Reactor Theory (Reactor Operations)
At low reactor power levels, changing the flow rate of the coolant through the reactor does not
result in a measurable reactivity change because fuel and moderator temperatures and the fraction
of steam voids occurring in the core are not changed appreciably.
When the flow rate is varied, however, the change in temperature that occurs across the core
(outlet versus inlet temperature) will vary inversely with the flow rate. At higher power levels,
on liquid cooled systems, increasing flow will lower fuel and coolant temperatures slightly,
resulting in a small positive reactivity insertion. A positive reactivity addition also occurs when
flow is increased in a two-phase (steam-water) cooled system. Increasing the flow rate decreases
the fraction of steam voids in the coolant and results in a positive reactivity addition. This
property of the moderator in a two-phase system is used extensively in commercial BWRs.
Normal power variations required to follow load changes on BWRs are achieved by varying the
coolant/moderator flow rate.
As a reactor is operated, atoms of fuel are constantly consumed, resulting in the slow depletion
of the fuel frequently referred to as core burnup. There are several major effects of this fuel
depletion. The first, and most obvious, effect of the fuel burnup is that the control rods must
be withdrawn or chemical shim concentration reduced to compensate for the negative reactivity
effect of this burnup.
Some reactor designs incorporate the use of supplemental burnable poisons in addition to the
control rods to compensate for the reactivity associated with excess fuel in a new core. These
fixed burnable poisons burn out at a rate that approximates the burnout of the fuel and they
reduce the amount of control rod movement necessary to compensate for fuel depletion early in
As control rods are withdrawn to compensate for fuel depletion, the effective size of the reactor
is increased. By increasing the effective size of the reactor, the probability that a neutron slows
down and is absorbed while it is still in the reactor is also increased. Therefore, neutron leakage
decreases as the effective reactor size is increased. The magnitude of the moderator negative
temperature coefficient is determined in part by the change in neutron leakage that occurs as the
result of a change in moderator temperature. Since the fraction of neutrons leaking out is less
with the larger core, a given temperature change will have less of an effect on the leakage.
Therefore, the magnitude of the moderator negative temperature coefficient decreases with fuel