2. OVERVIEW OF THE URANIUM INDUSTRY AND
2.3. Uranium mining and processing stages and techniques
The life cycle of a uranium mining and processing operation is a complex process which can extend over decades. The life cycle stages include exploration, planning, construction and operation, decommissioning, handover and surveillance (see Fig. 2). The mining method and design parameters have a significant bearing on the occupational exposures, control measures and monitoring that will be necessary.
The design stage of the life cycle is a critical stage of the process in which the mining and processing method and the plant design is optimized. In addition,
FIG. 2. Life cycle of a uranium mining operation.
the design stage needs to take account of conventional and radiation safety requirements, the methods of waste management and the decommissioning approach. There are a range of mining options available, including underground, surface and in situ mining. Processing also has a large range of options and some are integrally linked to the mining method, such as ISL. The mining and processing are generally closely linked and collectively can be called the operational phase.
Occupational exposure is associated with all of the above stages except for the design stage. Poor decisions in the design phase can have major negative impacts on occupational exposure, and these can be difficult to correct during the operational phase. The choice of mining and processing technique is heavily dependent on the ore grade and the characteristics of the ore body. Other important factors include topography, hydrogeology, geotechnical aspects, logistics and the perspectives of interested parties (e.g. the public, indigenous people, regulatory bodies). Therefore, awareness of the impact of the design approach on the control of occupational exposures is a critical aspect.
2.3.1. Exploration
Exploration is characterized by operations in the field to discover and assess the uranium resource. In most cases, the occupational exposures during exploration are expected to be low, due to the limited amount of radioactive material being handled (a few tonnes) and the usually low ore grades involved in most operations. However, there are exceptions where significantly higher grade ores and quantities are involved and, in some cases, where exploration involves trial mining including underground operations. In the past, the radiation protection aspects of exploration have often been ignored. The modern approach is to assess potential radiation hazards and doses through a prospective assessment and then implement an appropriate radiation protection programme.
2.3.2. Underground mines
Underground mines are designed to facilitate the safe and economic extraction of a mineral resource, and the mining approach will in large part be dictated by the geological constraints of the deposit. Uranium mines face the same safety challenges as mines for other minerals, with the additional constraint of dealing with the radiation associated with the ore. However, except in the case of high grade uranium deposits, it is usually typical mining constraints, such as ground conditions and the size and orientation of the ore zone, and not radiation issues, that determine the optimal mining method. Nevertheless, factors associated with controlling radiation need to be incorporated into the design of
the mine to extract the uranium ore safely. The exception is mine ventilation, where far more control of ventilation conditions is likely to be necessary than in conventional underground mines to prevent the buildup of radon concentrations.
The basic radiation protection approach of time–distance–shielding serves as a useful way to highlight some of the key issues that need to be considered in the design and operation of underground uranium mines. With regard to ‘time’, the goal is, to the extent possible, to minimize the amount of time workers are in direct contact with the ore. For low grade ore deposits, this design constraint is not as serious as it is for high grade deposits, where it can eliminate or at least severely restrict the use of some mining methods. Other strategies such as the use of remote controlled equipment and shielding (e.g. clean waste rock on floors and shotcrete on walls) can also be incorporated into the design and operation of an underground mine to reduce gamma doses. The choice of mining method and the layout of the mine will also have an impact on the ventilation system, which is a critical component in controlling radiation and dust exposures. Finally, careful consideration needs to be given to the handling and movement of ore out of the mine to the processing plant to minimize the spread of surface contamination and the creation of airborne LLRD. The typical mining methods that have been successfully used include:
— Room and pillar open stope mining;
— Sublevel stoping;
— Cut and fill stoping;
— Undercut and fill mining;
— Block caving;
— Non-entry mining.
It is also important to note that there are variations within each of these broad mining methods (see Section 6).
2.3.3. Surface mines
Open cut mining involves extracting the ore directly via a surface cutting [4]:
“This is most commonly used for ore bodies which are either on surface or relatively near surface. As depth to the deposit increases, the size and cost of the operation will increase as will the amount of waste rock generated.
[Open cut] operations are characterised by a high ratio between waste rock and ore and hence have the largest surface impact.”
This waste rock can then be a secondary source of radiological concern.
Waste rock can be a direct source of dust and radon and may also be an indirect source of radionuclides in the form of releases to surface water and groundwater and subsequent distribution. However, waste rock can be useful during the decommissioning stage by providing a cost effective source of cover material, enabling the effective isolation of the higher grade tailings material from the general environment.
During operation, open cut mines are generally a cost effective method for extracting large volumes of lower grade ore [5]. This means that there is potential for bulk extraction techniques (e.g. milling, leaching and extraction, or alternative techniques such as heap leaching), which would be uneconomical for underground operations [5]. For deeper deposits, there may be a need for bulk excavation of barren or low grade covering rock, and this can increase costs and reduce the speed at which the operation can be started.
