Nuclear chain reactions self-propagating

FIGURE 19.10 In a self-propagating nuclear chain reaction, the number of neutrons grows exponentially during fission. 

Figure 26-11 A self-propagating nuclear chain reaction. A stray neutron induces a single fission, liberating more neutrons. Each of them induces another fission, each of which is accompanied by release of two or three neutrons. The chain continues to branch in this way, very quickly resulting in an explosive rate of fission.

FIGURE 13.9 A self-propagating nuclear chain reaction initiated by capture of a neutron. The fission of uranium-235 produces a variety of products. Thirty-four elements have been detected among the fission products, including those shown here. Each fission event produces two lighter nuclei plus two or three neutrons. 

The two nuclei on the right side are just two of the many possible products of the fission process. Since more than one neutron is released in each process, the fission reaction is a self-propagating, or chain reaction. Neutrons released by one fission event may induce other fissions. When fission reactions are run under controlled conditions in a nuclear reactor, the energy released by... 

Perhaps the most striking feature of chain reactions is that some of them can result in detonation. If the propagation mechanism includes a step or steps that produce more chain carriers than they consume, the reaction is self-accelerating. This is called chain branching. The result may be an event much more violent than a thermal explosion, in which self-acceleration stems from temperature increase owing to the inability of heat transfer to keep up with the heat production of the reaction. The detonation of a nuclear bomb can be viewed as a chain reaction with neutrons as carriers and with chain branching. 

Chain reaction n. In general, any self-sustaining process, whether molecular or nuclear, the products of which are instrumental in, and directly contribute to the propagation of the process. Specifically, a fission chain reaction, where the energy liberated for particles produced (fission products) by the fusion of an atom cause the fusion of other atomic nuclei, which in turn propagate the fission reaction in the same manner. In other words, a reaction type characterized by the formation of products of a later step (chain carriers), which are reactants for an earlier step. 

A few months after the discovery of fission, it was clear that neutrons were set free in the process (von Halban jun et al. 1939). This initiation opened the way to speculations for a self-propagating chain reaction that would produce almost unlimited amounts of energy for peaceful and military uses (Fliigge 1939). With the beginning of World War II, this had the consequence that from 1940 to the end of the war results on the fission process were kept secret and could not be published. Meanwhile, enormous efforts and money were invested in the development of nuclear reactors and nuclear weapons, particularly in the USA and somewhat later also in the Soviet Union, in Great Britain, France, China, and in other countries. These developments will not be covered here. Further information may be found in O Chap. 1 of this Volume. The rest of this chapter will de dedicated to the physics of the fission process, i.e., to the experimental methods used, to the results, and their implications on the structure and behavior of nuclear matter. 

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