Neutronics basics - Neutron migration

Neutronics is the branch of physics that deals with the path of neutrons in a system and the nuclear reactions induced by these neutrons, in particular the fission reactions responsible for the release of energy. Because of its links with nuclear physics, it is a science of the microscopic; but the statistical treatment of the neutron population is akin to the kinetic theory of gases, and is therefore also related to the macroscopic.

Neutronics was born with the discovery of the free-state neutron by James Chadwick in 1932. In the years that followed, the essential physical aspects of neutronics were worked out, notably by Enrico Fermi, then a refugee in the United States. France also played a crucial role in this affair, first in the 1930s with the work of Frédéric Joliot-Curie and his collaborators, then after the creation in 1945 of the Commissariat à l'énergie atomique, headed by the same Frédéric Joliot. Although the first chain reaction did not take place in France, as might have been expected had war not broken out, but in the United States (Fermi's CP1 experiment on December 2, 1942), CEA pioneers were very active: as early as December 15, 1948, Zoé diverged at Fontenay-aux-Roses. The "French School of Neutronics", initiated by Joliot and his colleagues, has remained extremely active to this day. Although we can't list all the contributors, we can mention Jules Horowitz.

It is out of the question in these few pages to cover the whole of neutronics and its developments over the last 70 years or so. This and the next dossier are intended to provide just the neutronics basics needed to understand the other dossiers in "Techniques de l'Ingénieur" devoted to nuclear engineering, in particular those dedicated to nuclear reactors of the various types, and to give a few insights into neutronics calculation methods and their qualification, which have been the subject of a great deal of work. Even today, these topics are still the focus of most research and development efforts in neutronics.

This dossier is based on the concepts presented in the dossier "Bases de physique nucléaire nécessaires à la neutronique" (Bases of nuclear physics required for neutronics), which we strongly recommend you read first.

In a document intended for engineers, we thought it would be useful to introduce numerical examples to illustrate physical concepts: for the sake of simplicity and unless otherwise indicated, we have deliberately limited ourselves to the case of pressurized water reactors (see dossier "Nuclear reactors – general" and files devoted to water reactors).