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Environmental Aspects of Uranium Mining
Nuclear Issues Briefing Paper 10
June 2003
In many respects uranium mining is much the same as any other mining. Projects must have environmental approvals prior to commencing, and must comply with all environmental, safety and occupational health conditions applicable.
Once approved, open pits or shafts and drives are dug, waste rock and overburden is placed in engineered dumps. Tailings from the ore processing must be placed in engineered dams. Finally the whole site must be rehabilitated at the end of the project. Meanwhile air and water pollution must be avoided.
These processes are common to all metalliferous mining, and are well recognised and understood.
In the case of in situ leach (ISL) mining, there is much less disturbance - simply multiple boreholes, and rehabilitation is simpler.
URANIUM: Uranium itself is radioactive, though with the major isotope U-238 having a half-life equal to the age of the earth, it is certainly not strongly radioactive. U-235 has a half life one sixth of this and emits gamma rays as well as alpha particles. Hence a lump of pure uranium would give off some gamma rays, but less than those from a lump of granite. Its alpha radioactivity in practical terms depends on whether it is as a lump (or in rock as ore), or as a dry powder. In the latter case the alpha radioactivity is a potential, though not major, hazard. It is also toxic chemically, being comparable with lead. Uranium metal is commonly handled with gloves as a sufficient precaution. Uranium concentrate is handled and contained so as to ensure that people do not inhale or ingest it.
The gamma radiation detected by exploration geologists looking for uranium actually comes from associated elements such as radium and bismuth, which over geological time have resulted from the radioactive decay of uranium.
ENVIRONMENTAL APPROVALS
At an early stage of the feasibility study, environmental studies of the site begin. These escalate in detail and progressively focus on issues of concern in relation to the proposal, in consultation with state authorities (who in Australia generally operate under an agreement with the Commonwealth to ensure that its concerns are addressed).
Depending on the particular state, an environmental effects or impact statement is published and made available for public comment. After consideration of comments and in the light of judgements by a wide range of state authorities, approval may then be given by the state government for the project to proceed.
WASTES FROM MINING & MILLING
In most respects, conventional mining of uranium is the same as mining any other metalliferous ore, and well-established environmental constraints apply in order to avoid any off-site pollution.
From open cut mining, there are substantial volumes of barren rock and overburden waste. These are placed near the pit and either used in rehabilitation or shaped and revegetated where they are. At Ranger mine, the development of the first orebody involved a waste to ore ratio of slightly over 2:1.
However, uranium minerals are always associated with more radioactive elements such as radium and radon in the ore. Therefore, although uranium itself is not very radioactive, the ore which is mined, especially if it is high-grade, is handled with some care, for occupational health and safety reasons.
Mining methods, tailings and run-off management and land rehabilitation are subject to Government regulation and inspection. Mining operations are undertaken under relevant national health and radiation protection codes of practice. These set strict health standards for exposure to gamma radiation and radon gas. Standards apply to both workers and members of the public.
Tailings & Radon
Solid waste products from the milling operation are tailings. They comprise most of the original ore and they contain most of the radioactivity in it. In particular they contain all the radium present in the original ore. At an underground mine they may be first cycloned to separate the coarse fraction which is used for underground fill. The balance is pumped as a slurry to a tailings dam, which may be a worked-out pit as at Ranger and McClean Lake.
When radium undergoes natural radioactive decay one of the products is radon gas. Because radon and its decay products (daughters) are radioactive and because the tailings are now on the surface, measures are taken to minimise the emission of radon gas. During the operational life of a mine the material in the tailings dam is usually covered by water to reduce surface radioactivity and radon emission (though with lower-grade ores neither pose a hazard at these levels).
On completion of the mining operation, it is normal for the tailings dam to be covered with some two metres of clay and topsoil to reduce radiation levels to near those normally experienced in the region of the orebody, and for a vegetation cover to be established. At Ranger and Jabiluka in North Australia, tailings will be returned underground, as was done at the now-rehabilitated Nabarlek mine. In Canada, ore treatment is often remote from the mine that the new ore comes from, and tailings are emplaced in mined out pits wherever possible, and engineered dams otherwise.
