Nuclear Fusion & Fission (Cambridge (CIE) A Level Physics): Revision Note

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

• In the centres of stars, four hydrogen nuclei fuse to produce a helium nucleus , 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

Deuterium-tritium fusion

• 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

• 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

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 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