Fission and Fusion

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Fission and Fusion
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Change of Atomic Number
emit alpha particle
decrease by 2
emit beta- particle
increase by 1
emit proton
decrease by 1
no change
emit neutron
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Nuclear Stability
Up to Z30, NZ
For heavier nuclei, NgtZ
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1938 (Germany)
235 Bombarded U with neutrons.
141
92 Found Ba among the products.
56
Significant change in atomic .
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First Nuclear Fission

• 235 1 236
• U n ? ( U )
• 0 92

• 1
• n
• 0

141 92 ? Ba Kr
56 36
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What can happen to the 3 neutrons?
more fissions
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Fission Chain Reaction
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Fission Processes

• 90 different daughter nuclei possible.

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Energy Release

• Energy released from 1 U-235 atom
  200 MeV

7 million times the energy of one TNT molecule.
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Critical Mass

• Minimum amount of the fissionable material to
  make a chain reaction.

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CONTROL RODS
Neutron-absorbing material.
Allows reaction to continue at a controlled
rate. Ex. hafnium or cadmium
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MODERATOR
Slow neutrons are more easily absorbed.
The moderator slows down (thermalizes) the
neutrons. Ex. graphite, heavy water.
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Nuclear Fission Reactor
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How a Nuclear Reactor Works
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Enriched Fuel
Only 0.7 of natural uranium is fissionable
U-235. Most is U-238.
Need to extract U-235 from the natural uranium.
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The Breeder Reactor
U-238 in the reactor core absorbs neutrons.
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Breeding Plutonium-239
Pu-239 is fissionable
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Fission Benefits vs. Drawbacks

• Benefits

• Drawbacks

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Energy of a Fusion Reaction

• 1 2 3
• H H ? He ?
• 1 1 2

3.021927 u
mass of reactants (H-1 H-2) mass of product
(He-3)
3.016029 u
energy 5.494 MeV
mass defect 0.005898 u
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Other notation for nuclear reactions
1 2 3 H H ?
He ? 1 1 2
3 p(d,?) 2He
d deuterium (H-2)
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Nuclear Reaction a X ? Y b
If mass defect, exothermic. Q (reaction energy) gt
0
If Qlt0 (endothermic), minimum input energy is
the threshold energy.
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Conditions for Fusion

• Nuclei are positively charged (repel).

Must have high speed (high temp.)
Must be confined.
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Magnetic Confinement
Hot ionized gas (plasma) contained by magnetic
fields.
tokamak a donut- shaped magnetic bottle.
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Inertial Confinement
Many laser beams converge on pellet of fuel.
Intense heat and pressure cause fusion.
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Advantages of Fusion over Fission

• More efficient

No radioactive waste.
Unlimited supply.
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Proton-Proton Solar Fusion Chain