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Transcript of Fuel Cycle July 2012 New
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The Nuclear Fuel Cycle
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Uranium
URANIUM is a slightly radioactive metal that occurs throughout the earth's
crust.
It is about 500 times more abundant than gold and about as common astin.
It is present in most rocks and soils as well as in many rivers and in sea
water.
Most of the radioactivity associated with uranium in nature is due to other
materials derived from it by radioactive decay processes, and which are
left behind in mining and milling.
Economically feasible deposits of the ore, pitchblende, U3O8, range from
0.1% to 20% U3O8.
http://www.world-nuclear.org/info/inf14.html -
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Uranium Mining
Open pit mining is used where deposits are close to the surface
Underground mining is used for deep deposits, typically greater than 120m deep.
Insituleaching (ISL), where oxygenated groundwater is circulated through a veryporous ore body to dissolve the uranium and bring it to the surface. ISL may useslightly acidic or alkaline solutions to keep the uranium in solution. The uranium isthen recovered from the solution.
The decision as to which mining method to use for a particular deposit is governedby the nature of the ore body, safety and economic considerations.
In the case of underground uranium mines, special precautions, consisting primarily
ofincreased ventilation, are required to protect against airborne radiation exposure.
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Uranium Mine in Niger (Sahara Desert)
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Uranium Metallurgy
Yellowcake
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Yellowcake
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More than 200 pounds of byproduct material, tailings, are typically produced for
each pound of uranium.
After extraction of uranium from the ore, the tailings contain much of their
original radioactivity.
Toxic heavy metals, including chromium, lead, molybdenum, and vanadium, are also
present in this byproduct material in low, but significant, concentrations
Tailings from Uranium Mining and Milling
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Enriching Uranium for Reactor Fuel
Increase the concentration of fissionable U-235 isotope
Enrichment requires a physical process since
U-235 and U-238 have the same chemical properties
Physical processes require gases for separation
Uranium and its oxides are solids
Must convert uranium to UF6
Enriched UF6 must be converted back to solid
uranium or uranium oxide
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or centrifugation
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Enrichment
The two method of uranium enrichment are:
Gaseous diffusion (older)
Centrifugation (newer)
Both use small differences in the masses (< 1%) of the U-235F6 andU-238F6 molecules to increase the concentration of U-235.
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F6
F6
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Centrifuge Enrichment
FeedEnriched
exitDepleted
exit
U235F6is lighter andcollects in the center
(enriched)
U238F6 is heavier and
collects on the outsidewalls
(Depleted/Tails)
Feed toNext Stage
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The gas centrifuge process has three characteristics that make it economicallyattractive for uranium enrichment:
Proven technology: Centrifuge is a proven enrichment process, currently usedin several countries.
Low operating costs: Its energy requirements are less than 5% of the requirementsof a comparably sized gaseous diffusion plant.
Modular architecture: The modularity of the centrifuge technology allows for flexible
deployment, enabling capacity to be added in increments as demand increases.
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Fuel Fabrication
Reactor fuel is generally in the form of ceramic pellets.
These are formed from pressed uranium oxide which is sintered (baked)at a high temperature (over 1400C).
The pellets are then encased in metal tubes to form fuel rods, which are
arranged into a fuel assembly ready for introduction into a reactor.
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UF6 Gas to UO2 Powder to Pellets
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Fuel Pellets
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Nuclear Fuel Assembly
Fuel
Pellet
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Reprocessing: A Solution?
Reprocessing: The chemical separation ofspent nuclear fuel into its major components.
Wikipedia.org
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Reprocessing
Objectives
Recover U, Pu and Th to be used as fuel
Separate radioactive and neutron- absorbing
fission products
Convert the radioactive waste into suitable forms
for safe storage
The US does not have reprocessing nor a long
term storage facility.
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Lawrence Livermore National Laboratory
Reprocessing
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Products of Reprocessing
Uranium .6% U-235 and 99.4% U-238 Plutonium Minor Actinides
Americium
Major long-term heat source Neptunium
Major source of radiation
Curium
Fission Products
Strontium-90, Cesium-137 Generate large amounts of heat for the first 50-80 years after disposal Removal from the repository would reduce the amount of space needed
Iodine-129, Technetium-99 Mobile isotopes that can easily travel through geological formations
Major contributors of radiation to biosphere
U.S. DOE
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Three Nuclear Fuel Cycles
Once through thermal reactors
(current approach)
Recovery of plutonium for one additional
pass through thermal reactors
Repeated reprocessing and recycling ofuranium, plutonium, and other
transuranics in fast-neutron reactors
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Why Reprocess and Recycle?
Original reason (1940s-1970s):
Believed demand for nuclear power would be far greater than it
turned out to be Believed world uranium reserves were far smaller than they
turned out to be
Current reason:
Reduce amount of nuclear waste
Given the uncertainties surrounding the Yucca Mountainlicense application process
Additional uranium energy secondary
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Spent Fuel Storage Pools
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Reprocessing: Methods/Techniques
PUREX: Plutonium and Uranium Extraction Most widely used method
Results in a pure stream of plutonium
UREX: Uranium Reduction Extraction
Replacement for PUREX Results in pure uranium stream
The plutonium remains mixed with the fissionproducts and minor actinides
UREX+ Refinement of the UREX process
Pyroprocessing Reduces the liquid waste that remains in the UREX
process
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Reprocessing: Then what?
Storage in a repository, or . . .
. . . Waste Recycling
Definition: Transmuting or destroying the separatedwaste products of reprocessing. Transforms the long-lived radionuclides into short-lived ones. 12
Reprocessing alone is not sufficient to reduce thevolume and toxicity of used fuel, ensure adequatesupplies of uranium, and achieve proliferation resistance.
A transmutation program could transform the problem oflong-term isolation in a geological repository (for 10s ofthousands of years) to a less difficult problem of storagefor several decades or a few hundred years.
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Methods of Recycling
Transmutation involves inducing nuclear
reactions in some form of non-traditional
reactor.
Fast reactors
Breeder Reactors
Burner reactors
Fast Neutron Reactors
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Results of Recycling
Waste products:
No transmutation scheme is able to destroy
all of the components of spent nuclear fuel.
Most will require multiple passes through thereactor to recycle a significant amount.
Some of the components, although reduced
by volume, will be converted to moreradioactive forms.
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Benefits of Reprocessing andRecycling
The ability to reduce the volume and toxicity
of nuclear waste
A smaller, simpler repository Extension of time before a second repository is
necessary
Closed fuel cycle consistent uraniumsupply
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Costs of Reprocessing
Plutonium Stockpiles/WeaponsProliferation
Environmental and health harms
Terrorism Transportation of high level waste
Reactor safety, worker health
Economic cost
New regulatory schemes
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Mixed Oxide Fuel (MOX)
MOX is produced from the output of reprocessing plants and is a mixture of
plutonium and uranium oxides with a composition of 3% to 7% PuO2 and the rest
UO2
. The MOX is then mixed with ordinary LEU uranium-oxide fuel for use in light
water reactors. Mixture is 1/3 MOX and 2/3 LEU.
By 2001, over 20 power reactors in France were using MOX for one third of their
fuel In the US, MOX fuel is being used as a means of disposing of Pu from
dismantled nuclear weapons in the US and Russia.
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Fuel Reprocessing Plant, Marcoule, France
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Relative Costs
Process $/kg fuel
Uranium $500
Conversion $50
Enrichment $600
Fabrication $250
Wet storage Included in capital & O%M
Dry cast storage $200
Geological storage $400
Total Cost $2000