Ketjureaktio

Ketjureaktio is the Finnish term for a chain reaction, defined as a sequence of chemical or nuclear steps in which reactive intermediates produced in one step propagate the process by generating more intermediates, allowing the reaction to continue with little or no external input. A chain reaction proceeds through distinct stages and terminates when the reactive intermediates are removed or neutralized.

In nuclear physics, a chain reaction occurs when neutrons released by fission induce further fission in neighboring nuclei. If the process sustains itself, it is self-sustaining or critical; if it accelerates, it becomes supercritical, and if it dies out, subcritical. Controlling the neutron population with moderators, reflectors, and control rods is essential for safety and for maintaining a desired rate.

In chemistry, chain reactions involve reactive intermediates such as free radicals or ions. Initiation creates these intermediates, propagation uses them to form new intermediates and additional product, and termination removes radicals through recombination or scavenging. Chain reactions are common in radical halogenation, oxidation, and many combustion processes. In polymer chemistry, radical polymerization is a typical chain reaction, where growth of polymer chains occurs in propagation steps and ends when termination occurs by combination or disproportionation.

Key concepts include chain carriers (radicals or neutrons), chain length (average number of propagation steps), and branching (alternative pathways that can accelerate or complicate the process). Chain reactions can be highly exothermic and sensitive to temperature, impurities, and inhibitors, which can suppress or terminate the sequence. Ketjureaktio thus encompasses a range of phenomena united by the shared mechanism of self-propagating reactive intermediates.