SHIELDING MATERIALS
DOE-HDBK-1017/2-93
Plant Materials
SHIELDING MATERIALS
In the reactor plant, the principle source of radiation comes from the reactor
core. Attenuation of this radiation is performed by shielding materials
located around the core. This chapter discusses the various materials used
in a reactor plant for shielding.
EO 1.11
DESCRIBE the requirements of a material used to shield against
the following types of radiation:
a.
Beta
c.
High energy neutrons
b.
Gamma
d.
Low energy neutrons
Overview
Shielding design is relatively straightforward depending upon the type of radiation (gamma,
neutron, alpha, beta). For example, when considering the reactor core, it is first necessary to
slow down the fast neutrons (those not directly absorbed) coming from the core to thermal energy
by utilizing appropriate neutron attenuating shielding materials that are properly arranged. This
slowing down process is mostly caused by collisions that slow the neutrons to thermal energy.
The thermal neutrons are then absorbed by the shielding material. All of the gamma rays in the
system, both the gamma rays leaving the core and the gamma rays produced by neutron
interactions within the shielding material have to be attenuated to appropriate levels by utilizing
gamma ray shielding materials that are also properly arranged. The design of these radiation
shields and those used to attenuate radiation from any radioactive source depend upon the
location, the intensity, and the energy distribution of the radiation sources, and the permissible
radiation levels at positions away from these sources. In this chapter, we will discuss the
materials used to attenuate neutron, gamma, beta, and alpha radiation.
Neutron Radiation
The shielding of neutrons introduces many complications because of the wide range of energy
that must be considered. At low energies (less than 0.1 MeV), low mass number materials, such
as hydrogen in H2O, are best for slowing down neutrons. At these energies, the cross section for
interaction with hydrogen is high (approximately 20 barns), and the energy loss in a collision is
high. Materials containing hydrogen are known as hydrogenous material, and their value as a
neutron shield is determined by their hydrogen content. Water ranks high and is probably the
best neutron shield material with the advantage of low cost, although it is a poor absorber of
gamma radiation.
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