ABSTRACT
This chapter discusses the neutron diffusion equation to some geometries where an external neutron source denoted by the symbol S(r) is either imbedded in a material or is allowed to interact with a material that does not produce any additional neutrons. Thermal neutrons can be produced by a radioactive source that is injected directly into the material, or they can also be produced by another external device such as a neutron beam generator. The neutron source S that appears in the diffusion equation can assume a number of different configurations depending upon the way the source is deployed. If the neutrons emanate from a single point, it appears in the diffusion equation as a point source. The principle of superposition is a universal principle that is manifested in nuclear power plants in many different ways. In fact, it can be applied to both neutrons and photons.
