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Uranium Mining, Processing, and Enrichment
Uranium Mining, Processing, and Enrichment IAN HORE-LACY World Nuclear Association London, United Kingdom Uranium Information Centre Melbourne, Australia
1. Uranium Mining, Milling, and Processing 2. Uranium Refining and Conversion 3. Uranium Enrichment
other ores, such as the gold-bearing ores of South Africa. In these cases, the concentration of uranium may be as low as a tenth of those in orebodies mined primarily for their uranium content. Olympic Dam quotes ore reserves down to 0.045% U.
Glossary enrichment The physical process of increasing the proportion of U-235 to U-238. isotope An atomic form of an element having a particular number of neutrons. Different isotopes of an element have the same number of protons but different numbers of neutrons and hence different atomic mass (e.g., U-235, U-238). mill Treatment plant producing concentrate from ore. ore Rock containing economic concentrations of minerals. radon An inert but radioactive gas which is a decay product of radium. stope Cavity formed underground by drilling, blasting, and removing ore. tailings Crushed and ground rock from which economic minerals have been recovered. Uranium tailings are radioactive due to decay products that remain.
There are many uranium mines operating around the world, in some 20 countries, though almost threequarters of world production comes from just 10 mines. Most of the uranium ore deposits at present supporting these mines have average grades in excess of 0.10% of uranium. In the first phase of uranium mining to the 1960s, this would have been seen as a respectable grade, but today some Canadian mines have huge amounts of ore up to 20% U average grade. Some uranium is also recovered as a by-product with copper, as at Olympic Dam in Australia, or as by-product from the treatment of
Encyclopedia of Energy, Volume 6. r 2004 Elsevier Inc. All rights reserved.
1. URANIUM MINING, MILLING, AND PROCESSING 1.1 Overview Generally speaking, uranium mining is no different to other kinds of mining unless the ore is of a very high grade. Where orebodies lie close to the surface, they are usually accessed by open cut mining, involving a large pit and the removal of much overburden as well as a lot of waste rock. Where orebodies are deeper, underground mining is employed, involving construction of access tunnels and shafts but with less waste rock removed and less environmental impact. In either case, the grade is usually controlled by measuring radioactivity as a surrogate for uranium concentration. (The radiometric device detects associated radioactive minerals, which are decay products of the uranium rather than the uranium itself.) At Ranger in north Australia, Rossing in Namibia, and most of Canada’s Northern Saskatchewan mines through to McClean Lake, the orebodies have been accessed by open cut mining. Other mines such as Olympic Dam in Australia, McArthur River, Rabbit Lake and Cigar Lake in Northern Saskatchewan, and Akouta in Niger are underground, up to 600 m deep. At McClean Lake, mining will be completed underground. Table I lists the largest-producing uranium mines in the West.
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TABLE I The Largest-Producing Western World Uranium Mines in 2001 Mine
Country
McArthur R.
Canada
Olympic Dam
Australia
Ranger
Australia
Main owner
Type
Production (tU)
Percentage of world
Cameco
Underground
6937
19.4
WMC
By-product/underground
3693
10.3
ERA (Rio Tinto 68%)
Open pit
3564
10.0
McClean L.
Canada
Cogema
Open pit
2512
7.0
Rossing
Namibia
Rio Tinto (69%)
Open pit
2239
6.3
Akouta
Niger
Cogema/Onarem
Underground
2027
5.7
Rabbit Lake
Canada
Cameco
Underground
1755
4.3
Cluff Lake Arlit
Canada Niger
Cogema Cogema/Onarem
Open pit/underground Open pit
1306 1069
3.6 3.0
AngloGold
South Africa
Anglo American
By-product/underground
Top 10 total
Some orebodies lie in groundwater in porous material and may be accessed simply by oxygenating that groundwater and pumping it out—this is in situ leach (ISL) mining. ISL mining means that removal of the uranium minerals is accomplished without any major ground disturbance. Weakly acidified or alkaline groundwater with a lot of oxygen injected into it is circulated through an enclosed underground aquifer, which holds the uranium ore in loose sands. The leaching solution with dissolved uranium is then pumped to the surface treatment plant. Over half of the world’s uranium now comes from underground mines, about 30% from open cut mines and 16% from ISL. Conventional mines have a mill where the ore is crushed, ground, and then leached with sulfuric acid to dissolve the uranium oxides. Most of the ore, however, remains undissolved in the leaching process, and these solids or tailings are then separated from the uranium-rich solution, usually by allowing them to settle out. At the mill of a conventional mine, or the treatment plant of an ISL operation, the uranium is then separated by ion exchange before being dried and packed.
1.2 Hard Rock Mining With open cut mining, the overburden needs to be removed to expose the orebody, the part of the rock that contains economic concentrations of the minerals being sought. This, and subsequent mining, is undertaken by drilling and blasting, followed by removal of the broken rock. At open cut uranium mines, the drill cuttings from mine development may be tested radiometrically so that ore and waste (below ore-grade material) are blasted separately. In
840
2.9
25,942
72.5
addition, loads of mined material are often scanned radiometrically to ascertain average ore grade, and trucks are then directed to stockpiles, primary crusher, or waste dump accordingly. At Canada’s McClean Lake mine, waste is material with less than 0.025% U or less than 0.085% U if it has high acid-generating potential and requires special disposal. Anything over 0.085% U is unequivocally ore, but is stockpiled according to grade so that mill feed may be varied according to market conditions. With underground mining, the objective is to move as little waste rock as possible in sinking shafts or declines and digging tunnels (drives and adits) accessing the orebody as fully as possible. Mining the actual orebody is then generally more selective. Various mining techniques are employed, but open stoping is common. This involves carefully drilling and then blasting a large mass of rock so that the broken material can be removed. When the void is empty, it is usually filled from above with waste rock and coarse tailings material, with some cement and flyash binder, to maintain structural support and enable extraction of adjacent ore. At Olympic Dam, mining is by longhole open stoping, back-filled after ore extraction. This means that a block of ore is drilled from below and above, then blasted so that up to 80,000 tonnes is broken at once. Each stope will yield up to 500,000 tonnes. Ore is extracted from drawpoints at the bottom of the stope and taken to the underground primary crusher. Crushed ore is hoisted to the surface in skips. At McArthur River in Canada, the orebody is so high-grade (up to 20% U) that remote methods need to be employed for mining it because the minerals
Uranium Mining, Processing, and Enrichment
associated with the uranium are also so concentrated that they emit high levels of gamma radiation. It is also about 600 m deep and in wet, porous rock, so control of water under high pressure is another problem. Manned operations are confined to the barren rock surrounding the orebody, and water is controlled by freezing all around the orebody with some grouting. The orebody is than accessed remotely by raise boring, using a 3-m diameter reaming head. Remotely operated front-end loaders remove the ore cuttings that fall from this. Each load is radiometrically scanned, and waste rock or low-grade mineralized material is hoisted to the surface for disposal. While this whole exercise is high cost per tonne of ore removed, the ore is of such a grade that relatively little is involved, about 120 tonnes per day. The low volume also means that the ore cuttings are fed straight to an underground grinding circuit, and the ground slurry is then pumped to the surface. It is then loaded into special transportation tanks and road-hauled 80 km to the mill for further treatment. Meanwhile, the void from the raise boring is filled with concrete to provide ground support for subsequent raise boring operations. At the Cigar Lake underground mine in Canada, poor ground conditions mean that water jet boring will be employed instead of raise boring, with ground freezing. Below ore-grade material with significant mineralization from open cut mines has in the past been heaped up on an impervious pad and leached with sulfuric acid. This heap leaching produces pregnant liquor, which is fed to the mill for uranium extraction. However, the process can give rise to significant problems of rehabilitation, so it is apparently no longer used. At Australia’s Nabarlek mine in the 1980s, about 50% of the uranium recovery was achieved from 150,000 tonnes of heap leach material.
