Radon (Rn) as a tracer in natural ventilation efficiency assessment in underground workings - an example of "St John Mine" tourist complex in Krobica (the Sudetes, SW Poland).

The authors characterize the use of Rn as an effective tracer of natural ventilation of an underground site where air circulates within a whole system of workings and ventilation intensity (the number of air exchanges in the space) is determined by atmospheric factors. A radon-related database containing results of measurements conducted at various intervals and at different stages of site accessibility was compiled. During 8 months of the calendar year Rn activity concentration exceeds the mean annual reference value established by Polish law (300 Bq/m). These months correspond to periods with low intensity of natural ventilation of the workings and reduced efficiency of air exchange between the site and the atmosphere. They occur in autumn - in the second half of September, in October and November, and in May in spring, and persist for 7 to even 14 days. During these periods, the time spent inside the facility which is considered safe in terms of radiation protection is limited to an average of 6-8 h a day, i.e. from 6 a.m. to 6 p.m. in October, from 11 a.m. to 6 p.m. in November and from 11 a.m. to 5 p.m. in May. The length of a safe stay in the facility is determined by atmospheric factors, mainly the air and ground temperature. The concentrations of other gases in the atmosphere inside the facility comply with Polish mining regulations.