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Radioactivity - Nuclear Fission

This GCSE Physics quiz on radioactivity takes a look at nuclear fission. Fission means to split so during the process of nuclear fission atomic nuclei split. This process releases energy, which can be used to heat water and turn it into steam. The steam drives a turbine, which is connected to a generator which produces electricity that can be fed into the National Grid. Electricity generated in this way is often referred to as 'nuclear power'. Generating electricity from nuclear reactions has the advantage that the process does not release any carbon dioxide or nitrogen and sulphur oxides. These gases contribute to the environmental problems of global warming and acid rain. Although nuclear power stations are less polluting for the atmosphere, there are other problems such as how to make sure no radiation escapes into the environment and how to safely dispose of the used nuclear fuel which is highly radioactive.

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The two main fuels for nuclear power stations are the isotopes uranium-235 and plutonium-239. They are used because, when the nucleus of either isotope is hit by a neutron, they become unstable and split. We say that the two isotopes are fissionable. During fission, the nucleus splits into two smaller nuclei (referred to as daughter nuclei) and emits some neutrons, usually either two or three. Where you had one neutron to begin with, you end up with two or three. As well as this, some energy is released in the form of heat and gamma radiation.

One of the tasks you may be asked to do in the GCSE exam is to balance an equation involving nuclear fission. This is simply testing if you understand the principle of conservation of matter. Take for example the fission of a plutonium-239 nucleus. If this was hit by a neutron, two possible products of the fission would be cerium-148 and krypton-89 so how many neutrons would be produced? To work this out, you need to use your knowledge of atomic structure. The number 239 indicates that the nucleus of plutonium contains a total of 239 particles. One neutron is needed for the fission to occur so on the left hand side of the equation there are 240 particles in total. That means there must also be 240 particles on the right hand side too. Adding together the 148 particles of the cerium and the 89 of the krypton gives 237 so 3 neutrons are released.

The neutrons released in nuclear fission reactions will then hit other atoms and cause them to split and so on. This is called a chain reaction. If this is not controlled, it very quickly gets out of hand creating a nuclear explosion, that is how the first atomic bombs worked. Later, more powerful hydrogen bombs were developed - these work differently. In the core of a nuclear power station, the neutrons released during nuclear fission are carefully controlled by absorbing them using special control rods. If the neutrons are moving too fast or too slowly, fission will not occur - water is used in some types of reactor as a moderator to ensure that as many neutrons as possible are moving at the correct speed to keep the reaction going in the core.

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  1. Which two fissionable substances are commonly used in nuclear reactors?
    Not all isotopes of these two elements are fissionable
  2. What happens to the atom after it absorbs a neutron?
    The neutrons released start a chain reaction
  3. In nuclear power stations, a chain reaction is required. What is a chain reaction?
    It keeps going without the need for any extra help
  4. What is an advantage of nuclear fission?
    When compared with the amount of fossil fuel needed to release the same amount of energy, it is a tiny fraction
  5. What is nuclear fission?
    The smaller nuclei are called daughter nucleides
  6. What happens to the excess neutrons?
    As more neutrons are created by the fission reaction, more atoms absorb the now-free neutrons creating an avalanche effect until all of the fissionable material is used
  7. What is a disadvantage of nuclear fission?
    The waste produced by nuclear fission can stay radioactive for centuries and as such can be costly to dispose of
  8. What must a nucleus absorb for fission to occur?
    For fission to occur, neutrons are fired at the atomic nucleus
  9. If Uranium-235 and one neutron are collided together they can create one Kr92 and 1 Ba142. In this circumstance, how many free neutrons are released in this reaction?
    This question required you to balance the chemical equation in terms of the number of nuclear particles. There are 236 particles to begin with, so there must be 236 after the fission. Always remember that you will always have the same amount of particles on either side of the reaction. What goes in must always come out!
  10. In a nuclear reactor, water is used as a moderator. What is a moderator?
    If the neutrons are moving too quickly, fission will not occur

Author: Martin Moore

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