Boron-Containing Materials

CONTROL MATERIALS DOE-HDBK-1017/2-93 Plant Materials The control effectiveness of such alloys in water-moderated reactors can approach that of hafnium and is the control material commonly used in pressurized-water reactors. The alloys (generally 80% silver, 15% indium, 5% cadmium) can be readily fabricated and have adequate strength at water-reactor temperatures. The control material is enclosed in a stainless steel tube to protect it from corrosion by the high-temperature water. Boron-Containing Materials Boron is a useful control material for thermal (and other) reactors. The very high thermal- absorption cross section of ¹⁰B (boron-10) and the low cost of boron has led to wide use of boron-containing materials in control rods and burnable poisons for thermal reactors. The absorption cross section of boron is large over a considerable range of neutron energies, making it suitable for not only control materials but also for neutron shielding. Boron is nonmetallic and is not suitable for control rod use in its pure form. For reactor use, it is generally incorporated into a metallic material. Two of such composite materials are described below. Stainless-steel alloys or dispersions with boron have been employed to some extent in reactor control. The performance of boron-stainless-steel materials is limited because of the ¹⁰B (n,a) reaction. The absorption reaction is one of transmutation, ¹⁰B + ¹n ⁷Li + ⁴a, with the a-particle produced becoming a helium atom. The production of atoms having about twice the volume of the original atoms leads to severe swelling, hence these materials have not been used as control rods in commercial power reactors. The refractory compound boron carbide (B₄C) has been used as a control material either alone or as a dispersion in aluminum (boral). These materials suffer from burnup limitation. The preferred control rod material for boiling-water reactors is boron carbide. Long stainless-steel tubes containing the powdered boron carbide combined into assemblies with cruciform cross sections make up the control rods. Control rods of this nature have been used in PWRs, BWRs, and HTGRs and have been proposed for use in fast breeder reactors employing oxide fuels. Because of its ability to withstand high temperatures, boron carbide (possibly mixed with graphite) will probably be the control material in future gas-cooled reactors operating at high temperatures. In addition to its use in control elements, boron is widely used in PWRs for control of reactivity changes over core lifetime by dissolving boric acid in the coolant. When this scheme is used, the movable control elements have a reactivity worth sufficient to go from full power at operating temperature to zero power at operating temperature. At the beginning of life, enough boric acid is added to the coolant to allow the reactor to be just critical with all rods nearly completely withdrawn. As fuel burnup takes place through power operation, the boric acid concentration in the coolant is reduced to maintain criticality. If a cold shutdown is required, additional boric acid is added to compensate for the reactivity added as the moderator cools. This method is generally referred to as chemical shim control. MS-05 Page 16 Rev. 0