Tailings produced from the milling of uranium ore in the 1950s and 1960s were regarded as little or no different to other non-radioactive mill tailings in terms of their potential hazard.
The general poor level of knowledge as to the potential radiological effects is clearly demonstrated by use of uranium tailings as a building material in many places close to uranium mines and mills. The tailings were placed as a matter of convenience and cost in the nearest receptacle, which would commonly be a natural depression or lake if available. If no suitable natural depression was available, one would be constructed by building an embankment or low dam across a valley or across the lower slopes of gently falling ground.
Where no depressions or slopes were available the tailings were sometimes placed on flat ground in dry climates, or a ring dyke would be built in wetter areas. However, there was generally little or no attempt to prevent flow of waters that had contacted the tailings into streams and rivers. Scant regard was paid to dispersal as dust, and generally no action was taken to remediate areas affected by a containment failure or to collect and return the spilled tailings back to the containment.
Containment practices began to improve in the mid-1960s, and more substantial impoundments were commonly constructed, especially at new mine and mill sites where improved tailings dam designs were incorporated into tailings management plans – such as
the upstream method of construction where cycloned tailings would be used to build raises on the dam embankment. However, unsuitable practices for tailings disposal continued at some sites until the 1970s and later, especially where uranium production was subject to tight government control and secrecy.
Some of the options for placement of tailings that we would consider suitable today were used historically, but more by coincidence than design. An example is the backfilling of old open pits; however, no amendments were made to limit the potential for contamination of surface or ground waters. In general, no consideration was given to the different climatic situations in which the tailings were stored and the different levels of risk of containment failure or the consequences to human health or to the environment. This lack of concern was based on ignorance of the risks and hazards. Development of an understanding of these risks and hazards grew out of the growing body of knowledge about the effects of radiation on the human body. Legislation aimed primarily and protecting the health of workers was developed from the 1940s, but it was not until the mid 1960s that concern for impacts on the wider community and on the environment led to a major change in the way that the risks and hazards of uranium mill tailings were perceived, including the long term nature of those risks and hazards. From this time a major change took place in the approach to design of tailings containments. A new approach gradually developed that considered issues such as climate, possible agents for containment failure, long term containment, and the values and sensitivity of the surrounding environment in order to reduce the risk of containment failure and potential hazards to the environment.
9.2. Environmental impacts
Poor practices in the placement and management of mill tailings in the past have contributed significantly to the negative legacy of uranium mining. The dangers specific to uranium mill tailings relative to tailings from other types of mines were unknown and they were treated in the same manner. That manner was to dispose of the tailings cheaply and conveniently – in topographic depressions to contain tailings and liquor in wet environments (the depressions may have been valleys, lakes etc), or to pile them on flat areas in drier climates. The main vector for contamination in wetter regions is water erosion, leading to contamination of water courses and lakes, while in dry regions the main vector is dust. The contaminants comprised radioactive material as well as toxic metals and other chemical compounds, in many instances accompanied by acidity and salinity.
As was the general case in the mining industry in the first 60 years of last century, there was little or no maintenance of tailings piles; for example, if fences had been provided they were not mended or replaced if breached or stolen, and there was no rigorous restriction on access to and re-use of the tailings material.
Health and environmental impacts of a wide variety occurred, relating variously to:
Catastrophic failure/collapse of containments, in some cases causing deaths;
Uncontrolled public access to and inappropriate re-use, leading to increased radiation doses and cancer risks;
Overflow and/or seepage of tailings liquor into surface water systems, causing contamination and ecosystem impacts along rivers and in lakes;
Seepage into surface and groundwater systems, leading to contamination of water resources important for drinking, irrigation, public amenity and tourism.
The recognition of radiation exposure due to the re-use of tailings as a building material in the USA, mainly as aggregate for concrete production and for fill material, led to an understanding of the level of risk that tailings posed to human health. Subsequently, the first regulations applying specifically to uranium mill tailings were developed. An understanding of the risks posed to the environment has been slower to develop, starting from a concern that uptake and bio-accumulation in the food chain could increase the total dose to humans.
Assumptions using man as the indicator species to determine a safe radiological dose rate for all species appear to have been useful approximations upon which to base standards for protecting the natural environment from the risk of damage from ionizing radiation. However, the long term risks of chronic exposure are not understood, particularly in terms of potential long term (i.e. genetic) effects on species populations, density, ecosystem dynamics, and biodiversity. A new body of research is building in this area that focuses on risks and effects in the fabrication, power generation and disposal parts of the nuclear fuel cycle; it has yet to adequately address risks arising from lower activity levels from tailings impoundments.
Uranium mill tailings are made up of a cocktail of potentially toxic components, some of which are known to be significantly more toxic that the radioactive minerals. Studies of impacts in the past, as well as planning better tailings management in the future, needs to take into consideration the full suite of toxicants present in uranium mill tailings.
