Prior to absorption, a typical neutron will undergo many elastic scattering collisions with nuclei in a reactor. As a result, a neutron path consists of many straight line segments joining the points of collision. The combined effect of billions of neutrons darting in all directions is a cloudlike diffusion of neutrons throughout the reactor material.

Neutron flux is simply a term used to describe the neutron cloud. Neutron flux, f , is defined as the number of neutrons in 1 cubic centimetre multiplied by their average velocity.

Neutron flux is sometimes simply called nv. A clear theoretical picture of neutron flux may be had by considering a beam of neutrons of one square centimetre cross section all travelling in the same direction. Then the number of neutrons contained in one centimetre of length of the beam is n, and v is the length of the beam passing a plane in one second. Hence, flux is the number of neutrons passing through one square centimetre of the plane in one second. (However, remember that in reality the motion of the neutrons in a reactor is random, therefore, the above explanation is not really valid, but is a useful concept to help explain neutron flux.)