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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

Uranium Ore Mill Tailings Management

Radioactive Waste Management and Disposal

NUCP 2311

Lecture Materials contributed by Dr. John Poston.

objectives
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
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
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
“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 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
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
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 ores1
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
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
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
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
Uranium Mining Techniques
  • Open pit mines
  • Hard-rock mining
  • In situ leaching
leach mining
Leach Mining

Beverly, Austrilia

milling
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 milling1
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
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
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
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
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 milling2
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
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
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
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
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
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 radon1
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
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
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
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.