When a nucleus is bombarded with some sub-atomic particles such as a-particles, neutrons, protons, etc., these particles are either captured by the target nucleus or the nucleus disintegrates ejecting some other sub-atomic particles. The new element formed has mass either slightly greater than or slightly smaller than that of the parent element. In 1939, it was observed by Hahn and Strassmann that when uranium- 235 is hit by slow neutrons, it splits into a number of fragments each of which has mass much smaller than that of uranium. The two main fragments formed are barium and krypton. Usually 2 or 3 neutrons are also ejected from every target uranium atom. The process is accompanied by the liberation of a large amount of energy. The reaction may be written as
The process of splitting of a heavier nucleus like that of uranium-235 into a number of fragments of much smaller mass, by suitable bombardment with sub-atomic particles is called nuclear fission.
When uranium-235 is bombarded with slow neutron, uranium-236 is formed which being unstable, further breaks up in several different ways as described below:
Fig. 13.4. Fission of 235U .
The tremendous amount of energy released during nuclear fission is because of the loss in mass. The sum of the masses of the fragments produced and neutrons released as a result of fission is less than the sum of the masses of target 235U and bombarding neutron. The loss in mass gets converted into energy according to Einstein equation, E=mc2
where, m is mass, c is velocity of light, E is energy. For example, let us calculate the loss of mass when 235Unuclide splits up into 144Ba and 90Kr along with the release of two neutrons.
Thus, mass defect or mass converted into energy is given
Mass defect (Δm) = 236.127-235.846 = 0.281 amu
But we know, 1 amu = 931.48 MeV
:. Energy released = 931.48 x 0.281 = 261.75 MeV.
Hence, for every uranium atom undergoing fission, the energy released is approximately 261.75 MeV which corresponds to about 8.5 x 107 kJ per gram or 2 x 1010 kJ per mol of uranium.
We have seen that fission of U-235 nucleus results in the emission of 2 to 3 neutrons.
On the average, 2.5 neutrons per U-235 nucleus are emitted.
THE NUCLEAR CHAIN PROCESS
The neutrons emitted from the fission of first uranium atom hit other uranium nuclei and cause their fission resulting in the release of more neutrons which further continue the fission process. In this way, a nuclear chain reaction sets The up releasing tremendous amount of energy. A schematic view f nuclear chain process is shown in Fig. 13.5.
If the chain reaction takes place completely as mentioned above, the energy released would be extremely high. However, some of the released neutrons escape from the surface
unused and do not involve in the chain process. In spite of this, the net amount of energy released is quite high. It has been found that the energy available from 1 kg of uranium-235 is equivalent to that available from 2 x 104 kg of coal.
Concept of Critical mass. One of the important aspect of the chain process is that in order to continue or sustain the chain nuclear process there should be sufficient fissionable material. The minimum amount that the fissionable material must have so as to continue the chain reaction process under the given set of conditions is called the critical mass. If the mass of the material is more than the critical mass, it is referred to as super-critical mass whereas if the mass of the
fissionable material is smaller than the critical mass, it is called sub-critical mass. The critical mass of U-235 is between 1 kg to 100 kg.
It may be noted that the naturally occurring uranium contains mostly U-238 isotope (about 99.3%) which is not fissionable with the slow neutrons.
The tremendous amount of energy released during fission process if uncontrolled, can be used for destructive purposes as for example, in the atomic bomb. However, if the chain reaction is controlled, the energy released can be used for constructive purposes as for example, in the nuclear reactor. The principle of each of these devices is briefly explained as follows:
The basic principal of atomic bomb is nuclear chain fission reaction. It consists of several sub-critical samples of U-235 (or plutonium-239). A conventional explosive such as TNT is placed behind each sample. The explosion causes these sub-critical samples of U-235 to join together so as to constitute super critical mass. A stray neutron from Ra-Be source (S) initiate the fission process which starts the chain reaction finally leading to explosion and also releasing large
amount of heat energy. The rapid release of heat raises the temperature enormously and generates a very high pressure front in the surrounding atmosphere .
The arrangement in its simple form is shown in Fig. 13.6
We have studied that in fission reactions, a heavy nucleus splits into smaller fragments and· difference in mass being converted into energy. Nuclear fusion is opposite to nuclear fission in the sense that it is a reaction in which two or more nuclei combine to form a heavy nuclide. However, the similar aspect of both the processes is that there is liberation of large amount of energy. Thus, nuclear fusion may be defined as a process in which lighter nuclei fuse together to form a heavier nucleus. Like nuclear fission, mass defect also occurs in nuclear fusion reactions which is responsible for the tremendous amount of energy released. Some examples of nuclear fusion reactions are:
Nuclear fusion reactions require very high temperature, generally> 106 K in order to overcome electrostatic repulsion between the nuclei when they come together to fuse. The nuclear fusion reactions, therefore, are also called thermonuclear reactions.
The high temperature required for fusion to start can be made available only by nuclear fission process.
DIFFERENCE BETWEEN NUCLEAR FISSION AND NUCLEAR FUSION
The important points of differences between nuclear fission and nuclear fusion have been summed up as below in tabular form.