High grade ore mining and processing

In some areas, uranium has naturally concentrated to present very high ore grades. At these high grades (a few to tens of wt% of uranium), the controls and the associated monitoring programmes are significantly more rigorous than for lower grades. However, the general approaches to monitoring and control still apply.

The high concentration of uranium and its decay products in the ore means that all exposure pathways become far more significant. In particular, high gamma radiation fields will be present in areas close to the ore and any airborne dust can have significant radioactivity. The shielding of plant items and entry restrictions for certain areas become important control mechanisms. A key objective is to isolate workers from the high grade materials. Radon concentrations during entry

into areas of mineralization or those impacted by radon contaminated water can become significant. Furthermore, contamination control to prevent the potential for accidental intake through ingestion, inhalation, or wounds (i.e. wounds directly contaminated with radioactive materials) become far more important.

In high grade ore mining, more stringent controls on the handling and disposal of waste materials may also be necessary. Tailings retain most of the radioactivity of the ore and the waste rock can still retain significant quantities of uranium.

6.8.1.1. Underground mining

For underground high grade deposits, the general approach is to use non-entry mining methodologies (see Section 6.2.1.6). Three common mining types include boxhole boring, raise boring and jet boring.

Box hole boring uses a boring machine to grind out a cylindrical section of the ore body. The machine pushes the reaming equipment from below, leaving the cuttings to fall through the reaming head into the collection system installed below. The box hole machine and associated ore collection system needs to be ventilated adequately to contain RDP and LLRD, typically under negative pressure from a secondary ventilation system — radon and radon progeny can reach significant concentrations. Once a box hole chamber has been mined, it is backfilled so that a new box hole can be mined next to it.

Raise boring also involves using a large cutting drill to bore a cylinder through the ore zone. The main difference is that drifts are established above and below the ore zone and the raise boring machine pulls the drill head from the bottom chamber up through the ore zone. Similar to box hole boring, the raise bore machine in the upper chamber needs adequate ventilation to control RDP and LLRD. The mined out raises are backfilled.

Jet boring is conceptually very similar to box hole boring and can be a suitable alternative, depending on the local geological conditions. From a tunnel below the ore zone, a pilot hole is drilled up to the ore, through which a high pressure water jetting tool is installed. This water jet cuts out a cavity within the ore, and the ore cuttings and water drain to the drift below and are collected in a system that is attached to a secondary ventilation system to contain RDP and LLRD. The cavity is then backfilled before the next cavity is mined.

6.8.1.2. Surface mining

For high grade surface mining, it is more difficult to isolate the worker from the high grade ore, as the intervening waste rock cannot be used as a shielding material. The mining methods used are based around restricting the time over high

ore grades and by maximizing the distance of the worker from the ore. Remote controlled equipment can be used for drilling, charging, excavation and transport.

Shielding can be used if access to a mineralized area is necessary (e.g. by placing a cover of clean rock material over the surface of the ore) or within the equipment used to extract or transport the material (e.g. shielding between the driver cabin and the ore transport area). Radiometric sorting and control of the material being extracted can be a component of a surface operation.

6.8.1.3. Processing

The processing of high grade ores is chemically similar to other uranium processing methods but generally involves much smaller volumes of ore. Hence, the processing equipment is generally smaller and more compact and the overall size of the processing plant is reduced. Shielding and restriction of access to areas containing high grade ores are an integral part of the plant and process.

Special attention is needed to minimize exposures during non-routine work such as maintenance.

6.8.2. Design and operation

Radiation protection and safety is an integral part of the design and operation for high grade operations, the requirements of which can determine how an operation is performed and might be a limiting factor for some design decisions.

Design and operation methodologies from the nuclear fuel cycle are sometimes used in these operations owing to the high levels of containment needed.

The key control aspects for underground high grade operations are non-entry mining and isolation of personnel from the ore zone. This helps to limit direct exposure to gamma radiation from the ore and to minimize radon sources in the workplace. Additional practices to limit the movement of radon rich groundwater, such as grouting and freezing, can be important. The ventilation system is a critical control method in limiting exposure to radon progeny. Control of spilled material and the use of wet based loading and transport all contribute to reducing exposure. Non-routine work such as maintenance is generally performed in low background areas or, where this is not possible, in a controlled manner to minimize exposure.

The design of a high grade surface mine is generally determined by the geology of the deposit and how to utilize mining techniques to limit exposure while optimizing production. Different approaches may be needed for near surface rather than deep pits. In near surface mining, it is often appropriate to keep personnel at a distance from the high grade areas to minimize exposure. As depth increases, however, it is likely that work areas will be more restricted and there

is increased potential for personnel to be working on or near the ore. For tasks that involve working directly on the ore, such as drilling or explosive charging, remotely operated equipment may be justified. The use of large earthmoving equipment can provide sufficient distance and shielding to control doses for operators, but additional cab shielding may also be a viable dose optimization practice. There may also be concerns relating to radon in deeper pits, particularly during still or inversion conditions.

The standard uranium plant design is unlikely to be appropriate for the processing of high grade ores. One solution is to use blending or downgrading of the plant feed to enable conventional plants to process higher grade ore.

Care needs to be taken around the blending facilities to ensure that the higher potential for exposure from the high grade ore is addressed. The processing plant can also be designed specifically for the processing of high grade ores.

