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Welcome to our Physics lesson on More on Nuclear Fission. Nuclear Energy and Its Use, this is the sixth lesson of our suite of physics lessons covering the topic of Nuclear Reactions, you can find links to the other lessons within this tutorial and access additional physics learning resources below this lesson.
Earlier, we saw that some heavy nuclei split in two lighter nuclei after the capture of a slow neutron. This occurs because the parent nuclei acquire more energy than needed and as a result, they becomes excited. This is not a stable state, so the parent nuclei split to release this surplus of energy. During this process, at least two neutrons are also released for each single process. They have a high kinetic energy produced because the energy contained in the parent nucleus is higher than the total energy of daughter nuclei. This kinetic energy eventually turns into heat during particles deceleration when encountering a dense material on their way. In this way, nuclear reactions are used for obtaining heat energy, which eventually converts into other forms of energy such as electricity in nuclear power plants, thermal and chemical energy in nuclear bombs etc.
Obviously, there is a very large number of uranium nuclei participating in the process, so the energy obtained is very large. Therefore, the reaction must be controllable, otherwise it gets out of control and severe accidents may occur because of explosions arising when the energy delivered is higher than needed. The avalanche effect we explained earlier occurring when one neutron gives two new neutrons, two neutrons give four other neutrons and so on, must therefore be controllable to avoid troubles. The following conditions must meet in order to make this process take place safely:
There is a certain probability that the neutrons produced in the first stage of nuclear fission become effective "shells" to be used in new fission processes. If at least one new neutron is ensured from such a process, then it continues without the need for other neutrons coming from an external source.
If the number N2 of new neutrons produced during the nuclear fission is smaller than the number N1 of shell neutrons coming from the source to hit the uranium nuclei, then the process is not sustainable and it fades with time. In other words, if
then the process is not sustainable.
If the number N2 of new neutrons produced during the nuclear fission is equal the number N1 of shell neutrons coming from the source to hit the uranium nuclei, the system is known as critical and the process continues at a constant and controllable rate. In other words, if
then the process is sustainable and it takes place at constant rate.
If the number N2 of new neutrons produced during the nuclear fission is greater than the number N1 of shell neutrons coming from the source to hit the uranium nuclei, the system is known as supercritical and the process continues at an increasing and not controllable rate. In other words, if
then the process is not controllable despite the system is sustainable.
Controllable processes are better as we always are aware of the parameters involved in the process. This makes possible using the energy produced during the process in a practical way because the process can be stopped at any instant in case of needs fulfillment. A controllable and sustainable process of nuclear fission is known as chain process as explained earlier. The first controllable chain nuclear process was realized by Enrico Fermi in 1942.
There are a number of factors affecting the progress of a chain nuclear reaction such as temperature, type of reaction, as well as the amount of matter involved in the process. The geometrical shape of environment in which the reaction occurs also plays a role in this regard. However, the most relevant factor is the minimum mass of matter involved in the nuclear reaction, below which the reaction cannot occur. This minimum mass is known as the critical mass of nuclear reaction. Operating with values very close to critical mass is very important when nuclear reaction is involved, as this makes possible stopping the process very easily in case of anomalies. In addition, the self-activation of an uncontrollable (spontaneous) chain nuclear reaction is less likely to occur when working with such minimum amounts of nuclear material.
You have reached the end of Physics lesson 20.4.6 More on Nuclear Fission. Nuclear Energy and Its Use. There are 11 lessons in this physics tutorial covering Nuclear Reactions, you can access all the lessons from this tutorial below.
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