SHIELDING MATERIALSDOE-HDBK-1017/2-93Plant MaterialsSHIELDING MATERIALSIn the reactor plant, the principle source of radiation comes from the reactorcore. Attenuation of this radiation is performed by shielding materialslocated around the core. This chapter discusses the various materials usedin a reactor plant for shielding.EO 1.11DESCRIBE the requirements of a material used to shield againstthe following types of radiation:a.Betac.High energy neutronsb.Gammad.Low energy neutronsOverviewShielding 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 toslow down the fast neutrons (those not directly absorbed) coming from the core to thermal energyby utilizing appropriate neutron attenuating shielding materials that are properly arranged. Thisslowing 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 thesystem, both the gamma rays leaving the core and the gamma rays produced by neutroninteractions within the shielding material have to be attenuated to appropriate levels by utilizinggamma ray shielding materials that are also properly arranged. The design of these radiationshields and those used to attenuate radiation from any radioactive source depend upon thelocation, the intensity, and the energy distribution of the radiation sources, and the permissibleradiation levels at positions away from these sources. In this chapter, we will discuss thematerials used to attenuate neutron, gamma, beta, and alpha radiation.NeutronRadiationThe shielding of neutrons introduces many complications because of the wide range of energythat must be considered. At low energies (less than 0.1 MeV), low mass number materials, suchas hydrogen in H2O, are best for slowing down neutrons. At these energies, the cross section forinteraction with hydrogen is high (approximately 20 barns), and the energy loss in a collision ishigh. Materials containing hydrogen are known as hydrogenous material, and their value as aneutron shield is determined by their hydrogen content. Water ranks high and is probably thebest neutron shield material with the advantage of low cost, although it is a poor absorber ofgamma radiation. MS-05Page 18Rev. 0
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