A chain reaction is a process in which the products of one reaction event initiate additional reaction events, allowing the sequence to propagate through successive generations. In nuclear physics, the term commonly refers to a fission chain reaction in which neutrons released by one fission event cause further fissions in other fissile nuclei. A single fission can release multiple neutrons, but not every neutron necessarily causes another fission.
Some are absorbed by nonfissile materials, some are captured by fuel without causing fission, and some escape from the reactor or other system. A self sustaining chain reaction therefore depends on achieving an appropriate balance between neutron production and neutron losses. The concept is different from a simple sequence of independent reactions because there is feedback between generations: products from earlier events influence the probability of later events. Chain reactions can occur in nuclear systems and in some chemical processes, although the physical mechanisms differ substantially. In nuclear engineering, the term has a precise meaning associated with neutron multiplication and reactivity.
The most important quantity used to describe a fission chain reaction is the effective multiplication factor, commonly represented by the symbol k. It compares the number of neutrons in one generation that are available to produce fissions with the corresponding number in the preceding generation. If k is less than one, the neutron population decreases and the chain reaction is subcritical. If k equals one, the neutron population remains approximately constant and the system is critical.
If k is greater than one, the neutron population increases and the system is supercritical. These terms describe neutron population behaviour and should not automatically be interpreted as simple labels for safety or danger. A reactor operating at a steady power level is generally maintained close to critical conditions, with heat production balanced by heat removal and with reactivity controlled through engineered systems. A subcritical assembly cannot maintain a continuing fission chain reaction without an external neutron source. A supercritical system can increase in power, but the rate of increase depends on neutron physics, reactor design, temperature feedback and the distinction between prompt and delayed neutrons.
Neutrons are especially effective at sustaining fission chains because they have no electrical charge and can therefore approach atomic nuclei without being repelled by the positive charge of the nucleus. In a thermal reactor, a neutron may first be emitted at high energy and then undergo repeated scattering collisions in a moderator, losing kinetic energy until it reaches thermal energies. For some fissile isotopes, the probability of fission is substantially greater for slow neutrons.
The moderator therefore increases the likelihood that emitted neutrons will produce subsequent fissions. Water, heavy water and graphite have all been used as moderators in different reactor designs. Fast reactors use little or no moderation and instead exploit fission probabilities at higher neutron energies. The chain reaction consequently depends not merely on the number of neutrons released but also on their energies, trajectories, interaction probabilities and the composition and geometry of the surrounding material. Neutron leakage from the core is another important factor, which is why reactor shape, size and reflector systems influence criticality.
Nuclear chain reactions have two important temporal components because fission produces both prompt and delayed neutrons. Prompt neutrons are emitted almost immediately during fission. Delayed neutrons are produced by the radioactive decay of certain fission products and appear over longer timescales ranging from fractions of a second to minutes.
Although delayed neutrons represent only a small fraction of the total neutron population, they are extremely important for reactor control because they slow the effective response of the reactor to changes in reactivity. This provides operators and control systems with a manageable timescale for regulating reactor power. The concept of prompt criticality refers to a condition in which prompt neutrons alone are sufficient to sustain an increasing neutron population, producing much faster power changes. Reactor physics therefore distinguishes prompt neutron behaviour from delayed neutron behaviour rather than treating all emitted neutrons as identical. Delayed neutron fractions, neutron lifetimes and reactivity coefficients are central quantities in detailed reactor kinetics.
A chain reaction can be deliberately controlled by altering neutron absorption, neutron leakage, fuel arrangement or neutron energy. Control rods absorb neutrons and therefore reduce the number available for additional fissions. Changes in moderator density can alter neutron slowing and absorption characteristics. Temperature changes can also affect reaction probabilities and material densities, creating feedback that can naturally oppose changes in reactor power in many designs.
Fuel depletion changes the concentration of fissile material over time, while the accumulation of fission products can absorb neutrons and influence reactivity. Xenon 135 is particularly important because it is a strong neutron absorber produced through fission product decay pathways. Reactor operators and automated control systems account for these effects when managing power. The chain reaction is therefore not a simple uncontrolled multiplication process. It is the outcome of many interacting neutron transport, nuclear reaction, thermal and material processes that are intentionally engineered to remain within defined operating conditions.
Chain reactions are also relevant outside nuclear fission. In chemistry, a chain reaction can involve reactive intermediates such as free radicals, where one reaction produces a species that initiates another reaction. Combustion and polymerisation can involve chain mechanisms of this kind. In nuclear physics, however, the propagating agents in a fission chain reaction are primarily neutrons, and the initiating events are nuclear transformations rather than chemical reactions.
The two meanings share the general idea of self propagating reaction sequences but differ in microscopic mechanism. A nuclear fission chain reaction also differs from radioactive decay, because an isolated radioactive nucleus can decay independently without requiring products from a previous decay to trigger it. Chain reactions depend on coupling between reaction events. This distinction is important when explaining why a reactor can sustain a large number of nuclear transformations even though each individual fission event is probabilistic. The collective behaviour arises from many microscopic events connected through neutron transport and nuclear interaction probabilities.
The first controlled artificial nuclear chain reaction was achieved in the Chicago Pile 1 experiment in 1942. The experiment demonstrated that a carefully arranged assembly of fissile material, moderator and neutron control could sustain a controlled reaction. This was a major milestone in nuclear physics and led to the development of reactors for research, isotope production, propulsion and electricity generation.
The same underlying principle also became relevant to nuclear weapons, where the objective and engineering requirements are fundamentally different from those of a power reactor. A power reactor is designed to maintain a controlled reaction over long periods while continuously removing heat, whereas a weapon uses an extremely rapid change in nuclear configuration to produce a short intense energy release. The historical association between chain reactions and both peaceful and military nuclear technology is therefore important, but the physical concept itself is neutral and applies to the propagation of nuclear fission events under appropriate conditions.
Understanding chain reactions requires recognising that a reactor can have a continuing nuclear reaction without experiencing an uncontrolled increase in power. Criticality means that the neutron population is stable on the relevant timescale, not that every neutron causes fission or that the reactor is at maximum possible power. Reactor power depends on the absolute rate of fission, while criticality describes the balance between successive neutron generations.
A reactor can be critical at a low power level or at a high power level. Changing power while remaining near critical involves changing conditions gradually so that the neutron population reaches a new stable level. Thermal feedback, delayed neutrons, control systems and coolant behaviour all influence the dynamics. The chain reaction concept therefore connects nuclear physics with reactor engineering and explains how a collection of microscopic stochastic fission events can produce a controlled macroscopic source of heat and electricity.