1.3 In Situ Leach (ISL) Mining In situ leaching (ISL), also known as solution mining, involves leaving the ore where it is in the ground and using liquids that are pumped through it to recover the minerals out of the ore by leaching. Consequently there is little surface disturbance and no tailings or waste rock generated. However, the orebody needs to be permeable to the liquids used and located so that they do not contaminate groundwater away from the orebody. Some ISL mining in the past, notably in the eastern bloc, has occurred in broken rock with insecure containment of fluids, and this has resulted in considerable pollution. ISL mining was first tried
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on an experimental basis in Wyoming during the early 1960s. The first commercial mine began operating in 1974. About a dozen projects are licensed to operate in the United States (in Wyoming, Nebraska, and Texas), and most of the operating mines were less than 10 years old in the early 21st century. Most are small, but they supply some 85% of the U.S. uranium production. About 16% of world uranium production is by ISL (including all Kazakhstan and Uzbekistan output). ISL can also be applied to other minerals such as copper and gold. Uranium deposits suitable for ISL occur in permeable sand or sandstones, confined above and below by impermeable strata and below the water table. They may either be flat, or roll front—in cross section, C-shaped deposits within a permeable sedimentary layer. Such deposits were formed by the lateral movement of groundwater bearing oxidized uranium minerals through the aquifer, with precipitation of the minerals occurring when the oxygen content decreased, along extensive oxidation-reduction interfaces. The uranium minerals are usually uraninite (oxide) or coffinite (silicate) coatings on individual sand grains. The ISL process essentially reverses this ore genesis in a much shorter time frame. There are two operating regimes for ISL, determined by the geology and groundwater. If there is significant calcium in the orebody (as limestone or gypsum), alkaline (carbonate) leaching must be used. Otherwise, acid (sulfate) leaching is generally better. Techniques for ISL have evolved to the point where it is a controllable, safe, and environmentally benign method of mining, which can operate under strict environmental controls and which often has cost advantages. The mine consists of well fields, which are progressively established over the orebody as uranium is depleted from sections of the orebody after leaching. Well-field design is on a grid with alternating extraction and injection wells, each of identical design and typical of normal water bores. The spacing between injection wells is about 30 m with each pattern of four having a central extraction well with a submersible electric pump. A series of monitor wells are situated around each mineralized zone to detect any movement of mining fluids outside the mining area. The wells are cased to ensure that liquors only flow to and from the ore zone and do not affect any overlying aquifers. They are pressuretested before use. The submersible pumps initially extract native groundwater from the host aquifer prior to the addition of uranium complexing reagents (acid or alkaline) and an oxidant (hydrogen peroxide or
320
Uranium Mining, Processing, and Enrichment
oxygen) before injection into the well field. The leach liquors pass through the ore to oxidize and dissolve the uranium minerals in situ. While uranium production in Australia uses acid leaching of the crushed ore, ISL elsewhere normally uses alkaline leaching agents such as a combination of sodium bicarbonate and carbon dioxide. At Beverley and Honeymoon in South Australia the process is acid leaching, with weak sulfuric acid plus oxygen. The leach solution is at a pH of 2.0 to 3.0, about the same as vinegar. The pregnant solution from the production wells is pumped to the treatment plant where the uranium is recovered (see Section 1.4). Before the process solution depleted of uranium (i.e., barren liquor) is reinjected, it is oxygenated and if necessary recharged with sulfuric acid, or with sodium bicarbonate or carbon dioxide, to maintain its pH. Most of the solution is returned to the injection wells, but a very small flow (about 0.5%) is bled off to maintain a pressure gradient in the well field and this, with some solutions from surface processing, is treated as waste. It contains various dissolved minerals such as radium, arsenic, and iron from the orebody and is reinjected into approved disposal wells in a depleted portion of the orebody. This bleed of process solution ensures that there is a steady flow into the well field from the surrounding aquifer and serves to restrict the flow of mining solutions away from the mining area. In the United States, the production life of an individual ISL well pattern is usually less than 3 years, typically 6 to 10 months. Most of the uranium is recovered during the first 6 months of the operation of those wells. The most successful operations have achieved a total overall recovery of about 80% of the uranium from the ore. Over time, production flows decrease as clay and silt become trapped in the permeable sediments. These can be dislodged to some extent by using higher pressure injection or by reversing the flow between injection and production wells. At established operations in the United States, after ISL mining is completed, the quality of the remaining groundwater must be restored to a baseline standard determined before the start of the operation so that any prior uses may be resumed. In contrast to the main U.S. operations, the water quality at the Australian sites is very low to start with, and it is quite unusable. At Beverley the groundwater in the orebody is fairly saline and orders of magnitude too high in radionuclides for any permitted use. At Honeymoon, the water is even more saline and high in sulfates and radium. When
oxygen input and leaching are discontinued, the water quality soon reverts to its original condition. With ISL, no tailings are involved and very little waste is generated. ISL thus has clear environmental advantages in the places it can be applied.
1.4 Milling and Extraction Process Milling refers to the extraction of uranium product from the mined ore. The following is a simplified account. At conventional hard rock mines, the ore must first be crushed. At Olympic Dam and some other underground mines, the first stage is done underground. Primary crushing breaks up the largest pieces followed by secondary crushing to reduce the material to small pieces of about 20 mm and less. At some mines, after primary crushing the broken ore is fed through a radiometric ore sorter so that individual pieces of low-grade material are rejected. The broken ore is then ground in water to produce a slurry of fine ore particles suspended in the water. At McArthur River, the grinding is done underground. Because that is such a high-grade ore, it has to be diluted down to 4% U3O8 at the mill before being treated in order to limit radiation doses to staff at the mill. The U3O8 slurry, effectively a mixture of UO3 and UO2, is leached with sulfuric acid to dissolve the uranium oxides, this being the same process as in acid ISL: UO3 þ 2 Hþ ¼¼4UOþþ 2 þ H2 O 4 UOþþ 2 þ 3 SO4 ¼¼4UO2 ðSO4 Þ3 :
With an oxidant such as hydrogen peroxide or ferric iron present, the UO2 is oxidized to UO2þ þ . With some ores, and notably with ISL in the United States, carbonate leaching is used to form a soluble uranyl tricarbonate ion: UO2(CO3)43 . This can then be precipitated with an alkali (e.g., as sodium or magnesium diuranate). Alkaline leaching or treatment of ores is not undertaken in Australia or Canada. The liquid containing the uranium is filtered and the uranium then separated by an ion exchange process. Two methods are used for concentration and purification of uranium: solid ion exchange (IX) with resin and liquid ion exchange, better known as solvent extraction (SX). Early operations in Australia used ammonium type resins in polystyrene beads for ion exchange, but solvent extraction with amines in kerosene is now predominant.
Uranium Mining, Processing, and Enrichment
With pregnant solution from the ISL production wells, the choice is largely determined by the salinity of the groundwater. SX is better with high salinity, as at Honeymoon (17,000 to 20,000 ppm dissolved solids), while IX is most effective below 3000 ppm chloride, as presently at Beverley. With alkaline leaching, IX is effective to about 3000 ppm dissolved solids, but there are no commercially available resins that work at high chloride levels. In solvent extraction, tertiary amines (‘‘R’’ is an alkyl [hydrocarbon] grouping, with single covalent bond) are used in a kerosene diluent, and the phases move countercurrently: 2R3 N þ H2 SO4 ¼¼¼¼¼4ðR3 NHÞ2 SO4 2ðR3 NHÞ2 SO4 þ UO2 ðSO4 Þ4 3 ¼¼¼¼¼4 ðR3 NHÞ4 UO2 ðSO4 Þ3 þ 2SO2 4 : In the mill at Olympic Dam (which is primarily a copper mine; uranium is the major by-product), the ore is ground and treated in a copper sulphide flotation plant. About 80% of the uranium minerals remain in the tailings from the flotation cells, from which they are recovered by acid leaching. The copper concentrate is also processed through an acid leach to recover the other 20% of the uranium. The pregnant liquor is then separated from the barren tailings, and in the solvent extraction plant the uranium is removed using kerosene with an amine. The loaded solvents may then be treated to remove impurities. First, cations are removed at pH 1.5 using sulfuric acid, then anions are dealt with using injected gaseous ammonia. The solvents are then stripped in a countercurrent process using ammonium sulfate solution: ðR3 NHÞ4 UO2 ðSO4 Þ3 þ 2ðNH4 Þ2 SO4 ¼¼¼¼44R3 N þ ðNH4 Þ4 UO2 ðSO4 Þ3 þ 2H2 SO4 : Yellow ammonium diuranate is then precipitated from the loaded strip solution by adding gaseous ammonia to raise the pH and neutralize the solution (though in earlier operations caustic soda and magnesia were used). 2NH3 þ 2UO2 ðSO4 Þ4 3 ¼¼¼4ðNH4 Þ2 U2 O7 þ 4SO2 4 : The diuranate is then dewatered in a thickener and then by filter or centrifuge before being roasted in a furnace to produce a uranium oxide concentrate, about 99% U3O8, which is the form in which uranium is marketed and exported. This is sometimes known as yellowcake, though it is khaki.
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With IX, such as at Beverley, uranium is stripped from the ion exchange resin and precipitated chemically, usually with hydrogen peroxide. The uranium slurry is dewatered and dried to give hydrated uranium peroxide (UO4.2H2O) product. Either product is then packed into 200-liter steel drums, which are sealed for shipment. The U3O8 is only mildly radioactive and requires no special packaging on that account.
