Induced Fission by Thermal Neutrons and Critical Mass

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Induced Fission by Thermal Neutrons and Critical
Mass

• By Robert T Wilkin and David B Wyllie Esq.

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Fission Reactions
Spontaneous fission reactions occur for only the
very heaviest nuclides those with mass numbers
of 230 or more. Even when they do occur, these
reactions are often very slow.
The half-life for the spontaneous fission of
238U, for example, is 1016 years, or about two
million times longer than the age of our planet!
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Induced Fission Reactions

• By irradiating samples of heavy nuclides with
  slow-moving thermal neutrons it is possible to
  induce fission reactions.
• When 235U absorbs a thermal neutron, for example,
  it splits into two particles of uneven mass and
  releases an average of 2.5 neutrons, as shown in
  the figure below.

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Thermal Neutrons

• Neutrons from fission have very high speeds and
  must be slowed greatly by water "moderation" to
  maintain the chain reaction. This creates a
  thermal neutron
• A thermal neutron is a free neutron with a
  kinetic energy level of ca. 0.025 eV (approx.
  4.0e-21 J 2.4 MJ/kg, hence a speed of 2.2 km/s).
  They are named 'thermal' as this level of kinetic
  energy is similar to the average kinetic energy
  of the molecules in a room-temperature gas
• Thermal neutrons have a much larger effective
  cross-section than fast neutrons, and can
  therefore be absorbed more easily by any atomic
  nuclei that they collide with, creating a heavier
  - and often unstable - isotope of the element as
  a result.

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Critical mass

• For a chain reaction of nuclear fission, such as
  that of uranium-235, is to sustain itself, then
  at least one neutron from each fission must
  strike another U-235 nucleus and cause a fission.
  If this condition is just met, then the reaction
  is said to be "critical" and will continue.
• The mass of fissile material required to achieve
  this critical condition is said to be a critical
  mass.

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What it depends on

• The critical mass depends upon
• The concentration of U-235 nuclei in the fissile
  material
• Its geometry.
• It also depends upon the moderation used to slow
  down the neutrons.