Context: The fourth unit of the Kakrapar Atomic Power Project (KAPP-4) in Gujarat, with 700 MWe capacity, started controlled fission chain reaction and thus became critical.

About Kakrapar Atomic Power Project (KAPP)

• Kakrapar Atomic Power Project is built by the Nuclear Power Corporation of India Limited (NPCIL), a public sector undertaking of the Department of Atomic Energy.

• These reactors are Pressurised Heavy Water Reactors (PHWRs), which use natural uranium as fuel and heavy water as coolant and moderator.
• Coolant: A substance circulated through a nuclear reactor to remove or transfer heat. 
  • The most commonly used coolant in the is water. 
• Moderator: A material, such as ordinary water, heavy water, or graphite, that is used in a reactor to slow down high-velocity neutrons, thus increasing the likelihood of fission.

About Criticality and Nuclear Fission

• A reactor achieves criticality, when each fission event releases a sufficient number of neutrons to sustain an ongoing series of reactions.
  • The first criticality is the start of a controlled fission chain reaction.
  • This is what KAPS-4 achieved. After the fission, the heat is used to make steam that turns a turbine and produces electricity.
• Process: When a neutron (a particle with no electric charge) hits an atom like Uranium-235, and splits it into smaller atoms. This process is called nuclear fission, and it releases a lot of heat that can be converted into energy. 
• Fissile Choice: Uranium-235 is chosen for nuclear power due to its ease of splitting.
• Regulatory Approval: The criticality test was approved by India’s nuclear regulator, Atomic Energy Regulatory Board (AERB).

Significance:

• Signifies a crucial step in the operational readiness of the nuclear power plant.
• Efficient and continuous production of electricity through nuclear fission.

Also Read: Nuclear Power Plants In India 2023, here.