1.5 Wastes from Mining and Milling In most respects, conventional mining of uranium is the same as mining any other metalliferous ore, and well-established environmental constraints apply to avoid any offsite 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 2.2:1. 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 that 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. 1.5.1 Tailings and Radon Solid waste products from the milling operation are tailings. They constitute 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 workedout 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 minimize the emission of radon gas. During the operational life of a mine,
322
Uranium Mining, Processing, and Enrichment
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 2 m 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, totaling about 450 kBq/kg in ore with 0.3% U308 (e.g., 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 (Th234 and Pa-234) soon disappear, leaving the tailings with a little over 70% of the radioactivity 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 radium226 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. 1.5.2 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 that 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. 1.5.3 Rehabilitation 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 mine, the company worked together with government agencies, the Northern Land Council (NLC), and Aboriginal landowners to ensure a high standard of environmental management, culminating in its decommissioning and successful rehabilitation. Apart from groundwater considerations, rehabilitation of ISL mines is 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.
Uranium Mining, Processing, and Enrichment
1.6 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 2002–2003 (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 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 the following: *
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
*
*
*
*
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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 orebody’s radioactivity remains well underground and there is hence minimal increase in radon release and no ore dust.
2. URANIUM REFINING AND CONVERSION Most nuclear reactors require uranium to be enriched from its natural isotopic composition of 0.7% U-235 (most of the rest being U-238) to 3.5 to 4% U-235. The uranium therefore needs to be in a gaseous form, and the most convenient way of achieving this is to convert the uranium oxides to uranium hexafluoride, commonly referred to as hex. Conversion plants are operating commercially in United States, Canada, France, the United Kingdom, and Russia (see Table II). Uranium leaves the mine as the concentrate of a stable oxide known as U3O8 or as a peroxide. It still contains some impurities, and prior to enrichment it has to be further refined before being converted to uranium hexafluoride (UF6). The uranium oxide
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management of a conversion plant requires no special arrangements beyond those for any chemical processing plant involving fluorine chemicals.
TABLE II World Uranium Conversion Capacity in 2001 Tonnes U/yr Canada
10,950
France
14,000
UK
3. URANIUM ENRICHMENT
7200
USA
12,700
Russia
18,700
Source. OECD NEA (2002). Nuclear energy data. Nuclear Engineering International (2002). ‘‘World Nuclear Handbook.’’
concentrate received by the refinery is dissolved in nitric acid. The resulting solution of uranyl nitrate UO2(NO3)2.6H2O is fed into a countercurrent solvent extraction process, using tributyl phosphate dissolved in kerosene or dodecane. The uranium is collected by the organic extractant, from which it can be washed out by dilute nitric acid solution and then concentrated by evaporation. The solution is then calcined (heated strongly) in a fluidized bed reactor to produce UO3. The purified uranium oxide UO3 is then reduced in a kiln by hydrogen to UO2: UO3 þ H2 ¼¼¼4UO2 þ H2 O DH ¼ 109 kJ=mole: This reduced oxide is then reacted in another kiln with gaseous hydrogen fluoride (HF) to form uranium tetrafluoride (UF4), though in some places this is made with aqueous HF by a wet process: UO2 þ 4HF ¼¼¼4UF4 þ 2H2 O DH ¼ 176 kJ=mole: The tetrafluoride is then fed into a fluidized bed reactor with gaseous fluorine to produce uranium hexafluoride, UF6. Hexafluoride (‘‘hex’’) is condensed and stored: UF4 þ F2 ¼¼¼4UF6 : Removal of impurities takes place at several of these steps. The UF6, particularly if moist, is highly corrosive. When warm it is a gas, suitable for use in the enrichment process. At lower temperature and under moderate pressure, the UF6 may be liquefied and the liquid run into specially designed steel shipping cylinders, which are thick walled and weigh up to 15 tonnes when full. As it cools, the liquid UF6 within the cylinder becomes a white crystalline solid and is shipped in this form. The siting and environmental
3.1 General Uranium found in nature consists largely of two isotopes, U-235 and U-238. The production of energy in nuclear reactors is from the fission or splitting of the U-235 atoms, a process that releases energy in the form of heat. U-235 is the main fissile isotope of uranium. Natural uranium contains 0.7% of the U-235 isotope. The remaining 99.3% is mostly the U-238 isotope, which does not contribute directly to the fission process (though it does so indirectly by the formation of fissile isotopes of plutonium). Uranium-235 and U-238 are chemically identical but differ in their physical properties, particularly their mass. The nucleus of the U-235 atom contains 92 protons and 143 neutrons, giving an atomic mass of 235 units. The U-238 nucleus also has 92 protons but has 146 neutrons, three more than U-235, and therefore has a mass of 238 units. The difference in mass between U-235 and U-238 allows the isotopes to be separated and makes it possible to increase or enrich the percentage of U-235. All enrichment processes, directly or indirectly, make use of this small mass difference. Some reactors—for example, the Canadian-designed Candu and the British Magnox reactors—use natural uranium as their fuel. Most present day reactors (Light Water Reactors or LWRs) use enriched uranium, where the proportion of the U235 isotope has been increased from 0.7% to about 3 or up to 4%. (For comparison, uranium used for nuclear weapons would have to be enriched in plants specially designed to produce at least 90% U-235.) A number of enrichment processes have been demonstrated in the laboratory and some on a larger scale, but only two, the gaseous diffusion process and the centrifuge process, are operating on a commercial scale. In both of these, UF6 gas is used as the feed material. Molecules of UF6 with U-235 atoms are about 1% lighter than the rest, and this difference in mass is the basis of both processes. Large commercial enrichment plants are in operation in the United States, the United Kingdom, France, Germany, the Netherlands, and Russia, with smaller plants elsewhere (see Table III).
Uranium Mining, Processing, and Enrichment
3.2 Early Processes
TABLE III World Uranium Enrichment Capacity in 2001
Method
1000 kg SWU/year
France
Diffusion
10,800
Germany, Netherlands, United Kingdom
Centrifuge
5250
Japan
Centrifuge
1050
United States Russia
Diffusion Centrifuge
18,700 19,000
China Pakistan Total
325
Mostly centrifuge Centrifuge
400–800 5 55,000 approx.
Source. OECD NEA (2002). Nuclear energy data. Nuclear Engineering International (2002). ‘‘World Nuclear Handbook.’’
The capacity of enrichment plants is measured in terms of separative work units, or SWU. The SWU is a complex unit that is a function of the amount of uranium processed and the degree to which it is enriched (i.e., the extent of increase in the concentration of the U-235 isotope relative to the remainder). The unit is strictly a Kilogram Separative Work Unit, and it measures the quantity of separative work performed to enrich a given amount of uranium a certain amount. It is thus indicative of energy used in enrichment when feed and product quantities are expressed in kilograms. The unit tonnes SWU is also used. For instance, to produce one kilogram of uranium enriched to 3% U-235 requires 3.8 SWU if the plant is operated at a tails assay 0.25%, or 5.0 SWU if the tails assay is 0.15% (thereby requiring only 5.1 kg instead of 6.0 kg of natural U feed). About 100 to 120,000 SWU is required to enrich the annual fuel loading for a typical 1000 MWe light water reactor. Enrichment costs are related to electrical energy used. A gaseous diffusion plant typically demands 2500 kilowatt hours (9000 MJ) per SWU (kWh/SWU), while modern gas centrifuge plants require only about 50 kWh (180 MJ) per kg SWU. Enrichment accounts for almost half of the cost of nuclear fuel and about 5% of the total cost of the electricity generated. It can also account for the main greenhouse gas impact from the nuclear fuel cycle if the electricity used for enrichment is generated from coal. However, it still typically amounts to only 0.1% of the carbon dioxide from equivalent coal-fired electricity generation if gas centrifuge plants are used, or up to 3% in a worst case situation.
The electromagnetic isotope separation (EMIS) process was developed in the early 1940s in the Manhattan Project to make the highly enriched uranium used in the Hiroshima bomb, but was abandoned soon afterward. However, it reappeared as the main thrust of Iraq’s clandestine uranium enrichment program for weapons discovered in 1992. EMIS uses the same principles as a mass spectrometer (albeit on a much larger scale). Ions of uranium-238 and uranium-235 are separated because they describe arcs of different radii when they move through a magnetic field. Two aerodynamic processes were brought to demonstration stage. One is the jet nozzle process, with demonstration plant built in Brazil, and the other the Helikon vortex tube process developed in South Africa. Neither is in use now. They depend on a high-speed gas stream bearing the UF6 being made to turn through a very small radius, causing a pressure gradient similar to that in a centrifuge. The light fraction can be extracted toward the center and the heavy fraction on the outside. Thousands of stages are required to produce enriched product for a reactor. Both processes are energy-intensive—more than 3000 kWh/SWU.
