ABSTRACT
This chapter describes the neutron diffusion equation and discusses some of its limitations. The neutron diffusion equation can be written in two different forms—the time-dependent neutron diffusion equation and the steady-state or time-independent neutron diffusion equation. The chapter begins with neutron diffusion theory by introducing the concept of the neutron density, which represents the number of neutrons that can be found in a small volume of space. Neutrons always move from a region of high density to a region of low density. Adolf Fick introduced Fick's Law of Diffusion, which governed the diffusion of a gas across a fluid membrane. The chapter explains Fick's Law of diffusion and describes how it can be applied to electrically neutral particles such as neutrons. This will lead to a simple formula that can be used to determine the magnitude and the direction of the neutron current J.
