MP Board · Class 12 · Physics · NucleiExplain the phenomenon of nuclear fission. How is energy released in a nuclear fission reaction? Discuss the principle of a nuclear reactor with the help of a labeled diagram or description of its essential parts.

Definition and Process of Nuclear Fission\nNuclear fission is a reaction in which a heavy nucleus (such as Uranium-235 or Plutonium-239) splits into two or more smaller nuclei of comparable masses, accompanied by the release of a massive amount of energy. When a slow-moving thermal neutron is absorbed by a uranium-235 nucleus, it forms an unstable compound nucleus of Uranium-236, which immediately breaks apart.

Origin of Energy Release\nThe total mass of the fission products (lighter nuclei and emitted neutrons) is slightly less than the initial mass of the parent nucleus and the bombarding neutron. According to Albert Einstein's mass-energy equivalence relation ($E = \Delta m \cdot c^2$), this "missing" mass, known as the mass defect, is converted directly into energy. Each fission of a U-235 nucleus releases approximately 200 MeV of energy, mostly appearing as kinetic energy of the fission fragments, along with prompt neutrons and gamma rays.

Principle of a Nuclear Reactor\nA nuclear reactor operates on the principle of a controlled, self-sustaining chain reaction. When U-235 undergoes fission, it releases an average of 2 to 3 neutrons. These secondary neutrons can trigger fissions in other U-235 nuclei, producing more neutrons and creating a chain reaction.

Essential Parts of a Nuclear Reactor

1. Nuclear Fuel: Typically enriched uranium ($_{92}U^{235}$) which undergoes fission.
2. Moderator: Materials like heavy water ($D_2O$), graphite, or ordinary water used to slow down the fast neutrons produced during fission so they become thermal neutrons capable of further fission.
3. Control Rods: Rods made of neutron-absorbing materials such as cadmium, boron, or hafnium that are inserted into the core to regulate the reaction rate by absorbing excess neutrons.
4. Coolant: A fluid (such as water, liquid sodium, or gas) used to transfer the heat generated in the reactor core to a steam generator for electricity production.
5. Shielding: Thick concrete and lead walls surrounding the reactor to protect operators and the environment from harmful radiation.