3.3 Gaseous Diffusion Process Commercial uranium enrichment was first carried out by the diffusion process in the United States. It has since been used in Russia, the United Kingdom, France, China and Argentina as well. Only the United States and France use the process on any significant scale. The remaining large USEC plants in the United States was originally developed for weapons programs and have a capacity of some 19 million kg SWU per year. At Tricastin, in southern France, a more modern diffusion plant with a capacity of 10.8 million kg SWU per year has been operating since 1979. This plant can produce enough 3.7% enriched uranium a year to fuel some 90 1000MWe nuclear reactors. The gaseous diffusion process accounts for over 57% of world enrichment capacity. However, though they have proved durable and reliable, most gaseous diffusion plants are now nearing the end of their design life and the focus is on which enrichment technologies will replace them. The process involves forcing uranium hexafluoride gas under pressure through a series of porous membranes or diaphragms. As U-235 molecules are
326
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lighter than the U-238 molecules they move faster and have a slightly better chance of passing through the pores in the membrane. The UF6 that diffuses through the membrane is thus slightly enriched, while the gas which did not pass through is depleted in U-235. This process is repeated many times in a series of diffusion stages called a cascade. Each stage consists of a compressor, a diffuser, and a heat exchanger to remove the heat of compression. The enriched UF6 product is withdrawn from one end of the cascade and the depleted UF6 is removed at the other end. The gas must be processed through some 1400 stages to obtain a product with a concentration of 3% to 4% U-235.
3.4 Centrifuge Process The gas centrifuge process was first demonstrated in the 1940s but was shelved in favor of the simpler diffusion process. It was then developed and brought on stream in the 1970s as the second-generation enrichment technology. It is economic on a smaller scale (e.g., less than 2 million kg SWU/year, which enables staged development of larger plants). It has been deployed at a commercial level in Russia and in Europe by Urenco, an industrial group formed by British, German and Dutch companies. Russia’s three plants at Seversk, Zelenogorsk and Novouralsk account for some 30% of world capacity. Urenco operates enrichment plants in the United Kingdom, the Netherlands, and Germany and is participating in a proposal for one in the United States. In Japan, JNC and JNFL operate small centrifuge plants, the capacity of JNFL’s at Rokkasho planned to be 1.5 million SWU/year. China also has a small centrifuge plant imported from Russia at Lanzhou, which is expected to reach 0.5 million SWU/year about 2005. Another small plant at Hanzhong is operating and was to reach 0.5 million SWU/year by 2003. Brazil has a small plant that is being developed to 0.2 million SWU/year. Pakistan has developed centrifuge enrichment technology based on Urenco designs, and this appears to have been sold to North Korea. Iran has sophisticated centrifuge technology, which had not been commissioned as of early 2003. Both France and the United States are now considering centrifuge technology to replace their aging diffusion plants, not least because they are more economical to operate. As noted, a centrifuge plant requires as little as 50 kWh/SWU power (Urenco at Capenhurst, United Kingdom, input 62.3 kWh/SWU for the whole plant in 2001–2002, including infrastructure and capital works).
Like the diffusion process, the centrifuge process uses UF6 gas as its feed and makes use of the slight difference in mass between U-235 and U-238. The gas is fed into a series of vacuum tubes, each containing a rotor about 1 to 2 m long and 15 to 20 cm diameter. When the rotors are spun rapidly, at 50,000 to 70,000 rpm, the heavier molecules with U-238 increase in concentration toward the cylinder’s outer edge. There is a corresponding increase in concentration of U-235 molecules near the center. These concentration changes are enhanced by inducing the gas to circulate axially within the cylinder. The enriched gas is drawn off and goes forward to further stages while the depleted UF6 goes back to the previous stage. To obtain efficient separation of the two isotopes, centrifuges rotate at very high speeds, with the outer wall of the spinning cylinder moving at between 400 and 500 m per second to give a million times the acceleration of gravity. Although the capacity of a single centrifuge is much smaller than that of a single diffusion stage, its capability to separate isotopes is much greater. Centrifuge stages normally consist of a large number of centrifuges in parallel. Such stages are then arranged in cascade similarly to those for diffusion. In the centrifuge process, however, the number of stages may only be 10 to 20 instead of a thousand or more for diffusion.
3.5 Laser Processes Laser enrichment processes have been the focus of interest for some time. They are a possible thirdgeneration technology promising lower energy inputs, lower capital costs, and lower tails assays, hence significant economic advantages. None of these processes is yet ready for commercial use, though one is well advanced. Development of the Atomic Vapour Laser Isotope Separation (AVLIS and the French SILVA) began in the 1970s. In 1985, the U.S. government backed it as the new technology to replace its gaseous diffusion plants as they reached the end of their economic lives early in the 21st century. However, after some US$2 billion in R&D, it was abandoned in United States in favor of SILEX, a molecular process. French work on SILVA has now ceased. Atomic vapor processes work on the principle of photo-ionization, whereby a powerful laser is used to ionize particular atoms present in a vapor of uranium metal. (An electron can be ejected from an atom by light of a certain frequency. The laser techniques for uranium use a frequency that is tuned to ionize a
Uranium Mining, Processing, and Enrichment
U-235 atom but not a U-238 atom.) The positively charged U-235 ions are then attracted to a negatively charged plate and collected. Atomic laser techniques may also separate plutonium isotopes. The main molecular processes that have been researched work on a principle of photo-dissociation of UF6 to solid UF5, using tuned laser radiation as noted earlier. Any process using UF6 fits more readily within the conventional fuel cycle than the atomic process. The leading molecular laser process on the world stage is SILEX, an Australian development that also utilizes UF6. In 1996, USEC secured the rights to evaluate and develop SILEX for uranium (it is also usable for silicon and other elements). The SILEX process is now at prototype stage in Sydney and applications to silicon and zirconium are being developed.
Feed, product, and depleted material are all in the form of UF6, though the depleted uranium may be stored long-term as the more stable U3O8. Uranium is only weakly radioactive, and its chemical toxicity—especially as UF6—is more significant than its radiological toxicity. The protective measures required for an enrichment plant are therefore similar to those taken by other chemical industries concerned with the production of fluorinated chemicals. Uranium hexafluoride is a corrosive material, therefore any leakage is particularly undesirable. Hence, *
*
3.6 Enrichment of Reprocessed Uranium In some countries, spent fuel is reprocessed to recover its uranium and plutonium and to reduce the final volume of high-level wastes. The plutonium is normally recycled promptly into mixed-oxide (MOX) fuel by mixing it with depleted uranium. Where uranium recovered from reprocessing spent nuclear fuel is to be reused, it needs to be converted and re-enriched. This is complicated by the presence of impurities and two new isotopes in particular: U232 and U-236, which are formed by neutron capture in the reactor. Both decay much more rapidly than U235 and U-238, and one of the daughter products on U-232 emits very strong gamma radiation, which means that shielding is necessary in the plant. U-236 is a neutron poison, which impedes the chain reaction and means that a higher level of U-235 enrichment is required in the product to compensate. Both isotopes tend to report with the enriched (rather than depleted) output, so reprocessed uranium that is re-enriched for fuel must be segregated from enriched fresh uranium. Both diffusion and centrifuge processes are used for re-enrichment, though contamination issues are more readily managed with the latter. A laser process would theoretically be ideal as it would ignore all but the desired U-235, but this remains to be demonstrated with reprocessed feed.
3.7 Environmental Aspects With the minor exception of reprocessed uranium, enrichment involves only natural, long-lived radioactive materials; there is no formation of fission products or irradiation of materials, as in a reactor.
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*
In most areas of a centrifuge plant the pressure of the UF6 gas is maintained below atmospheric pressure and thus any leakage could only result in an inward flow. Double containment is provided for those few areas where higher pressures are required. Effluent and venting gases are collected and returned to the process.
3.8 After Enrichment The enriched UF6 is converted back to UO2 and made into fuel pellets—ultimately a sintered ceramic—which are encased in metal tubes to form fuel rods, typically up to 4 m long. A number of fuel rods make up a fuel assembly, which is ready to be loaded into the nuclear reactor.
SEE ALSO THE FOLLOWING ARTICLES Coal Mining, Design and Methods of Nuclear Engineering Nuclear Fission Reactors: Boiling Water and Pressurized Water Reactors Radiation, Risks and Health Impacts of Uranium and Thorium Resource Assessment Uranium Mining: Environmental Impact
Further Reading Hore-Lacy, I. (2003). ‘‘Nuclear Electricity,’’ 7th Ed. World Nuclear Association, London and Uranium Information Centre, Melbourne. [Available at http:/ www.world-nuclear.org.] Heriot, I. D. (1988). ‘‘Uranium Enrichment by Centrifuge,’’ Report EUR 11486, Commission of the European Communities, Brussels. Kehoe, R. B. (2002). ‘‘The Enriching Troika, a History of Urenco to the Year 2000.’’ Urenco, Marlow UK.
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Pollock, R., and Rowson, J. (2002). COGEMA Resources’ McClean Lake and other mines. The Nuclear Engineer 43(1), 9–13. Quick, M. (2001). New uranium mines in Canada: McArthur River. The Nuclear Engineer 42(6), 179–183.
Wilson, P. D. (ed.). (1996). ‘‘The Nuclear Fuel Cycle—From Ore to Wastes.’’ Oxford University Press, Oxford, UK. Woodcock, J. T., and Hamilton, J. K. (1993). ‘‘Australasian Mining and Metallurgy, the Sir Maurice Mawby Memorial Volume.’’ Four papers, pp. 1160–1170. Aust IMM, Melbourne.
