Nuclear fission

• Nuclear fission occurs when a nucleus splits into two or more pieces. Usually triggered by the absorption of a neutron
• Fissile: the nuclide is capable of nuclear fission

  • all have high atomic numbers
• Nuclear fission does not occur when an alpha particle is exerted towards the nucleus as it is repelled by the nucleus. Whereas works when a neutron is absorbed as they have neutral charge.
• Electronvolts (eV)

  : the measurement of the energy/radiation of subatomic particles

  1 eV = 1.6 x 10 ^-19 J
• Converting

  eV -> J

  eV x 1.6 x 10^-19

  J -> eV

  J / 1.6 x 10^-19
• Binding energy: the energy holding the nucleus together and is where the absence of mass is used
• Chain Reactions

  • when undergoing fission different numbers of neutrons are released. Each of these neutrons are able to be absorbed by another nucleus causing fission, which will release more neutrons. Creating a chain reaction, escalating the numbers of neutrons being released and the amount of reactions occuring
• Naturally occuring nuclear fuel

  • The Uranium mined consists of 99.3% U-238 which is non-fissile, 0.7% U-235 which is readily fissile
  • the natural sample must be enriched with further U-235 as they are their fissile proportion is too low and must be of 4%
• Nuclear weapons require fissile material >90%

  Nuclear reactors require fissile material of 4%
• CRITICAL MASS

  refers to the explosive ability of a fissile material (whether sub/super/critical)

  dependent on - purity, shape, size

  effects the possibility of a chain reaction occuring
• Critical mass : the minimum amount of fissile material necessary to sustain a chain reaction

  Supercritical mass: is a mass greater than what is required to sustain a chain reaction

  Subcritical mass: is a mass that is less than the required amount
• Shape of the sample's effect

  • Flat sample - has a large SA, a greater amount of neutrons are able to escape into air not causing another fission -> chain reaction dies out
  • Spherical sample - has a smaller SA, greater amount of neutrons remain in the sample -> sustaining the chain reaction
• Physical size of the sample effects

  • Smaller piece may have SUBCRITICAL mass - it has more SA when compared to its volume -> reaction dies out
  • Larger piece may have SUPERCRITICAL mass - it has a higher proportion of neutron remaining -> continuing the chain reaction to the extent of an explosion