Monday, December 21, 2009
Author: Gavin M. Mudd
What is Uranium and How is it Mined?
Uranium is the heaviest, naturally occurring element. It consists of two principal isotopes – uranium-238 (238U) with 238 neutrons in its nucleus, and uranium-235 (235U) with 235 neutrons. The 235U isotope is the desired isotope for nuclear reactors or nuclear weapons due to its ability to fission or split apart and release vast quantities of energy in the process. Natural uranium consists of 99.3% 238U and about 0.7% 235U. Uranium is unstable – it decays into slightly lighter elements, which are also unstable and further decay. The process of decay releases energy and a small atomic particle, and is known as radioactivity.
There are two principal types of radioactive decay – alpha decay, the release of a charged helium atom, and beta decay, the release of an electron. This decay chain progresses through until a stable isotope is achieved (i.e. lead-206 or 206Pb from 238U and 207Pb from 235U). The rate at which an isotope decays is a characteristic of that isotope, and the time taken for 50% of an isotope to decay is known as its ‘halflife’.
The various decay products from uranium have half-lives ranging from fractions of a second to billions of years, shown in Table 1.
As uranium is mostly present in oxide form, it is commonly reported as either uranium (U) or its oxide ‘U3O8’. Averageconcentrations of uranium in typical soils and rocks are about 3 mg/kg U3O8 or parts per million U3O8 (ie. about 3 grams per tonne). This background uranium is partly responsible for natural background radiation. In order to mine uranium economically using existing technology, this concentration has to reach at least 300 mg/kg or 0.03% U3O8, with most uranium mines historically ranging between 0.1 to 0.5% U3O8. Due to uranium’s variable chemistry, it can be concentrated to mineable ore grades and deposits by numerous geologic processes. The most common types of mineable economic uranium ores are found in sandstone deposits, unconformity deposits, breccia complex deposits,
intrusive deposits, metamorhpic deposits and surficial deposits.
Uranium is mined using traditional techniques such as open cut or underground mining, but sandstone deposits can also be mined by ‘in situ leaching’ (also known as solution mining).Once the ore is mined it is finely ground and the uranium is chemically extracted through conventional processes involving leaching with acid or alkali, concentration and then purification to uranium oxide. Acid leaching is the most common. An oxidising chemical is commonly also used, such as pyrolusite (MnO2) or hydrogen peroxide (H2O2), to
ensure the leaching is rapid. For in situ leaching, the acid or alkali is injected directly into the ore zone and pumped back to the surface (no ore is excavated). After leaching from the ore, the uranium is further concentrated usingsolvent extraction or ion exchange, followed by chemical precipitation to an impure oxide using ammonia (this product is ‘yellowcake’). Finally, the yellowcake is heated at high temperature to remove the ammonia and leave relatively pure uranium oxide (>97% U3O8).
Is Uranium Mining Like Any Other Mining?
Uranium ore is significantly radioactive – a property that is very uncommon across the mining industry. There are some other mineral deposits that also contain elevated uranium or thorium (also radioactive), however these aregenerally very few. A uranium ore deposit may have outcrops at the surface, presenting a major localised radiological risk, although more commonly uranium deposits are not visible at the surface and hence have negligible radiological risk. The geologic structure that holds the uranium is relatively stable. The process of mining and milling uranium ore involves severe disturbance to this natural equilibrium, especially as crystalline rocks are broken up during mining, ground for milling and aggressively chemically treated to liberate the uranium. An ore grade of 0.3% U3O8 means that 99.7% of the ore is left as solid waste, known as tailings (the minor loss of uranium is easily made up by the amount of chemicals added during leaching). Uranium mill tailings retain about 85% of the original radioactivity of the ore, and must be managed so as to minimise releases of radioactive decay products such as radium and radon as well as heavy metals (eg. arsenic, copper, lead).
What are the environmental impacts of uranium mining?
The environmental impacts of uranium mining include the traditional impacts associated with gold or copper mining, as well as additional radiological impacts. Depending on the type of deposit and method of mining, the environmental impacts are associated with solid waste management, water management, and chemicals and emissions from milling.
In open cut mining large quantities of waste rock are excavated to access the ore, with much of this waste rock also containing low grade, uneconomic quantities of uranium. Additionally, this waste rock may also contain sulphide minerals such as pyrite. When undisturbed in situ this rock is stable. However, the process of mining increases the cracks present and allows water and oxygen to diffuse into the waste. The oxygen and water reacts with the sulphide to produce sulphuric acid. This in turn dissolves much of the heavy metals and radionuclides present in the waste, allowing it to leach out of the rock into the surrounding environment.
This leachate, known as acid mine drainage (AMD), is extremely toxic to aquatic ecosystems and will cause major, long-lasting environmental impacts. AMD is a major problem in the mining of many metals, but presents an additional problem when combined with uranium mining.
Infamous sites where environmental impacts from AMD have been extensive include Rum Jungle, near Darwin in Australia, as well as the Elliot Lake district in northern Ontario, Canada.
Since the late 1970’s, in Australia at least, more stringent requirements have been placed on solid waste and water management at uranium mines.
At Ranger, all tailings will be required to be emplaced within former open cuts and all waste rock re-contoured to a landform which is intended to be stable. For Olympic Dam, however, the present planning is for all tailings to remain above ground and then covered with engineered soils to minimise erosion, infiltration and radiological releases.