1. Uranium Mining, Milling, and Processing 2. Uranium Refining and Conversion 3. Uranium Enrichment
other ores, such as the gold-bearing ores of South Africa. In these cases, the concentration of uranium may be as low as a tenth of those in orebodies mined primarily for their uranium content. Olympic Dam quotes ore reserves down to 0.045% U.
Glossary enrichment The physical process of increasing the proportion of U-235 to U-238. isotope An atomic form of an element having a particular number of neutrons. Different isotopes of an element have the same number of protons but different numbers of neutrons and hence different atomic mass (e.g., U-235, U-238). mill Treatment plant producing concentrate from ore. ore Rock containing economic concentrations of minerals. radon An inert but radioactive gas which is a decay product of radium. stope Cavity formed underground by drilling, blasting, and removing ore. tailings Crushed and ground rock from which economic minerals have been recovered. Uranium tailings are radioactive due to decay products that remain.
There are many uranium mines operating around the world, in some 20 countries, though almost threequarters of world production comes from just 10 mines. Most of the uranium ore deposits at present supporting these mines have average grades in excess of 0.10% of uranium. In the first phase of uranium mining to the 1960s, this would have been seen as a respectable grade, but today some Canadian mines have huge amounts of ore up to 20% U average grade. Some uranium is also recovered as a by-product with copper, as at Olympic Dam in Australia, or as by-product from the treatment of
Encyclopedia of Energy, Volume 6. r 2004 Elsevier Inc. All rights reserved.
1. URANIUM MINING, MILLING, AND PROCESSING 1.1 Overview Generally speaking, uranium mining is no different to other kinds of mining unless the ore is of a very high grade. Where orebodies lie close to the surface, they are usually accessed by open cut mining, involving a large pit and the removal of much overburden as well as a lot of waste rock. Where orebodies are deeper, underground mining is employed, involving construction of access tunnels and shafts but with less waste rock removed and less environmental impact. In either case, the grade is usually controlled by measuring radioactivity as a surrogate for uranium concentration. (The radiometric device detects associated radioactive minerals, which are decay products of the uranium rather than the uranium itself.) At Ranger in north Australia, Rossing in Namibia, and most of Canada’s Northern Saskatchewan mines through to McClean Lake, the orebodies have been accessed by open cut mining. Other mines such as Olympic Dam in Australia, McArthur River, Rabbit Lake and Cigar Lake in Northern Saskatchewan, and Akouta in Niger are underground, up to 600 m deep. At McClean Lake, mining will be completed underground. Table I lists the largest-producing uranium mines in the West.
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TABLE I The Largest-Producing Western World Uranium Mines in 2001 Mine
Country
McArthur R.
Canada
Olympic Dam
Australia
Ranger
Australia
Main owner
Type
Production (tU)
Percentage of world
Cameco
Underground
6937
19.4
WMC
By-product/underground
3693
10.3
ERA (Rio Tinto 68%)
Open pit
3564
10.0
McClean L.
Canada
Cogema
Open pit
2512
7.0
Rossing
Namibia
Rio Tinto (69%)
Open pit
2239
6.3
Akouta
Niger
Cogema/Onarem
Underground
2027
5.7
Rabbit Lake
Canada
Cameco
Underground
1755
4.3
Cluff Lake Arlit
Canada Niger
Cogema Cogema/Onarem
Open pit/underground Open pit
1306 1069
3.6 3.0
AngloGold
South Africa
Anglo American
By-product/underground
Top 10 total
Some orebodies lie in groundwater in porous material and may be accessed simply by oxygenating that groundwater and pumping it out—this is in situ leach (ISL) mining. ISL mining means that removal of the uranium minerals is accomplished without any major ground disturbance. Weakly acidified or alkaline groundwater with a lot of oxygen injected into it is circulated through an enclosed underground aquifer, which holds the uranium ore in loose sands. The leaching solution with dissolved uranium is then pumped to the surface treatment plant. Over half of the world’s uranium now comes from underground mines, about 30% from open cut mines and 16% from ISL. Conventional mines have a mill where the ore is crushed, ground, and then leached with sulfuric acid to dissolve the uranium oxides. Most of the ore, however, remains undissolved in the leaching process, and these solids or tailings are then separated from the uranium-rich solution, usually by allowing them to settle out. At the mill of a conventional mine, or the treatment plant of an ISL operation, the uranium is then separated by ion exchange before being dried and packed.
1.2 Hard Rock Mining With open cut mining, the overburden needs to be removed to expose the orebody, the part of the rock that contains economic concentrations of the minerals being sought. This, and subsequent mining, is undertaken by drilling and blasting, followed by removal of the broken rock. At open cut uranium mines, the drill cuttings from mine development may be tested radiometrically so that ore and waste (below ore-grade material) are blasted separately. In
840
2.9
25,942
72.5
addition, loads of mined material are often scanned radiometrically to ascertain average ore grade, and trucks are then directed to stockpiles, primary crusher, or waste dump accordingly. At Canada’s McClean Lake mine, waste is material with less than 0.025% U or less than 0.085% U if it has high acid-generating potential and requires special disposal. Anything over 0.085% U is unequivocally ore, but is stockpiled according to grade so that mill feed may be varied according to market conditions. With underground mining, the objective is to move as little waste rock as possible in sinking shafts or declines and digging tunnels (drives and adits) accessing the orebody as fully as possible. Mining the actual orebody is then generally more selective. Various mining techniques are employed, but open stoping is common. This involves carefully drilling and then blasting a large mass of rock so that the broken material can be removed. When the void is empty, it is usually filled from above with waste rock and coarse tailings material, with some cement and flyash binder, to maintain structural support and enable extraction of adjacent ore. At Olympic Dam, mining is by longhole open stoping, back-filled after ore extraction. This means that a block of ore is drilled from below and above, then blasted so that up to 80,000 tonnes is broken at once. Each stope will yield up to 500,000 tonnes. Ore is extracted from drawpoints at the bottom of the stope and taken to the underground primary crusher. Crushed ore is hoisted to the surface in skips. At McArthur River in Canada, the orebody is so high-grade (up to 20% U) that remote methods need to be employed for mining it because the minerals
Uranium Mining, Processing, and Enrichment
associated with the uranium are also so concentrated that they emit high levels of gamma radiation. It is also about 600 m deep and in wet, porous rock, so control of water under high pressure is another problem. Manned operations are confined to the barren rock surrounding the orebody, and water is controlled by freezing all around the orebody with some grouting. The orebody is than accessed remotely by raise boring, using a 3-m diameter reaming head. Remotely operated front-end loaders remove the ore cuttings that fall from this. Each load is radiometrically scanned, and waste rock or low-grade mineralized material is hoisted to the surface for disposal. While this whole exercise is high cost per tonne of ore removed, the ore is of such a grade that relatively little is involved, about 120 tonnes per day. The low volume also means that the ore cuttings are fed straight to an underground grinding circuit, and the ground slurry is then pumped to the surface. It is then loaded into special transportation tanks and road-hauled 80 km to the mill for further treatment. Meanwhile, the void from the raise boring is filled with concrete to provide ground support for subsequent raise boring operations. At the Cigar Lake underground mine in Canada, poor ground conditions mean that water jet boring will be employed instead of raise boring, with ground freezing. Below ore-grade material with significant mineralization from open cut mines has in the past been heaped up on an impervious pad and leached with sulfuric acid. This heap leaching produces pregnant liquor, which is fed to the mill for uranium extraction. However, the process can give rise to significant problems of rehabilitation, so it is apparently no longer used. At Australia’s Nabarlek mine in the 1980s, about 50% of the uranium recovery was achieved from 150,000 tonnes of heap leach material.
