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Uranium Mining and Milling Wastes:
An Introduction

by Peter Diehl

(last updated 18 May 2011)

Contents:


URANIUM MINES

Most uranium ore is mined in open pit or underground mines. The uranium content of the ore is often between only 0.1% and 0.2%. Therefore, large amounts of ore have to be mined to get at the uranium. In the early years up until the 1960's uranium was predominantly mined in open pit mines from ore deposits located near the surface.

image (51k) new window: Rössing open pit mine, Namibia (Thomas Siepelmeyer 1987)

image (50k) new window: Ranger open pit mine, Australia

image (32k) new window: 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)

 

WASTE ROCK

Waste rock is produced during open pit mining when overburden is removed, and during underground mining when driving tunnels through non-ore zones. Piles of so-called waste rock often contain elevated concentrations of radioisotopes compared to normal rock. Other waste piles consist of ore with too low a grade for processing. The transition between waste rock and ore depends on technical and economic feasibility.

Uranium Concentrations in Rock

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): new window 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.

 

HEAP LEACHING

In some cases uranium has been removed from low-grade ore by heap leaching. This may be done if the uranium contents is too low for the ore to be economically processed in a uranium mill. The leaching liquid (often sulfuric acid) is introduced on the top of the pile and percolates down until it reaches a liner below the pile, where it is caught and pumped to a processing plant.
During leaching, piles present a hazard because of release of dust, radon gas and leaching liquid.
After completion of the leaching process, a longterm problem may result from naturally induced leaching if the ore contains the mineral pyrite (FeS2), as with the uranium deposits in Thuringia, Germany) or Ontario, Canada. Then, acces of water and air may cause continuous bacterially induced production of sulfuric acid inside the pile, which results in the leaching of uranium and other contaminants for centuries and possibly permanent contamination of ground water.

 

IN SITU LEACHING

With the in situ leaching technology, a leaching liquid (e.g. ammonium-carbonate or sulfuric acid) is pumped through drill- holes into underground uranium deposits, and the uranium bearing liquid is pumped out from below. This technology can only be used for uranium deposits located in an aquifer in permeable rock, confined in non-permeable rock.

Scheme of ISL operation

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:

The disadvantages are: After finishing the in situ leaching, the waste sludge must be dumped in a final deposit and the ore zone aquifer must be restored to pre-leaching conditions. Ground water restoration is a very protracted and troublesome process, which is not yet completely understood. It is still impossible to establish pre- leach levels for all parameters.

(for details, see Impacts of Uranium In-Situ Leaching)

 

MILLING OF THE ORE

Ore mined in open pit or underground mines is crushed and leached in a uranium mill. A uranium mill is a chemical plant designed to extract uranium from ore. It is usually located near the mines to limit transportation. In the most cases, sulfuric acid is used as the leaching agent, but alkaline leaching is also used. As the leaching agent not only extracts uranium from the ore, but also several other constituents like molybdenum, vanadium, selenium, iron, lead and arsenic, the uranium must be separated out of the leaching solution. The final product produced from the mill, commonly referred to as "yellow cake" (U3O8 with impurities), is packed and shipped in casks.
When closing down a uranium mill, large amounts of radioactively contaminated scrap are produced, which have to be disposed in a safe manner. In the case of Wismut's Crossen uranium mill, to reduce cost some of the scrap is intended to be disposed in the Helmsdorf tailings, but there it can produce gases and thus threaten the safe final disposal of the sludge.

 

URANIUM MILL TAILINGS DEPOSITS

Characteristics of uranium mill tailings

Uranium mill tailings are normally dumped as a sludge in special ponds or piles, where they are abandoned.. The largest such piles in the US and Canada contain up to 30 million tonnes of solid material. In Saxony, Germany the Helmsdorf pile near Zwickau contains 50 million tonnes, and in Thuringia the Culmitzsch pile near Seelingstädt 86 million tonnes of solids.

(image (160k) new window: Atlas Co. uranium mill tailings, Moab, Utah, USA - U.S. DOE Sep. 2010)

(image (46k) new window Rio Algom Quirke Tailings (water covered): Aerial view - BHP Billton Aug. 1999)

(image (39k) new window: Ranger uranium mill tailings pond, Australia)

(image (144k) new window: Olympic Dam tailings, Australia - Strahlendes Klima 2008)

(image (116k) new window: 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.

Uranium Mill Tailings Activity

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)

Uranium Mill Tailings Hazards

Potential hazards from uranium mill tailings

Radionuclides contained in uranium tailings emit 20 to 100 times as much gamma-radiation as natural background levels on deposit surfaces. Gamma radiation levels decrease rapidly with distance from the pile.

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) new window
(Split Rock uranium mill tailings site, Wyoming)

> View extension of groundwater plumes new window
(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:

(see also Chronology of uranium tailings dam failures)

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.

Concepts for tailings disposal

In most cases, uranium mill tailings are disposed in some form or another, to limit contaminant release into the environment. There are, however, two known exceptions, where the tailings were simply released into the environment without any control:

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) new window: 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.

Standards for uranium mill tailings management

In the early years of uranium mining after World War II, the mining companies often left sites without any clean up after the ore deposits were exhausted: often, in the United States, the mining and milling facilities were not even demolished, not to mention reclamation of the wastes produced; in Canada, uranium mill tailings were often simply dumped in one of the numerous lakes.

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 new window · Aerial view Aug. 1999 (BHP Billiton) new window

Water covers for uranium mill tailings dams are also used by Cogéma at Mounana (Gabon) and at St-Priest-la-Prugne (Loire, France).

Reclamation of uranium mill tailings deposits

To reclaim an uranium mill tailings pile according to principles of a safe long-term isolation, detailed investigations have to be performed in advance to assess the site.
If the tailings pile presents an immediate hazard, then intermediate protective measures can be taken in parallel, such as installation of a cover against windblown dust, or collection of seepage waters. These measures, however, should not conflict with long-term measures to be taken later.

The site must be appropriate for tailings disposal from the view of geology and hydrology:

During investigation of the site, ground water flow has to be monitored, to allow development of computer based three- dimensional ground water models. These models can be used for prediction of effects of supposed or real contaminant releases.

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.

 

DISPOSAL OF OTHER MATERIALS

The lack of sites for disposal of toxic and nuclear waste has led to proposals to dump these hazardous wastes in uranium mill tailings piles or in former uranium mines. For example: If the mixing of uranium mill tailings and other wastes is allowed, then the reclamation of the tailings piles becomes even more difficult, if not impossible, because a best fit method always can be found for a single contaminant only. (for details see: Disposal of Other Wastes in Uranium Mines and Mill Tailings)

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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