19.4 Nuclear Fusion

19.4 Nuclear Fusion

The curve of the graph in Fig. 19.1 also shows that the binding energy per nucleon increases up to $A = 50$. Hence, when two light nuclei merge together to form a heavy nucleus whose mass numbers $A$ is less than $50$, then energy is given out. In the topic on mass defect and binding energy, we have observed that when two protons and two neutrons merge to form a helium nucleus, then about $27\,\mathrm{MeV}$ energy is given out.

A nuclear reaction in which two light nuclei merge to form a heavy nucleus is called fusion reaction. 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 more difficult to produce fusion. Two positively charged 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 has to be spent. The probability of occurring fusion of two lighter nuclei is great where one proton or one neutron is produced as given below:

or

In both of these reactions, about $1.0\,\mathrm{MeV}$ energy per nucleon is produced which is equal to the energy produced during fission. If $_{1}^{2}\mathrm{H}$ and $_{1}^{3}\mathrm{H}$ are forced to fuse, then $17.6\,\mathrm{MeV}$ energy is obtained. i.e.,

In above reactions, $_{1}^{2}\mathrm{H}$, $_{1}^{3}\mathrm{H}$ are termed as fusion fuels. We know 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 collided towards one another at a very high speed. One method to do so is to give these nuclei a very large velocity with the help of an accelerator as shown in Fig. 19.6.

(Fig. 19.6: Fusion reaction)

Key

• Neutron
• Proton
• Deuterium
• Tritium
• Helium

This method has been used in the research study of nuclear fusion of $_{1}^{2}\mathrm{H}$ and $_{1}^{3}\mathrm{H}$. But this method of nuclear fusion for getting continuously 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 $10$ million degrees Celsius. At such extraordinarily high temperature, the reaction that takes place is called thermonuclear reaction. Ordinarily such a high temperature cannot be achieved. However, during the explosion of an atom bomb this temperature can be had for a very short time. Until now the fusion reaction has been observed in the experimental testing of hydrogen bomb by triggering it by an atomic bomb. 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. However, the fusion process is taking place in the core of stars including our Sun.

For your information

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An "artificial Sun" refers to experimental nuclear fusion reactors like China's EAST and HL-$2$M/Hl-$3$. They heat hydrogen isotopes to over $100$ million $^{\circ}\mathrm{C}$, hotter than the sun's core to mimic solar fusion for clean, limitless energy.