Continuous diurnal radon measurements in Bulgarian caves and dose assessment

Bulgaria is a country in the eastern part of the Balkan Peninsula, in South-eastern Europe. The presence of many mountains and karst areas is a prerequisite for a large number of caves in the country, but only some of them are managed and accessible to tourists. The purpose of the study is continuous measurement of radon concentration in six popular Bulgarian tourist caves (Bacho Kiro, Magura, Venetsa, Saeva dupka, Snejanka, and Uhlovica), and analysis of the diurnal variation. The direct, continuous radon measurements were performed with TERA (TSR4) system (Tesla, Czech Republic). 29 radon concentration time series were considered for evaluation of radon variations during the day and night. The period of the measurement was the autumn of 2019, and only one cave (Bacho Kiro Cave) was measured during the winter. The measurements were performed at different points (gallery, halls, or other cave formations) in each of the caves. The average diurnal radon concentration in the different caves varied from 531 Bq m−3 (Magura cave) to 5472 Bq m−3 (Venetsa cave). The variation of radon concentration between distinct places in most caves was approximately 1–2 k Bq m−3. No significant difference in radon concentrations was observed between night and day in a 24–96 h period measurement. The large fluctuations between the values measured at the entry (10–640 Bq m−3) and exit (165–927 Bq m−3) compared to those inside the cave were found only in cave Magura. The radon exposure of workers and visitors was assessed, using long-term measurements in the caves, based on the ICRP recommendation