The radon gas emanates from the rock and tailings as the radium or thorium decays. It then decays itself to (solid) radon daughters, which are significantly alpha radioactive*.
* About 95% of the radioactivity in the ore is from the U-238 decay series, totalling about 450 kBq/kg in ore with 0.3% U308 (eg from Ranger). The U-238 series has 14 radioactive isotopes in secular equilibrium, thus each represents about 32 kBq/kg (irrespective of the mass proportion). When the ore is processed, the U-238 and the very much smaller masses of U-234 (and U-235) are removed. The balance becomes tailings, and at this point has about 85% of its original intrinsic radioactivity. However, with the removal of most U-238, the following two short-lived decay products (Th-234 & Pa-234) soon disappear, leaving the tailings with a little over 70% of the radio-activity of the original ore after several months. The controlling long lived isotope then becomes Th-230 which decays with a half life of 77,000 years to radium-226 followed by radon-222.
Radon occurs in most rocks and traces of it are in the air we all breathe. However, at high concentrations it is a health hazard.
A 1998 paper looks at the long-term population dose due to radon from uranium mining and shows that it is insignificant.
Water
Run-off from the mine stockpiles and waste liquors from the milling operation are collected in secure retention ponds for isolation and recovery of any heavy metals or other contaminants. The liquid portion is disposed of either by natural evaporation or recirculation to the milling operation. Most Australian mines adopt a "zero discharge" policy for any pollutants.
Process water discharged from the mill contains traces of radium and some other metals which would be undesirable in biological systems downstream. This water is evaporated and the contained metals are retained in secure storage. During the operational phase, such water may be used to cover the tailings while they are accumulating.
With in situ leach (ISL) operations, the orebody stays in the ground and uranium is recovered by circulating oxygenated and acidified groundwater through it, using injection and recovery wells. The saline quality of this groundwater in Australian ISL mines makes it far from potable in the first place, and after the uranium is recovered, oxygen input and circulation are discontinued, leaving the groundwater much as it was.
The main environmental consideration with ISL is avoiding pollution of any groundwater away from the orebody, and leaving the immediate groundwater no less useful than it was initially.
Descriptions of how environmental mangement is undertaken at Australia's three uranium mines, Ranger, Olympic Dam, and Beverley are under the Environmental Management headings of Australia's Uranium Mines, in the sections on the respective mines.
In relation to Ranger, the Office of the Supervising Scientist was established by the Commonwealth Government in 1979 to oversee environmental protection at uranium mines in the Alligator Rivers region of the Northern Territory.
Rehabilitation
Apart from tailings, other solid wastes at a mine include equipment which is not able to be sold at the conclusion of the operation. This is usually buried with the tailings.
At the conclusion of mining, tailings are covered permanently with enough clay and soil to reduce both gamma radiation levels and radon emanation rates to levels near those naturally occurring in the region, and enough rock to resist erosion. A vegetation cover is then established.
Mary Kathleen in Queensland was the site of Australia's first major rehabilitation project of a uranium mine. It involved the plant site, a 28 hectare tailings dam, and a 60 ha evaporation pond area. All this has now returned to being a cattle station, with unrestricted access. The rehabilitation project was completed at the end of 1985 at a cost of some $19 million, and won an award for engineering excellence.
The Nabarlek uranium mine in the Northern Territory, c 270 km east of Darwin, was the first of the "new generation" of uranium mines to commence operations and the first to be rehabilitated. Environmental protection was stressed at Nabarlek since before mining commenced, and everything proceeded with eventual rehabilitation very much in mind. During the life of the operation the company worked together with government agencies, the Northern Land Council (NLC) and Aboriginal land owners to ensure a high standard of environmental management, culminating in its decommissioning and successful rehabilitation.
