Assessment of the radiological impact of gamma and radon dose rates at former U mining sites in Tajikistan

An assessment of the radiological situation due to exposure to gamma radiation, radon ((222)Rn) and thoron ((220)Rn) was carried out at former uranium (U) mining and processing sites in Taboshar and at Digmai in Tajikistan. Gamma dose rate measurements were made using various field instruments. (222)Rn/(220)Rn measurements were carried out with field instruments for instantaneous measurements and then discriminative (222)Rn/(220)Rn solid state nuclear track detectors (SSNTD) were used for longer representative measurements. The detectors were exposed for an extended period of time in different outdoor and indoor public and residential environments at the selected U legacy sites. The results showed that gamma, (222)Rn and (220)Rn doses were in general low, which consequently implies a low to relatively low radiological risk. The radiation doses deriving from external radiation (gamma dose rate), indoor (222)Rn and (220)Rn with their short-lived progenies did not exceed national or international standards. At none of the sites investigated did the average individual annual effective doses exceed 10 mSv, the recommended threshold value for the general public. A radiation hazard could be associated with exceptional situations, such as elevated exposures to ionizing radiation at the Digmai tailings site and/or in industrial facilities, where gamma and (222)Rn/(220)Rn dose rates could reach values of several 10 mSv/a. Current doses of ionizing radiation do not represent a hazard to the health of the resident public, with the exception of some specific situations. These issues should be adequately addressed to further reduce needless exposure of the resident public to ionizing radiation.

Environmental impact assessment of radionuclide and metal contamination at the former U sites Taboshar and Digmai, Tajikistan

Uranium (U) ore mining and processing were initiated in the former Soviet Republics of Tajikistan after the Second World War as part of the USSR nuclear weapon programme. The U mine in Taboshar was opened in 1936, and mining took place from 1945 to 1965, while the Digmai tailings dump was exploited during 1963-1993. The mining, milling and extraction activities have resulted in large amounts of waste rock deposits and U tailing materials placed in the vicinity of inhabited areas. To assess the environmental impact of radionuclides and trace metals in the Taboshar and Digmai mining and tailing sites in Tajikistan, field expeditions were performed in 2006 and 2008. In addition to in situ gamma and radon dose rate measurements, sampling of water, fish, sediments, soils and vegetation including in situ fractionation of water were performed. The U concentrations in water from Taboshar Pit Lake (2.0 mg U/L) were higher than in waters collected in the Digmai area. The Pit Lake and the stream water from the tailing mountain were also characterised by elevated concentrations of As, Mo, Mn and Fe, exceeding the WHO recommended values for drinking water. Uranium, As, Mo and Ni were present as low molecular mass species in the waters, and are therefore considered mobile and potentially bioavailable. The (238)U concentrations in sediments and soils varied between the sites; with peak concentrations (6 kBq/kg dw) in sediments from the Pit Lake, while the soil concentrations were significantly lower (296-590 Bq/kg dw). In contrast, high levels of the radium isotopes ((226)Ra and (228)Ra) were found in the Digmai soil (17-32 kBq/kg dw). Based on sequential extraction results, both U and Pb were found to be quite mobile at the Pit Lake site, showing that these elements were associated with the pH sensitive and redox sensitive amorphous fractions. In tailings, U was found to be quite mobile, but here Pb was rather inert. The transfer of radionuclides and metals from sediments to waters was in general low. In the Pit Lake, U was quite mobile (Kd = 90 L/kg), followed by Ni (1.5 × 10(3) L/kg) and As (6 × 10(3) L/kg), Cu and Cd (1.5 × 10(4) L/kg), while Pb (3 × 10(5) L/kg) was rather inert. The transfer from soil to plant, TFs (kg/kg dw), was in general low, while the bioconcentration factor for water living Poaceae and for fish from water was relatively high (Pb 1.8 × 10(5) and Cd 1 × 10(4)). These legacy sites, containing enhanced levels of natural radioactive material (TENORM) as well as heavy metals, may represent a hazard having a potential radiological and chemical impact on man and the environment, and measures should be taken to reduce the environmental risk to man and biota.