What is Nuclear Chain Reaction?

Nuclear chain reaction is a sequence of single nuclear reactions, each of which is caused by a particle that appeared as a reaction product at the previous step of the sequence. An example of a nuclear chain reaction is the nuclear chain reaction of the fission of nuclei of heavy elements, in which the majority of fission events are initiated by neutrons produced by nuclear fission in the previous generation.

Chain reactions are widespread among chemical reactions, where free atoms or radicals play the role of particles with unused bonds. The mechanism of the nuclear chain reaction in nuclear transformations can provide neutrons that do not have a Coulomb barrier and excite the nucleus during absorption. The appearance of the necessary particle in the medium causes a chain of the following, one after the other reactions, which continues until the chain breaks due to the loss of the carrier particle of the reaction.

There are two main causes of losses: the absorption of a particle without emitting a secondary one and the departure of a particle beyond the limits of the volume of the substance supporting the chain process. If in each act of reaction only one carrier particle appears, then the nuclear chain reaction is called branched. An unbranched nuclear chain reaction cannot produce large amounts of energy.

If more than one particle appears in each reaction event or in some links of the chain, then a branched nuclear chain reaction occurs, because one of the secondary particles continues the chain that has been started, while others produce new chains that branch again. True, the processes leading to chain breaks compete with the branching process, and the current situation gives rise to limiting or critical phenomena that are specific to branched chain reactions. If the number of chain breaks is greater than the number of emerging new chains, then a self-sustaining nuclear chain reaction (self-sustaining chain reaction) is impossible. Even if it is artificially excited, by introducing some necessary particles into the medium, then, since the number of chains in this case can only decrease, the process that has begun quickly fades out. If the number of generated new chains exceeds the number of breaks, the nuclear chain reaction quickly spreads over the entire volume of the substance when at least one initial particle appears.

The region of states of matter with the development of a chain self-sustaining reaction is separated from the region where a nuclear chain reaction is impossible at all, by a critical state. The critical state is characterized by equality between the number of new chains and the number of breaks.

Achieving a critical state is determined by a number of factors. The fission of a heavy nucleus is excited by one neutron, and as a result of a fission event, more than one neutron appears (for example, for 235 U, the number of neutrons born in one fission event is on average from 2 to 3). Consequently, the fission process can produce a branched chain reaction, the carriers of which will be neutrons. If the neutron loss rate (captures without fission, departures from the reaction volume, etc.) compensates for the neutron multiplication rate in such a way that the effective neutron multiplication factor is exactly one, then the nuclear chain reaction proceeds in a stationary mode. The introduction of negative feedbacks between the effective multiplication factor and the rate of energy release allows a controlled chain reaction, which is used, for example, in nuclear power engineering. If the multiplication factor is greater than one, the nuclear chain reaction develops exponentially; unmanaged fission nuclear chain reaction is used in nuclear weapons.