Open Pit/Underground Mining
Open pit development method is used where a deposit is shallow. Open pit mining method includes the use of bulldozers to strip and develop the deposit, bulldozers and shovel loaders for loading and dump trucks to remove the ore from the open pit. After completion of mining operations the pit is then backfilled with overburden and reclamation work on restored surface is performed.
It is considered that radiation hazard of open pits for the personnel is much lower than that of the underground mines. However, contamination of surface and ground waters often generates problems due to necessity of backfilling the pit with waste rock and reclamation upon completion of mining operations. There are rules and laws which provide for the environmental protection measures as a mandatory element of a mine design. They contain the following requirements:
- performance of a preliminary environmental impact assessment;
- gradual implementation of rehabilitation program, including landscape restoration and restoration of forestlands;
- planting of endogenous flora;
- restoration of endogenous wildlife;
- spot and continuous audits of compliance of the environmental condition with the existing norms.
The most challenging task in underground mine is how to bring ore to the surface. For this purpose, the miners either use a horizontal passage from a slope to the depth of the mountain (mine adit) or a vertical passage (sometimes pitching) to the depth of a mine working (mine).
Underground method is commonly used to mine a higher grade ore as compared to open pit method: higher costs of mining deep under ground are only set off by a high grade of ores. Technically, there is no limit on depth of ore occurrence for the underground mining method, but today the deepest uranium mines are no more than 2 km deep. Larger depths of a mine result in excessive increase of cost of uranium mined.
The most dangerous factor associated with the underground method is radon which may travel over substantial distances and form high concentrations in the underground air. The average life of a radon atom is 5.5 days. The main problem affecting organization of radiation protection is that as the reserves are developed the total share of radon emitted from the developed mine workings increases. If ventilation arrangements are adequate most of it is directed right to the surface. Another peculiarity of formation of the general volume of this gas in the mine is that radon is lifted from the ore massif mainly with filtration air flows, and from the developed sites — with convection air flows. Thus, in order to decrease the volume of radon released to the active mine workings the ventilation system must be arranged in such a way that these flows go directly to the emanating mine air flow. Currently, all mines have a system of artificial mine venting. Thus, at the Priargunsky mine 1,410 m3/s of fresh air are injected in the mine by three main ventilation machines.
Air diffusers and ventilation doors work without interruption. These measures allowed to substantially decrease radon release to the operating space of the mines and maintain the value of equivalent equilibration activity concentration of radon at an acceptable level which is consistent with the permitted norms. At the underground mines the stope is monitored at least three times a month, at other mines — once a month. On a daily basis, at the end of the shift the results of all measurements and laboratory assays of long-lived nuclides and radon are recorded, and upon processing of data documents in the established form are issued.