13.4 the Nucleus 3 Nuclear Fission and Nuclear Fusion

13.4 THE NUCLEUS 3 – NUCLEAR FISSION AND NUCLEAR FUSION

Fission means breaking apart and fusion means joining together.

Nuclear fissionrefers to the breaking apart of the nucleus of an atom. The best known example of nuclear fission occurs when the nucleus of the uranium isotope 235U captures an extra neutron. This makes the nucleus unstable and it breaks into two roughly equal parts called fission fragments; these are isotopes of twomuch lighter elements.The fission process also producesa fewneutrons and a lot of energy in the form of γ-rays and heat.The equation below summarises one example of this fission process.

The equation shows a neutron combining with a 235U nucleus. The nucleus breaks down to form isotopes of the elements barium and krypton, together with three neutrons and a huge amount of energy. Less than 1% of natural uranium is 235U; however, this can be increased by modern technology to yield enriched uranium. If the percentage of 235U is sufficient, then the neutrons produced by the fission of one nucleus, will be captured by another nucleus, which will break down releasing more neutrons, which will be captured by more nuclei and so on. This is called a chain reaction. If this chain reaction is not controlled, it will release vast quantities of heat and γ-rays in a devastating explosion. This uncontrolled chain reaction fuelled the atomic bombs that were dropped on Hiroshima and Nagasaki to end World War 2. In Nuclearpower stations, heat from the same chain reaction is used to make steam to generate electricity. The chain reaction is controlled by using uranium that has not been enriched so much, and by using control rods of materials that absorb neutrons. In nuclear power stations, workers must be protected from γ-rays by thickshields of lead and concrete.

Nuclear fusionis the process of joining together two nuclei to create a new nucleus.The new nucleus will be an isotope of a new, heavier element. Nuclear fusion produces even more energy than nuclear fission.

Nuclear fusion is the process that fuels the stars, including thesun (see Module 11.2).A series of nuclear fusion reactions result in the nuclei of four hydrogen atoms coming together to form the nucleus of a helium atom.As stars get older, they startproducing heavier elements by nuclear fusion. All the 90 different elements that we find on Earth were made by nuclear fusion in ancient stars.

Energy from nuclear fusion reactions in the sun reach us as heat, light and γ-rays.Gamma-rays from stars are often called cosmic rays. Nuclear fusion has been used in hydrogen bombs,however it has not yet been controlled for peaceful uses such as generating electricity.

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• 1.What are the similarities and differences as between nuclear fission and nuclear fusion?
• 2. What is a chain reaction? Why does 235U explode in an atomic bomb, but not in a nuclear power station?

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