Nuclear Fusion & Fission (CIE A Level Physics)

Nuclear fusion

• Fusion is defined as:

The fusing together of two small nuclei to produce a larger nucleus

• Low mass nuclei (such as hydrogen and helium) can undergo fusion and release energy
• When two protons fuse, the element deuterium is produced
• In the centre of stars, the deuterium combines with a tritium  nucleus to form a helium nucleus, plus the release of energy, which provides fuel for the star to continue burning

Fusion of hydrogen isotopes

The fusion of deuterium and tritium to form helium with the release of energy

• For two nuclei to fuse, both nuclei must have high kinetic energy
  • This is because nuclei must be able to get close enough to fuse

• However, two forces acting within the nuclei make this difficult to achieve

• Electrostatic repulsion 
  • Protons inside the nuclei are positively charged, which means that they electrostatically repel one another

• Strong nuclear force
  • The strong nuclear force, which binds nucleons together, acts at very short distances within nuclei
  • Therefore, nuclei must get very close together for the strong nuclear force to take effect

• It takes a great deal of energy to overcome the electrostatic force, hence fusion can only be achieved in an extremely hot environment, such as the core of a star

Nuclear fission

• Fission is defined as:

The splitting of a large atomic nucleus into smaller nuclei

• High mass nuclei (such as uranium) can undergo fission and release energy

Fission reaction

The fission of a target nucleus, such as uranium, to produce smaller daughter nuclei with the release of energy

• Fission must first be induced by firing neutrons at a nucleus
  • When the nucleus is struck by a neutron, it splits into two, or more, daughter nuclei
  • During fission, neutrons are ejected from the nucleus, which in turn, can collide with other nuclei and trigger a cascade effect
  • This leads to a chain reaction which lasts until all of the material has undergone fission or the reaction is halted by a moderator

• Nuclear fission is the process which produces energy in nuclear power stations, where it is well-controlled
• When nuclear fission is not controlled, the chain reaction can cascade to produce the effects of a nuclear bomb

• At low values of A:
  • attractive nuclear forces between nucleons dominate over repulsive electrostatic forces between protons
  • in the right conditions, nuclei undergo fusion

• In fusion, the mass of the nucleus that is created is slightly less than the total mass of the original nuclei
  • The mass defect is equal to the binding energy that is released since the nucleus that is formed is more stable

• At high values of A:
  • repulsive electrostatic forces between forces begin to dominate, and these forces tend to break apart the nucleus rather than hold it together
  • in the right conditions, nuclei undergo fission

• In fission, an unstable nucleus is converted into more stable nuclei with a smaller total mass
  • This difference in mass, the mass defect, is equal to the binding energy that is released