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.
STATE the five common poisons used as control rod material.
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.
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.
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