Question

What is nuclear fission? Discuss the source of energy released during fission.

Answer 1

Nuclear fission: A process in which a heavy nucleus disintegrates into two lighter nuclei of nearly equal masses, is called nuclear fission

Slow neutrons were bombarded on a heavy nucleus of ₉₂U²³⁵ yielding two lighter nuclei of barium (₅₆Ba¹⁴¹) are krypton (₃₆Kr⁹²) along with three neutrons and tremendous amount of energy Fig. The nuclear reaction can be represented as follows:

₉₂U²³⁵ + ₀n¹ → ₅₆Ba¹⁴¹ + ₃₆Kr⁹² + 3 ₀n¹ + Q

Energy released in fission. This huge energy is provided by the difference of mass of the nuclei before and after fission.

Before fission:

Mass of ₉₂U²³⁵ = 235.0439 a.m.u.

Mass of one ₀n¹ = 1.0087 a.m.u.

Total mass of reactants = 236.0526 a.m.u.

After fission. Mass of ₅₆Ba¹⁴¹ = 140.9139 a.m.u.

Mass of ₃₆Kr⁹² = 91.8973 a.m.u.

Mass of 3 neutrons = 3.0261 a.m.u

Total mass of reaction products = 235.8373 a.m.u.

Mass lost in fission = 236.0526 - 235.8373 = 0.2153 a.m.u.

This mass is converted into energy in the fission process. Thus, energy released in a single fission = 2153 x 931 = 200 MeV

This energy appears mainly in the form of kinetic energy of fission fragments and secondary neutrons. It may be subsequently converted into heat. The nuclear fission is important reaction for two reasons.

• A large amount of energy about 200 MeV is released in fission of each nucleus.
• The reaction is accompanied by emission of secondary neutrons.

Theory. Bohr and Wheeler gave the explanation of the nuclear fission based on the liquid drop model of the nucleus. The nucleus is considered to be spherical in shape and is acted upon by two kinds of forces.

• The surface tension force (maintaining its spherical shape)
• The coulomb's repulsive force (tending to deform the shape).

When these spherical nuclei are bombarded by slow neutrons, results in its capture and form a compound nucleus and sets oscillations in the nucleus tending to deform its shape and the surface tension forces try to keep it spherical, but if the effect of coulomb’s force is increased, the nucleus ultimately splits into two parts of comparable masses and few neutrons. The sequence of steps leading to fission is as shown in Fig.