19.2 Nuclear Fission

19.2 Nuclear Fission

Otto Hahn and Fritz Strassmann of Germany while working upon the nuclear reactions made a startling discovery. They observed that when slow moving neutrons are bombarded on $_{92}^{235}\mathrm{U}$, then as a result of the nuclear reaction $_{92}^{235}\mathrm{Ba}$, $_{36}^{92}\mathrm{Kr}$ and an average of three neutrons are obtained. It may be remembered that the mass of both krypton and barium is less than that of the mass of uranium. This nuclear reaction was different from other nuclear reactions, in two ways: Firstly as a result of the breakage of the uranium nucleus, two nuclei of almost equal size are obtained, whereas in the other nuclear reactions the difference between the masses of the reactants and the products was not large. Secondly, a very large amount of energy is given out in this reaction.

A reaction in which a heavy nucleus like that of uranium splits up into two nuclei of roughly equal size along with the emission of energy during the reaction is called fission reaction. Fission reaction of $_{92}^{235}\mathrm{U}$ can be represented by the equation:

here $Q$ is the energy given out in this reaction. By comparing the total energy on the left side of the equation with total energy on the right side, we find that in the fission of one uranium nucleus, about $200\,\mathrm{MeV}$ energy is given out. It may be kept in mind that there is no difference between the sum of the mass and the charge numbers on both sides of the equation. Fission reaction is shown in Fig. 19.2. Fission reaction can be easily explained with the help of graph of Fig. 19.1. This graph shows that the binding energy per nucleon is greatest for the middle elements of the periodic table and this binding energy per nucleon is a little less for the light or very heavy elements i.e., the nucleons in the light or very heavy elements are not so rigidly bound.

(Fig. 19.2: Process of fission reaction)

For example, the binding energy per nucleon for uranium is about $7.6\,\mathrm{MeV}$ and the products of the fission reaction of uranium, namely barium and krypton, have binding energy of about $8.5\,\mathrm{MeV}$ per nucleon. Thus, when a uranium nucleus breaks up into barium and krypton, as a result of fission reaction, an energy at the rate of $(8.5-7.6) = 0.9\,\mathrm{MeV}$ per nucleon is given out. This means that an energy $235 \times 0.9 \approx 211.5\,\mathrm{MeV}$ is given out in the fission of one uranium nucleus. The fission process of uranium does not always produce the same fragments (Ba, Kr). In fact, any of the two nuclei present in the upper horizontal part of binding energy could be produced. Two possible fission reactions of uranium are given below as an example:

Hence, in the uranium fission reaction, several products may be produced. All of these products (fission fragments) are radioactive. Fission reaction is not confined to uranium alone; it is possible in many other heavy elements. However, it has been observed that fission takes place very easily with the slow neutrons in uranium-$235$ and plutonium-$239$, and mostly these two are used for fission purposes.