Transcript 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
• 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 claylike 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
Fuel Cycle
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.
Uranium-Bearing Minerals
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)
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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 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
Uranium
Mining
Uranium
Mining
In-Situ Leach Mining
Leach Mining
Beverly, Austrilia
Leach Mining
A Typical Uranium Mill Operation
Milling
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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 (U3O8)
Uranium
Conversion
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
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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
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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
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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
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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.
Questions
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