Recent analyses have examined the energy and water costs and greenhouse emissions associated with uranium production.
Energy is measured in Joules, and a GJ is one thousand million Joules. (About 1GJ of heat would be produced by 500 typical electric radiator bars operating for an hour.) The analyses show that the energy cost of extracting uranium is between 170 to 350 GJ per tonne of U3O8, with higher values from lower grade ores,
while for water it takes between 46,000 to 2,900 litres/t U3O8 (eg at Beverley, an acid leach mine, consumes an average 7.7 million litres of water per tonne of U3O8). The corresponding greenhouse emissions of carbon dioxide ranges from 8.5 to 51 t CO2/t U3O8. These environmental costs are particularly sensitive to ore grade, with higher values from lower grade ores.
How much radioactive waste does uranium mining produce?
The radioactive nature of uranium means that any mining leads to the production of significant quantities of
radioactive wastes – principally waste rock and tailings. The extent of waste will depend on the specifics of a
particular deposit and mine plan, but in general open cut mining produces significantly more waste than underground mining.
By December 2005 in Australia, on average, each tonne of uranium extracted has led to the production of 848 tonnes of mill tailings and 1,152 tonnes of combined low-grade ore and waste rock (excluding in situ leach production). The total quantity of tailings is about 128 million tonnes (grading about 0.03% U3O8) with about 175 million tonnes of combined low-grade ore and waste rock. In comparison to the volumes of radioactive waste in the nuclear fuel chain, the largest quantity is easily produced in the mining and milling of uranium.
What are the radioactivity releases from uranium mining?
The releases of radioactivity from uranium mining are sourced from tailings, low-grade ore and, to a lesser extent water management. The principal release is that of radon – a noble gas that is a radioactive decay product of uranium. Radon has a high rate of radioactivity per mass, and is implicated in lung cancers in long-term health studies of former uranium mineworkers.
Can uranium mines be operated safely?
The most recent experience of Australia’s operating uranium mines demonstrates the challenges involved in uranium mining, which are distinct and unique. There have been numerous incidents at the now closed Nabarlek mine and the operating Ranger, Olympic Dam and Beverley projects.
The most common examples include mismanagement of water, sometimes leading to unauthorised releases to adjacent creeks, significant risks to mine/mill workers, waste rock leaching, and ongoing seepage impacts from tailings. Some relatively recent examples include:
• despite being expected to operate under a “no-release” water management system, incidents involving misplaced low grade ores or failures in water control bunds have led on numerous occasions to contaminated runoff waters being leaked into adjacent creeks (especially Corridoor Creek, a tributary of Magela Creek).
• in early 2004 incorrect plumbing saw the process water circuit being connected to the potable drinking water circuit – leading to rapid and significant toxic process water being mixed with drinking water, and much of the Ranger workforce being potentially exposed to both acute chemical and radiological exposure.
• after operating for nearly a decade, a major ongoing leak from the tailings dam was revealed, amounting to the loss of billions of litres of tailings water to groundwater.
• in March 1999, and again October 2001, major explosions and fires caused substantive damage to the mill and smelter complexes, including major releases of noxious fumes – though the extent of radiological releases
remains highly contentious, the fact that the uranium solvent extraction circuit in the 2001 incident was on fire
raises serious concerns about how these incidents are handled by current regulators.
• numerous spills and leaks from pipelines have occurred.
Nabarlek (now closed):
• due to the need to reduce the inventory of contaminated mine site waters, evaporation pond water was irrigated over an area adjacent to the mine/mill and led to significant tree deaths and lasting impacts on water quality in the adjacent creek which have taken nearly two decades to flush through.
Can uranium mines be satisfactorily rehabilitated?
The experience of rehabilitating uranium mines to date in Australia is questionable. The first generation of uranium mines from the Cold War, namely Rum Jungle, Radium Hill, Mary Kathleen and the South Alligator group of mines, all still present environmental and radiological management problems and require constant vigilence and maintenance.
• Rum Jungle – despite extensive remediation/rehabilitation works in the early 1980’s, including excavating
remnant tailings and disposal into former pits, re-contouring and engineering soil covers over low grade ore and waste rock dumps, acid mine drainage continues to pollute the Finniss River, and the complete site still urgently requires more remediation/rehabilitation works.
• Radium Hill – after being abandoned in early 1962, minimal earth works were undertaken in the early 1980’s, mainly just engineering soil covers over the tailings piles – erosion is a continual problem and tailings requires ongoing maintenance.
• Mary Kathleen – operating in both the Cold War phase of the late 1950’s to mid-1960’s as well as again in the commercial era of the late 1970’s, the mid-1980’s rehabilitation of the mine won an engineering excellence award for its perceived quality – despite internal concerns by the regulators about potential for long-term seepage from the tailings dam. Recent field studies in the late 1990’s have validated this concern and shown ongoing seepage of saline, metal and radionuclide rich waters from the tailings dam – well above the quantities predicted at the time of rehabilitation – impacting on the local creek.
Overall, the experience to date with uranium mining does not give rise to any sufficient degree of confidence, as past sites – even after significant rehabilitation works – are still showing problems with erosion and seepage and ongoing impacts on water quality.
About the author:
Gavin Mudd holds a PhD in Environmental Engineering and is a lecturer in the Department of Civil Engineering, Monash University. His research interest include the environmental impacts from uranium mining and milling in Australia.