Reactor Theory (Reactor Operations)
There is also another effect that is a consideration only on reactors that use dissolved boron in
the moderator (chemical shim). As the fuel is burned up, the dissolved boron in the moderator
is slowly removed (concentration diluted) to compensate for the negative reactivity effects of fuel
burnup. This action results in a larger (more negative) moderator temperature coefficient of
reactivity in a reactor using chemical shim. This is due to the fact that when water density is
decreased by rising moderator temperature in a reactor with a negative temperature coefficient,
it results in a negative reactivity addition because some moderator is forced out of the core.
With a coolant containing dissolved poison, this density decrease also results in some poison
being forced out of the core, which is a positive reactivity addition, thereby reducing the
magnitude of the negative reactivity added by the temperature increase. Because as fuel burnup
increases the concentration of boron is slowly lowered, the positive reactivity added by the above
poison removal process is lessened, and this results in a larger negative temperature coefficient
The following effect of fuel burnup is most predominant in a reactor with a large concentration
of uranium-238. As the fission process occurs in a thermal reactor with low or medium
enrichment, there is some conversion of uranium-238 into plutonium-239. Near the end of core
life in certain reactors, the power contribution from the fission of plutonium-239 may be
comparable to that from the fission of uranium-235. The value of the delayed neutron fraction
(b) for uranium-235 is 0.0064 and for plutonium-239 is 0.0021. Consequently, as core burnup
progresses, the effective delayed neutron fraction for the fuel decreases appreciably. It follows
then that the amount of reactivity insertion needed to produce a given reactor period decreases
with burnup of the fuel.
A reactor is considered to be shut down when it is subcritical and sufficient shutdown reactivity
exists so there is no immediate probability of regaining criticality. Shutdown is normally
accomplished by insertion of some (or all) of the control rods, or by introduction of soluble
neutron poison into the reactor coolant.
The rate at which the reactor fission rate decays immediately following shutdown is similar for
all reactors provided a large amount of negative reactivity is inserted. After a large negative
reactivity addition the neutron level undergoes a rapid decrease of about two decades (prompt
drop) until it is at the level of production of delayed neutrons. Then the neutron level slowly
drops off as the delayed neutron precursors decay, and in a short while only the longest-lived
precursor remains in any significant amount.
This precursor determines the final rate of
decrease in reactor power until the neutron flux reaches the steady state level corresponding to
the subcritical multiplication of the neutron source.