Large volumes of low activity milling residues, such as mill tailings, are produced – sometimes exceeding millions of tonnes at a single uranium mining/milling facility, in particular, when uranium is only a by-product. The common mode of disposal is in near-surface impoundments in the vicinity of the respective mine or mill [1]. Such impoundments were often arranged in a haphazard fashion, utilizing geomorphological depressions or by filling-in valleys. As a result, there was (is) little or no care taken to isolate the tailing materials from their environment.
While geomechanical aspects, such as the stability of pile slopes, dikes and retaining dams, are standard engineering problems, for which in most countries provisions are made in the relevant building or mining regulations, environmental and radiological impacts have often been neglected. It should be mentioned, however, that mill tailings as such can pose serious engineering challenges, owing to the geomechanical and physico-chemical characteristics of the sediments.
Fig. 1. Typical uranium mill tailings pond (Urgeiriça, Portugal).
Typical environmental problems arising from mill tailings are radon emanation, windblown dust dispersal, and the leaching of contaminants, including radionuclides, heavy metals and arsenic, into surface and groundwaters. Radon (Rn) emissions are due to exhalation from the waste materials and the Rn can reach the ambient atmosphere when free circulation of air in the material and its cover is possible. Emissions to water bodies occur when infiltration of precipitation is unhindered, bottom-liners are absent, and no collection of drainage waters is installed. The leaching of contaminants is usually exacerbated by acid formation from pyrite oxidation under conditions of varying degrees of saturation with water. Additional effects from acid rain have also been observed. In many instances contaminants other than radionuclides may be the real problem, and a comprehensive and holistic assessment of the impoundment inventory and all processes may be necessary.
A range of technical measures can be employed to prevent or reduce the extent of these processes. Capping can be used to control radon emanation, moisture infiltration and chemical reactions that may promote leaching. The physical and chemical properties of the tailings can be improved in situ or by reprocessing to enhance long term stability. Containment structures can be improved to meet the minimum factor of safety. Tailings drainage canbe collected and treated in the short term, until the discharge standards set by the appropriate regulator(s) are met. If such measures are determined not to meet long term objectives, relocation of tailings may be considered.
Any engineering solution has a finite life-span, which may be shorter than desirable from a radiological or toxicological safety point of view. Apart from the structural degradation and/or weathering of the material impounded, failure of retaining structures, such as dams, must be considered. Erosion of cappings and other engineered structures may be a problem in certain settings. Engineering solutions, therefore, may need to consider long term care and maintenance as an integral part of planning and design. In turn, this may require active institutional control and stewardship over very long periods of time. Engineering solutions, long term care and maintenance and institutional control should together strive for an optimization of economic, technical, risk reduction and societal factors.
The aim of searching for long term solutions is to limit risk to future generations and minimise the commitment of future resource requirements [2]. Design requirements for disposal longevity generally range from a few hundred to a thousand or more years. For example, the USA EPA promulgated standards for long term stabilization and control of uranium mill tailings [3] require that the remediation: “Be designed to be effective for up to 1000 years to the extent reasonably achievable, but at a minimum for 200 years.”
Based on the objective to keep environmental emissions to a minimum over long times, the task, therefore, is to find conceptual and technical solutions
that render tailings more inert over prolonged time-spans,
that render impounded materials and engineered structures stable over prolonged time spans,
that minimize the need for active maintenance,
and that are technically and economically feasible and acceptable to society.
The emphasis of this CRP is on technical solutions that can be applied in a restoration/
remediation context. Of crucial importance in this particular context are costs, as these frequently have to be borne by the taxpayer and can no longer be included in the product price. Any proposed expenditure has to be carefully balanced against the likely benefit from such measures, implying that a comparison of forecast environmental and radiological impacts with and without the measure is to be undertaken beforehand.
1.2. Objectives
1.2.1. Overall objective
Dozens of uranium mining/milling sites have been shut down over the last few years. To ensure long term stabilization and isolation of residues is but one element in sustainable and environmentally responsible plant operation schemes. However, legal requirements, environmental targets and standards, economic resources available, and hence the actual management and remediation/restoration practices may vary considerably from Member State to Member State. This CRP is proposed as one step towards raising the awareness of potential
problems and assisting Member States in the development of efficient procedures and processes for the sustainable long term management and, if deemed appropriate, remediation of uranium mining/milling waste sites, and to encourage a harmonized and systematic approach where feasible.
1.2.2. Scientific research objectives
The overall objective of stabilization and isolation of mill tailings and other uranium mining residues is to minimize exposure of target groups from radiation and contaminants in the various environmental media. This can be achieved by creating conditions resulting in low source terms for solid, aqueous, and gaseous releases, and by designing disposal facilities resistant to failure.
Long term stabilization and isolation of mill tailings is an active R&D area, covering inter alia the development of new techniques for tailings deposition, the geomechanical and geochemical stabilization of waste materials, and the design of advanced barriers, both at the bottom and as cappings. A closely related field that has seen rapid technological advances over the past decade is the restoration/decontamination of contaminated land, and the remediation of engineered landfills.
