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SHIELDING MATERIALS

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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.

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) 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 gamma­ray 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.

 



   


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