1.3 In Situ Leach (ISL) Mining In situ leaching (ISL), also known as solution mining, involves leaving the ore where it is in the ground and using liquids that are pumped through it to recover the minerals out of the ore by leaching. Consequently there is little surface disturbance and no tailings or waste rock generated. However, the orebody needs to be permeable to the liquids used and located so that they do not contaminate groundwater away from the orebody. Some ISL mining in the past, notably in the eastern bloc, has occurred in broken rock with insecure containment of fluids, and this has resulted in considerable pollution. ISL mining was first tried
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on an experimental basis in Wyoming during the early 1960s. The first commercial mine began operating in 1974. About a dozen projects are licensed to operate in the United States (in Wyoming, Nebraska, and Texas), and most of the operating mines were less than 10 years old in the early 21st century. Most are small, but they supply some 85% of the U.S. uranium production. About 16% of world uranium production is by ISL (including all Kazakhstan and Uzbekistan output). ISL can also be applied to other minerals such as copper and gold. Uranium deposits suitable for ISL occur in permeable sand or sandstones, confined above and below by impermeable strata and below the water table. They may either be flat, or roll front—in cross section, C-shaped deposits within a permeable sedimentary layer. Such deposits were formed by the lateral movement of groundwater bearing oxidized uranium minerals through the aquifer, with precipitation of the minerals occurring when the oxygen content decreased, along extensive oxidation-reduction interfaces. The uranium minerals are usually uraninite (oxide) or coffinite (silicate) coatings on individual sand grains. The ISL process essentially reverses this ore genesis in a much shorter time frame. There are two operating regimes for ISL, determined by the geology and groundwater. If there is significant calcium in the orebody (as limestone or gypsum), alkaline (carbonate) leaching must be used. Otherwise, acid (sulfate) leaching is generally better. Techniques for ISL have evolved to the point where it is a controllable, safe, and environmentally benign method of mining, which can operate under strict environmental controls and which often has cost advantages. The mine consists of well fields, which are progressively established over the orebody as uranium is depleted from sections of the orebody after leaching. Well-field design is on a grid with alternating extraction and injection wells, each of identical design and typical of normal water bores. The spacing between injection wells is about 30 m with each pattern of four having a central extraction well with a submersible electric pump. A series of monitor wells are situated around each mineralized zone to detect any movement of mining fluids outside the mining area. The wells are cased to ensure that liquors only flow to and from the ore zone and do not affect any overlying aquifers. They are pressuretested before use. The submersible pumps initially extract native groundwater from the host aquifer prior to the addition of uranium complexing reagents (acid or alkaline) and an oxidant (hydrogen peroxide or
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Uranium Mining, Processing, and Enrichment
oxygen) before injection into the well field. The leach liquors pass through the ore to oxidize and dissolve the uranium minerals in situ. While uranium production in Australia uses acid leaching of the crushed ore, ISL elsewhere normally uses alkaline leaching agents such as a combination of sodium bicarbonate and carbon dioxide. At Beverley and Honeymoon in South Australia the process is acid leaching, with weak sulfuric acid plus oxygen. The leach solution is at a pH of 2.0 to 3.0, about the same as vinegar. The pregnant solution from the production wells is pumped to the treatment plant where the uranium is recovered (see Section 1.4). Before the process solution depleted of uranium (i.e., barren liquor) is reinjected, it is oxygenated and if necessary recharged with sulfuric acid, or with sodium bicarbonate or carbon dioxide, to maintain its pH. Most of the solution is returned to the injection wells, but a very small flow (about 0.5%) is bled off to maintain a pressure gradient in the well field and this, with some solutions from surface processing, is treated as waste. It contains various dissolved minerals such as radium, arsenic, and iron from the orebody and is reinjected into approved disposal wells in a depleted portion of the orebody. This bleed of process solution ensures that there is a steady flow into the well field from the surrounding aquifer and serves to restrict the flow of mining solutions away from the mining area. In the United States, the production life of an individual ISL well pattern is usually less than 3 years, typically 6 to 10 months. Most of the uranium is recovered during the first 6 months of the operation of those wells. The most successful operations have achieved a total overall recovery of about 80% of the uranium from the ore. Over time, production flows decrease as clay and silt become trapped in the permeable sediments. These can be dislodged to some extent by using higher pressure injection or by reversing the flow between injection and production wells. At established operations in the United States, after ISL mining is completed, the quality of the remaining groundwater must be restored to a baseline standard determined before the start of the operation so that any prior uses may be resumed. In contrast to the main U.S. operations, the water quality at the Australian sites is very low to start with, and it is quite unusable. At Beverley the groundwater in the orebody is fairly saline and orders of magnitude too high in radionuclides for any permitted use. At Honeymoon, the water is even more saline and high in sulfates and radium. When
oxygen input and leaching are discontinued, the water quality soon reverts to its original condition. With ISL, no tailings are involved and very little waste is generated. ISL thus has clear environmental advantages in the places it can be applied.
1.4 Milling and Extraction Process Milling refers to the extraction of uranium product from the mined ore. The following is a simplified account. At conventional hard rock mines, the ore must first be crushed. At Olympic Dam and some other underground mines, the first stage is done underground. Primary crushing breaks up the largest pieces followed by secondary crushing to reduce the material to small pieces of about 20 mm and less. At some mines, after primary crushing the broken ore is fed through a radiometric ore sorter so that individual pieces of low-grade material are rejected. The broken ore is then ground in water to produce a slurry of fine ore particles suspended in the water. At McArthur River, the grinding is done underground. Because that is such a high-grade ore, it has to be diluted down to 4% U3O8 at the mill before being treated in order to limit radiation doses to staff at the mill. The U3O8 slurry, effectively a mixture of UO3 and UO2, is leached with sulfuric acid to dissolve the uranium oxides, this being the same process as in acid ISL: UO3 þ 2 Hþ ¼¼4UOþþ 2 þ H2 O 4 UOþþ 2 þ 3 SO4 ¼¼4UO2 ðSO4 Þ3 :
With an oxidant such as hydrogen peroxide or ferric iron present, the UO2 is oxidized to UO2þ þ . With some ores, and notably with ISL in the United States, carbonate leaching is used to form a soluble uranyl tricarbonate ion: UO2(CO3)43 . This can then be precipitated with an alkali (e.g., as sodium or magnesium diuranate). Alkaline leaching or treatment of ores is not undertaken in Australia or Canada. The liquid containing the uranium is filtered and the uranium then separated by an ion exchange process. Two methods are used for concentration and purification of uranium: solid ion exchange (IX) with resin and liquid ion exchange, better known as solvent extraction (SX). Early operations in Australia used ammonium type resins in polystyrene beads for ion exchange, but solvent extraction with amines in kerosene is now predominant.
Uranium Mining, Processing, and Enrichment
With pregnant solution from the ISL production wells, the choice is largely determined by the salinity of the groundwater. SX is better with high salinity, as at Honeymoon (17,000 to 20,000 ppm dissolved solids), while IX is most effective below 3000 ppm chloride, as presently at Beverley. With alkaline leaching, IX is effective to about 3000 ppm dissolved solids, but there are no commercially available resins that work at high chloride levels. In solvent extraction, tertiary amines (‘‘R’’ is an alkyl [hydrocarbon] grouping, with single covalent bond) are used in a kerosene diluent, and the phases move countercurrently: 2R3 N þ H2 SO4 ¼¼¼¼¼4ðR3 NHÞ2 SO4 2ðR3 NHÞ2 SO4 þ UO2 ðSO4 Þ4 3 ¼¼¼¼¼4 ðR3 NHÞ4 UO2 ðSO4 Þ3 þ 2SO2 4 : In the mill at Olympic Dam (which is primarily a copper mine; uranium is the major by-product), the ore is ground and treated in a copper sulphide flotation plant. About 80% of the uranium minerals remain in the tailings from the flotation cells, from which they are recovered by acid leaching. The copper concentrate is also processed through an acid leach to recover the other 20% of the uranium. The pregnant liquor is then separated from the barren tailings, and in the solvent extraction plant the uranium is removed using kerosene with an amine. The loaded solvents may then be treated to remove impurities. First, cations are removed at pH 1.5 using sulfuric acid, then anions are dealt with using injected gaseous ammonia. The solvents are then stripped in a countercurrent process using ammonium sulfate solution: ðR3 NHÞ4 UO2 ðSO4 Þ3 þ 2ðNH4 Þ2 SO4 ¼¼¼¼44R3 N þ ðNH4 Þ4 UO2 ðSO4 Þ3 þ 2H2 SO4 : Yellow ammonium diuranate is then precipitated from the loaded strip solution by adding gaseous ammonia to raise the pH and neutralize the solution (though in earlier operations caustic soda and magnesia were used). 2NH3 þ 2UO2 ðSO4 Þ4 3 ¼¼¼4ðNH4 Þ2 U2 O7 þ 4SO2 4 : The diuranate is then dewatered in a thickener and then by filter or centrifuge before being roasted in a furnace to produce a uranium oxide concentrate, about 99% U3O8, which is the form in which uranium is marketed and exported. This is sometimes known as yellowcake, though it is khaki.
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With IX, such as at Beverley, uranium is stripped from the ion exchange resin and precipitated chemically, usually with hydrogen peroxide. The uranium slurry is dewatered and dried to give hydrated uranium peroxide (UO4.2H2O) product. Either product is then packed into 200-liter steel drums, which are sealed for shipment. The U3O8 is only mildly radioactive and requires no special packaging on that account.
1.5 Wastes from Mining and Milling In most respects, conventional mining of uranium is the same as mining any other metalliferous ore, and well-established environmental constraints apply to avoid any offsite 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 2.2:1. 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 that 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. 1.5.1 Tailings and Radon Solid waste products from the milling operation are tailings. They constitute 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 workedout 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 minimize the emission of radon gas. During the operational life of a mine,
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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 2 m 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, totaling about 450 kBq/kg in ore with 0.3% U308 (e.g., 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 (Th234 and Pa-234) soon disappear, leaving the tailings with a little over 70% of the radioactivity 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 radium226 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. 1.5.2 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 that 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. 1.5.3 Rehabilitation 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 mine, the company worked together with government agencies, the Northern Land Council (NLC), and Aboriginal landowners to ensure a high standard of environmental management, culminating in its decommissioning and successful rehabilitation. Apart from groundwater considerations, rehabilitation of ISL mines is 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.
