1. Introduction to neutronics

1.1 Principle of the fission chain reaction

The neutronics of nuclear reactors is concerned with the fission chain reaction and the conditions of its equilibrium. Remember that energy release is based on a chain of reactions: neutrons give fissions, fissions give neutrons, neutrons give fissions, fissions give neutrons, and so on.

Experience shows that during fission, in addition to the two fragments, a few neutrons are emitted in the free state (on average between 2 and 3, the precise value depending on the fissioned nuclei). This is easy to understand, if we remember that the proportion of neutrons in stable nuclei is greater the heavier the nuclei, and therefore too great in relation to stability in the two fission fragments. Despite this neutron emission,...