Reactor Theory (Nuclear Parameters)
The material used for the control rods varies depending on reactor design. Generally, the
material selected should have a good absorption cross section for neutrons and have a long
lifetime as an absorber (not burn out rapidly). The ability of a control rod to absorb neutrons
can be adjusted during manufacture. A control rod that is referred to as a "black" absorber
absorbs essentially all incident neutrons. A "grey" absorber absorbs only a part of them. While
it takes more grey rods than black rods for a given reactivity effect, the grey rods are often
preferred because they cause smaller depressions in the neutron flux and power in the vicinity
of the rod. This leads to a flatter neutron flux profile and more even power distribution in the
If grey rods are desired, the amount of material with a high absorption cross section that is
loaded in the rod is limited. Material with a very high absorption cross section may not be
desired for use in a control rod, because it will burn out rapidly due to its high absorption cross
section. The same amount of reactivity worth can be achieved by manufacturing the control rod
from material with a slightly lower cross section and by loading more of the material. This also
results in a rod that does not burn out as rapidly.
Another factor in control rod material selection is that materials that resonantly absorb neutrons
are often preferred to those that merely have high thermal neutron absorption cross sections.
Resonance neutron absorbers absorb neutrons in the epithermal energy range. The path length
traveled by the epithermal neutrons in a reactor is greater than the path length traveled by
thermal neutrons. Therefore, a resonance absorber absorbs neutrons that have their last collision
farther (on the average) from the control rod than a thermal absorber. This has the effect of
making the area of influence around a resonance absorber larger than around a thermal absorber
and is useful in maintaining a flatter flux profile.
Types of Control Rods
There are several ways to classify the types of control rods. One classification method is by the
purpose of the control rods. Three purposes of control rods are listed below.
used for coarse control and/or to remove reactivity in relatively large
Regulating rods -
used for fine adjustments and to maintain desired power or temperature.
provide a means for very fast shutdown in the event of an unsafe condition.
Addition of a large amount of negative reactivity by rapidly inserting the
safety rods is referred to as a "scram" or "trip."