FUSION AND FISSION

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FUSION AND FISSION
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THE SUN
Every second, the sun converts 500 million metric
tons of hydrogen to helium. Due to the process of
fusion, 5 million metric tons of excess material
is converted into energy in each second. This
means that every year, 157,680,000,000,000 metric
tons are converted into energy.
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Nuclear Fusion

• Nuclear fusion is the process by which multiple
  nuclei join together to form a heavier nucleus.
• It is accompanied by the release or absorption of
  energy depending on the masses of the nuclei
  involved.

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Nuclear Fusion

• Iron and nickel nuclei have the largest binding
  energies per nucleon of all nuclei and therefore
  are the most stable.

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Nuclear Fusion

• The fusion of two nuclei lighter than iron or
  nickel generally releases energy.
• The fusion of nuclei heavier than them absorbs
  energy.

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Complete the Reaction
1H element atomic number
(protons)
4Be
1H
2He
6C
2He
1H
1H
6C
2He
4Be
8O
2He
2He
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Learning Check

• What process creates energy in the Sun?
• Fusion of hydrogen into helium in the Suns
  core generates the Suns energy.
• How long ago did fusion generate the energy we
  now receive as sunlight?
• Fusion created the energy we receive today
  about a million years ago. This is the time it
  takes for photons and then convection to
  transport energy through the solar interior to
  the photosphere. Once sunlight emerges from the
  photosphere, it takes only about 8 minutes to
  reach Earth.

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NUCLEAR FISSION

• A reaction in which an atomic nucleus of a
  radioactive element splits by bombardment from an
  external source, with simultaneous release of
  large amounts of energy, used for electric power
  generation

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Nuclear Fission
Neutron induced in U235
Fission is Exothermic The sum of the masses of
the resulting nuclei is less than the original
mass (about 0.1 less) The missing mass is
converted to energy according to Emc2
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Neutrons may

• 1 - Cause another fission by colliding with a
  U235 nucleus
• 2 - Be absorbed in other material
• 3 - Lost in the system
• If sufficient neutrons are present,
• we may achieve a chain reaction

• Creates two smaller nuclides and free neutrons
• The free neutrons potentially collide with
  nearby U235 nuclei
• May cause the nuclide to split as well

Each split (fission) is accompanied by a large
quantity of E-N-E-R-G-Y
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Fission can be controlled

• So energy is released more slowly.
• Neutron moderation is a process that sows down
  neutrons so the reactor fuel (uranium or
  plutonium) captures them to continue the chain
  reaction
• Neutron absorption decreases the number of slow
  moving neutrons

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Fission can be controlled

• Why must neutrons in a reactor be slowed down?
• So the reactor fuel can capture them to continue
  the chain reaction thus more energy is produced

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So why is this so important to us?
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U.S. Electrical Power Production by Source
Source EIA
(2004)
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Nuclear Fuel Costs

• Nuclear Fuel Costs Include
• Uranium
• Enrichment
• Manufacturing
• Waste Disposal
• Total Nuclear Fuel Cost is Only About 0.5 cents
  per kilowatt-hour
• Uranium accounts for only about 20 of this cost
  or 0.1 cents per kilowatt-hour
• Increasing Uranium Cost has Minimal Impact

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Review
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One final thought

• What about nuclear waste from the process?

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Fuel rods are a hugh source of nuclear waste

• The rods are made from uranium-235 or
  plutonium-239, which are both fissionable
  isotopes
• Once the rods are spent they are still
  considered high-level nuclear waste because they
  continue to emit radiation
• They are placed in holding tanks or storage pools

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Make a list of growing concerns
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the growing concerns

• The rods spend years in the storage tanks
• Its cheaper to mine new sources of isotope than
  it is to recycle the isotope remaining in the
  rods
• The rods are moved to an off-site storage
  facility and are taking up LOTS of room
• The DOE is responsible for cleaning up nuclear
  sites and managing their waste