Design of a reactor core requires detailed prediction of the balance between neutron production and neutron loss. The rates of neutron production, transport and absorption are key information not only for core design and analysis but also for thermal–hydraulic, heat–mass transfer, accident scenarios and radioactivity release estimates. After neutrons are born in fission reactions, they move through the reactor core and undergo collisions of various types (absorption and scattering). There are two main absorption processes which may occur, radiative capture and fission. In fission, the target isotope splits and releases additional neutrons. In radiative capture, the neutron is parasitically absorbed and does not contribute to sustaining the chain reaction. In scattering collisions (elastic or inelastic) neutrons change their energy, spatial position and direction of motion in a process known as slowing down. In general, the interaction of neutrons with nuclei in medium may be considered as neutrons being transferred or transported from one location to another, from one energy to another and from one direction of motion to another. A schematic diagram of the various paths for a neutron born in a thermal reactor is depicted in Fig. 8-1. The details of neutron interactions as well as the concept of the sustained (and controlled) chain reaction are described in the succeeding sections.