9.3. Remediation programmes
Remediation of uranium mill tailings piles has been undertaken and/or is in progress in virtually every country, where uranium has been or is being produced. Early attempts at remediation have been variable in their success. Assessments of effectiveness are few in number and generally limited in quantitative and comparable information. Even with the most careful planning, there is room for certain critical concerns to be overlooked. As knowledge on environmental sensitivity grows and regulations and standards evolve to reflect this knowledge in response to public concern, the design standards for remediation will become more demanding.
Much is being learned from the remediation programmes as they are undertaken and as monitoring data are collected after completion. A distinctive message that comes out of the work done to date is that there is no single best approach. Each site must be studied closely to determine the optimal approach. The amount and type of work, and the associated costs, will differ markedly from site to site. Short-cuts will probably lead to parts of the work having to be re-done. Methods of construction and design that are harmonious with and emulate nature are preferable to engineered designs that are built to resist, rather than to harness natural processes for long term stability.
The challenge of remediating areas of environmental impact from old mines has proven difficult on technological and economic grounds. Even the well-funded Uranium Mill Tailings Remediation Action Programme (UMTRA) in the USA took almost twenty years to undertake (Annex XIII). Remediation works and costs are most demanding when planning and incremental preparation for closure and remediation are not integral to the operational life of the mine and tailings facilities. Where uranium mining is undertaken by the private sector, regulations may be developed and imposed to ensure adequate and timely planning and funding for remediation to reduce or remove the probability of liabilities being transferred to government. Where uranium mining is undertaken by government institutions, similar arrangements may be put in place to ensure that remediation planning and implementation are
identified as the responsibility of the operators. Funding normally takes place on the basis of approved programmes and is subjected to comprehensive control.
9.4. Present day practices
In general terms the technologies used in the uranium mining industry are no different from those used for other mill tailings. Most of the environmental problems experienced with uranium tailings are similar to those found at other mines and mills. In fact, a significant source of environmental hazard related to uranium mill tailings may come from the non-radioactive components of the contaminant inventory. Hence, the principles developed for other types of mining can generally be applied to uranium mill tailings. Examples of most of the different types of containment and method of their construction can be found in the uranium mining industry. For tailings facilities developed during the last 25 years, detailed information is readily available and in the public domain.
The containment of mill tailings is closely related to the site specific setting of the mine and mill. The main considerations for defining the most appropriate practice are technical, environmental, economic and societal. While engineering design provides a wide range of options for the containment of mill tailings, the nature of the site where they are located generally determines the containment method employed. What can be considered the best or the most cost-effective containment will differ from site to site, depending on climate, ore/tailings type, impounded volume, seismicity, remoteness, available transportation and logistics, and availability of natural or constructed openings. Clearly, a careful analysis needs to be undertaken for each individual site to ensure that the most appropriate option is chosen.
Tailings dam failures have occurred in the past, but their occurrence and frequency are not readily related to certain types of dam or construction methods. There is statistical evidence to support the view that most failures relate to the saturation, liquefaction and rheological characteristics of wet tailings held in dams, and that the risk is much higher during the operational phase. Clearly, water management is a critical issue for risk reduction. Dams can fail after closure, mainly as a result of earthquakes, geotechnical factors, and overland flooding. Often, poor or non-existent care and maintenance are at the root of these failures.
Accordingly, close-out designs should pay particular attention to mitigating these hazards. A large body of information exists with respect to tailings dam failures outside the uranium industry that is directly relevant to uranium tailings management.
Containments other than dams that utilise natural depressions, such as lakes, or constructed voids, such as mine pits and underground openings, offer certain advantages because of their inherent physical stability after closure (Annex XII). Risk from earthquakes and overtopping is mostly removed. The main mechanism for environmental pollution from below ground containment is the contamination of groundwater.
Methods available to reduce the risk of pollution from containments involve reduction of possible contaminant source terms by tailings preparation and treatment (Annex II and VI)), isolation from the environment by covers and seals, as well as active and passive effluent treatment. In some cases relocation may be the most appropriate remedial solution, particularly, if the tailings are in the immediate proximity of populated areas.
A large body of work is available on capping design. Cover designs and construction needs to be site-specific, taking into account the local climate, availability of construction materials, nature of the tailings impoundments, regulatory constraints, as well as local community demands and acceptability. Although site specific information is needed for each site, it is
probable that research work into designs is being repeated unnecessarily in several countries.
It seems to be time for this large body of work to be assimilated so that it is readily available and comprehensible to all. Nevertheless, the local availability or non-availability of generally preferred construction materials, such as clay for cappings, and the limited financial resources to obtain these materials on the world market, may make it necessary to develop local designs in some Member States (see Annex III-China for instance).
Research specific to uranium mill tailings containment goes beyond research into containment of non-uranium mill tailings, because of the focus on longevity and the problems specific to radioactivity (particularly radon emanation). Specific areas of research into longevity include, climate change and variability, erosion, long term geotechnical stability, monitoring, and long term model development and calibration.
