Chain reaction

A chain reaction is a physical or chemical transformation ( reaction), consisting of identical, mutually conditional partial reactions. It is a product of a single reaction starting product ( reactant, reactant) for a subsequent reaction. The reaction chain may be linear or branched.

Typical of branched chain reaction is an exponential growth law. The reaction rate ( number of individual reactions per unit time) can increase or decrease, or even be in equilibrium ( see below for examples ). "Chain reaction" thus means - contrary to a common misconception, - not that the process is constantly accelerate or spread and must lead to a disaster. Also, not everyone is growing like an avalanche process a chain reaction, especially thermonuclear explosions not.

The term chain reaction is used in everyday language in the same sense, see domino effect or chain letter.

Chemical chain reactions

Chain reactions need not consist of a single type of tissue reaction. It may be about a reaction that alone would lead to only linear reaction chains combined with another reaction that leads to chain branching. Are important chemical chain reactions such as in the polymerization or free-radical substitution. In the polymerase chain reaction ( PCR) clocked by temperature changes branching reactions occur, in which the turnover doubled in each step. A known application is the determination of the PCR DNA fingerprinting.

Processes with energy, but not product disclosure, in its pure form as in the detonation are not mentioned chain reaction. Often the transitions are but fluent. To start many combustion processes with predominantly linear reaction chains, see chlorine and hydrogen gas, but achieve more or less rapidly ( ignition delay ) by release of energy a temperature dominate the thermal dissociation reactions about possibly concurrent -chain branches. That is the usual state at a burning flame. The reaction rate can be controlled by the supply of the reaction materials.

Physical chain reactions

Neutron -induced fission

The triggered by absorption of a free neutron fission of a suitable atomic nucleus is in turn some neutrons ( on average between two and three neutrons, depending on the split nuclide and energy of the neutron inducing ). These can be further split cores, again new neutrons are released, etc.

This chain reaction is controlled in the normal operation of a nuclear reactor, the reaction rate remains constant ( criticality), i.e., in the middle right of the new neutron again initiates a cleavage. For increasing the power is increased, the reaction rate is reduced to decrease. Changes in the reaction rate can be expressed by the multiplication factor k; 1 each free neutron has an average of k free neutrons as a successor in the next generation. At a constant reaction rate - and thus a constant reactor power - is k = 1.00.

The control in the reactor is usually done by removing neutrons from the reaction by means of suitable non- fissile materials ( neutron absorber ). For example, dissolved in the cooling water amount of a boron compound is changed in pressurized water reactors for slow and permanent changes; to short-term fixes and shutdown are the control rods.

A nuclear weapon is, however, designed to possible rapid and high increase in the reaction rate down. Neutron absorber than the fissionable material itself are avoided here in the design.

The picture shows a fission chain reaction with fast increasing reaction rate, here: doubling in each generation. ( The direction from left to right represents only the time, can not represent spatial sequence In reality, the released neutrons move in a space containing fissile material in all directions. )

Other types of nuclear reaction with neutrons passing

Even before the discovery of nuclear fission Leo Szilard has thought about the possibility that other types of nuclear reactions induced by neutrons and even re- release neutrons, could run as chain reactions. But this is not the case. To a multiplication k = 1 or higher must be able to achieve that release reaction more neutrons than it uses; to propagate automatically, without energy supply, it must be exothermic. Nuclear fission is, as we know today, the only response that satisfies both conditions. Any (n, 2n) -, (n, 3n) -, ( n x 2 n ) -, and the like reactions ( x is any one or more particles ) are endothermic.

Nuclear fusion

Although some fusion reactions form reaction sequences referred to in subsequent reactions within a reaction mechanism, the different reactions of the proton-proton reaction that results in stars in the formation of helium, but no chain reaction, since the reaction steps are not identical.

Charge multiplication by impact processes

Chain reactions occur in semiconductor diodes in spontaneous gas discharge and as avalanche breakdown. This accelerated in the electric field the charge carriers in the collision produces a further charge carrier pair.

• Reactor Technology
• Nuclear weapons technology
• Polymer -forming reaction