Gamma Radiation - h1017v2

Plant Materials DOE-HDBK-1017/2-93 SHIELDING MATERIALS 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 radiation. 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 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. Rev. 0 Page 19 MS-05