chainreaction

A chain reaction is a process in which a single initiating event triggers a series of subsequent reactions, such that the outcome propagates and can continue with little or no additional input.

In chemistry, chain reactions involve reactive intermediates called chain carriers (such as free radicals or ions) that form in an initiation step, then propagate the reaction by successive steps, and are eventually terminated by side reactions. The overall rate depends on the balance of initiation, propagation, and termination. Common examples include radical polymerization, where growing chains generate more radicals to continue growth, and certain combustion processes.

In nuclear physics, a nuclear chain reaction occurs when fission of a fissile nucleus releases neutrons that cause further fissions. If the number of neutrons in successive generations remains constant or increases, the reaction is critical or supercritical; if it dies out, subcritical. Moderators slow neutrons, while absorbers or control rods limit the reaction. Nuclear chain reactions power most reactors and can release large energy in weapons; safety relies on maintaining subcriticality under normal conditions.

Outside chemistry and physics, the term describes any cascade where one event triggers a sequence of events, such as information or economic cascades in networks, or cascading failures in infrastructure.

Since the early 20th century, the concept has been central to theories of reaction kinetics and, in the 1930s, to the understanding of nuclear fission.