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The results of the study on speciation and mobility of uranium in the ore processing residues in the Centralny Yar tailings (CY, former uranium processing site - Pridneprovsky Chemical Plant in Ukraine) are presented. Due to poor neutralization sludge material was dumped into the tailings body in acidic state. Several incidents with breakage in the pipeline transporting complex radiochemical solutions caused radioactive material spillover onto the tailings surface. Two features of radiological concern were identified - secondary contamination of the tailings surface amid elevated gamma dose rates, and excessive migration of radionuclides of U/Th decay series in strong acidic conditions within the tailings body. The monitoring data collected during 2005-2017 showed fast migration of uranium from the tailings body into the groundwater with specific activity varied in the range from 1 to 20 Bq/L. To support this finding the experimental studies aimed to obtain physical and chemical speciation of uranium in the tailings materials in existing and simulated conditions were undertaken. This was conducted by application of modified BCR sequential extraction methods followed by assessment of uranium speciation in equilibrium conditions, using the geochemical modeling tool MEDUSA coupled with the HYDRA database. [ABSTRACT FROM AUTHOR]

Results are presented of the characterization of radioactivity inventory of Zapadnoe uranium mill tailings impoundment situated at Pridneprovsky Chemical Plant (PChP; Dneprodzerginsk, Ukraine). Analyses of radioactivity data set based on analytical studies of core material from 15 characterization boreholes allowed significantly refining waste volume and radioactivity inventory estimates. Geostatistical analyses using variogram function have established that radioactivity distribution in Zapadnoe tailings is characterized by regular spatial correlation patterns. Ordinary kriging method was applied to assess distribution of radioactivity in 3D. Results of statistical analyses suggest significant redistribution of uranium in the dissolved form in the residues (presumably due to water infiltration process). The developed structural model for radioactivity distribution is used for further risk assessment analyses. Derived radioactivity correlation scales can be used for optimization of sample collection when characterizing the PChP Site and similar contaminated sites elsewhere. [ABSTRACT FROM AUTHOR]

Results of long-term observations on 137Cs and 90Sr concentration in water mass of the ChNPP Cooling Pond are presented. Drastic transformation of the intrinsic trend in changes with time of averaged radionuclides content in water is clearly demonstrated alongside with the spatial heterogeneity of radioactive contamination of water body. The cycling character of seasonal changes of 137Cs activity concentration in water is being linked to hydrochemical, temperature and oxygen regimes of the Cooling Pond. [ABSTRACT FROM AUTHOR]

During the years after the Chernobyl accident the radioceasium activity concentration in most contaminated aquatic ecosystems decreased markedly. Lakes with no permanent inflows and outflows (closed lakes), however, still present a radioecological problem which is expected to continue for some time. In this paper, a mechanistic model for the long-term prediction of radiocaesium behaviour in closed lakes is developed. The model of Prokhorov (Radiokhimiya (Radiochemistry) 11 (1969) 317) was modified to describe the effects of bottom sediment bioturbation, surface runoff from the catchment and suspended solids formation and sedimentation. The model input parameters are the effective diffusion coefficient in bottom sediments, depth of the completely mixed layer, the distribution coefficient in the sediment-water system, the runoff coefficient, sedimentation rate, and deposition density. Values of all these parameters can be independently estimated or measured in a short-term experiment. Given negligible runoff and sedimentation, the dynamics of radiocaesium in lake water is described by a simple equation with only one unknown parameter. This allows us to make long-term predictions on the basis of a series of measurements carried out during the relatively short period. The model was tested against 137Cs activity concentrations measured between 1993 and 1999 in Svyatoe lake in the Bryansk region of Russia. Calculated and measured activity concentrations are in good agreement.