Presentation on theme: "Uranium Ore Mill Tailings Management Radioactive Waste Management and Disposal NUCP 2311 Lecture Materials contributed by Dr. John Poston."— Presentation transcript:
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.
Uranium Material Production - Manhattan Project
Crustal Abundance of Selected Elements
“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 UO 2 ), 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% U 3 O 8 Recovery rate from ores has ranged from 87% to 97% Total mass of tailings produced is about 1.9 x 10 8 tons Total volume of tailings produced is about 1.2 x 10 8 m 3
Uranium Mines and Mills
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
An Underground Mine
In-Situ Leach Mining
Leach Mining Beverly, Austrilia
A Typical Uranium Mill Operation
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.”
Uranium Mill Product (U 3 O 8 )
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 SiO 2 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% U 3 O 8. 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 SiO 2. May contain complex clay-like silicates of Na, Ca, Mn, Mg, Al, and Fe. May also contain metallic oxides. The concentrations of U 3 O 8 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 +, NH 4+, SO 4 -2, Cl - and PO 4 -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 – CO 3 -2, and HCO 3 – Dissolved solids up to 10 wt% – 200 pCi 226Ra/L – Essentially no 230Th
Uranium Milling In the U.S., 1.2 x 10 8 m 3 of tailings have been accumulated These tailings are distributed among 24 sites About 9 x 10 4 m 3 of tailings are created for each gigawatt year of operation of a nuclear power plant
Department of Energy DOE manages about 32 x 10 6 m 3 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 230 Th. Assume a 0.1% ore body and the extraction of 105 tons of uranium – 1 x 10 8 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/cm 3. Production rate of 222Rn is about 1,000 pCi/m 3 /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/m 2 /s. Emanation rates for non-uranium areas are typically 1-2 pCi/m 2 /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.