Radiological safety in mining of low grade uranium ores: Four decades of monitoring and control in Indian mines

Full text: Mining of low grade uranium ore involves deployment of large man power in many stopes simultaneously to achieve the production target. The first uranium mine in India commenced commercial operation in 1968 with production from shallower haulage levels. The mine is now operating up to a vertical depth of about 905 meters. Radiation exposure of workers is mainly from external gamma radiation and inhalation of radon progeny. The long lived alpha emitters in the airborne ore dust are relatively small in such mines. While gamma radiation is not amenable to control the radon and its progeny can be effectively reduced by adequate ventilation and a judicious distribution of ventilating air to the working zones and sealing of worked out areas. Workplace monitoring for radiological parameters in the mines commenced right from the beginning of the operations. Initially the effective dose to the workers was evaluated from the area monitoring and occupancy period of workers in different zones. Subsequently, SSNTD and TLD based personal dosimeters were developed and deployed in a phased manner. Average dose to the workers in the early stages was around 10 mSv/y. Ventilation was progressively improved by widening of air passages and increasing the fan capacity. The system itself was modified from the series to a parallel system of ventilation to supply fresh air to each operating haulage level and allow the used air to join the return air stream. The modifications had positive impact and the average doses have shown a downward trend are now around 5 mSv/y. Progressive mechanization of mining operations over the years has resulted in reduction of manpower and consequently in a reduction of the collective dose. Subsequently, three additional underground uranium mines have been opened with low grade uranium ores. Increasing ventilation has resulted in reduction of radon concentration to an average of around 0.3 KBq/m3 EER in Jaduguda mine. The internal and external radiation dose fractions for this mine has been around 0.45 and 0.55, respectively, in recent years with some variations for the different mines. The paper gives an overview of the monitoring for external radiation, radon and the long-lived alpha emitters in the mines. The trend of doses in Jaduguda mines is given for the last four decades and recent data for the other mines are summarised. Efforts made to increase the ventilation and its impact on radon control is also discussed. (author)