Uranium Mining, Processing, and Enrichment
1.6 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 2002–2003 (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 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 the following: *
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
*
*
*
*
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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 orebody’s radioactivity remains well underground and there is hence minimal increase in radon release and no ore dust.
2. URANIUM REFINING AND CONVERSION Most nuclear reactors require uranium to be enriched from its natural isotopic composition of 0.7% U-235 (most of the rest being U-238) to 3.5 to 4% U-235. The uranium therefore needs to be in a gaseous form, and the most convenient way of achieving this is to convert the uranium oxides to uranium hexafluoride, commonly referred to as hex. Conversion plants are operating commercially in United States, Canada, France, the United Kingdom, and Russia (see Table II). Uranium leaves the mine as the concentrate of a stable oxide known as U3O8 or as a peroxide. It still contains some impurities, and prior to enrichment it has to be further refined before being converted to uranium hexafluoride (UF6). The uranium oxide
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management of a conversion plant requires no special arrangements beyond those for any chemical processing plant involving fluorine chemicals.
TABLE II World Uranium Conversion Capacity in 2001 Tonnes U/yr Canada
10,950
France
14,000
UK
3. URANIUM ENRICHMENT
7200
USA
12,700
Russia
18,700
Source. OECD NEA (2002). Nuclear energy data. Nuclear Engineering International (2002). ‘‘World Nuclear Handbook.’’
concentrate received by the refinery is dissolved in nitric acid. The resulting solution of uranyl nitrate UO2(NO3)2.6H2O is fed into a countercurrent solvent extraction process, using tributyl phosphate dissolved in kerosene or dodecane. The uranium is collected by the organic extractant, from which it can be washed out by dilute nitric acid solution and then concentrated by evaporation. The solution is then calcined (heated strongly) in a fluidized bed reactor to produce UO3. The purified uranium oxide UO3 is then reduced in a kiln by hydrogen to UO2: UO3 þ H2 ¼¼¼4UO2 þ H2 O DH ¼ 109 kJ=mole: This reduced oxide is then reacted in another kiln with gaseous hydrogen fluoride (HF) to form uranium tetrafluoride (UF4), though in some places this is made with aqueous HF by a wet process: UO2 þ 4HF ¼¼¼4UF4 þ 2H2 O DH ¼ 176 kJ=mole: The tetrafluoride is then fed into a fluidized bed reactor with gaseous fluorine to produce uranium hexafluoride, UF6. Hexafluoride (‘‘hex’’) is condensed and stored: UF4 þ F2 ¼¼¼4UF6 : Removal of impurities takes place at several of these steps. The UF6, particularly if moist, is highly corrosive. When warm it is a gas, suitable for use in the enrichment process. At lower temperature and under moderate pressure, the UF6 may be liquefied and the liquid run into specially designed steel shipping cylinders, which are thick walled and weigh up to 15 tonnes when full. As it cools, the liquid UF6 within the cylinder becomes a white crystalline solid and is shipped in this form. The siting and environmental
3.1 General Uranium found in nature consists largely of two isotopes, U-235 and U-238. The production of energy in nuclear reactors is from the fission or splitting of the U-235 atoms, a process that releases energy in the form of heat. U-235 is the main fissile isotope of uranium. Natural uranium contains 0.7% of the U-235 isotope. The remaining 99.3% is mostly the U-238 isotope, which does not contribute directly to the fission process (though it does so indirectly by the formation of fissile isotopes of plutonium). Uranium-235 and U-238 are chemically identical but differ in their physical properties, particularly their mass. The nucleus of the U-235 atom contains 92 protons and 143 neutrons, giving an atomic mass of 235 units. The U-238 nucleus also has 92 protons but has 146 neutrons, three more than U-235, and therefore has a mass of 238 units. The difference in mass between U-235 and U-238 allows the isotopes to be separated and makes it possible to increase or enrich the percentage of U-235. All enrichment processes, directly or indirectly, make use of this small mass difference. Some reactors—for example, the Canadian-designed Candu and the British Magnox reactors—use natural uranium as their fuel. Most present day reactors (Light Water Reactors or LWRs) use enriched uranium, where the proportion of the U235 isotope has been increased from 0.7% to about 3 or up to 4%. (For comparison, uranium used for nuclear weapons would have to be enriched in plants specially designed to produce at least 90% U-235.) A number of enrichment processes have been demonstrated in the laboratory and some on a larger scale, but only two, the gaseous diffusion process and the centrifuge process, are operating on a commercial scale. In both of these, UF6 gas is used as the feed material. Molecules of UF6 with U-235 atoms are about 1% lighter than the rest, and this difference in mass is the basis of both processes. Large commercial enrichment plants are in operation in the United States, the United Kingdom, France, Germany, the Netherlands, and Russia, with smaller plants elsewhere (see Table III).
Uranium Mining, Processing, and Enrichment
3.2 Early Processes
TABLE III World Uranium Enrichment Capacity in 2001
Method
1000 kg SWU/year
France
Diffusion
10,800
Germany, Netherlands, United Kingdom
Centrifuge
5250
Japan
Centrifuge
1050
United States Russia
Diffusion Centrifuge
18,700 19,000
China Pakistan Total
325
Mostly centrifuge Centrifuge
400–800 5 55,000 approx.
Source. OECD NEA (2002). Nuclear energy data. Nuclear Engineering International (2002). ‘‘World Nuclear Handbook.’’
The capacity of enrichment plants is measured in terms of separative work units, or SWU. The SWU is a complex unit that is a function of the amount of uranium processed and the degree to which it is enriched (i.e., the extent of increase in the concentration of the U-235 isotope relative to the remainder). The unit is strictly a Kilogram Separative Work Unit, and it measures the quantity of separative work performed to enrich a given amount of uranium a certain amount. It is thus indicative of energy used in enrichment when feed and product quantities are expressed in kilograms. The unit tonnes SWU is also used. For instance, to produce one kilogram of uranium enriched to 3% U-235 requires 3.8 SWU if the plant is operated at a tails assay 0.25%, or 5.0 SWU if the tails assay is 0.15% (thereby requiring only 5.1 kg instead of 6.0 kg of natural U feed). About 100 to 120,000 SWU is required to enrich the annual fuel loading for a typical 1000 MWe light water reactor. Enrichment costs are related to electrical energy used. A gaseous diffusion plant typically demands 2500 kilowatt hours (9000 MJ) per SWU (kWh/SWU), while modern gas centrifuge plants require only about 50 kWh (180 MJ) per kg SWU. Enrichment accounts for almost half of the cost of nuclear fuel and about 5% of the total cost of the electricity generated. It can also account for the main greenhouse gas impact from the nuclear fuel cycle if the electricity used for enrichment is generated from coal. However, it still typically amounts to only 0.1% of the carbon dioxide from equivalent coal-fired electricity generation if gas centrifuge plants are used, or up to 3% in a worst case situation.
The electromagnetic isotope separation (EMIS) process was developed in the early 1940s in the Manhattan Project to make the highly enriched uranium used in the Hiroshima bomb, but was abandoned soon afterward. However, it reappeared as the main thrust of Iraq’s clandestine uranium enrichment program for weapons discovered in 1992. EMIS uses the same principles as a mass spectrometer (albeit on a much larger scale). Ions of uranium-238 and uranium-235 are separated because they describe arcs of different radii when they move through a magnetic field. Two aerodynamic processes were brought to demonstration stage. One is the jet nozzle process, with demonstration plant built in Brazil, and the other the Helikon vortex tube process developed in South Africa. Neither is in use now. They depend on a high-speed gas stream bearing the UF6 being made to turn through a very small radius, causing a pressure gradient similar to that in a centrifuge. The light fraction can be extracted toward the center and the heavy fraction on the outside. Thousands of stages are required to produce enriched product for a reactor. Both processes are energy-intensive—more than 3000 kWh/SWU.
