CBSE · Class 12 · Physics · NucleiExplain the phenomenon of Nuclear Fission and Nuclear Fusion. Discuss how energy is released in these processes on the basis of the binding energy per nucleon curve.

Introduction to Nuclear Reactions\nNuclear reactions involve changes in the nucleus of an atom, resulting in the transformation of elements and massive release of energy. The two primary types of nuclear reactions are nuclear fission and nuclear fusion.

1. Nuclear Fission

• Definition: Nuclear fission is the process in which a heavy nucleus (such as Uranium-235) splits into two or more smaller, medium-sized nuclei when bombarded with low-energy neutrons.
• Mechanism: When a slow neutron is absorbed by a $^{235}_{92}U$ nucleus, it forms an unstable compound nucleus which immediately breaks down into fission fragments (e.g., Barium and Krypton), accompanied by the emission of 2 to 3 neutrons and a tremendous amount of energy (about 200 MeV per fission event).
• Chain Reaction: The emitted neutrons can trigger further fissions in surrounding uranium nuclei, leading to a self-sustaining chain reaction which forms the principle of nuclear reactors and atomic bombs.

2. Nuclear Fusion

• Definition: Nuclear fusion is the process in which two or more light nuclei combine together to form a heavier nucleus, accompanied by the release of energy.
• Mechanism: For instance, four hydrogen nuclei (protons) fuse to form a helium nucleus under extreme temperature and pressure conditions (stellar interiors), releasing vast amounts of energy (approx. 26.7 MeV).
• Requirements: High temperature (order of $10^7$ K) and high pressure are required to overcome the electrostatic repulsion between positively charged nuclei.

3. Explanation Based on Binding Energy Curve

• Binding Energy per Nucleon ($BE/A$): The stability of a nucleus depends on its binding energy per nucleon. The $BE/A$ curve shows a rising trend for light nuclei, a broad maximum around $A = 56$ (iron region) with a value of about 8.8 MeV/nucleon, and a gradual decrease for heavy nuclei down to about 7.6 MeV/nucleon for uranium.
• Energy Release in Fission: When a heavy nucleus ($A \approx 240$) splits into two medium-sized nuclei ($A \approx 120$), the total binding energy increases because the fragments lie higher up on the $BE/A$ curve. This increase in total binding energy manifests as the release of kinetic energy and radiation.
• Energy Release in Fusion: When two very light nuclei ($A \le 10$) combine to form a heavier nucleus, the product nucleus has a higher binding energy per nucleon than the reactants. The mass defect resulting from this increased stability is converted into energy according to Einstein's mass-energy equivalence relation ($E = \Delta mc^2$). \nThus, both processes result in an increase in total binding energy, explaining the massive energy release.