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 1.9STATE 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 Allovs

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.

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 thermalabsorption cross section of ¹B(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 reaction. The absorption reaction is one of transmutation, , 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.

Boron may also be used as a burnable poison to compensate for the change in reactivity with lifetime. In this scheme, a small amount of boron is incorporated into the fuel or special burnable poison rods to reduce the beginning-of-life reactivity. Burnup of the poison causes a reactivity increase that partially compensates for the decrease in reactivity due to fuel burnup and accumulation of fission products. Difficulties have generally been encountered when boron is incorporated directly with the fuel, and most applications have used separate burnable poison rods.

Summary

The important information in this chapter is summarized below.

Control Materials Summary

Hafnium

Advantages: Excellent control for water-cooled, water-moderated reactors due to neutronic, mechanical, and physical properties.

Disadvantages: Limited availability and high cost. Silver-Indium-Cadmium Alloys

Advantages: Highly effective neutron absorber.

Control effectiveness in water-moderated reactors is close to hafnium. Used in pressurized-water reactors.

Easily fabricated and adequate strength

Disadvantages: Must be enclosed in stainless steel tube to protect it from corrosion.

Boron

Advantages: Very high thermal-absorption cross-section and low cost.

Commonly used in thermal reactors for control rods and burnable poison.

Disadvantages: Nonmetallic thus must be incorporated into a metallic material for use as control rod.