Upon closure, open cut operations can be the most expensive to remediate due to the large number of disturbed areas and greater waste rock and tailings volume. Remediation options are likely to be heavily dependent on site specific factors, such as climate and topography.
2.3.4. In situ leaching mines and processing
The ISL process for uranium mining and milling involves dissolving the uranium within the ore body itself by circulating groundwater fortified with oxygen and a chemical additive (slightly alkaline in the United States of America, acidic in Australia and Kazakhstan) into the formation through injection wells, dissolving the uranium in situ and extracting the pregnant uranium solution through recovery wells. The final steps in processing (separation via ion exchange or solvent extraction, precipitation, drying, packaging) may be partially or totally carried out at the in situ facility near to the well fields, or an intermediate product (loaded resin or slurry precipitates) may be shipped to another ISL facility or a conventional uranium mill for final processing. Some ISL operations in the United States of America are referred to as satellite plants in that they load uranium onto ion exchange resin at the well fields and/or produce intermediate products that are then shipped to another uranium recovery facility some distance away for further processing [6].
The absence of production scale acidic ISL in the United States of America is on account of the practical limitations of geochemistry and concern about a greater environmental impact relative to the alkaline leach method [6]. However, studies indicate that the environmental impact from alkaline leach processes (in the United States of America [6]) and acid leach processes (in Australia [7]) is low.
Section 6 details the typical ISL processes and the radiation protection and radiological monitoring programmes appropriate to adequately monitor and control doses to workers. Although many radiological characteristics are similar to those of conventional mills, conventional type tailings as such are not generated. However, liquid and solid by-product materials can be generated and impounded, which can result in a source of occupational exposure; and some specific monitoring considerations are necessary due to the manner in which radon gas is released in the process [8].
2.3.5. Heap leaching
Heap leaching is an alternative method of extracting uranium rich liquor from extracted ore. The mining of the ore is conventional (either underground or surface) and the ore is placed on surface pads where extractive liquors (acid or alkaline) are pumped over and through the material. This process can be repeated until liquor of sufficient uranium content is transferred for further processing to extract the uranium.
The highest occupational exposures will occur in individuals who spend a high proportion of their time near to the heap leaching pads. Gamma exposure will be the dominant exposure pathway; LLRD and RDP exposures are usually much lower.
2.3.6. Processing
The processing facility is designed to extract the uranium from the incoming stream (either ore or liquor), purify and concentrate it, and produce a solid final product for sale and transport. The general approach is to prepare the ore (by crushing, grinding, milling), extract the uranium (by acid or alkali leaching), separate out the uranium bearing liquid, and then concentrate, purify, precipitate, dry and pack the product. The final products include U3O8, UO4, UO2 and ammonium diuranate.
Occupational exposure during processing is best controlled by the design of the plant. The material is wet for most of the process, so gamma exposure usually dominates. However, during final product drying and packaging, the material is dry and inhalation of LLRD is likely to dominate the dose. The final product area generally has the highest occupational doses.
2.3.7. Non-conventional uranium extraction
Most of the world’s uranium production comes from facilities dedicated to uranium extraction. However, a small percentage is a by-product of mining for
copper, gold, nickel, phosphate, silver, vanadium and rare earth elements and also from water treatment facilities. Occupational exposures from these facilities are heavily influenced by the specifics of the extraction processes and the quantity of the uranium being produced.
2.3.8. Tailings facilities
After the processing of uranium ore, the residual material or tailings still contain about 85% of the original radioactivity. This material has to be stored and isolated from the environment. The options available include the use of purpose built impoundments or natural features, or disposal back into the mine pit or underground workings.
Occupational exposures at tailings facilities are usually low due to the low grade of many uranium ores. The dominant exposure pathway is generally gamma exposure, although both RDP and LLRD exposures can become significant in specific circumstances involving high grade ore tailings.
2.3.9. Transport
The transport of material containing uranium by road can result in occupational radiation exposures. The material can include both low and high grade ores, the high grade final product and some types of waste and contaminated plant items. Transport can be on-site or involve the transfer of material between sites on public roads. Transport also occurs once the final uranium product is shipped to the customer. On-site transport of uranium material is generally covered in the site radiation protection programme. The transport of radioactive material on public roads is subject to specific national and international transport regulations. During transport the dominant exposure pathway is usually gamma exposure. The other pathways are normally only of concern during accident or emergency situations.
2.3.10. Decommissioning
The final stage of the uranium mine life cycle is decommissioning, which involves the demolition or removal of plant structures, the rehabilitation of tailings and waste rock structures, and other longer term activities to process wastes arising from the decommissioned facility (mainly contaminated surface water and groundwater). Occupational exposures from the three main exposure pathways will occur and a radiation protection programme is needed. The highest exposures will occur around the contaminated plant and land, during dusty
operations, during entry into plant vessels, and during the decontamination, cutting and grinding of contaminated objects.