Edexcel IGCSE Physics: Double Science 复习笔记:7.3.2 Nuclear Fission

Edexcel IGCSE Physics: Double Science 复习笔记:7.3.2 Nuclear Fission

Nuclear Fission


  • 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


Induced Fission Reactions


  • 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
  • Usually, for fission to occur the unstable nucleus must first absorb a neutron
  • This makes the nuclei more unstable, so that it decays almost immediately


Fission of Uranium-235

  • Uranium-235 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 a 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





A uranium-235 nucleus is struck by a neutron, breaking it into two smaller daughter nuclei and 2 or 3 neutrons



Exam Tip

You need to remember that uranium and plutonium are possible elements for fission, but you do not need to know the specific daughter nuclei that are formed.Use your knowledge of balancing nuclear equations to work these out.

Products of Fission


  • 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


  • These products of the fission reaction move away very quickly
    • Energy transferred is from nuclear potential energy to kinetic energy
    • Eventually this energy can be converted, within the nuclear power station, into electrical energy



Worked Example

During a particular spontaneous fission reaction, plutonium-239 splits as shown in the equation below:


Which answer shows the section missing from this equation?



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