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Uranium Ore Mill Tailings Management. Radioactive Waste Management and Disposal NUCP 2311 Lecture Materials contributed by Dr. John Poston. Objectives. Review general approaches to uranium mining. Provide a general overview of mill tailings management.
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Uranium Ore Mill Tailings Management Radioactive Waste Management and Disposal NUCP 2311 Lecture Materials contributed by Dr. John Poston.
Objectives • Review general approaches to uranium mining. • Provide a general overview of mill tailings management. • Provide some general understanding of the constituents of mill tailings.
Uranium Mill Tailings • The residual wastes from milled ore after the uranium has been extracted. • May result from an acid leach process or an alkaline leach process. • Mills in the U.S. are designed to use the acid leach process. • Tailings consist of slurries of sands and clay-like particles (called “slimes”).
Uranium Milling • The starting point in the nuclear fuel cycle whether U-cycle or Th-cycle. • Uranium is ubiquitous at low concentrations. • In the past, uranium was a waste product in mining for other materials. • Uranium ores are normally classified as high, medium, and low grade.
“Fun Facts” • Uranium is a common substance which is found throughout the earth’s crust. As it is present virtually everywhere, it contributes to what is called natural background radiation. • A typical backyard (in Canada), with dimensions of 10 metres by 10 metres (about 33 feet by 33 feet) and a soil depth of one metre (slightly more than three feet), contains approximately 300 grams (0.7 pounds) of uranium. • There are only a few places in the world where major uranium deposits have been found and where it is mined—Australia, Canada, Kazakhstan, Namibia, and the United States.
Uranium Exploration • Uranium is one of the more common elements in the earth’s crust • it is more common than tin • ~40 times more common than silver • ~500 times more common than gold. • More than 200 minerals which contain uranium • Uraninite is the most common. • Uranium concentrations vary from substance to substance and place to place. • Photo: A geologist examines core samples for uranium at an exploration site in northern Saskatchewan.
High Grade Ores • Contain a few percent of uranium (1-4%), in unusual cases, up to 10% • Typically in the form of uraninite (largely UO2), or pitchblende. • These ores are found primarily in central Africa (Zaire) and in Canada (Big Bear Lake). Pitchblende Sample
Medium Grade Ores • Contain 0.1 to 1.0% uranium • Found on the Colorado plateau region (Colorado, Utah, New Mexico and Arizona), also found in California, Nevada, Texas, and Washington • Found in Canada, Australia, and Czechoslovakia • Typically carnotite, thorianite, phosphates, and carbonates
Medium Grade Ores • Assume a typical ore with a concentration of 0.25% • Concentration = 0.0021 g U/g ore, which means ~ 0.0021 g 238U/g ore • All daughters in equilibrium with both the 238U and 235U • Activity = 26 Bq/g ore • Activity = 700 pCi/g ore
Typical 0.1 % Ore (low grade) • 1000 tons of ore • Uranium content – 1 ton • U-238 activity – 0.3 Ci • Ra-226 activity – 0.3 Ci • Ra-226 mass – 0.3 g • Found in many parts of the world. • Have found some commercial feasibility in recent years. • However, generally ores less than 0.1% may not be processed efficiently.
U.S. Uranium Processing • Since 1978, concentration in ores has ranged from 0.112% to 0.531% U3O8 • Recovery rate from ores has ranged from 87% to 97% • Total mass of tailings produced is about 1.9 x 108 tons • Total volume of tailings produced is about 1.2 x 108 m3
Uranium Mining and Milling • At the end of 1996, there were no uranium mills in the U.S. that were operational. • Six mills have been put into a standby status. • Twenty-one mills were either decommissioned or scheduled for decommissioning. • Primary source of tailings is activities called “nonconventional” production .
Uranium Mining Techniques • Open pit mines • Hard-rock mining • In situ leaching
Leach Mining Beverly, Austrilia
Milling • General Approach • Grind to powder to increase surface area (Comminution) • Oxidize and dissolve U-oxide in acid (Leaching) [U(IV) to U(VI)] • Separate liquids and solids and concentrate U solution (multiple steps) • Reduce with a strong base and precipitate (precipitation) [U(VI) to U(IV)] • Dry Product for further processing.
Uranium Milling • Many mined ores in the U.S. are in the range of 0.04 to 0.42%. • About 2 kg (~5 lbs.) of uranium is obtained from a ton of ore. • The remainder is called “tailings” or “mill tailings.” • Transferred as a slurry into “tailings ponds.”
