Surface mining

Surface (or open cut) mining extracts the ore via surface cutting. It has the largest surface signature because of the high ratio between waste rock and ore, the latter of which can be of radiological concern.

6.3.2. Design and operation

Surface mining utilizes a range of different techniques and methodologies dependent on local site specific factors such as topography, geology and geomorphology. The term surface mining can be misleading, as some surface mines are very large and can extend over a kilometre in depth. Other mines might only be surface scrapes and not extend any significant distance into the underlying strata. Some surface operations become hybrid mines as they are transformed into an underground mine because of the depth of the ore body (or some other topographical constraint).

Open cut pits are used when the ore extends from the near surface down to depth. The typical design is based on there being several benches around the perimeter of the main pit, with mining occurring both downwards and laterally.

The design of the benches depends on the nature of the rock, geological factors such as local faulting and the scale of the operations. Bench height and angle are generally altered to address local conditions. The overall depth of the pit is determined by the nature of the ore body but other factors can limit the depth, such as groundwater inflow. Within the pit, there are haul roads to transport ore and waste rock to the surface. Adjacent to the pit are waste and ore stockpiles, the location of which is generally guided by the optimization of cost and how the mine develops, and so they are often close to the haul road exits. Other features include ponds for the storage or evaporation of surface water and groundwater from the pit and rock pile surrounds.

The general mining methodology for open cut pits is a combination of drilling and blasting followed by extraction and transport. During drilling, sampling is usually taken for grade control purposes. Ore and waste rock is extracted by using shovels to lift the blasted rock into trucks for haulage to the surface. Some operations use in-pit crushers to reduce the size of the blasted rock and skips or conveyors to transport the material to the surface.

Near surface mining (strip mining) is used when the ore body does not extend to a significant depth. For some operations, the process involves a continuous mine and fill process, in which, after an area is mined, it is progressively filled by the overburden from newly mined areas. Tailings can also be deposited as part of this process. This adds greatly to the complexity of the mining operation but reduces the impact on the surface (e.g. it minimizes surface facilities such as waste rock piles) and the potential for long term legacy issues.

One form of surface mining is where the uranium mineralization only occurs directly on the surface. Although rare, this does occur; and in such cases, mining can be as simple as extraction of only the uppermost layer of the soil (<1 m).

6.3.3. Principal exposure pathways

For surface mining, occupational exposure is generally at the lower end of exposures in uranium mining. Doses in the order of a couple of mSv are the norm and higher doses are rare during routine mining operations. The major exposure pathway is generally direct gamma, with a smaller contribution coming from the inhalation of radionuclides in airborne dust. The inhalation of radon progeny is generally only significant in deep pits and where natural ventilation processes cannot remove exhaled radon. For gamma exposure, there is a direct relationship between the uranium grade of the exposed rock in the mine and the time workers spend close to it. The dominant contribution is from the rock directly below the working area, as personnel are generally restricted from working adjacent or in close proximity to the bench vertical faces (due to rock fall risk). The gamma dose to individual workers is easy to predict based on the expected ore grades.

Inhalation of radionuclides in airborne dust is a direct function of the amount of dust generated by mine operations. Some stages are dustier and can require additional monitoring or controls to maintain low doses. Drilling operations generally offer the highest risk from dust owing to a combination of the inherent dust generated by some drill rigs (i.e. percussion, air core, reverse circulation) and the close proximity of the operator to the area being drilled.

Drillers often bypass control mechanisms owing to a perceived need to ‘feel’ the drill to maximize performance. Dust may also be an issue during the maintenance of mining equipment, but good practice is for equipment to be cleaned prior

to maintenance and practices that generate large quantities of dust (e.g. using compressed air to clean air filters) may be restricted.

Inhalation of radon progeny is generally a minor pathway for surface mining. Natural dispersal of radon exhaled from the ore is normally sufficient to keep occupational exposures low. The exception is where natural ventilation is curtailed by either pit design, surrounding structures or meteorological conditions. Deep pits are the most likely to be affected, as the air in the bottom might not have sufficient circulation, which results in a layer of high radon and radon progeny concentrations. Where atmospheric temperature inversions occur frequently (e.g. arid regions), it may be necessary to consider the impact of reduced radon dispersal on occupational exposure.

