: Rössing open pit mine, Namibia (Thomas Siepelmeyer 1987)| HOME WISE Uranium Project > Mining & Milling > Impacts > |
(last updated 18 May 2011)
image (51k) : Rössing open pit mine, Namibia (Thomas Siepelmeyer 1987)
image (50k) : Ranger open pit mine,
Australia
image (32k) : former Lodève open pit
mine, France, 1992
Later, mining was continued in underground mines.
After the decrease of uranium prices since the 1980's on the
world market, underground mines became too expensive for most
deposits; therefore, many mines were shut down.
New uranium deposits discovered in Canada have uranium grades of
several percent.
To keep groundwater out of the mine during operation, large
amounts of contaminated water are pumped out
and released to rivers and lakes. When the pumps are shut down
after closure of the mine, there is a risk of groundwater
contamination from the rising water level.
(see also Uranium Ore Radiation Properties)
All these piles threaten people and the environment after shut down of the mine due to their release of radon gas and seepage water containing radioactive and toxic materials.
(image (36k): The former waste rock "pyramids" of Ronneburg, Germany, 1990)
Waste rock was often processed into gravel or cement and used for road and railroad construction. VEB Hartsteinwerke Oelsnitz in Saxony has processed 200,000 tonnes of material per year into gravel containing 50 g/t uranium. Thus, gravel containing elevated levels of radioactivity were dispersed over large areas.
In situ leaching gains importance with a decrease in price of
uranium. In the USA, in situ leaching is often used. In 1990, in
Texas alone in situ leaching facilities for uranium were
operated at 32 sites. In Saxony, Germany, an underground mine
converted to an underground in situ leaching mine was operated
until end of 1990 at Königstein near Dresden. In the Czech
Republic, the in situ leaching technology was used at a large
scale at Stráz pod Ralskem in Northern Bohemia.
The advantages of this technology are:
(for details, see Impacts of Uranium In-Situ Leaching)
(image
(160k) : Atlas Co. uranium mill tailings, Moab, Utah, USA - U.S. DOE Sep. 2010)
(image (46k) Rio Algom Quirke Tailings (water covered): Aerial view - BHP Billton Aug. 1999)
(image (39k) : Ranger uranium mill tailings pond, Australia)
(image (144k) : Olympic Dam tailings, Australia - Strahlendes Klima 2008)
(image (116k) : Olympic Dam tailings, Australia - Strahlendes Klima 2008)
The amount of sludge produced is nearly the same as that of the
ore milled. At a grade of 0.1% uranium, 99.9% of the material is
left over.
Apart from the portion of the uranium removed, the sludge
contains all the constituents of the ore. As long lived decay
products such as thorium-230 and radium-226 are not removed, the
sludge contains 85% of the initial
radioactivity of the ore. Due to technical limitations,
all of the uranium present in the ore can not be extracted.
Therefore, the sludge also contains 5% to 10% of the uranium
initially present in the ore.
In addition, the sludge contains heavy metals and other
contaminants such as arsenic, as well as chemical reagents used
during the milling process.
Mining and milling removes hazardous constituents in the ore from their relatively safe underground location and converts them to a fine sand, then sludge, whereby the hazardous materials become more susceptible to dispersion in the environment. Moreover, the constituents inside the tailings pile are in a geochemical disequilibrium that results in various reactions causing additional hazards to the environment. For example, in dry areas, salts containing contaminants can migrate to the surface of the pile, where they are subject to erosion. If the ore contains the mineral pyrite (FeS2), then sulfuric acid forms inside the deposit when accessed by precipitation and oxygen. This acid causes a continuous automatic leaching of contaminants.
Radon-222 gas emanates from tailings piles and has a half life of 3.8 days. This may seem short, but due to the continuous production of radon from the decay of radium-226, which has a half life of 1600 years, radon presents a longterm hazard. Further, because the parent product of radium-226, thorium-230 (with a half life of 80,000 years) is also present, there is continuous production of radium-226. (view Uranium decay series)
After about 1 million years, the radioactivity of the tailings and thus its radon emanation will have decreased so that it is only limited by the residual uranium contents, which continuously produces new thorium-230.
If, for example, 90% of
the uranium contained in an ore with 0.1% grade was extracted
during the milling process, the radiation of the tailings
stabilizes after 1 million years at a level 33 times that of
uncontaminated material. Due to the 4.5 billion year half-life
of uranium-238, there is only a minuscule further decrease.
