Nuclear Energy: Benefits and Risks

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Chapter 11

• Nuclear Energy Benefits and Risks

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Fission and Fusion

• Atoms nucleus have positively charged protons
  and neutrons
• Protons are repelled from each other, so energy
  is required to keep them within nucleus
• If atom is radioactive, nucleus decomposes and
  energy is released as an alpha, beta, or gamma
  ray. Neutrons, protons, and electrons may be
  released when this happens

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Rate of Decomposition

• Rate of decomposition is expressed as
  half-life. Well known decomposition pathways
• radium-226 gt radon-222 gt lead 206 (half-life
  1620 years)
• If a neutron is released and it strikes another
  nucleus, the nucleus splits and releases energy -
  Nuclear fission.

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Radiation

• alpha - 2 neutrons and 2 protons - big particle,
  so can cause a lot of damage to tissues, but easy
  to stop (paper, skin)
• beta - electron - harder to stop clothes / glass
• gamma - electromagnetic radiation. Needs several
  cm of concrete to stop

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Nuclear Fission

• Splitting nuclei release neutrons, which can
  strike the nuclei of other atoms and cause the
  same reaction.
• Most common fissionable materials used by humans
  are Uranium 235 and Plutonium 239

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Fusion Reactions

• If hydrogen isotope deuterium (1 proton and 1
  neutron) or tritium (1 proton and 2 neutrons)
  combine, helium isotopes plus energy is formed
  --- nuclear fusion
• Enough energy from deuterium in one cubic km of
  ocean water is greater than the worlds entire
  fossil fuel supply

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Why Dont We Use Fusion?

• Reaction has to occur at very high temps - close
  to center of the sun
• Vessel must be able to withstand this temperature
  - technologically impossible today.
• Nuclei repel one another, so hard to contain
• Some research on fields to contain

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Nuclear Reactors

• Controls nuclear fission chain reaction
• U-235 isotope provides fuel (neutrons)
• Rods of uranium (fuel) are lowered into the
  reactor, which allows chain reaction to occur
• Rods of non-fissionable material can be lowered
  into the reactor to absorb neutrons and slow the
  rate of reaction

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Nuclear Reactors

• Water is a moderator, slowing neutrons down
  (better for splitting nuclei), absorbs the energy
  from fission, and heats until steam forms, which
  drives turbines
• After turning the turbine, steam is cooled and
  returned to the reactor to form steam again.

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Types of Reactors

• 1. Light water reactor - uses ordinary water for
  a moderator. 90 of reactors in use today.
• 2. Heavy water (deuterium) reactor.
• 3. Other types.
• There are two types of light water
• reactors boiling water and
• pressurized water reactors

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1. Light Water Reactors

• Boiling water - water is in contact with the
  radioactive material (20 of worlds reactors)
• Pressurized water reactor the moderator (water)
  is sealed and heat is transferred to water for
  steam generation via a secondary loop. (70 of
  worlds reactors)

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2. Heavy Water Reactors

• Deuterium is used as a moderator
• Does not require enriched uranium fuel can use
  naturally occurring uranium
• Lower costs , not as much environmental problems
  from enrichment process.

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3a. Gas Cooled Reactors

• Graphite is used as a moderator
• These reactors can also use naturally occurring
  uranium
• Carbon dioxide or helium is used as a coolant.

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3b. Breeder Reactors

• Uranium is fissioned, producing heat and a new
  fission material - plutonium
• It takes fast neutrons to do this, so water cant
  be used as a moderator
• Use liquid sodium as moderator, which allows
  reaction to occur
• After about 10 years, enough plutonium-239 will
  be formed to fuel a second reactor

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Problems with Breeder Reactors

• sodium is explosive in air or water
• sodium must be cooled to prevent melt down, but
  cant be pumped until it melts.
• Reaction rates are very fast and hard to control
• Plutonium is very dangerous (toxicity and bombs)

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Nuclear Fuel Cycle

• Low grade uranium ore (0.2) mined
• U-235 is extracted with solvents to form
  yellowcake (0.7)
• Yellowcake is enriched by gas centrifugation or
  gas diffusion (3). Note an expensive process.
• Enriched material is crushed, formed into
  pellets, and sealed in metal fuel rods

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Nuclear Fuel Cycle (cont)

• Fuel rods are consumed in reactors (down to 1
  U-235 and 1 P-239) in 3 years or so.
• Spent fuel rods were reprocessed for a time in
  the U.S., but now are stored as waste.
• Big storage problem - long half-life

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Concerns

• Reactor safety Chernobyl and Three Mile Island
  have raised concerns
• Long term effects of exposure to radiation
  (mutations in sex cells, cancer).
• Safe levels of exposure, measured in rems
  (roentgen equivalents to man - measure of tissue
  damage) at mining, operations, transport, and
  disposal sites

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Concerns (continued)

• Thermal pollution (only 1/3 of heat generated is
  converted into electricity). Fossil fuel plant
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• Decommissioning nuclear power plants (most had 30
  year design life)
• Radioactive waste disposal both high level and
  low level waste.
• Low level cooling water, medical isotopes, hard
  to control disposal
• High level 380,000 cubic meters in U.S.

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Decommissioning

• 3 real options
• Decontaminate and dismantle dangerous!!!!
• Wait 20-100 years and decontaminate and dismantle
• Cover it all with concrete.
• Typically 200 to 400 million per plant.

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