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Nuclear power


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Nuclear power

  1. 1. Electricity from Nuclear
  2. 2. Advantages of Nuclear Power <ul><li>Nuclear electricity is reliable and relatively cheap (with an average generating cost of 2.9 cents per kW/h) once the reactor is in place and operating. </li></ul><ul><li>Large reserves of Uranium in United States - Fuel for nuclear power plants will not run out for tens of thousands of years </li></ul><ul><li>Nuclear power plants contribute no greenhouse gasses and few atmospheric pollutants </li></ul>
  3. 3. Disadvantages of Nuclear Power <ul><li>Uranium is ultimately a nonrenewable resource. </li></ul><ul><li>Nuclear power plants are extremely costly to build. </li></ul><ul><li>The slight possibility that nuclear power plants can have catastrophic failures. </li></ul><ul><li>Large environmental impact during the mining and processing stages of uranium are numerous. </li></ul><ul><li>Nuclear waste (Spent nuclear fuel) is extremely hazardous and must be stored safely for thousands of years. </li></ul>
  4. 4. Comparing Uranium to Coal <ul><li>*1 kg of uranium-235 will generate as much energy as 3,000 tons of coal without CO 2 emissions </li></ul>
  5. 5. Comparing Uranium to Coal
  6. 6. Coal Power Plant Environmental Concerns
  7. 7. Nuclear Power Plant Environmental Concerns
  8. 8. Nuclear Power in the U.S. <ul><li>The U.S. has 104 nuclear power plants (more than any other country) </li></ul><ul><li>Combined, these reactors produce about 100 billion watts of electricity (20% of the total) </li></ul>
  9. 9. Nuclear Power in the U.S. <ul><li>No new reactors have been completed in the U.S. since 1979. </li></ul><ul><li>Why??? </li></ul><ul><ul><li>High Operating Costs </li></ul></ul><ul><ul><li>Liability Problems </li></ul></ul><ul><ul><li>Wastes </li></ul></ul><ul><ul><li>Health Concerns </li></ul></ul><ul><ul><li>Terrorism </li></ul></ul><ul><ul><li>Public Misconceptions </li></ul></ul>
  10. 10. Illinois Nuclear Power <ul><li>Nuclear power typically accounts for nearly one-half of Illinois electricity generation and more than one-tenth of all the nuclear power generated in the United States. </li></ul><ul><li>With 11 operating reactors at six nuclear power plants, Illinois ranks first among the States in nuclear generation. </li></ul><ul><li>The growth of the Illinois nuclear industry is due largely to State government initiatives, which began encouraging nuclear power development in the 1950s </li></ul>
  11. 11. Illinois Nuclear Power <ul><li>The origin of all of the commercial and military nuclear industries in the world can be traced back to December 2, 1942 at the University of Chicago. </li></ul><ul><li>On that day, a team of scientists under Dr. Enrico Fermi initiated the first controlled nuclear chain reaction. </li></ul>
  12. 12. Illinois Nuclear Power <ul><li>There are 6 operating nuclear power plants in Illinois: Braidwood, Byron, Clinton, Dresden, LaSalle, and Quad Cities. </li></ul><ul><li>With the sole exception of the single-unit Clinton plant, each of these facilities has two reactors. </li></ul><ul><li>The two reactors at Braidwood and both reactors at Byron are pressurized light water reactors (PWR). </li></ul><ul><li>The reactors at the other facilities (Clinton, Dresden, LaSalle, and Quad Cities are boiling water reactors (BWR). </li></ul>
  13. 13. Fission and Fusion <ul><li>Fission: A heavy nucleus is split into smaller nuclei, releasing energy </li></ul><ul><li>Fusion: Two light nuclei fuse into a heavy nucleus, releasing energy </li></ul><ul><ul><li>Fusion generates much more energy than fission </li></ul></ul><ul><ul><li>Fusion of hydrogen into helium is what provides power to the sun (and thus to the Earth) </li></ul></ul>
  14. 14. Energy from Fission <ul><li>When a neutron strikes a uranium-235 nucleus, it splits into two nuclei, releasing energy </li></ul><ul><li>Example: n + 235 U -> 93 Kr + 141 Ba + 2n </li></ul><ul><li>One neutron goes in, two neutrons come out </li></ul>
  15. 15. Chain Reactions <ul><li>If at least one neutron from each fission strikes another nucleus, a chain reaction will result </li></ul><ul><li>An assembly of uranium-235 that sustains a chain reaction is critical </li></ul>
  16. 16. Power Plants and Bombs <ul><li>Natural uranium contains 99.3% uranium-238 and 0.7% uranium-235 </li></ul><ul><li>Only uranium-235 can create fission chain reactions </li></ul><ul><ul><li>Nuclear power plants use uranium that has been enriched to 3% uranium-235 </li></ul></ul><ul><ul><li>Nuclear bombs use uranium that has been enriched to 95% uranium-235 </li></ul></ul><ul><li>3% enriched uranium can never explode! </li></ul>
  17. 17. Nuclear Bombs <ul><li>Since neutrons initiate fission, and each fission creates more neutrons, there is potential for a chain reaction </li></ul>
