SCIENTIFIC AND TECHNICAL
BACKGROUND
Large volumes of low activity mining/milling residues, such as mill tailings,
are produced - sometimes exceeding millions of tonnes at a single uranium
mining/milling facility. The common mode of disposal is in near-surface
impoundments in the vicinity of the respective mine or mill. Such impoundments
were (are) 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 are often 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.
Typical environmental problems arising from mill tailings are radon
emanation and the leaching of contaminants, including radionuclides, heavy
metals and arsenic, into surface and groundwaters. Radon emission is due
to the free circulation of air in the waste materials and emission to water
bodies occurs when infiltration of precipitation is unhindered, bottom-liners
are absent, and no collection of drainage waters is installed. The leaching
of contami-nants is usually exacerbated by acid formation from pyrite oxidation
under conditions of varying 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 geotechnical engineering measures can be employed to prevent
or reduce the extent of these processes. Capping with inert soil, compound
or multi-layer barriers and similar means are used to control radon emanation.
Low-permeability cappings also reduce the infiltrating fraction of precipitation
and, hence the leaching and acid formation from the waste materials. Seepage
waters can be collected at the bottom - provided adequate bottom liners
and drainage layers are installed. The water can be treated to remove dissolved
radionuclides and/or to remove contaminated colloids and solid particles.
The treatment has to be continued until the discharge standards set by
the appropriate regulatory body are met.
The latter implies maintenance activities for a prolonged period of
time, extending over decades if not longer. In turn, this requires active
institutional control over very long periods of time, which is clearly
not desirable and certainly not cost-effective.
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, and may be triggered by burrowing animals. Intrusion,
in particular human intrusion following the loss of institutional control,
is frequently raised as an issue. To the contrary, this issue and the finite
life-span of engineered features is rarely addressed in the context of
performance assessments for conventional hazardous waste landfills, where
the toxicological relevance may persist ad infinitum.
Based on the objective to keep environmental emissions to a minimum
over long times, the task, therefore, is to find conceptual and technical
solutions
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which, render tailings more inert over prolonged time-spans,
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which render impounded materials and engineered structures stableover prolonged
time spans,
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which minimize the need for active maintenance,
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and which are technically and economically feasible.
The emphasis of this CRP is on solutions, which can be applied in retrospective,
i.e. in a restoration/remediation context. Of crucial importance in this
particular context are costs, as these frequently have to be borne by the
tax payer 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 impacts
with and without the measure is to be undertaken beforehand.