Two highly desirable properties of both neutron reflectors and moderators are efficient neutron slow­ing and low neutron absorption. The first requires effective slowing of neutrons over short distances, thus reducing the required volume of the reflector or moderator in the reactor core. Moreover, in a reactor core of a given shape and volume, this reduces the leakage of neutrons in the course of their slowing.

For reflectors in particular, the key requirements include a high reflectivity, a large macroscopic cross­section, and efficient neutron slowing. The reflectivity of a material is inversely proportional to its diffusion ratio (D/L), which is the ratio of its diffusivity (D) to its diffusion length (L). This ratio is generally considered to decrease as scattering becomes large in comparison with absorption. It is essential, moreover, to obtain high reflectivity without excessive thickness, and for this purpose, to use a material with a large macroscopic total cross-section. In a thermal reactor, the performance of the reflector is enhanced if it does not simply reflect the neutrons but rather slows and then reflects them, and for this reason, the same material is often used as both reflector and moderator.

In general, materials whose nuclides have low mass number and neutron absorption may be used as mod­erators and reflectors. The most commonly used materials are light water (H2O), heavy water (D2O), and graphite (C). In addition, hydrocarbons, zirco­nium hydride, and other such materials are often used as moderators. Heavy water is particularly effec­tive because of its very low absorption level. Graphite is second to heavy water in its low absorption level, is lower in cost, and has the added advantage of suitabil­ity for use at high temperatures. Beryllium is gener­ally used as a reflector rather than as a moderator.

In addition to the aforementioned materials, there exist other candidates as neutron reflectors. For exam­ple, Commissariat a l’Energie Atomique (CEA) is studying zirconium silicide as the reflector for next generation reactors.1 Tungsten carbide has also been used as neutron reflectors (http://en. wikipedia. org/ wiki/Tungsten_carbide). For fusion reactors, various materials such as titanium carbide and boron carbide are considered as reflectors.2

This chapter outlines the basic properties of be­ryllium and zirconium hydride that are fundamental to their utilization as neutron reflectors and modera­tors in nuclear reactors.