BharatNotes What is Chain Reaction?
A chain reaction is a self-propagating process in which the products of one reaction initiate further reactions of the same type, allowing the process to continue without external input. In the nuclear context, when a fissile nucleus such as Uranium-235, Plutonium-239 or Uranium-233 absorbs a neutron and splits (fission), it releases energy along with two to three fresh neutrons. If these neutrons strike other fissile nuclei and cause further fissions, a fission chain reaction is established. The first human-made self-sustaining nuclear chain reaction was achieved by Enrico Fermi's team in the graphite-pile reactor Chicago Pile-1 at the University of Chicago on 2 December 1942.
Key Features: Criticality and Control
The behaviour of a chain reaction is described by the neutron multiplication factor (k) — the ratio of neutrons in one generation to those in the previous generation.
| Condition | Multiplication factor | Outcome | Application |
|---|---|---|---|
| Sub-critical | k < 1 | Reaction dies out | Shut-down reactor |
| Critical | k = 1 | Self-sustaining, steady | Operating nuclear reactor |
| Super-critical | k > 1 | Reaction grows rapidly | Reactor start-up; nuclear weapon (uncontrolled) |
In a reactor, the chain reaction is kept controlled using control rods (neutron absorbers such as boron or cadmium) and a moderator (heavy water, light water or graphite) that slows neutrons to speeds at which fission is more probable. An atomic bomb, by contrast, is an uncontrolled super-critical chain reaction in a mass of fissile material exceeding the critical mass.
Significance for India
India's mastery of the controlled chain reaction dates to Apsara, Asia's first research reactor, which attained criticality at Trombay on 4 August 1956 under Homi J. Bhabha's leadership. Today, the controlled chain reaction powers India's nuclear fleet of about 25 reactors with roughly 8.8 GW installed capacity (as of 2025), contributing around 3% of electricity generation. Bhabha's three-stage nuclear power programme — PHWRs, fast breeder reactors, and thorium-based reactors — is essentially a roadmap of progressively advanced chain-reaction technologies suited to India's vast thorium reserves.
Current Status (as of June 2026)
The 500 MWe Prototype Fast Breeder Reactor (PFBR) at Kalpakkam, built by BHAVINI with technology from IGCAR, attained first criticality on 6 April 2026 (at 8:25 pm, per the Department of Atomic Energy) — marking India's formal entry into Stage II of its nuclear programme. The Union Budget 2025-26 announced a Nuclear Energy Mission targeting at least 100 GW of nuclear capacity by 2047, along with proposed amendments to the Atomic Energy Act, 1962 and the Civil Liability for Nuclear Damage Act, 2010 to enable private participation.
UPSC Angle
This is a foundational concept that underpins questions on nuclear reactor working, fissile vs fertile materials (U-238 and Th-232 are fertile, not fissile), the role of moderators and control rods, criticality, the three-stage programme, and small modular reactors. Aspirants should be able to distinguish a controlled chain reaction (reactor) from an uncontrolled one (weapon), and link the concept to current developments such as the PFBR criticality and the 100 GW nuclear target.