3.3 Gaseous Diffusion Process Commercial uranium enrichment was first carried out by the diffusion process in the United States. It has since been used in Russia, the United Kingdom, France, China and Argentina as well. Only the United States and France use the process on any significant scale. The remaining large USEC plants in the United States was originally developed for weapons programs and have a capacity of some 19 million kg SWU per year. At Tricastin, in southern France, a more modern diffusion plant with a capacity of 10.8 million kg SWU per year has been operating since 1979. This plant can produce enough 3.7% enriched uranium a year to fuel some 90 1000MWe nuclear reactors. The gaseous diffusion process accounts for over 57% of world enrichment capacity. However, though they have proved durable and reliable, most gaseous diffusion plants are now nearing the end of their design life and the focus is on which enrichment technologies will replace them. The process involves forcing uranium hexafluoride gas under pressure through a series of porous membranes or diaphragms. As U-235 molecules are
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lighter than the U-238 molecules they move faster and have a slightly better chance of passing through the pores in the membrane. The UF6 that diffuses through the membrane is thus slightly enriched, while the gas which did not pass through is depleted in U-235. This process is repeated many times in a series of diffusion stages called a cascade. Each stage consists of a compressor, a diffuser, and a heat exchanger to remove the heat of compression. The enriched UF6 product is withdrawn from one end of the cascade and the depleted UF6 is removed at the other end. The gas must be processed through some 1400 stages to obtain a product with a concentration of 3% to 4% U-235.
3.4 Centrifuge Process The gas centrifuge process was first demonstrated in the 1940s but was shelved in favor of the simpler diffusion process. It was then developed and brought on stream in the 1970s as the second-generation enrichment technology. It is economic on a smaller scale (e.g., less than 2 million kg SWU/year, which enables staged development of larger plants). It has been deployed at a commercial level in Russia and in Europe by Urenco, an industrial group formed by British, German and Dutch companies. Russia’s three plants at Seversk, Zelenogorsk and Novouralsk account for some 30% of world capacity. Urenco operates enrichment plants in the United Kingdom, the Netherlands, and Germany and is participating in a proposal for one in the United States. In Japan, JNC and JNFL operate small centrifuge plants, the capacity of JNFL’s at Rokkasho planned to be 1.5 million SWU/year. China also has a small centrifuge plant imported from Russia at Lanzhou, which is expected to reach 0.5 million SWU/year about 2005. Another small plant at Hanzhong is operating and was to reach 0.5 million SWU/year by 2003. Brazil has a small plant that is being developed to 0.2 million SWU/year. Pakistan has developed centrifuge enrichment technology based on Urenco designs, and this appears to have been sold to North Korea. Iran has sophisticated centrifuge technology, which had not been commissioned as of early 2003. Both France and the United States are now considering centrifuge technology to replace their aging diffusion plants, not least because they are more economical to operate. As noted, a centrifuge plant requires as little as 50 kWh/SWU power (Urenco at Capenhurst, United Kingdom, input 62.3 kWh/SWU for the whole plant in 2001–2002, including infrastructure and capital works).
Like the diffusion process, the centrifuge process uses UF6 gas as its feed and makes use of the slight difference in mass between U-235 and U-238. The gas is fed into a series of vacuum tubes, each containing a rotor about 1 to 2 m long and 15 to 20 cm diameter. When the rotors are spun rapidly, at 50,000 to 70,000 rpm, the heavier molecules with U-238 increase in concentration toward the cylinder’s outer edge. There is a corresponding increase in concentration of U-235 molecules near the center. These concentration changes are enhanced by inducing the gas to circulate axially within the cylinder. The enriched gas is drawn off and goes forward to further stages while the depleted UF6 goes back to the previous stage. To obtain efficient separation of the two isotopes, centrifuges rotate at very high speeds, with the outer wall of the spinning cylinder moving at between 400 and 500 m per second to give a million times the acceleration of gravity. Although the capacity of a single centrifuge is much smaller than that of a single diffusion stage, its capability to separate isotopes is much greater. Centrifuge stages normally consist of a large number of centrifuges in parallel. Such stages are then arranged in cascade similarly to those for diffusion. In the centrifuge process, however, the number of stages may only be 10 to 20 instead of a thousand or more for diffusion.
3.5 Laser Processes Laser enrichment processes have been the focus of interest for some time. They are a possible thirdgeneration technology promising lower energy inputs, lower capital costs, and lower tails assays, hence significant economic advantages. None of these processes is yet ready for commercial use, though one is well advanced. Development of the Atomic Vapour Laser Isotope Separation (AVLIS and the French SILVA) began in the 1970s. In 1985, the U.S. government backed it as the new technology to replace its gaseous diffusion plants as they reached the end of their economic lives early in the 21st century. However, after some US$2 billion in R&D, it was abandoned in United States in favor of SILEX, a molecular process. French work on SILVA has now ceased. Atomic vapor processes work on the principle of photo-ionization, whereby a powerful laser is used to ionize particular atoms present in a vapor of uranium metal. (An electron can be ejected from an atom by light of a certain frequency. The laser techniques for uranium use a frequency that is tuned to ionize a
Uranium Mining, Processing, and Enrichment
U-235 atom but not a U-238 atom.) The positively charged U-235 ions are then attracted to a negatively charged plate and collected. Atomic laser techniques may also separate plutonium isotopes. The main molecular processes that have been researched work on a principle of photo-dissociation of UF6 to solid UF5, using tuned laser radiation as noted earlier. Any process using UF6 fits more readily within the conventional fuel cycle than the atomic process. The leading molecular laser process on the world stage is SILEX, an Australian development that also utilizes UF6. In 1996, USEC secured the rights to evaluate and develop SILEX for uranium (it is also usable for silicon and other elements). The SILEX process is now at prototype stage in Sydney and applications to silicon and zirconium are being developed.
Feed, product, and depleted material are all in the form of UF6, though the depleted uranium may be stored long-term as the more stable U3O8. Uranium is only weakly radioactive, and its chemical toxicity—especially as UF6—is more significant than its radiological toxicity. The protective measures required for an enrichment plant are therefore similar to those taken by other chemical industries concerned with the production of fluorinated chemicals. Uranium hexafluoride is a corrosive material, therefore any leakage is particularly undesirable. Hence, *
*
3.6 Enrichment of Reprocessed Uranium In some countries, spent fuel is reprocessed to recover its uranium and plutonium and to reduce the final volume of high-level wastes. The plutonium is normally recycled promptly into mixed-oxide (MOX) fuel by mixing it with depleted uranium. Where uranium recovered from reprocessing spent nuclear fuel is to be reused, it needs to be converted and re-enriched. This is complicated by the presence of impurities and two new isotopes in particular: U232 and U-236, which are formed by neutron capture in the reactor. Both decay much more rapidly than U235 and U-238, and one of the daughter products on U-232 emits very strong gamma radiation, which means that shielding is necessary in the plant. U-236 is a neutron poison, which impedes the chain reaction and means that a higher level of U-235 enrichment is required in the product to compensate. Both isotopes tend to report with the enriched (rather than depleted) output, so reprocessed uranium that is re-enriched for fuel must be segregated from enriched fresh uranium. Both diffusion and centrifuge processes are used for re-enrichment, though contamination issues are more readily managed with the latter. A laser process would theoretically be ideal as it would ignore all but the desired U-235, but this remains to be demonstrated with reprocessed feed.
3.7 Environmental Aspects With the minor exception of reprocessed uranium, enrichment involves only natural, long-lived radioactive materials; there is no formation of fission products or irradiation of materials, as in a reactor.
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*
In most areas of a centrifuge plant the pressure of the UF6 gas is maintained below atmospheric pressure and thus any leakage could only result in an inward flow. Double containment is provided for those few areas where higher pressures are required. Effluent and venting gases are collected and returned to the process.
3.8 After Enrichment The enriched UF6 is converted back to UO2 and made into fuel pellets—ultimately a sintered ceramic—which are encased in metal tubes to form fuel rods, typically up to 4 m long. A number of fuel rods make up a fuel assembly, which is ready to be loaded into the nuclear reactor.
SEE ALSO THE FOLLOWING ARTICLES Coal Mining, Design and Methods of Nuclear Engineering Nuclear Fission Reactors: Boiling Water and Pressurized Water Reactors Radiation, Risks and Health Impacts of Uranium and Thorium Resource Assessment Uranium Mining: Environmental Impact
Further Reading Hore-Lacy, I. (2003). ‘‘Nuclear Electricity,’’ 7th Ed. World Nuclear Association, London and Uranium Information Centre, Melbourne. [Available at http:/ www.world-nuclear.org.] Heriot, I. D. (1988). ‘‘Uranium Enrichment by Centrifuge,’’ Report EUR 11486, Commission of the European Communities, Brussels. Kehoe, R. B. (2002). ‘‘The Enriching Troika, a History of Urenco to the Year 2000.’’ Urenco, Marlow UK.
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Pollock, R., and Rowson, J. (2002). COGEMA Resources’ McClean Lake and other mines. The Nuclear Engineer 43(1), 9–13. Quick, M. (2001). New uranium mines in Canada: McArthur River. The Nuclear Engineer 42(6), 179–183.
Wilson, P. D. (ed.). (1996). ‘‘The Nuclear Fuel Cycle—From Ore to Wastes.’’ Oxford University Press, Oxford, UK. Woodcock, J. T., and Hamilton, J. K. (1993). ‘‘Australasian Mining and Metallurgy, the Sir Maurice Mawby Memorial Volume.’’ Four papers, pp. 1160–1170. Aust IMM, Melbourne.