At Ranger the planning of final restoration is well-established, and each year the company prepares a full-costed plan which assumes that mining will cease that year. All rehabilitation objective must be achieved, including ecosystem viability, radiological safety, and landform stability (re erosion). The costings are checked and the sum involved becomes the adjusted amount of the company's bond held by the government (most recently about US$ 17 million). Such bonds are a routine requirement for mines today.
Apart from groundwater considerations discussed above, rehabilitation of ISL mines is very straightforward, making this a technique with remarkably low environmental impact. Upon decommissioning, wells are sealed or capped, process facilities removed, any evaporation pond revegetated, and the land can readily be returned to its previous uses.
Experience at many mine sites is networked throughout the industry and available to present and future operators.
HEALTH OF WORKERS
In Australia all uranium mining and milling operations are undertaken under the Code of Practice on Radiation Protection in the Mining and Milling of Radioactive Ores. This was drawn up by the Commonwealth in line with recommendations of the International Commission on Radiological Protection (ICRP), but it is administered by state health and mines departments. This Health Code, which was updated in 1995 and again in 2002-03 (as part of a combined Health-Waste Code), sets strict health standards for radiation and radon gas exposure, for both workers and members of the public.
In Canada the Canadian Nuclear Safety Commission is responsible for regulating uranium mining as well as other aspects of the nuclear fuel cycle. In Saskatchewan, provincial regulations also apply concurrently, and set strict health standards for both miners and local people. Similar standards are set in other countries.
While uranium itself is only slightly radioactive, radon, a radioactive inert gas, is released to the atmosphere in very small quantities when the ore is mined and crushed. Radon is one of the decay products of uranium and radium, and occurs naturally in most rocks - minute traces of it are present in the air which we all breathe.
Australian uranium mines have mostly been open cut and therefore naturally well ventilated. The Olympic Dam and Canadian underground mines are ventilated with powerful fans. Radon levels are kept at a very low and certainly safe level in uranium mines. (Radon in non-uranium mines also may need control by ventilation.)
Gamma radiation may also be a hazard to those working close to high-grade ores. It comes principally from radium in the ore, so exposure to this is regulated as required. In particular, dust is suppressed, since this represents the main potential exposure to alpha radiation as well as a gamma radiation hazard.
At the concentrations associated with uranium (and some mineral sands) mining, radon is a potential health hazard, as is dust. Precautions taken during the mining and milling of uranium ores to protect the health of the workers include:
- Good forced ventilation systems in underground mines to ensure that exposure to radon gas and its radioactive daughter products is as low as possible and does not exceed established safety levels.
- Efficient dust control, because the dust may contain radioactive constituents and emit radon gas.
- Limiting the radiation exposure of workers in mine, mill and tailings areas so that it is as low as possible, and in any event does not exceed the allowable dose limits set by the authorities. In Canada this means that mining in very high-grade ore is undertaken solely by remote control techniques and by fully containing the high-grade ore where practicable.
- The use of radiation detection equipment in all mines and plants.
- Imposition of strict personal hygiene standards for workers handling uranium oxide concentrate.
At any mine, designated employees (those likely to be exposed to radiation or radioactive materials) are monitored for alpha radiation contamination and personal dosimeters are worn to measure exposure to gamma radiation. Routine monitoring of air, dust and surface contamination is undertaken.
Canadian mine and mill facilities are designed to handle safely ore grades of up to 26% U.
If uranium oxide is ingested it has a chemical toxicity similar to that of lead oxide. Similar hygiene precautions to those in a lead smelter are therefore taken when handling it in the drying and packing areas of the mill.
The usual radiation safeguards are applied at an ISL mining operation, despite the fact that most of the orebody¹s radioactivity remains well underground and there is hence minimal increase in radon release and no ore dust.
SOURCES
Hore-Lacy, Ian, Nuclear Electricity, UIC, 7th edition 2003.
Mary Kathleen Uranium Ltd, Review Report: Rehabilitation of the Mary Kathleen Mine, 1986.
For further information
Uranium Information Centre Ltd
A.C.N. 005 503 828
GPO Box 1649N, Melbourne 3001, Australia
phone (03) 9629 7744
fax (03) 9629 7207