However, non-uranium tailings also pose long term hazards, particularly due to acid mine drainage generation [202]. There is potential for the benefits of research into uranium tailings management to feed into other, non-uranium tailings research areas and vice versa, and for co-operative research on shared issues of concern, such as the longevity of physical and chemical performance of containments.
In summary, considerable progress has been made in the safe containment of uranium tailings over the past 20–30 years.
9.5. Outstanding issues
The outstanding issues relating to stabilization and isolation of uranium mill tailings concern the confidence in long term secure containment, prevention of seepage from the containments, and chemical mobility of contaminants in the short and long term. A new concept is developing that seeks to embrace natural processes as much as possible in improving physical and chemical stability by understanding and taking advantage of natural assimilation and attenuation (ecological design). This could lead to a fundamental change in the way that impoundments are designed, away from structures engineered to keep their contents out of contact with the environment, to structures that somehow utilise natural processes to assimilate the entire structure and its contents with the surrounding environment over the long term.
9.6. New approaches
A hiatus in the level of research occurred in the 1990s following the conclusion of much of the research undertaken in north America under the US Uranium Mill Tailings Remediation Action Programme and the Canadian National Uranium Tailings Programme. The results and lessons learned from those programmes are still relevant today, and form the basis of basic research into design and planning of remedial works in other countries today. However, the results of the earlier work are not always readily accessible and a comprehensive critical evaluation is not available, so that valuable funds and research resources are not spent on un-necessary repetition and duplication of research.
The main foci of current research are methods to stabilise tailings (Annexes II,VI, IX, and X).
Whilst the above discussion attempts to divide the description of this work into physical stabilization and chemical stabilization of tailings piles, and stabilization of covers, much of the work being done cuts across all three areas. This is because the majority of the work is on methods to ‘cement’ together the tailings with agents that are able to absorb or co-precipitate the contaminants (with emphasis on the metals) as a result of redox reactions and
modifications to pH. Therefore both chemical and physical stability result from the one process. The agents vary from tried and tested Portland cements, to other inorganic and organic reagents, and high-tech polymers (Annex XI). Some harness and enhance natural diagenetic processes, and others take no account of them.
The result is a large body of work offering tantalizing potential opportunities for application in the isolation and stabilization if uranium mill tailings, at the placement stage, during non-operational pre-closure stages, during remediation preparation, remediation works, and ex post facto. However, much of the work is not sufficiently developed and demonstrated to allow the potential benefits, costs, and operational requirements to be evaluated. Also, techniques that have been developed and implemented successfully for non-uranium tailings have not been adequately tested on uranium tailings for their suitability to be fully apparent (e.g. acid drainage mitigation techniques that would also reduce uranium mobility in the case of acidic/sulfidic tailings, paste and dry cake technology for dewatering and pacing tailings).
The requirement for high security tailings isolation for periods of time in the order of thousands of years is peculiar to the uranium mining industry, even though sulfidic tailings dumps are now known to pose environmental risks for up to a thousand years. There is a growing level of interest in a new approach to designing mine closure in a way which is cognisant of the effectively infinite time frame, rather than approaches based on finite design lives and, therefore, probable future failure. Similarly, nature provides various mechanisms that assist in stabilization of contaminants, and there is now high interest and research activity into understanding these processes so that their potential can be maximised in order to develop better ‘passive’ systems and reduce the probability of failure inherent in interventionist systems.
A new generation of stabilization techniques based on high-tech polymers and sophisticated organic chemistry is developing (Annex IX), and promises to provide more effective and durable techniques for operational and ex post stabilization than those based on Portland cement paste technology. This field of research is developing rapidly. Much of the information is based on laboratory scale testing with only limited field trails, and information on its feasibility for application in the field is generally lacking.
9.7. Performance assessment
Performance assessment normally includes the verification of compliance with applicable regulations, as well as site specific performance objectives. A performance assessment also provides a tool for the optimization of remediation measures and the identification of shortcomings in design, operating procedures and management systems. These assessment cover radiological and non-radiological, as well as conventional geotechnical aspects.
As the design and operation of many tailings facilities relies on assumptions and models, performance assessment supplies information that is useful in the verification and calibration of these assumptions and models (Annexes I and V). Future research and development priorities can be identified on the basis of deficiencies within a design or system thus assessed.
Two basic approaches exist, risk-based assessment and design-based assessment. The actual approach adopted for a performance assessment will depend on the conceptual starting point, site specific parameters, as well as possible requirements by the licensing authorities.
A performance assessment would comprise a variety of elements, including the establishing of site baseline conditions, monitoring for the short term performance, and predictive
A performance assessment would comprise a variety of elements, including the establishing of site baseline conditions, monitoring for the short term performance, and predictive