  18. 18. Nuclear Bombs <ul><li>Have to have enough fissile material around to intercept liberated neutrons </li></ul><ul><li>Critical mass for 235 U is about 15 kg, for 239 Pu it’s about 5 kg </li></ul><ul><li>Bomb is dirt-simple: separate two sub-critical masses and just put them next to each other when you want them to explode! </li></ul><ul><ul><li>difficulty is in enriching natural uranium to mostly 235 U </li></ul></ul>
  19. 19. Nuclear Fuel Cycle
  20. 20. Mining and Milling <ul><li>Uranium is usually mined by either surface (open cut) or underground mining techniques, depending on the depth at which the ore body is found. </li></ul><ul><li>From these, the mined uranium ore is sent to a mill which is usually located close to the mine. </li></ul>
  21. 21. Mining and Milling <ul><li>At the mill the ore is crushed and ground to a fine slurry which is leached in sulfuric acid to allow the separation of uranium from the waste rock. </li></ul><ul><li>It is then recovered from solution and precipitated as uranium oxide (U 3 0 8 ) concentrate. </li></ul><ul><ul><li>Sometimes this is known as &quot;yellowcake“ </li></ul></ul>
  22. 22. Conversion <ul><li>Because uranium needs to be in the form of a gas before it can be enriched, the U 3 0 8 is converted into the gas uranium hexafluoride (UF 6 ) at a conversion plant. </li></ul>
  23. 23. Enriching <ul><li>Need to enrich uranium to at least 3% U-235 for a power plant </li></ul><ul><li>Gaseous Diffusion Method </li></ul><ul><ul><li>UF 6 (hexafluoride) gas heated </li></ul></ul><ul><ul><li>U-238 is heavier than U-235 </li></ul></ul><ul><ul><li>Hexafluoride Gas can be separated into two streams </li></ul></ul><ul><ul><ul><li>Low velocity U-238 </li></ul></ul></ul><ul><ul><ul><li>High Velocity U-235 </li></ul></ul></ul><ul><li>Centrifuge Method </li></ul><ul><ul><li>Gas spun in centrifuge </li></ul></ul><ul><ul><li>Lighter U-235 will separate from heavier U-238 </li></ul></ul>
  24. 24. Fuel Conversion <ul><li>Enriched Uranium transported to a fuel fabrication plant where it is converted to uranium dioxide (UO 2 ) powder and pressed into small pellets. </li></ul><ul><li>These pellets are inserted into thin tubes, usually of a zirconium alloy (zircalloy) or stainless steel, to form fuel rods. </li></ul><ul><li>The rods are then sealed and assembled in clusters to form fuel assemblies for use in the core of the nuclear reactor. </li></ul>
  25. 25. Fuel Packaging in the Core <ul><li>Rods contain uranium enriched to ~3% 235 U </li></ul><ul><li>Need roughly 100 tons per year for a 1 GW plant </li></ul><ul><li>Uranium stays in three years, 1/3 of the fuel rods are cycled annually </li></ul>
  26. 26. The Reactor Core <ul><li>The reactor core consists of fuel rods and control rods </li></ul><ul><ul><li>Fuel rods contain enriched uranium </li></ul></ul><ul><ul><li>Control rods are inserted between the fuel rods to absorb neutrons and slow the chain reaction </li></ul></ul><ul><li>Control rods are made of cadmium or boron, which absorb neutrons effectively </li></ul>
  27. 27. Moderators <ul><li>Neutrons produced during fission in the core are moving too fast to cause a chain reaction </li></ul><ul><ul><li>Note: This is not an issue with a bomb, where fissile uranium is so tightly packed that fast moving neutrons can still do the job. </li></ul></ul><ul><li>A moderator is required to slow down the neutrons </li></ul><ul><li>In Nuclear Power Plants water (light or heavy) or graphite acts as the moderator </li></ul><ul><ul><li>Graphite increases efficiency but can be unstable (Chernobyl) </li></ul></ul>
  28. 28. Light vs. Heavy Water <ul><li>99.99% of water molecules contain normal hydrogen (i.e. with a single proton in the nucleus) </li></ul><ul><li>Water can be specially prepared so that the molecules contain deuterium (i.e. hydrogen with a proton and a neutron in the nucleus) </li></ul><ul><li>Normal water is called light water while water containing deuterium is called heavy water </li></ul><ul><li>Heavy water is a much better moderator but is very expensive to make </li></ul>
  29. 29. Boiling Water Reactor (BWR) <ul><li>Heat generated in the core is used to generated steam through a heat exchanger </li></ul><ul><li>The steam runs a turbine just like a normal power plant </li></ul>
  30. 30. Pressurized Water Reactor (PWR)
  31. 31. Pressurized Water Reactor (PWR) <ul><li>Water in the core heated top 315 ° C but is not turned into steam due to high pressure in the primary loop. </li></ul><ul><li>Heat exchanger used to transfer heat into secondary loop where water is turned to steam to power turbine. </li></ul><ul><li>Steam used to power turbine never comes directly in contact with radioactive materials. </li></ul>
  32. 32. PWR vs. BWR
