Plant MaterialsDOE-HDBK-1017/2-93CONTROL MATERIALSCONTROL MATERIALSFour general methods have been used or proposed for changing the power orneutron flux in a nuclear reactor; each involves the temporary addition orremoval of (a) fuel, (b) moderator, (c) reflector, or (d) a neutron absorber orpoison. This chapter discusses the materials used as poisons in a reactor plant.EO 1.9STATE the five common poisons used as control rod material.EO 1.10IDENTIFY the advantage(s) and/or disadvantage(s) of the fivecommon poisons used as control rod material.Overviewof PoisonsThe most commonly used method to control the nuclear reaction, especially in power reactors,is the insertion or withdrawal of control rods made out of materials (poisons) having a largecross section for the absorption of neutrons. The most widely-used poisons are hafnium, silver,indium, cadmium, and boron. These materials will be briefly discussed below.HafniumBecause of its neuronic, mechanical, and physical properties, hafnium is an excellent controlmaterial for water-cooled, water-moderated reactors. It is found together with zirconium, andthe process that produces pure zirconium produces hafnium as a by-product. Hafnium isresistant to corrosion by high-temperature water, has adequate mechanical strength, and can bereadily fabricated. Hafnium consists of four isotopes, each of which has appreciable neutronabsorption cross sections. The capture of neutrons by the isotope hafnium-177 leads to theformation of hafnium-178; the latter forms hafnium-179, which leads to hafnium-180. The firstthree have large resonance-capture cross sections, and hafnium-180 has a moderately large crosssection. Thus, the element hafnium in its natural form has a long, useful lifetime as a neutronabsorber. Because of the limited availability and high cost of hafnium, its use as a controlmaterial in civilian power reactors has been restricted.Silver-Indium-CadmiumAlloysBy alloying cadmium, which has a thermal-absorption cross section of 2450 barns, with silverand indium, which have high resonance absorption, a highly-effective neutron absorber isproduced. Rev. 0Page 15MS-05
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