2.9 physics

• Nuclear fission


  The process of splitting a fissile nucleus after the absorption of a slow neutron
• Nuclear reactors


  • Use the process of nuclear fission to generate electricity
  • Fission is the process of splitting a fissile nucleus after the absorption of a slow neutron
  • This makes the nucleus unstable so it splits apart into two smaller nuclei, also releasing two or three neutrons as well as a lot of energy
• Chain reaction in nuclear fission

  Neutrons released are subsequently absorbed by other nuclei causing their splitting into smaller nuclei as well as the expulsion of another two or three neutrons
• Uranium-235 and Plutonium


  The most commonly used fissile materials in fission reactors as they have large nuclei which are easily split
• The chain reaction in nuclear reactors has to be controlled to prevent it becoming out of control and thus dangerous
• Generating power from nuclear fission

  1. Heat energy released from nuclear fission reactions is used to boil water and generate steam

  2. The steam is then used to drive a turbine to generate electricity

  3. All components are sealed inside a reactor with steel lined concrete walls to prevent ionising radiation from escaping
• Control features in nuclear reactors

  • Control rods and a moderator are used to control the rate of the reaction occurring in the reactor, by changing how much of the nuclear material is exposed
  • Water is typically used, as it may act as both a moderator and a coolant
• Control rods


  Made of graphite or boron, they absorb excess radiation to ensure the number of neutrons in the reactor is low enough that a sustainable reaction is maintained
• Used control rods can remain radioactive for a very long time so have to be disposed of very carefully as high level radioactive waste
• Containment of nuclear reactors

  • Nuclear reactors are contained within a pressurised steel container inside a concrete building
  • This containment building is designed to make sure the radioactive material is confined under all conditions and shields the outside world from penetrative gamma radiation
• Nuclear fusion


  Lighter elements can undergo nuclear fusion to produce heavier elements
• Conditions for nuclear fusion

  • Nuclei are positively charged so would ordinarily repel each other at close proximities, however if they are moving very fast, they can overcome this electrostatic repulsion and collide
  • High energy collisions that result in fusion are more likely under very hot conditions where molecules move very quickly
• Hydrogen fusion


  Hydrogen and deuterium (H-2) nuclei fuse to form a helium-3 nucleus
• Nuclear fusion requires very high temperatures and pressures which are very hard to recreate in a reactor, especially for prolonged periods of time
• Fusion reactors being developed use deuterium (H-2) and tritium (H-3) nuclei which are accelerated and collided to produce helium (2-4He) and a neutron
• The excess neutron can make the products of such a fusion reaction radioactive, meaning a lot of safety precautions have to be taken around the reactor
• At present, fusion reactors require far more energy to run than is generated in the reaction, so nuclear fusion is not yet viable as a sustainable energy generation method