Nuclear Fusion & Fission (Cambridge International A-Level Physics)

Nuclear fusion & fission

Nuclear fusion

• Nuclear fusion is when:

Two nuclei combine to form a single nucleus

• Low mass nuclei, such as hydrogen and helium, can undergo fusion and release energy

• For example, when two hydrogen nuclei (protons) fuse, a deuterium nucleus is produced

  • A positron and an electron neutrino are also produced as one of the protons converts into a neutron through beta-plus decay

H _ 1 ^ 1 + H _ 1 ^ 1 -> H _ 1 ^ 2 + e ^ + + nu _ e

• In the centres of stars, four hydrogen nuclei ( H _ 1 ^ 1 ) fuse to produce a helium nucleus $( He_{2}^{4} )$, plus the release of energy

  • This provides fuel for the star to continue burning

• On Earth, research is focused on achieving the deuterium-tritium (D-T) reaction

• This involves fusing a deuterium nucleus and a tritium nucleus together to produce a helium nucleus and a neutron

H _ 1 ^ 2 + H _ 1 ^ 3 -> He _ 2 ^ 4 + n _ 0 ^ 1

Deuterium-tritium fusion

The fusion of deuterium and tritium nuclei to form a helium nucleus and a neutron, 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

• Nuclear fission is when:

A single large nucleus divides to form smaller nuclei

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

Induced fission

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

• Fission must be induced by firing neutrons at a nucleus

  • When a neutron strikes a nucleus, it splits into two or more daughter nuclei
  • During fission, neutrons are ejected from the nucleus, which in turn can collide with other nuclei, triggering 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

Exam Hint When an atom undergoes nuclear fission, take note that extra neutrons are ejected by the nucleus and not from the fission products

Significance of Binding Energy per Nucleon

• 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 protons 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