Radioecological assessment of the consequences of hydronuclear testing at the Semipalatinsk test site: Radionuclide distribution, soil-to-plant transfer, and radiological risk.

Although the radioecological status of several areas within the Semipalatinsk Test Site (STS) has been extensively investigated, the Aktan-Berli hydronuclear testing area has remained insufficiently studied, limiting the assessment of its radioecological condition and associated potential radiological risks. The aim of this study was to provide a comprehensive assessment of the activity concentrations of naturally occurring and artificial radionuclides in soils and vegetation, their accumulation characteristics, and the radiological hazard associated with the site. Radionuclide activity concentrations in soil and plant samples were determined by γ-ray spectrometry together with α/β-particle spectrometry following preliminary radiochemical separation. The activity concentrations of the naturally occurring radionuclides 40K, 232Th, and 226Ra were consistent with their typical values reported for soils in Kazakhstan, and their activity concentrations in vegetation were lower than or comparable to those in the corresponding soils. Localised elevated activity concentrations of 241Am (up to 93 Bq kg-1) and 137Cs (up to 100 Bq kg-1) were identified in the south-western and northern parts of the study area, respectively. The calculated transfer factors (Tfs) indicate a limited migration of most investigated radionuclides from soil to vegetation. The Tfs for naturally occurring radionuclides agree with published data, whereas among the artificial radionuclides, 90Sr exhibited the greatest bioavailability, 137Cs showed substantially lower bioavailability, and the accumulation of transuranic elements remained limited. These patterns are consistent with findings previously reported for other areas of the STS. The calculated radiological hazard parameters and the projected annual effective dose to the public (not exceeding 0.3 mSv) were below the recommended reference levels, while dose rates to plants remained below internationally accepted criteria. The results provide the first comprehensive characterisation of the radioecological status of the Aktan-Berli testing area and may be used to assess potential changes in radiological conditions associated with the redistribution of long-lived artificial radionuclides. These results provide a scientific basis for long-term monitoring and radioecological management of the Aktan-Berli testing area.