Radiological survey on radon entry path in an underground mine and implementation of an optimized mitigation system

Background The European Union council has introduced the basic safety standards (EU-BSS) for protection against the dangers arising from exposure to ionizing radiation by laying down a new radon reference level at workplaces. In this regard, all European state members must establish a national reference level based on all pre-defined requirements. After implanting the directive 2013/59/Euratom by European state members, new challenges have been revealed to mitigate radon appropriately in underground workplaces due to the exciting limitations (e.g., ventilation system, dust dispersion, air injection, etc.). Therefore, a conceptual design of an environmental radiological survey was defined and implemented by examining the hypothesis to find practical solutions following EU-BSS. The main objectives of this study were to identify the potential radon entry paths, utilize an optimized ventilation system, and carry on long-term radon monitoring in an operational underground manganese mine. Results The mullock rocks (the geological structure of the mine walls) contained a small amount of Ra-226 (2-4 Bq kg(-1)). On the other hand, the mine ore (black shale, underlayer black shale, and carbonate ore) has shown the highest concentration of Ra-226 (12-16 Bq kg(-1)) and the highest radon exhalation (1.2-1.6 mBq s(-1) m(-2)). The surface radon exhalation from the mine walls was in the range of 0.7 +/- 0.1 and 1.5 +/- 0.2 mBq m(-2) s(-1). It was found that shortly after mining activity was undertaken, radon concentration increased dramatically with an average of about 5900 +/- 420 Bq m(-3) near the freshly broken walls. The optimized mobile mitigation system reduced radon concentration to 250 +/- 41 Bq m(-3) on average. Conclusion Apart from the fact that aged walls were involved in the radon accumulation, considering the mine ventilation performance and the total active surface area, the exhaled radon from the aged walls could not be the primary potential source of high radon concentration when mining activity was undertaken. According to the obtained results, the ores, recently fragmented during the course of mining operations, were the primary path. Therefore, after successfully identifying the radon entry path, radon concentration could be reduced to meet the EU-BSS requirement by implementing the developed mitigation system.