Reactor Theory (Reactor Operations)
For most reactor designs, the only factors that change significantly after the reactor is shut down
are the average reactor temperature and the concentration of fission product poisons. The
reactivities normally considered when calculating an ECP include the following.
Basic Reactivity of the Core-
The reactivity associated with the critical control rod
position for a xenon-free core at normal operating
temperature. This reactivity varies with the age of the core
(amount of fuel burnup).
Direct Xenon Reactivity -
The reactivity related to the xenon that was actually present
in the core at the time it was shutdown. This reactivity is
corrected to allow for xenon decay.
Indirect Xenon Reactivity -
The reactivity related to the xenon produced by the decay
of iodine that was present in the core at the time of
Temperature Reactivity -
The reactivity related to the difference between the actual
reactor temperature during startup and the normal operating
To arrive at an ECP of the control rods, the basic reactivity, direct and indirect xenon reactivity,
and temperature reactivity are combined algebraically to determine the amount of positive control
rod reactivity that must be added by withdrawing control rods to attain criticality. A graph of
control rod worth versus rod position is used to determine the estimated critical position.
Core Power Distribution
In order to ensure predictable temperatures and uniform depletion of the fuel installed in a
reactor, numerous measures are taken to provide an even distribution of flux throughout the
power producing section of the reactor. This shaping, or flattening, of the neutron flux is
normally achieved through the use of reflectors that affect the flux profile across the core, or
by the installation of poisons to suppress the neutron flux where desired. The last method,
although effective at shaping the flux, is the least desirable since it reduces neutron economy by
absorbing the neutrons.
A reactor core is frequently surrounded by a "reflecting" material to reduce the ratio of peak
flux to the flux at the edge of the core fuel area. Reflector materials are normally not
fissionable, have a high scattering cross section, and have a low absorption cross section.
Essentially, for thermal reactors a good moderator is a good reflector. Water, heavy water,
beryllium, zirconium, or graphite are commonly used as reflectors. In fast reactor systems,
reflectors are not composed of moderating materials because it is desired to keep neutron energy
high. The reflector functions by scattering some of the neutrons, which would have leaked from
a bare (unreflected) core, back into the fuel to produce additional fissions.