32.6 Fission

Nuclear fission splits heavy atomic nuclei, releasing energy and neutrons. This process powers nuclear reactors and weapons. Fission reactions can be controlled for electricity generation or unleashed in atomic bombs, demonstrating the immense potential and risks of nuclear technology.

Understanding fission is crucial for grasping nuclear energy's role in modern society. It highlights the delicate balance between harnessing atomic power for beneficial uses and managing its destructive potential, shaping global energy and security landscapes.

Nuclear Fission

Process of nuclear fission

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1. Nuclear fission involves the splitting of a heavy atomic nucleus (uranium or plutonium) into lighter nuclei
2. Occurs when a neutron collides with a fissile nucleus, causing it to become unstable and split
3. Key components of nuclear fission:
   • Fissile material capable of undergoing fission ($^{235}U$ or $^{239}Pu$)
   • Neutron initiates the fission reaction by colliding with the fissile nucleus
   • Fission fragments are lighter nuclei produced from the splitting of the heavy nucleus
   • Prompt neutrons are additional neutrons released during the fission process, which can trigger further fission reactions
   • Significant amount of energy released in the form of kinetic energy of fission fragments and gamma radiation
   • Neutron absorption by the fissile nucleus is crucial for initiating the fission process

Products of fission reactions

• Fission reactions produce various products and release energy:
  • Fission fragments are two or more lighter nuclei formed from the split of the heavy nucleus ($^{141}Ba$ and $^{92}Kr$ from the fission of $^{235}U$)
  • Typically 2-3 prompt neutrons are released per fission event, which can initiate further fission reactions if absorbed by nearby fissile nuclei
  • Approximately 200 MeV (million electron volts) of energy is released per fission event
    • Majority of energy is in the kinetic energy of fission fragments (about 168 MeV)
    • Prompt gamma radiation and neutron kinetic energy account for the remaining energy
• Fission reactions demonstrate Einstein's famous equation, $E=mc^2$
  • Total mass of fission products is less than the initial mass of the fissile nucleus
  • The "missing" mass is converted into the released energy according to $E=mc^2$
  • The difference in binding energy between the initial nucleus and the fission products accounts for the energy release

Controlled vs uncontrolled chain reactions

• Fission chain reaction occurs when prompt neutrons from one fission event trigger further fission events
  • Can be controlled or uncontrolled, depending on the application
• Controlled fission chain reaction:
  • Used in nuclear power plants to generate electricity
  • Fission rate is carefully regulated using control rods and moderators
    • Control rods absorb excess neutrons to prevent runaway chain reactions
    • Moderators slow down neutrons to increase their likelihood of causing fission
  • Allows for a steady, sustainable release of energy
• Uncontrolled fission chain reaction:
  • Occurs in nuclear weapons (atomic bombs)
  • Fission rate rapidly increases, leading to a massive release of energy in a short time
  • No control mechanisms are used to regulate the reaction
  • Can cause significant destruction and radioactive fallout

Nuclear reactors and safety considerations

• Nuclear reactors harness the energy from controlled fission reactions to generate electricity
• Key components of a nuclear reactor include:
  • Fuel rods containing fissile material
  • Control rods to regulate the reaction rate
  • Moderator to slow down neutrons
  • Coolant to remove heat and generate steam for power generation
• Safety considerations in nuclear reactors:
  • Proper containment to prevent release of radioactive materials
  • Monitoring of reactor core temperature and pressure
  • Emergency shutdown systems
  • Management of radioactive waste products
• Half-life of fission products is an important factor in waste management and environmental impact