CBSE · Class 12 · Physics · NucleiExplain the phenomenon of nuclear fission. How does the release of neutrons lead to a chain reaction? Discuss the difference between a controlled and an uncontrolled chain reaction with the help of examples.

Introduction to 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 mass, releasing a tremendous amount of energy along with a few neutrons. The discovery of fission by Otto Hahn and Fritz Strassmann in 1938 marked the dawn of the nuclear age.

Mechanism of Fission and Chain Reaction\nWhen a slow-moving (thermal) neutron strikes a uranium-235 nucleus, it is absorbed, forming an unstable compound nucleus of Uranium-236. This compound nucleus immediately splits into smaller fragments (like Barium and Krypton) and releases typically 2 to 3 fast neutrons along with roughly 200 MeV of energy per fission event.

• Secondary Neutrons: The neutrons released during fission can go on to strike other U-235 nuclei, causing them to undergo fission as well.
• Chain Reaction: This continuous process of neutron production and absorption resulting in successive fissions is known as a chain reaction. For a chain reaction to sustain itself, the reproduction factor ($k$) must be equal to or greater than 1.

Controlled Chain Reaction\nIn a controlled chain reaction, the number of neutrons available to cause further fission is carefully regulated so that the reaction proceeds at a steady, manageable rate.

• Mechanism: Control rods made of neutron-absorbing materials such as cadmium or boron are inserted into the reactor core to absorb excess neutrons.
• Example: Nuclear reactors used for power generation operate on controlled chain reactions, where heat is extracted to produce electricity.

Uncontrolled Chain Reaction\nIn an uncontrolled chain reaction, the number of fissions multiplies exponentially in a fraction of a second, releasing massive amounts of destructive energy almost instantaneously.

• Mechanism: There are no control mechanisms to absorb excess neutrons, leading to a rapidly escalating avalanche of fission events.
• Example: An atomic bomb (fission bomb) is a practical application of an uncontrolled chain reaction.