  33. 33. Spent Nuclear Storage <ul><li>Fuel rods must be replaced every 3 years. </li></ul><ul><li>Spent Fuel Rods are temporarily stored in special ponds which are usually located at the reactor site. </li></ul><ul><li>The water in the ponds serves the dual purpose of acting as a barrier against radiation and dispersing the heat from the spent fuel. </li></ul>
  34. 34. Spent Fuel Rods Radiation <ul><li>Spent fuel assemblies taken from the reactor core are highly radioactive </li></ul><ul><li>Contain plutonium-239, which is radioactive. </li></ul><ul><li>Forms from the reaction of U-238 and neutrons in the core </li></ul>
  35. 35. Fate of Spent Fuel <ul><li>There are two alternatives for spent fuel: </li></ul><ul><li>Reprocessing to recover the usable portion of it </li></ul><ul><li>Long-term storage and final disposal without reprocessing. </li></ul>
  36. 36. Uranium Reprocessing <ul><li>Spent fuel still contains approximately 96% of its original uranium, of which the fissionable U-235 content has been reduced to less than 1%. </li></ul><ul><li>About 3% of spent fuel comprises waste products and the remaining 1% is plutonium produced while the fuel was in the reactor </li></ul><ul><li>Reprocessing extracts useable fissile U-238 and Plutonium from the spent fuel rods </li></ul>
  37. 37. Uranium Reprocessing <ul><li>Most of the spent fuel can be reprocessed but isn’t in the U.S. </li></ul><ul><li>Federal law prohibits commercial reprocessing because it will produce plutonium (which can be used both as a fuel and in constructing bombs) </li></ul>
  38. 38. Nuclear Waste Disposal <ul><li>In the U.S., no high-level nuclear waste is ever disposed of --it sits in specially designed pools resembling large swimming pools (water cools the fuel and acts as a radiation shield) or in specially designed dry storage containers.  </li></ul><ul><li>Spent nuclear fuel must be isolated for thousands of years </li></ul><ul><li>After 10,000 years of radioactive decay, according to EPA standards, the spent nuclear fuel will no longer pose a threat to public health and </li></ul>
  39. 39. Yucca Mountain <ul><li>The United States Department of Energy's long range plan is for this spent fuel to be stored deep in the earth in a geologic repository, at Yucca Mountain, Nevada.  </li></ul>
  40. 40. Concerns about Yucca Mountain <ul><li>Possible health risks to those living near Yucca Mountain and transportation routes </li></ul><ul><li>Eventual corrosion of the metal barrels containing the waste </li></ul><ul><li>Located in an earthquake region and contains many interconnected faults and fractures </li></ul><ul><ul><li>These could move groundwater and any escaping radioactive material through the repository to the aquifer below and then to the outside environment </li></ul></ul>
  41. 41. Reactor Safety <ul><li>These are the only major accidents (Three Mile Island and Chernobyl) to have occurred in more than 12,700 cumulative reactor-years of commercial operation in 32 countries.  </li></ul><ul><li>The risks from nuclear power plants, are minimal compared with other commonly accepted risks </li></ul>
  42. 42. Breeder Reactor <ul><li>In a fission reactor, both uranium-235 and plutonium-239 can be used as nuclear fuel </li></ul><ul><li>Most of the uranium in a typical reactor is uranium-238, which cannot be used as nuclear fuel </li></ul><ul><li>When a high-energy neutron collides with the nucleus of uranium-238, plutonium-239 is sometimes produced </li></ul><ul><li>Could a nuclear reactor create more fuel while it generates energy? Yes, this is a breeder reactor </li></ul>
  43. 43. Breeder Reactor <ul><li>To convert uranium-238 to plutonium-239, you need fast neutrons </li></ul><ul><li>Therefore, water (which acts as a moderator) cannot be used as the coolant </li></ul><ul><li>Liquid sodium or high pressure gasses are used instead </li></ul>Uranium-238 only captures fast (high-energy) neutrons
  44. 44. The Core of a Breeder Reactor <ul><li>A breeder reactor core consists of a normal uranium-235 reactor (moderated by graphite rods) surrounded by a jacket of uranium-238 </li></ul><ul><li>The uranium-238 will be converted to plutonium-239 by neutrons escaping from the core </li></ul>Orange: uranium-238 Green: enriched uranium-235 Blue: graphite rods
  45. 45. A Typical Breeder Reactor <ul><li>Because liquid sodium becomes radioactive, it is separated from the rest of the power plant using a heat exchanger </li></ul><ul><li>High pressure gas-cooled reactors are also being researched </li></ul>
  46. 46. Comments on Breeder Reactors <ul><li>Breeder reactors must be tightly controlled, because plutonium-239 can be used to build atomic weapons </li></ul><ul><li>Liquid sodium is extremely reactive (and dangerous) thus high-pressure gasses are preferred as the coolant </li></ul><ul><li>Only France and Japan are actively pursuing breeder reactor technology </li></ul><ul><li>Breeder reactors are Illegal in the united States due to safety and terrorist concerns. </li></ul>