The Nuclear Fuel Cycle
Presentation
Components of the Fuel Cycle
Front End
Service Period (conversion of fuel to energy in a reactor)
Back end
Storage (open cycle)
Reprocessing (closed cycle)
Alternatives and Economics
Proliferation Concerns
The Front End of the CycleFor Light Water Reactor Fuel
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 as tin. 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.
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.
In situ leaching (ISL), where oxygenated groundwater is circulated through a very porous ore body to dissolve the uranium and bring it to the surface. ISL may use slightly acidic or alkaline solutions to keep the uranium in solution. The uranium is then 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 primarilyof increased ventilation, are required to protect against airborne radiation exposure.
Uranium Mine in Niger (Sahara Desert)
Uranium Metallurgy
Yellowcake
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
Uranium Global Resources
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
or centrifugation
COMURHEX Pierrelatte, FranceUF4 UF6
EnrichmentThe two method of uranium enrichment are:
Gaseous diffusion (older)
Centrifugation (newer)
Both use small differences in the masses (< 1%) of the U-235F6 and U-238F6 molecules to increase the concentration of U-235.
F6F6
Loading uranium hexafluoride containersGaseous diffusion plantPaducah, Kentucky
Centrifuge EnrichmentFeedEnriched exitDepleted exitU235F6is lighter and collects in the center(enriched)U238F6 is heavier and collects on the outside walls (Depleted/Tails)Feed to Next Stage
The gas centrifuge process has three characteristics that make it economically attractive 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 requirements of 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.
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.
UF6 Gas to UO2 Powder to Pellets
Fuel Pellets
Nuclear Fuel AssemblyFuel Pellet
Basic Pressurized Water Reactor (PWR)FuelRods
Fuel Assemblies are Inserted in Reactor Vessel
U-235Pu-239Pu-240AmountTime in reactorRemoval of fuel elementsfor making weaponsProduction of plutonium in a nuclear reactor92U238 + 0n1 => 94Pu239 + 2(-10)In addition to the fission of U-235 atoms, some U-238 atom absorb neutrons and emit beta particles to become plutoniumFission of Pu produces about 1/3 energy from the reactor
Back End of the Fuel Cycle(Open vs. Closed Cycles)Open CycleClosed Cycle
Composition of Spent fuel Rods from a Light Water ReactorMaterialInitial FuelSpent FuelType of WasteTransuranic elements0.0000.065% TRUU-2360.0000.46%Pu isotopes0.0000.89% TRUFission products0.0000.35%High LevelU-235 3.3%0.08%U-238 96.7%94.3%TUR = transuranic
The actinides are the fifteen elements with atomic numbers 89 to 103. Fission products have shorter half-lives and higher activities.
Actinides have longerhalf-lives and loweractivities
The spent fuel removed from the reactors continues to release heat and is still radioactive. It is, for those reasons, that the fuel is initially stored under water in the spent fuel storage pools.
Spent Fuel Storage Pools
Dry Cask Storage on Reactor Sites
Transport of Spent Fuel
Solidifying high-level waste in borosilicate glass for long term storage in a repository
Reprocessing Closed Fuel CycleRecovers of uranium and plutonium from spent fuel
Reduces volume and radioactivity of waste
France, the UK, Japan, and Russia currently reprocess spent fuel
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 lightwater 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 fromdismantled nuclear weapons in the US and Russia.
Fuel Reprocessing Plant, Marcoule, France
Relative Costs
Process$/kg fuelUranium$500Conversion$50Enrichment$600Fabrication$250Wet storageIncluded in capital & O%MDry cast storage$200Geological storage$400Total Cost$2000
Proliferationof Nuclear Materials and Weapons
HEUPu-239
Iranian Nuclear Complex
Yongbyon Site
Presentation
Background
Components of the Fuel Cycle
Front End
Service Period (conversion of fuel to energy)
Back end
Open (Storage)
Closed (Reprocessing)
Alternatives and Economics
Proliferation Concerns
Three Useful Educational Resources
The Alsos Digital Library for Nuclear Issues
Nuclear Chemistry in the Community
Concept Map for Nuclear Power
Concept Map for Civilian and Military Uses of Nuclear Energyhttp://www.chemcases.com/nuclear/index2.html#concept
http://alsos.wlu.edu/
http://alsosconceptmap.wlu.edu/nuclearpower/main/index.html
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