Water also provides a ready means for removing the heat generated by radiation absorption. At
higher energies (10 MeV), the cross section for interaction with hydrogen (1 barn) is not as
effective in slowing down neutrons. To offset this decrease in cross section with increased
neutron energy, materials with good inelastic scattering properties, such as iron, are used. These
materials cause a large change in neutron energy after collision for high energy neutrons but have
little effect on neutrons at lower energy, below 0.1 MeV.
Iron, as carbon steel or stainless steel, has been commonly used as the material for thermal
shields. Such shields can absorb a considerable proportion of the energy of fast neutrons and
gamma rays escaping from the reactor core. By making shields composed of iron and water, it
is possible to utilize the properties of both of these materials. PWRs utilize two or three layers
of steel with water between them as a very effective shield for both neutrons and gamma rays.
The interaction (inelastic scattering) of high energy neutrons occurs mostly with iron, which
degrades the neutron to a much lower energy, where the water is more effective for slowing
down (elastic scattering) neutrons. Once the neutron is slowed down to thermal energy, it
diffuses through the shield medium for a small distance and is captured by the shielding material,
resulting in a neutron-gamma (n,g) reaction. These gamma rays represent a secondary source of
Iron turnings or punchings and iron oxide have been incorporated into heavy concrete for
shielding purposes also. Concrete with seven weight percent or greater of water appears to be
adequate for neutron attenuation. However, an increase in the water content has the disadvantage
of decreasing both the density and structural strength of ordinary concrete. With heavy concretes,
a given amount of attenuation of both neutrons and gamma rays can be achieved by means of
a thinner shield than is possible with ordinary concrete. Various kinds of heavy concretes used
for shielding include barytes concrete, iron concrete, and ferrophosphorus concrete with various
modified concretes and related mixtures.
Boron compounds (for example, the mineral
colemanite) have also been added to concretes to increase the probability of neutron capture
without high-energy gamma-ray production.
Boron has been included as a neutron absorber in various materials in addition to concrete. For
example, borated graphite, a mixture of elemental boron and graphite, has been used in
fast-reactor shields. Boral, consisting of boron carbide (B4C) and aluminum, and epoxy resins
and resin-impregnated wood laminates incorporating boron have been used for local shielding
purposes. Boron has also been added to steel for shield structures to reduce secondary gamma-
ray production. In special situations, where a shield has consisted of a heavy metal and water,
it has been beneficial to add a soluble boron compound to the water.
Gamma radiation is the most difficult to shield against and, therefore, presents the biggest
problem in the reactor plant. The penetrating power of the gamma is due, in part, to the fact that
it has no charge or mass. Therefore, it does not interact as frequently as do the other types of
radiation per given material.