CONTROL MATERIALS

Plant Materials DOE-HDBK-1017/2-93 CONTROL MATERIALS CONTROL MATERIALS Four general methods have been used or proposed for changing the power or neutron flux in a nuclear reactor; each involves the temporary addition or removal of (a) fuel, (b) moderator, (c) reflector, or (d) a neutron absorber or poison. This chapter discusses the materials used as poisons in a reactor plant. EO 1.9 STATE the five common poisons used as control rod material. EO 1.10 IDENTIFY the advantage(s) and/or disadvantage(s) of the five common poisons used as control rod material. Overview of Poisons The 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 large cross 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. Hafnium Because of its neuronic, mechanical, and physical properties, hafnium is an excellent control material for water-cooled, water-moderated reactors. It is found together with zirconium, and the process that produces pure zirconium produces hafnium as a by-product. Hafnium is resistant to corrosion by high-temperature water, has adequate mechanical strength, and can be readily fabricated. Hafnium consists of four isotopes, each of which has appreciable neutron absorption cross sections. The capture of neutrons by the isotope hafnium-177 leads to the formation of hafnium-178; the latter forms hafnium-179, which leads to hafnium-180. The first three have large resonance-capture cross sections, and hafnium-180 has a moderately large cross section. Thus, the element hafnium in its natural form has a long, useful lifetime as a neutron absorber. Because of the limited availability and high cost of hafnium, its use as a control material in civilian power reactors has been restricted. Silver-Indium-Cadmium Alloys By alloying cadmium, which has a thermal-absorption cross section of 2450 barns, with silver and indium, which have high resonance absorption, a highly-effective neutron absorber is produced. Rev. 0 Page 15 MS-05