Reactor physics is the discipline devoted to the study of interactions between neutrons and
matter in a nuclear reactor. Such an interaction is produced when a neutron collides with
the nucleus of a specific nuclide (or isotope). Interactions between neutrons and nuclei are
described by nuclear physics models as a function of neutron energy and nuclide characteristics.
A statistical mechanics approach is also used to describe the distribution of neutrons in phase
space (position and velocity vectors) as a function of time. The neutronic number density (or
neutron distribution) can be obtained as the solution of a transport equation similar to the
equation used to describe photon populations.