Presentation on theme: "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."— Presentation transcript:
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 Cycle For 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, U 3 O 8, range from 0.1% to 20% U 3 O 8.
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 governed by the nature of the ore body, safety and economic considerations. In the case of underground uranium mines, special precautions, consisting primarily of increased ventilation, are required to protect against airborne radiation exposure.
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
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 UF 6 Enriched UF 6 must be converted back to solid uranium or uranium oxide
Enrichment The two method of uranium enrichment are: Gaseous diffusion (older) Centrifugation (newer) Both use small differences in the masses (< 1%) of the U-235F 6 and U-238F 6 molecules to increase the concentration of U-235.
Centrifuge Enrichment Feed Enriched exit Depleted exit U235F 6 is lighter and collects in the center (enriched) U238F 6 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 used in 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 1400°C). 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.
U-235 Pu-239 Pu-240 Amount Time in reactor Removal of fuel elements for making weapons Production of plutonium in a nuclear reactor 92 U 238 + 0 n 1 => 94 Pu 239 + 2( -1 β 0 ) In addition to the fission of U-235 atoms, some U-238 atom absorb neutrons and emit beta particles to become plutonium Fission of Pu produces about 1/3 energy from the reactor
Back End of the Fuel Cycle (Open vs. Closed Cycles) Open Cycle Closed Cycle
Composition of Spent fuel Rods from a Light Water Reactor MaterialInitial FuelSpent FuelType of Waste Transuranic elements0.0000.065% TRU U-2360.0000.46% Pu isotopes0.0000.89% TRU Fission products0.0000.35%High Level U-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 longer half-lives and lower activities
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% PuO 2 and the rest UO 2. 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.
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 Energy http://www.chemcases.com/nuclear/index2.html#concept