Reactor Theory (Nuclear Parameters)
Control Rods Summary
A black neutron-absorbing material absorbs essentially all incident neutrons. A
grey neutron-absorbing material absorbs only part of the incident neutrons.
A grey neutron-absorbing material may be preferable to a black neutron-
absorbing material in the construction of control rods because the grey absorber
causes smaller depressions in neutron flux and power in the vicinity of the rod.
Resonance absorbers are sometimes preferred to thermal absorbers as control
rod materials because they have a larger area of influence and result in a flatter
Integral control rod worth is the total reactivity worth of the control rod at a
particular degree of withdrawal from the core.
Differential control rod worth is the reactivity change per unit movement of a
The typical differential control rod worth curve has a bell shape. It has very
low values at the top and bottom of the core and a maximum value at the center
of the core. The curve has this shape because rod worth is related to neutron
flux, and flux is highest in the center of the core.
The typical integral control rod worth curve has an "S" shape. It has a
relatively flat slope at the top and bottom of the core and a maximum slope at
the center of the core.
Integral or differential control rod worth curves can be used to determine the
reactivity change due to a control rod movement between two positions.
Integral or differential control rod worth curves can be plotted based on
measured control rod worth data.