Nuclear Fission (OCR A Level Physics)

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Katie M


Katie M



Nuclear Fission

  • There is a lot of energy stored within the nucleus of an atom
    • This energy can be released in a nuclear reaction such as fission or fusion

  • Nuclear fission is defined as:

The splitting of a large, unstable nucleus into two smaller nuclei

  • Isotopes of uranium and plutonium both undergo fission and are used as fuels in nuclear power stations
  • During fission, when a neutron collides with an unstable nucleus, the nucleus splits into two smaller nuclei (called daughter nuclei) as well as two or three neutrons
    • Gamma rays are also emitted

nuclear-fission, IGCSE & GCSE Physics revision notes

Large nuclei can decay by fission to produce smaller nuclei and neutrons with a lot of kinetic energy

  • The products of fission move away very quickly
    • Energy transferred is from nuclear potential energy to kinetic energy

Spontaneous Fission

  • It is rare for nuclei to undergo fission without additional energy being put into the nucleus
  • When nuclear fission occurs in this way it is called spontaneous fission

Induced Fission

  • Usually, for fission to occur the unstable nucleus must first absorb a neutron
  • Take, for example, uranium-235, which is commonly used as a fuel in nuclear reactors
  • It has a very long half-life of 700 million years
  • This means that it would have low activity and energy would be released very slowly
    • This is unsuitable for producing energy in a nuclear power station

  • During induced fission, a neutron is absorbed by the uranium-235 nucleus to make uranium-236
  • This is very unstable and splits by nuclear fission almost immediately

Chain Reactions

  • Only one extra neutron is required to induce a Uranium-235 nucleus to split by fission
  • During the fission, it produces two or three neutrons which move away at high speed
  • Each of these new neutrons can start another fission reaction, which again creates further excess neutrons
  • This process is called a chain reaction

Chain reaction analogy, downloadable IGCSE & GCSE Physics revision notes

The neutrons released by each fission reaction can go on to create further fissions, like a chain that is linked several times – from each chain comes two more

Worked example

During a particular spontaneous fission reaction, plutonium-239 splits as shown in the equation below:WE Fission equation example, downloadable IGCSE & GCSE Physics revision notesWhich answer shows the section missing from this equation?

WE Spontaneous Fission Question image, downloadable IGCSE & GCSE Physics revision notes


Step 1: Identify the different mass and atomic numbers

    • Pu (Plutonium) has mass number 239 and atomic number 94
    • Pd (Palladium) has mass number 112 and atomic number 46
    • Cd (Cadmium) has mass number 124 and atomic number 48

Step 2: Calculate the mass and atomic number of the missing section

    • Mass number is equal to the difference between the mass numbers of the reactants and the products

239 – (112 + 124) = 3

    • Atomic number is equal to the difference between the atomic numbers of the reactants and the products

94 – (46 + 48) = 0

    • The answer is therefore not B or C

Step 3: Determine the correct notation

    • Neutrons have a mass number of 1
    • The answer is therefore not A
    • Therefore, this must be three neutrons, which corresponds to D

Exam Tip

Fission and fusion are very different processes. Fusion comes from the word "fuse" as in bind/stick together. This brings small nuclei together. Fission is the opposite. It is the breaking down of large nuclei like Uranium.

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Katie M

Author: Katie M

Katie has always been passionate about the sciences, and completed a degree in Astrophysics at Sheffield University. She decided that she wanted to inspire other young people, so moved to Bristol to complete a PGCE in Secondary Science. She particularly loves creating fun and absorbing materials to help students achieve their exam potential.

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