Waste Characteristics • Dry weight of tailings about equal to dry weight of the ore processed. • Dry tailings contain 70 to 80 wt% sand-sized particles and 20 to 30 wt% finer-sized particles. • Waste liquid accompanying tailings to ponds is about 1.5 times the weight of the processed ore.
Characteristics of Sands • Particle size range is 75 to 500 m. • Typically SiO2 • Contains <1 wt% complex silicates of Al, Fe, Mg, Ca, Na, K, Se, Mn, Ni, Mo, Zn, U and V. • May also contain metallic oxides. • Contains 0.004 to 0.01 wt% U3O8. • Contains 26 to 100 pCi 226Ra/g and 70 to 600 pCi 230Th/g.
Characteristics of Slimes • Particle size range is 45 to 75 m. • Contains small amounts of SiO2. • May contain complex clay-like silicates of Na, Ca, Mn, Mg, Al, and Fe. • May also contain metallic oxides. • The concentrations of U3O8 and 226Ra are twice that in the sands. • Contains 150 to 400 pCi 226Ra/g and 70 to 600 pCi 230Th/g.
Characteristics of Liquids • Acid leaching • pH from 1.2 to 2.0 • Na+, NH4+, SO4-2, Cl- and PO4-3 • Dissolved solids up to 1 wt% • 20 to 7,500 pCi 226Ra/L • 2,000 to 22,000 pCi 230Th/L • Alkaline leaching • pH from 10 to 10.5 • CO3-2, and HCO3 • Dissolved solids up to 10 wt% • 200 pCi 226Ra/L • Essentially no 230Th
Uranium Milling • In the U.S., 1.2 x 108 m3 of tailings have been accumulated • These tailings are distributed among 24 sites • About 9 x 104 m3 of tailings are created for each gigawatt year of operation of a nuclear power plant
Department of Energy • DOE manages about 32 x 106 m3 of material • Called 11e(2) byproduct material • 65% nuclear weapons, 27% supporting NNPP, and 8% other activities
Mill Tailings • Contains almost all of the 226Ra and 230Th. • Assume a 0.1% ore body and the extraction of 105 tons of uranium • 1 x 108 tons of mill tailings • 30,000 Ci of 226Ra • 30,000 Ci of 230Th • This situation existed before the processing as well.
Tailings Hazards • Only 90-95% uranium extracted (50 pCi/g remain). • Contains all the daughter radionuclides in the decay chain. • Activity can exceed 1000 pCi/g. • Radon gas is released from the pile. • May contain low concentrations of toxic heavy metals (e.g., Cr, Pb, Mo, and V).
Mill Tailings - Hazards • Leaching resulting in contamination of surface and ground water. • Blowing (dispersal) of tailings material. • Radiation exposure. • Radon emanation. • Human intervention.
Mill Tailings - Radon • Assume typical concentration of 300 pCi/g of 226Ra. • Emanation rate is 0.0006 pCi/g/sec of 222Rn. • Typical density of tailings pile is 1.6 g/cm3. • Production rate of 222Rn is about 1,000 pCi/m3/s • Only 222Rn is the first meter or so will be released to the atmosphere – rule of thumb is that about one-fourth of the volume will be released. • Actual emanation rate of 222Rn is about 250 pCi/m2/s. • Emanation rates for non-uranium areas are typically 1-2 pCi/m2/s. • Dilution of factors of 200-300 within a few hundred meters downwind.
Mill Tailings - Radon • During operation, tailings covered by water. • Results in a factor of 25 reduction in emanation rate. • After operation, tailings dry out – emanation increases. • Usually cover piles with rocks and soil and plant grasses to stabilize the piles. • Results in factor of 4 reduction in emanation rate.
Control of Tailings • Stabilization of piles • Return to mine shaft • Cover the exposed piles • Keeping the tailings above the groundwater levels. • Restricting public access to piles.
Mill Tailings – External Exposure • 1 m above pile are less than 1 mrem/h – typical values in the range of 0.5 mrem/h. • Hot spots may exist, however – dose rates of 20 mrem/h have been measured. • Reaches background levels within 50 m of the pile. • 60 cm of packed earth very effective.
Long-Term Management • Stabilize piles against wind and water erosion. • Control public access to piles. • Restrict habitation in controlled area. • Restrict and/or control removal of materials. • Maintain tailings dams to reduce runoff. • Mill tailings contain radionuclides from the series chains that have existed in equilibrium with uranium. • These tailings must be managed as radioactive waste.