It is recognized, however, that the above objectives cannot exclusively be achieved by engineering design, but must involve also adequate management and planning procedures.
Hence, the long term stabilization of uranium mill includes, inter alia, the following topical areas:
Planning and management
Site characterization;
assessment of likely and probable environmental impacts due to radiological and non-radiological contaminants;
identification of processes relevant to the long term performance;
design features that improve long term performance;
conceptualization of time-frame for closure;
conceptualization of remediation goals and techniques;
definition of factors affecting long term care and maintenance and the need for institutional control;
methodologies for quality control and quality assurance (QA/QC);
design of cost-effective long term surveillance and monitoring programmes for
environmental performance;
geotechnical performance.
Technologies
identification of properties relevant to the long term environmental and geotechnical performance of tailings and structural materials;
structural integrity of impoundment, viz.
design features controlling the long term stability of engineered structures, e.g.
dams;
techniques for ex post improvement of isolation, e.g. bottom seals;
design features controlling erosion resistance;
in situ/on site techniques for ex post treatment of existing tailings, e.g.
solidification, de-watering, capping;
techniques for (ex post) improvement of the long term geotechnical performance of waste materials, viz. biochemical and geochemical resistance of
sealants/additives with respect to structural degradation;
techniques for cost-effective characterization of radionuclide inventory, viz.
determination of source term characteristics;
techniques to minimize long term contaminant release and to improve geochemical stability of tailing materials including in situ/on-site techniques for ex post treatment of existing tailings, i.e. to reduce leacheability and/or permeability, or to reduce Rn emanation;
Low maintenance/cost or maintenance-free drainage systems and drainage treatment systems for removal of radionuclides and other contaminants;
tools (models) for the assessment/prediction of long term environmental and geotechnical performance;
mechanistic models
systems analyses
fault tree analyses
incident sequence analyses;
Institutional, legal and economic aspects
site release criteria and use restriction criteria;
applicable legislative and regulatory regime for radiological and non-radiological issues;
funding of and liability for remediation/restoration activities.
1.3. The focus of the CRP
The projects for this CRP were selected to provide a number of focal areas and clusters of related projects. The main emphasis, however, was on the technological aspects and the design aspects as relevant for the development of appropriate technologies.
The influence of institutional, legal, management and socio-economic aspects on decision making in remediation/restoration projects and the problem of site and source-term characterization is being addressed by other IAEA projects [4][5], while the environmental issues in uranium mining and milling in general are discussed in joint reports by OECD/IAEA [6][7].
It is expected that this CRP will contribute to the transfer of technologies and know-how within the international (uranium) mining/milling, waste disposal and contaminated land communities. The specific problems arising from the properties of relevant radionuclides and the properties of tailing materials have to be addressed. Special emphasis has been given to the development of innovative methods and techniques for stabilization.
The objective of the proposed CRP was to encourage the sharing of practical experience (adaptive research) and (applied) R&D work by Member States on topics relevant to the long term stabilization/isolation of mill tailings.
1.4. The structure of the CRP
The projects composing the co-ordinated research project were grouped into four subject areas (Table 1). Projects whithin these subject areas are intended to complement each other.
The final reports on the individual projects are given in Annexes I to XIII.
Table 1. The project composing the CRP
Project Title Principal
Investigator Country Subject area I: Tailings remediation case studies
A study case on the uranium tailings dam of Poços de
Caldas uranium mining and milling site H. Fernandes Brazil Cameco Research and Development Projects for Tailings
Disposal Technology P. Landine Canada
Subject area II: Capping of tailings
Studies of bentonite and red soils as capping of the
uranium mill tailing impoundments Z. Wen China
Development of Method of Covering Raising Dust
Beaches of Radioactive Wastes Storage Out of Operation A. Gagarin Kazachstan Polymeric Coats for Contaminated Surfaces Localization S. Mikheykin Russian
Federation Improvement of Soil Properties Applied to Capping and
Multi-Layer Barriers J. Koszela Poland
Subject area III: In situ conditioning of materials Remediation of Uranium Mill Tailings Using Natural and
Organo-Clays S. Choi Korea
Development of Technologies for In-Situ Remediation of Contaminated Sites by Directed Formation of Naturally Occurring Slightly Soluble Minerals
G. Ziegenbalg Germany Room Temperature Ceramics, the Breakthrough Material
for Long Term Stabilization and Isolation of Low-Waste Uranium
A. Piestrzynski Poland Research and Development of Measures to be Taken for
Long Term Stabilization of Uranium Liquid Wastes G. Maslyakov Ukraine Subject area IV: Management of tailings in remediation situations
Predicting the Long –Term Stabilization of Uranium Mill
Tailings J. Trojacek Czech
Republic Harmonization of Radiological Impact Assessment
Methodologies of Uranium Mill Tailings Repositories A.-C. Servant France Holistic Approach to Remediating Uranium Mill Tailings
and Contaminated Groundwater D. Metzler USA
2. HISTORICAL PRACTICES