CIE A Level Physics复习笔记23.1.4 Nuclear Fusion & Fission

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

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 the protons inside the nuclei are positively charged, which means that they repel one another
• It takes a great deal of energy to overcome the electrostatic force, so this is why it is can only be achieved in an extremely high-energy environment, such as star’s core
• 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

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

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 which triggers 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