6.3.4. Control mechanisms

As the radiological risk associated with surface mining is relatively low, control mechanisms are usually incorporated into controls for conventional hazards. Radiological specific controls are generally only necessary for mines with very high ore grades or activities with very close and continual interaction with the material.

For direct gamma exposure, the most common control mechanisms are the physical shielding and distance from the rock surface provided by the mining equipment and reducing the time spent in active areas. For the excavation and haulage of blasted rock (mechanical shovels), the operator can be over 5 m from the rock surface and shielded by several centimetres of steel. In high grade operations, further shielding can be added at low cost if gamma becomes a significant pathway. Workers such as geologists, explosive charge hands and, to a lesser degree, drillers work directly on the surface of the material. In these cases, active controls are minimal and focus on minimizing the time spent over higher ore grades and the distance from vertical faces. In particularly high grade areas, a cover of inert material can be placed over the ore (~0.5 m) to reduce the gamma field at the drill location.

One critical lesson from past practices is to ensure that non-mining facilities (e.g. offices, workshops, cleaning bays) are kept remotely from the ore stockpiles.

Historically, they have often been placed near the active mining area, with mine material being sometimes used as the building base. The enhanced exposure can mean that the clean area has similar gamma dose rates to the active mining area.

Positioning auxiliary structures on low dose rate areas can result in a significant decrease in potential occupational exposures. This will also reduce the number of monitored workers and provide a safe work area for personnel with special requirements, such as apprentices and pregnant workers.

To reduce exposures to radionuclides in inhaled dust, the usual control methods are to minimize the amount of dust generated, restrict access during dust generating conditions or provide filtered air to worker cabins. Dust is often an issue for surface mining and can be critical for a range of non-radiological aspects (e.g. environmental damage, dispersal, silica exposure). Controls include: wetting or dust resistant agents on roads; altered blasting to reduce dust;

stockpile design to reduce dispersion; wind breaks; and minimized drop heights during loading. These controls are generally applied irrespective of radiological risk and sometimes it is necessary to prevent access to the mining area. Blasting is almost universally dust generating, so personnel are restricted from accessing the area until the dust settles. During high dust levels, drilling personnel can remain upwind of the dust plume. Most modern mining equipment (e.g. shovels, trucks, drill rigs, cars) have air-conditioned cabins for the operator. Appropriate filtration of the intake air can significantly reduce potential exposure and improve the comfort of the operator. The use of PPE (e.g. respirators) is generally only necessary when there are other non-radiological requirements or there is a particularly dusty operation.

Control of RDP exposures is very rarely needed in surface mining operations. Natural ventilation processes, aided by the increased air movement caused by vehicle heat and movement and ground–air temperature differentials, are almost always sufficient to prevent the buildup of RDP to significant levels.

Higher radon gas levels may be found in deep open pit mines under temperature inversion conditions, and it may be necessary either to limit access or use full cab filtered ventilation.

6.3.5. Monitoring and dose assessment

The monitoring approach in surface mining is similar to that used in all other stages of uranium mining. For gamma exposures, normal practice is to use either individual personal dosimeters (e.g. TLDs, OSLDs, electronic personal dosimeters, EPDs) or workgroup SEG averaging of personal dosimeters.

Electronic dosimeters are likely to be only necessary in areas of either high dose rates or for non-standard work tasks. The work area can be surveyed periodically and the results used for future mine and exposure planning. The correlation between ore grade and dose rate can enable the ore grade to be used as a surrogate for direct gamma surveying.

For dust, personal (on a workgroup averaged basis) or work area sampling is normal practice. For jobs that involve the frequent movement of workers, personal dust sampling is appropriate; for jobs which are mainly cabin based (e.g. shovel operator, truck driver), an in-cabin sampler may be better. However, if in-cabin monitoring is required, it may be necessary to check that the behaviour

of the operator is consistent with remaining in the cabin — drill operators can often leave cabins (or lean out of the door) to be closer to the rig.

For RDP, periodic area monitoring can be enough to confirm that it is not a significant exposure pathway, employing: passive sampling (e.g. track etch monitors); grab sampling (e.g. Rolle, Borak); or electronic sampling. From the dosimetry standpoint, the only variation from other stages of uranium mining is that equilibrium can be assumed in almost all cases for radionuclides in airborne dust (except for radon and radon progeny). However, there are rare cases where this might not apply, such as in an active roll front deposit.

6.4. IN SITU LEACH MINING AND PROCESSING