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Alternative Energy Sources

Alternative Energy Sources. Nuclear Energy. Nuclear Fission. The source of nuclear energy. 1. Artificial Transmutation 2. Decay The neutron is the key!. Transmutation. Radioactive Decay. U-236 is very unstable.

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Alternative Energy Sources

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  1. Alternative Energy Sources Nuclear Energy

  2. Nuclear Fission • The source of nuclear energy. 1. Artificial Transmutation 2. Decay The neutron is the key!

  3. Transmutation

  4. Radioactive Decay • U-236 is very unstable. • The attraction between the new neutron and the nucleus throws off the balance between the strong nuclear force and electromagnetic force. • The nucleus splits into two smaller nuclei, releasing about 200MeV in the form of neutrons and gamma rays.

  5. U-235 U-236 γ Kr Ba γ

  6. The Products • The two daughter nuclei (usually Kr and Ba). • Neutrinos. • β particles. • γ rays. • Fast neutrons.

  7. Sustaining a Chain Reaction • Neutrons • The fast neutrons ( v = 2x107 ms-1 ) produced by the reaction can start a chain reaction. • Mass • Critical Mass: The minimum amount of fissionable material required to sustain a chain reaction. • Shape • Uranium pellets in zirconium fuel rods.

  8. Controlled vs. Uncontrolled Nuclear Fission • Two or three neutrons are produced in each fission reaction. • The neutrons usually have too much kinetic energy to start another reaction. • The neutrons must be slowed down by a moderator (usually water).

  9. Uncontrolled Fission • All or most of the neutrons produced start their own reaction. • The chain reaction quickly goes out of control, causing an explosion. A NUCLEAR WEAPON

  10. Controlled Fission • In nuclear reactors. • Only one of the neutrons produced by each reaction can go on to start another reaction. • The extra neutrons are absorbed by control rods:

  11. If… • less than one neutron (on average) moves on, the chain reaction stops and the reactor shuts down. • more than one neutron moves on, the fuel melts, setting fire to the reactor (a meltdown).

  12. Fuel Enrichment • Naturally occurring Uranium: 99.3% U-238, 0.7% U-235, 0.006 % U-234 • Only U-235 is fissionable. • Enrichment: U-235 concentration is increased to about 4% using a centrifuge.

  13. Nuclear Power Stations

  14. Sankey Diagram:

  15. The Thermal Fission Reactor • Thermal Fission: Slow neutrons. • Essential features: - The fuel - The coolant - A moderator - Radiation - The control rods shielding

  16. Moderator: • Slows down neutrons to thermal speed. • Placed around core and between fuel rods. • Possible moderator materials: • Water • Heavy water(D2O) • Graphite

  17. Coolant: • Water, which takes the heat from the core and uses it to produce steam. Control Rods: • Absorb the extra neutrons in the core. • Regulating rods moved in and out as needed. • Extra rods used for emergency shut-down.

  18. Heat Exchanger • An engine used to convert the heat from the core into work.

  19. Low T reservoir High T reservoir

  20. High T source: 570K • Low T source: 310K • Efficiency:

  21. Plutonium-239 • Start with U-238, which is not fissionable, but is fertile (can be changed into fissionable elements). • Next, two-step β decay to make Pu-239:

  22. Used in breeder reactors (ones that create more fissionable material than they consume). • On average, 2.4 neutrons are produced in U-235 fission. • Blanket of U-238 absorbs 1.4 neutrons per reaction and produces Pu-239. • Usually, for every 100 U-235 fissions, 110 Pu-239 atoms are produced.

  23. Safety and Risks • Nuclear energy is less efficient than fossil fuel. • U-235 can last for thousands of years. • Creates less pollution. • Mining for Uranium is difficult. • Disposal of nuclear waste. • Risk of thermal meltdown. • Risk of nuclear power programs being used to produce nuclear weapons.

  24. Nuclear Fusion • Could provide a clean energy source. • Could be fueled by Deuterium and Tritium • Deuterium extracted from seawater. • Tritium bred from Lithium. • In the reaction, a very high energy neutron is produced, and the energy is converted to heat. • Requires T = 100 000 oC, which turns the H into a plasma.

  25. Plasma must be confined at 500 trillion atoms per cubic centimeter for one second. • Not a chain reaction, so the density must be maintained: - A magnetic field (“magnetic bottle”) could be used to do this.

  26. Unfortunately… • We still don’t have the technology. • The cost may be very high compared to other energy sources. • Currently, more energy is required to produce the fusion than is produced by the fusion.

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