Because of the comparatively low volume throughput, it is possible to design a plant with radiation control mechanisms as an integral part of the design. This can include the shielding of vessels which contain significant quantities of ore, and dedicated ventilation systems to keep vessels under negative pressure and exhaust any generated radon. This process can also be applied to non-routine and maintenance tasks where design and work practices can assist in dose control.

Examples include the use of quick decoupling systems to minimize change out times for critical equipment (e.g. pumps) and increased slope on bonded areas to speed up the wash down of collected slurries.

6.8.3. Principal exposure pathways

Due to the higher concentrations of uranium, all exposure pathways have increased significance when handling high grade ores. While doses that are well over the limit are possible, experience has shown that good design and operating practices can reduce doses to levels that are comparable to those of low grade mines. With high grade ore, gamma doses in excess of 1 mSv/h are possible, so both passive and active controls are likely to be necessary. Irrespective of the process, the isolation of personnel from the ore (or tailings), as far as is possible, will assist in dose minimization. The amount of material that can give rise to significant doses is also greatly reduced, which means that more work areas are likely to need control.

Radon generation from the ore will be higher than for lower grade ores, but due to the range of factors influencing radon exhalation into the workplace, the range of potential conditions is highly dependent on site specific conditions.

Furthermore, because less material is being handled, it is possible to design more efficient ventilation systems and keep air residence times low (thereby decreasing the ingrowth of radon progeny). However, the potential for rapid

changes in airborne concentrations remains and needs to be considered in the design, operational monitoring and work practices.

The potential dose from the inhalation of LLRD increases with ore grade and hence is of more significant concern at high grade operations than those processing low grade ore. For example, in a low grade deposit (~100 ppm U), a worker would need to inhale in the order of 10–100 g to reach the occupational dose limit. In practical terms, this would be very unlikely to occur, even with minimal dust control. For a high grade ore (~10% U), however, inhaling less than 1 g of ore dust would be sufficient to reach the occupational limit. This means that control of airborne dust is far more important with high grade operations.

Similarly for ingestion and wound pathways, situations normally not important become more significant when handling high grade ore. For low grade ores, these pathways normally become significant when handling the final uranium product. For example, a wound directly contaminated with tens to hundreds of milligrams of high grade ore that is not cleaned out could result in an appreciable dose (i.e. >1 mSv). The potential for appreciable doses from wounds contaminated with low grade ore is significantly lower. Even for high grade operations, the ingestion of uranium ore would normally be a minor radiological issue and the dominant concern is likely to be a chemical toxicity issue, as when handling the final uranium product.

6.8.4. Control mechanisms

The primary control method for the mining and processing of high grade ores is isolation of the material from the workforce. In practice, this means a strong commitment to radiation protection being an integral part of both the design and day to day operation of the facility. However, a lack of appropriate controls has the potential to result in situations where doses approaching or exceeding the occupational dose limits could occur in a relatively short period of time (e.g. days to weeks). Hence, it is important that the proper design, operational and administrative controls are in place.

Shielding and separation of the workforce from the ore and waste materials is commonly used to control exposures. For mining, non-entry or remote mining become the most appropriate methods, while active ventilation and dust controls are also used. Mine planning is the key to dose control and having a strong knowledge of the geological distribution of the high grade ore is often the key to controlling doses. Ore handling is generally performed using wet material to minimize dust, and material to be transported may need additional shielding to protect the driver. Ventilation and isolation of ore areas are often the best means of ensuring that radon progeny exposure is minimized. For open

cut operations, natural dispersal might be sufficient, but there may be a need for forced ventilation or work restrictions during periods of still air.

Worker access in close proximity to vessels containing ore has to be controlled. Gamma dose rates in the mSv/h range are possible, so access control is a key safety component. Areas where worker occupancy is higher may need shielding or, alternatively, the ore vessels may be shielded.

With respect to maintenance, these activities are usually planned, and dose assessments are used to optimize exposures. Good design features which reduce either the need or the time required for maintenance can have strong benefits for reducing doses. For example, having pumps outside of the tank area can enable maintenance to be performed in a lower dose area. Quick release fittings and pre-existing lifting and attachment systems can greatly reduce the time for removal of equipment. Easy to clean, hoseable surfaces can increase the speed of the cleanup after spillages and increase the distance of the worker from the spilt material (a small change in the slope of sumps can greatly affect the cleanup speed). Administrative controls to authorize entry into areas containing ore are also used to control and minimize dose.

6.8.5. Monitoring and dose assessment

Monitoring programmes in high grade mining and processing are like those used in all other stages of uranium mining, with the exception that there may be more use of real time monitoring (such as EPDs for gamma dose and real time RPD monitoring in critical airways). For gamma monitoring, the use of individual personal dosimeters (TLDs, OSLDs, EPDs) is normal practice. For workers close to the ore, alarm electronic dosimeters might be necessary. In some high grade mines, workers wear personal dosimeters that record radon and thoron progeny, gamma and dust exposures in one unit.

For radon progeny exposure, a combination of real time area and airway monitoring and personal monitoring can be used. The real time monitoring is used to confirm that ventilation systems are operating as designed and to allow quick response in the event of any change in exposure conditions. Personal monitoring of RPD is likely to be used due to the high potential for variation in both radon concentration and equilibrium factors in underground situations.

Personal monitoring of dust is likely to be the monitoring method for LLRD exposure. This personal sampling may be combined with the RPD monitoring (and gamma monitoring). Area sampling may be utilized to confirm that dust control mechanisms are effective. In the event of any increase in dust exposure, additional sampling may be performed to identify the source of dust.

6.9. URANIUM TAILINGS FACILITIES