IGCSE Physics / Fission and fusion
Nuclear fission revision notes
Edexcel International GCSE Physics (4PH1), chapter 25: Fission and fusion, specification point 7.17, 7.18, 7.19. Free lesson.
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Nuclear fission is the splitting of a large nucleus, such as uranium-235, after it absorbs a slow neutron. It releases energy as the kinetic energy of the products, so understanding the products and describing the process accurately are important.
Three nuclear reactions release energy
Energy can be released from the nucleus, the tiny centre of an atom made of protons and neutrons. There are three types of nuclear reaction that do this: Radioactive decay happens when an unstable nucleus emits radiation and energy. Carbon-14 is an example. Fission is when a large nucleus splits into smaller nuclei. Uranium-235 is an example, and fission is used in nuclear power stations. Fusion is when small nuclei join to make a larger nucleus. Hydrogen nuclei in the Sun provide an example.
How a uranium-235 nucleus splits
A slow neutron is more likely to be absorbed by a uranium-235 nucleus than a fast neutron. Absorbing the neutron makes the nucleus unstable, so it splits into two smaller daughter nuclei. The daughters are radioactive. Two or three neutrons are released when the nucleus splits. The daughter nuclei and the released neutrons fly apart quickly. Their movement is kinetic energy. This is the energy released by the fission reaction.
Describe the process using the right word
Fission happens to a nucleus, not an atom. Use “nucleus” throughout your answer, because calling it an atom splitting can lose the marks for describing the process. A neutron is absorbed by a uranium-235 nucleus. The nucleus splits into two radioactive daughter nuclei and releases two or three neutrons. Energy is released as the kinetic energy of the products. These products can collide with surrounding material and heat it.
Find how many neutrons are released
You can work out the number of neutrons by conserving the total number of nucleons, meaning protons and neutrons, in the reaction. Compare the total before and after the split.