Nuclear Fission: Reactions and Power Plants

• Energy is stored within the nucleus of the atom and can be harnessed through nuclear fission
• Students should understand energy released in spontaneous and neutron-induced fission (7 hours)
• Students should understand the role of chain reactions in nuclear fission reactions
• Students should understand the role of control rods, moderators, heat exchangers and shielding in a nuclear power plant
• Students should understand the properties of the products of nuclear fission and their management.

Guidance for Calculations and Considerations

• Calculations to determine the energy released in fission reactions are required
• Consider the impact of long-term storage of nuclear waste

Linking Questions for Fission

• Energy is released as a result of nuclear fission
• Binding energy determines the rate of energy production in a nuclear power plant
• Fission has a role to play in addressing climate change

Key takeaway on Nuclear Physics

• Iron (A= 56) features the maximum binding energy nucleon
• Iron has the most stable nucleus; eventually, all nuclei will form iron
• Products of nuclear reactions possess higher binding energy than reactants.
• The mass defect releases energy as kinetic energy of the product particles
• Binding energy differences are most significant at the curve's extremes
• Fusion of small nuclei (H → He) yields enormous energy
• Fission of large nuclei (U → Ba + Kr) releases significant energy

Fission Reactions

• Fission is the splitting of a large parent nucleus into smaller, more stable daughter nuclei
• Fission reactions are exothermic, releasing energy
• Spontaneous fission is a rare radioactive decay in large nuclei (A>230)
  • Large nuclei split into two smaller nuclei larger than helium (alpha decay)
• Neutron-induced fission involves shooting neutrons at large unstable nuclei
  • A large, metastable nucleus absorbs a neutron and becomes even more unstable

Fission Products and Nuclear Stability

• Multiple possible products exist for each reactant nucleus
• The two nuclei rarely have the same number of protons.
• Uranium-235, used in CANDU reactors, is an example
• Daughter nuclei are more stable than Uranium-235 due to greater binding energies per nucleon

Mass Defect and Energy Release

• Mass defect is the difference between the total mass of reactants and products
• Calculate energy using the mass defect; do not use individual proton and neutron masses
• Released energy is equivalent to the mass defect

Binding Energy and Stability

• Daughter nuclei have more binding energy than the parent nucleus
• Total binding energy increases as products stabilize

Energy Distribution in U-235 Fission

• Roughly 200 MeV of energy is released per U-235 nucleus during fission.
  • Most (85-90%) is immediately released as kinetic energy of particles and gamma radiation.
    • 165 MeV (80-85%) is from the kinetic energy of daughter nuclei.
    • 5 MeV (2-3%) is from the kinetic energy of neutrons.
    • 7 MeV (3-4%) is from gamma photons.
  • Some (10-15%) is released from fission products undergoing radioactive decay.
    • 7 MeV (3-4%) is from beta particles.
    • 7 MeV (3-4%) is from gamma radiation.
    • 9 MeV (4-5%) is from antineutrinos.
• Plutonium-239 is a key feature of fission reactions

Chain Reactions

• Chain reaction: a self-sustaining sequence where a reaction product causes additional reactions
• Fission rate remains constant when the total number of neutrons in the next set of fission reactions is constant

Conditions and Control

• Many fission reactions produce more than one neutron (excess neutrons)
• Some excess neutrons leave the nuclear fuel without initiating fission
  • Other nuclei in the environment
• Excess neutrons can be absorbed by other elements, such as control rods
• Critical mass: the minimum mass of nuclear fuel needed to sustain a chain reaction

Energy Released in Reactions

• Mass Defect: the difference between the total mass of reactants and the total mass of the products.
  • Do not use the masses of individual protons and neutrons of each element in the calculation Released energy is the energy equivalent of the mass defect
• Fusion
  • 3.26 MeV of energy is produced by the fusion of 2 deuterium nuclei.
• Fission
  • 173 MeV of energy is produced by the fission of 1 uranium-235 nucleus.