“Such a nuclear reaction in which two light nuclei merge to form a heavy nucleus is called fusion reaction”
We known that the energy given out per nucleon per fission of heavy element like that of uranium is 0.9 MeV. It is due to the fact that the binding energy per nucleon of the fission fragments is greater than uranium. In fact energy is obtained from any nuclear reaction in which the binding energy per nucleon of the products increases. Is there any other reaction besides the fission reaction from which energy could be obtained? In order to answer this question must ponder over again. This graph shows that the binding energy per nucleon increases upto A = 50. Hence when two light nuclei merge together to from a heavy nucleus whose mass number A is less than 50, then energy is given out. In section on “Mass Defect and Binding Energy” we have observed that when two protons and two neutrons merge to form a helium nucleus, then about 28 MeV energy is given out.
During a fusion reaction some mass is lost and its equivalent energy is given out. In a fusion reaction, more energy per nucleon can be obtained as compared to the fission reaction. But unfortunately it is comparatively more difficult to produce fusion. Two positively charge light nuclei must be brought very close to one another . To do so work has to be done against the electrostatic force of repulsion between the positively charged nuclei. Thus a very large amount of energy is required to produce fusion reaction. It is true that a greater amount of energy can be obtained during a fusion reaction compared to that produced during a fission reaction,but in order to start this reaction a very large amount of energy is spent. On the contrary no difficult is faced to start the fission reaction because neutron has no charge on it and it has to face no repulsive force while reaching the nucleus.
Let us take the example of fusion reaction when two deuterons are merged to form a helium nucleus, 24 MeV energy is released during this process i.e.,
But there is very little chance of the formation of nucleus by the merger of two deuterons. The probability occurring such a reaction is great one proton where one proton or one neutron is produced as given below:
In both of these reactions about 1.0 MeV energy per nucleon is produced which is equal to the energy produced during fission. If and are forced to fuse then 17.6 MeV energy obtained I.e.,
We known that for fusion of two light nuclei the work has to be done to overcome the repulsive force which exists between them. For this the two nuclei are hurled towards one another at a very speed. One method to do so is to give these nuclei a very large velocity with the help of an accelerator. This method has been used in the research study of nuclear fusion of and . But this method of nuclear fusion for getting energy cannot be used on a large scale.
There is another method to produce fusion reaction. It is based upon the principle that the speed of atoms of a substance increases with the increase in the temperature of that substance. To start a fusion reaction the temperature at which the required speed of the light nuclei can be obtained is about 10 millions degrees Celsius. As such extraordinary high temperature the reaction that takes place is called thermo-nuclear reaction. Ordinary such a high temperature cannot be achieved. However during the explosion of am atom bomb this temperature can be had for a very short time.
Until now the fusion reaction is taking place only in a hydrogen bomb.That extraordinary high temperature is obtained during the explosion of an atom bomb,due to this high temperature the fusion reaction between and sets in.In this way a very large amount of energy is given out with the explosion.
A very large amount of energy can be had from a fusion reaction,but till now this reaction has not been brought under control like a fission reaction and so is not being used to produce electricity.Efforts are in full swing in this field and it is hoped that in near future some method would be found to control this reaction as well.