(see also Uranium Mill Tailings Radiation Properties)
The radium-226 in tailings continuously decays to the
radioactive gas radon-222, the decay products
of which can cause lung cancer. Some of this radon escapes from
the interior of the pile. Radon releases are a major hazard that
continues after uranium mines are shut down. The U.S.
Environmental Protection Agency (EPA) estimates the lifetime
excess lung cancer risk of residents living nearby a bare
tailings pile of 80 hectares at two cases per hundred.
Since radon spreads quickly with the wind, many people receive
small additional radiation doses. Although the excess risk for
the individual is small, it cannot be neglected due to the large
number of people concerned. EPA estimates that the uranium
tailings deposits existing in the United States in 1983 would
cause 500 lung cancer deaths per century, if no countermeasures
are taken.
See also:
Tailings deposits are subject to many kinds of
erosion. Due to the long half-lives of the
radioactive constituents involved, safety of the deposit has to
be guaranteed for very long periods of time.
After rainfall, erosion gullies can form; floods can destroy the
whole deposit; plants and burrowing animals can penetrate into
the deposit and thus disperse the material, enhance the radon
emanation and make the deposit more susceptible to climatic
erosion.
When the surface of the pile dries out, the fine sands are blown
by the wind over adjacent areas. The sky has darkened from
storms blowing up radioactive dust over villages located in the
immediate vicinity of Wismut's uranium mill tailings piles.
Subsequently, elevated levels of radium-226 and arsenic were
found in dust samples from these villages.
Seepage from tailings piles is another major hazard. Seepage poses a risk of contamination to ground and surface water. Residents are also threatened by radium-226 and other hazardous substances like arsenic in their drinking water supplies and in fish from the area. The seepage problem is very important with acidic tailings, as the radionuclides involved are more mobile under acidic conditions. In tailings containing pyrite, acidic conditions automatically develop due to the inherent production of sulfuric acid, which increases migration of contaminants to the environment.
> View animation of modeled contaminant plume dispersion in groundwater (153k)
(Split Rock uranium mill tailings site, Wyoming)
> View extension of groundwater plumes
(Church Rock uranium mill tailings site, New Mexico)
Tailings dams are often not of stable construction. In most cases, they were made from sedimentation of the coarse fraction of the tailings sluge. Some, including those of Culmitzsch and Trünzig in Thuringia, were built on geologic faults. Therefore, they are subject to the risk of an earthquake. As the Thuringian tailings deposits are located in the center of an area of earthquake risk in the former GDR, they suffer a risk of dam failure. Moreover, strong rain or snow storms can also cause dam failures. (for details see: Safety of Tailings Dams)
It is of no surprise that again and again dam failures have occured. Some examples are:
Occasionally, because of their fine sandy texture, dried tailings have been used for construction of homes or for landfills. In homes built on or from such material, high levels of gamma radiation and radon were found. The U.S. Environmental Protection Agency (EPA) estimates the lifetime excess lung cancer risk of residents of such homes at 4 cases per 100.
The obvious idea of bringing the tailings back to where the ore has been taken from, does not in the most cases lead to an acceptable solution for tailings disposal. Although most of the uranium was extracted from the material, it has not become less hazardous, quite to the contrary. Most of the contaminants (85% of the total radioactivity and all the chemical contaminants) are still present, and the material has been brought by mechanical and chemical processes to a condition where the contaminants are much more mobile and thus susceptible to migration into the environment. Therefore, dumping the tailings in an underground mine cannot be afforded in most cases; there, they would be in direct contact with groundwater after halting the pumps.
The situation is similar for deposit of tailings in former open pit mines. Here also, immediate contact to ground water exists, or seepage presents risks of contamination of ground water. Only in the case of the presence of proven impermeable geologic or man-made layers can the contamination risk to ground water be prevented. An advantage of in-pit deposition is relatively good protection from erosion.
(image (35k) : Tailings disposal in Bellezane open pit, France, 1992)
In France and Canada, on the other hand, the concept of dumping
the tailings in former open pits in groundwater
is pursued or proposed at several sites in recent years.
