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 :

• fuel,
• moderator,
• reflector, or
• a neutron absorber or poison. 

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 neutronic, 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.

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 or aluminum tube to protect it from corrosion by the high-temperature water.

Here at MIT we use Cadmium as the material for the "low worth" regulating rod.

Boron-Containing Materials

Boron is a useful control material for thermal (and other) reactors. The very high thermal ­absorption cross section of ¹⁰B 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.

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.

Here we use this material in the "high worth" shim blades.

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.