Radon release at a rehabilitated uranium mine site and dose assessment (case of the former Beshtaugorskiy mine, North Caucasus)

The uranium mines both developed and abandoned appear to be one of the most significant sources of radon exhalation in the world. Therefore, the study of radon exposure of the population in the areas around rehabilitated uranium mines is very important. This article presents the results of the radon release studies at the rehabilitated Beshtaugorsky uranium mine site, which is now used by local people for hiking and picnicking. A dose assessment is also provided. The following parameters were measured: (i) radon flux density from the soil by the open charcoal chamber method using the beta radiometer “Camera-01”; (ii) concentration of radon and its progeny in the air using the radiometer “Alpha-AERO” based on a semiconductor detector; (iii) radon in groundwater by accumulation of radon in charcoal using the “Camera-01” equipment; (iv) content of natural radionuclides in soils and rocks by NaI(Tl) spectrometry; and (v) gamma dose rate using the Geiger-Muller counter. Measurements were taken at 38 points along the Beshtaugorskaya ring road, as well as at the mouths of abandoned adits. The measurements showed significant radon release at the mouths of the former adits and in some sections of tectonic faults. In our opinion, the abnormal radon release is due to convective thermally induced air convection in permeable zones of the mountain massif; radon transport by groundwater does not affect surface radon emissions. The most important source of radiation risk is the incompletely sealed mine openings, which periodically blow out radon-enriched air. In places around the mouth of these mines, the equivalent equilibrium concentration of radon progeny in the open air reaches 35,000–45,000 Bq/m³, and the gamma dose rate also exceeds 10 µSv/h, which is more than 30 times the permissible level. A person resting for 4 h in such an area may, in the worst case, receive a dose of more than 2 mSv. It is advisable to implement measures to prevent or reduce the time people spend in the area around the former mine openings.