bringing this site in a safe state.

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