Fission and fusion for Edexcel GCSE Physics
This page covers the following topics:
2. Net decline
4. Chain reactions
Due to the random nature of radioactive decay, it can never be known which nucleus will decay next. However, half-life gives the time taken for half the nuclei of a sample to decay and thus the activity of the sample to halve. Although half-life is not affected by chemical and physical reactions, different isotopes have different values for half lives, which come with different hazards. Isotopes with shorter half-lives will be more intense due to the fact that more decays will occur per second, which can be extremely dangerous when ingested. Isotopes with longer half-lives will contaminate areas for very long periods of time.
The number of decaying nuclei in a sample reduces with time due to radioactive decay. This reduction can be expressed as net decline after a certain number of half-lives. Net decline can be expressed as (1/2)ˣ where x is the number of half-lives elapsed.
Fission is the process by which a large atomic nucleus is split up into smaller nuclei. A neutron hits a nucleus and is absorbed by it. This causes the nucleus to become unstable and thus it splits up into smaller nuclei, called the daughter nuclei, and more neutrons. The neutrons released carry most of the energy of the fission reaction as kinetic energy, and thus have to be slowed down before colliding with other nuclei. Fission has an advantage in generating energy over fossil fuels due to the fact that it requires less fissionable material to produce the same amount of energy.
Chain reactions are caused by fission. The additional neutrons produced in a fission reaction collide with other nuclei and cause further fission reactions, which are called chain reactions. Nuclear reactions thus must take place in nuclear reactors to control the speed and temperature of the reactions. The reactor is placed inside a concrete shield so as to protect from the hazardous radioactive nuclei released in the fission reactions. The nuclear fuel, which are the initial nuclei, are held in rods so that the neutrons released in the fission reactions will fly out and cause further ones in other rods. There are also control rods in reactors to control the speed of the chain reactions, which are raised and lowered into it to stop neutrons from travelling between rods. The rods are all in a graphite core, since graphite helps in slowing down the neutrons so that they are absorbed by a nearby fuel rod. The energy released by the fission reactions is used to heat up the coolant and is used to boil water, which in turn will be used to drive turbines and generate electricity.
Nuclear fusion is the process by which small and light nuclei join together to form a bigger and heavier nucleus. Comparing the combined mass of the smaller nuclei to the mass of the resulting nucleus may show that the latter is smaller. This missing mass is converted into energy and is radiated away. Fusion reactions must be quick to occur, so that the repulsion between the two positively charged nuclei does not have time to act. Nuclear fusion must also occur in very high temperatures and pressures to overcome the repulsion between the nuclei.
Define nuclear fusion.
Nuclear fusion is the process by which two small and light nuclei join together to form a bigger and heavier nucleus.
How does absorbing a neutron cause a nucleus to split?
Absorbing a neutron makes the nucleus unstable, which causes it to split.
Define net decline.
The number of decaying nuclei in a sample reduces with time due to radioactive decay. This reduction can be expressed as net decline after a certain number of half-lives.
Draw a diagram to represent nuclear fission.
What happens to the energy of a nuclear fission reaction?
The energy of the fission reaction is transferred to kinetic energy in the neutrons formed by the reaction.
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