In this case, a highly permeable layer is installed around the
tailings, to allow free groundwater circulation around the
tailings. Since the permeability of the tailings themselves is
lower, it is anticipated (by the proponents) that nearly no
exchange of contaminants between tailings and groundwater takes
place. A similar method is being tested in Canada for the
disposal of uranium mill tailings in lakes
(called "pervious surround disposal").
Recent proposals even deny the necessity of an artificial
permeable layer around the tailings, since the surrounding rock
would provide a high enough permeability.
In most cases, tailings have to be dumped on the surface for lack of other options. Here, the protection requirements can more easily be controlled by appropriate methods, but additional measures have to be performed to assure protection from erosion.
The untenability of this situation was for the first time
recognized by U.S. legislation, which defined legal requirements
for the reclamation of uranium mill tailings in 1978 (UMTRCA). On the basis of this law,
regulations were promulgated by the Environmental Protection
Agency (EPA: 40 CFR 192) and the
Nuclear Regulatory Commission (NRC: 10 CFR 40). These regulations not
only define maximum contaminant concentrations for soils and
admissible contaminant releases (in particular for radon), but
also the period of time, in which the reclamation measures taken
must be effective: 200 - 1000 years. The reclamation action thus
not only has to assure that the standards are met after
completion of the reclamation work; but for the first time, a
long-term perspective is included in such
regulations. A further demand is that the measures taken must
assure a safe disposal for the prescribed period of time
without active maintenance. If these conditions
cannot be met at the present site, the tailings must be
relocated to a more suitable place.
Considering the actual period of time the hazards from uranium
mining and milling wastes persist, these regulations are of
course only a compromise, but they are a first step, at least.
Regulations for the protection of groundwater were not included
in the initial legislation; they were only promulgated in
January 1995.
Last but not least, public involvement is given an important role in planning and control of the reclamation action.
Based on these regulations, various technologies for the safe and maintenance-free confinement of the contaminants were developed in the United States during subsequent years. The reclamation efforts also include the decontamination of homes in the vicinity built from contaminated material or on contaminated landfills.
In Canada, on the contrary, authorities decide on a site-by-site
basis on the measures to be taken for reclamation; there are no
legal requirements. The Atomic Energy Control Board (AECB) has
only promulgated rough guidelines; and it decides, together with
the mine and mill operators, on the necessity of measures to be
taken. Therefore, it is no surprise that the Canadian approach
results in a much lower level of protection. The proposals for
the management of the uranium mill tailings
in the Elliot Lake area, Ontario, for example, include no
other "protective barrier" than a water cover.
> View Rio Algom Quirke Tailings (water covered): Schematic profile · Aerial view Aug. 1999 (BHP Billiton)
Water covers for uranium mill tailings dams are also used by Cogéma at Mounana (Gabon) and at St-Priest-la-Prugne (Loire, France).
The site must be appropriate for tailings disposal from the view of geology and hydrology:
In some circumstances, it may become necessary, to move all of
the material to an intermediate storage place to allow for the
installation of a liner below the final deposit. An example for
this procedure was the tailings deposit at Canonsburg,
Pennsylvania, USA.
In some very unfortunate circumstances, it even may become
necessary to move the whole material to a safer site for
permanent disposal. This procedure was preferred at 11 sites in
the U.S., involving a total of 14.36 million cubic meters of
tailings.
To prevent seepage of contaminated water, a liner must be
installed below the deposit in many cases, if no natural
impermeable layer is present. For this purpose, appropriate
lining materials have to be selected. A multi-layer liner may
become necessary.
To increase mechanical stability, the following management
options may be applied: dewatering of the sludge, smoothing of
the slopes, and installation of erosion protection.
On top of the pile, an appropriate cover has to be installed for
protection against release of gamma radiation and radon gas,
infiltration of precipitation, intrusion of plants and animals,
and erosion. This cover in most cases consists of several
different layers to meet all requirements.
Moreover, the catchment, collection and treatment of seepage
water is necessary to release purified waters to the surface
water only. In the long term however, water treatment should no
longer be necessary.
Finally, it has to be determined if, and to what extent,
contaminated material was used in the surrounding area for
construction or landfill purposes. Such contaminated properties
should be included in the reclamation program.
Former uranium mine and mill sites often have very poor properties for the isolation of contaminants. Detailed investigations have to be performed at such sites by independent experts, before such disposal can be considered.
> See also: Environmental impacts of uranium mining and milling - Slide Talk
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