Nuclear fusion is a process in which two or more lighter nuclei combine to form a heavier nucleus. The mass of the product nucleus is always less than the sum of the masses of the individual lighter nuclei. The difference in mass is converted into energy. The fusion process can be carried out only at a extremely high temperature of the order of 107 K because, only at these very high temperatures the nuclei are able to overcome their mutual repulsion. Therefore before fusion, the lighter nuclei must have their temperature raised by several million degrees. The nuclear fusion reactions are known as thermo-nuclear reactions.
The principle of nuclear fusion is used in hydrogen bomb. It is an explosive device to release a very large amount of energy by the fusion of light nuclei. The temperature required for the purpose of fusion is produced by fission reactions. The explosion of an atom bomb produces temperature of the order of 50 million degree celcius. A suitable assembly of deuteron and triton is arranged at the sight of the explosion of the atom bomb. Favourable temperature initiates the fusion of light nuclei in an uncontrolled manner. This releases enormous amount of heat energy.
The fusion reaction in hydrogen bomb is
1H3 + 1H2 -- -- > 2He4 + 0n1 + energy
Fusion is the source of stellar energy. The temperature of the interior of the sun is about 1.4 × 107 K and the temperature of some stars is of the order 108 K. It has been estimated that the total energy radiated by sun is about 3.8 × 1026 joule per second. The origin of such a tremendous amount of energy is neither chemical nor gravitational. The fusion of protons into helium is supposed to release energy in sun and stars. All the elements like hydrogen and helium are in a highly ionised state called plasma at such a high temperature. The energy produced in fusion is responsible for the maintenance of high temperature of the sun and stars and also for the emission of energy in the form of heat and light.
Proton−proton cycle and carbon−nitrogen cycle are the two important types in which nuclear fusion takes place in sun and stars.
Proton - Proton cycle
1H1 + 1H 1 → 1H2 +1e 0 + ν (emission of positron and neutrino) 1H1 + 1H2 → 2He3 + γ (emission of gamma rays)
22He3 → 2He4 + 21H1
The reaction cycle is written as
41H1 → 2He4 + 21e0 + 2ν + energy (26.7 MeV)
Thus four protons fuse together to form an alpha particle and two positrons with a release of large amount of energy.
Carbon - Nitrogen Cycle
The following cycle of reactions take place in carbon - nitrogen cycle in which carbon acts as a catalyst.
1H1 + 6C12 → 7N13* + γ (emission of gamma rays)
7N13* → 6C 13 + 1e0 + ν (emission of positron and neutrino) 1H1 + 6C13 → 7N14 + γ (emission of gamma rays)
1H1 + 7N14 → 8015* + γ (emission of gamma rays)
8015* → 7N15 + 1e0 + ν (emission of positron and neutrino)
1H1 + 7N15 → 6C12 + 2He4
The overall reaction of the above cycle is given as 4 1H1 → 2 He4 + 21e0 + 2ν + energy (26.7 MeV)
Copyright Â© 2018-2020 BrainKart.com; All Rights Reserved. Developed by Therithal info, Chennai.