Class 12 Physics Chapter 13 Nuclei
In this blog we will discuss about nuclear fission and fusion. Before starting with the talk, let’s first speak about nuclear energy. Day by day need of energy is increasing. We consume energy by burning fossil fuels. But we also know that, these fuels will finish one day. Therefore, there is need think about some alternative source. Solution to this problem can be given in form of nuclear energy. It is the energy released during the time of nuclear fission or fusion. So,let’s first know what is nuclear fission.
It is defined as, “Breaking of heavy nucleus into two middleweight nuclei with the release of energy.” It is believed that, energy obtained from nuclear process is about million times larger than the energy obtained from same quantity of conventional source. Enrico Fermi, an Italian – American physicist, found that, when a neutron strikes over uranium nucleus, the uranium nucleus splits into two nearly equal fragments, thereby, releasing enormous amount of energy. Such a reaction is termed as nuclear fission. Following steps shows nuclear fission of Uranium.
1. 01n + 92235U ⟶ 92236U ⟶ 56144Ba + 3689Kr + 301n
2. 01n + 92235U ⟶ 92236U ⟶ 51133Sb + 4199Nb + 4 01n
3. 01n + 92235U ⟶ 54140Xe + 3894Sr + 201n
Fragments produced have large number of neutrons and therefore these are unstable. However, these fragments undergo successive beta emission until each reaches a stable end product. Have you ever imagined, how much of energy is released during the nuclear fission reaction?
Energy released during nuclear fission reaction: Consider a heavy nucleus containing nucleons A = 240 and it is transforming to daughter nuclei each having A = 120, then the binding energy per nucleon A= 240 nucleus is about 7.6 MeV. However, binding energy per nucleon of A= 120 fragment nuclei is 8.5 MeV. Thus gain in binding energy per nucleon is 0.9 MeV. Therefore, for 240 nucleons gain will be 240 × 0.9 = 216 MeV. This energy appears as kinetic energy of the fission product. This energy is immediately transferred to surrounding in form of heat. Such high amount of energy could burn everything in the surrounding. Thus, scientist have developed special device in which controlled nuclear fission is carried out in order to harness energy from nucleus.
Now we know about nuclear fission. But, there is also the possibility that two lighter nuclei combine into larger and more tightly bound nucleus, thereby, releasing energy. This is called as nuclear fusion. Following are some examples of nuclear fusion.
1. 11H + 11H ⟶ 12H + e+ + v + 0.42 MeV
2. 12H + 12H ⟶ 23He + n + 3.27 MeV
3. 12H + 12H ⟶ 13H + 11H + 4.03 MeV
The process of nuclear fusion is opposed by coulomb’s repulsive forces existing between the positive charges of the two combining nuclei. Thus, there exist an energy barrier due to opposing force, which the combining nuclei must overcome and come in range of attractive nuclear force and thus facilitating fusion process. The extent or height of coulomb barrier depends on the charges and radii of two interacting nuclei. In short,higher the charge, greater will be the coulomb’s repulsion. Thus, higher will be the barrier height. The essential way to achieve fusion of two lighter nuclei is carrying out the nuclear fusion in very high temperature environment, such that the temperature of the material can be raised, the particle gain energy by thermal motion. By gaining enough energy they can pass through the energy barrier and get fused. Such a process is called thermonuclear fusion. In return it releases large amount of energy.
Controlled thermonuclear fusion: The first thermonuclear reaction on earth occurred on 1st November 1952, where USA exploded a fusion device, generating energy equivalent to 10 million tons of TNT. One ton of TNT explosion releases 2.6 × 1022 MeV of energy. To manage such large amount of energy is difficult. Therefore, a sustained and controllable source of fusion power is more difficult to achieve. Fusion reactor is regarded as the future power source and so world is continuously trying to harness energy from nuclear fusion.
Keywords: Fusion, Fission, Nuclear fusion, Nuclear fission, Thermonuclear fusion, Uranium