Neutron Chain Reactions

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Neutron Chain Reactions

• Dr. Steve Reese
• NE 113

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Consider the reactor as a lump
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Fate Of the Neutron

• The neutron moves in a straight line until it
  hits (collides) with something.
• If it leaves or escapes the lump, it is lost.
• If it hits something, it will either be
• Scattered
• Absorbed

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Neutron Scattering and Absorption

• Scattering may result in changing the neutrons
• direction
• Energy
• Instead of releasing the neutron, the nucleus may
  absorb the neutron.
• If the nucleus is fissionable, it may split into
  two pieces and give off more neutrons. The
  number of extra neutrons is identified as (?
  new) .
• ? ? 2.5 (2.43 for thermal neutrons)

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Fission

• The neutron moves in a straight line until it
  hits (collides) with something.
• If it leaves or escapes the lump, it is lost.
• If it hits something, it will either be
• Scattered
• Absorbed

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Starting the Chain Reaction

• If we want to have a steady state energy source,
  the number of neutrons must remain constant.
• Since the energy (200 MeV/fission) comes from
  fission, we need one neutron from the previous
  fission event to go on and produce another
  fission event.
• The problem is, many neutrons are being lost or
  absorbed.

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Chain Reaction
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Starting the Chain Reaction

• Each fission produces 2.43 new neutrons
• Then 1 goes on to produce another fission then,
  1.43 (on average) leak, are absorbed in non-fuel,
  or are absorbed in the fuel but do not cause
  fission.

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Criticality
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Life History of Neutrons

• Follow the life of 10 neutrons until all are
  absorbed or escape.
• 2 escape
• 8 absorbed
• 6 absorbed in fuel
• 5 cause fission

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Rewrite k
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Definitions
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Back to the 10 Neutrons
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Criticality

• k 1.000 Critical
• k gt 1 Supercritical
• k lt 1 Subcritical

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k?

• k? is the value computed in the absence of
  leakage (e.g., an infinite reactor)
• k??f
• From the example
• k? (2.0833)(0.75) 1.5625
• For the TRIGA Reactor
• k? ? 1.4
• LNL ? 0.7 (30 of the neutrons leak out!)

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Criticality
These numbers are actually reaction rates.
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Looking at ?
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Looking at f
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Looking at LNL