Reactor Theory (Reactor Operations)
As reactor power increases to a level above the level of the new energy demand, the temperature
of the moderator and fuel increases, adding negative reactivity and decreasing reactor power
level to near the new level required to maintain system temperature. Some slight oscillations
above and below the new power level occur before steady state conditions are achieved. The
final result is that the average temperature of the reactor system is essentially the same as the
initial temperature, and the reactor is operating at the new higher required power level. The
same inherent stability can be observed as the energy demand on the system is decreased.
If the secondary system providing cooling to the reactor heat exchanger is operated as an open
system with once-through cooling, the above discussion is not applicable. In these reactors, the
temperature of the reactor is proportional to the power level, and it is impossible for the reactor
to be at a higher power level and the same temperature.
The pressure applied to the reactor system can also affect reactor operation by causing changes
in reactivity. The reactivity changes result from changes in the density of the moderator in
response to the pressure changes. For example, as the system pressure rises, the moderator
density increases and results in greater moderation, less neutron leakage, and therefore the
insertion of positive reactivity. A reduction in system pressure results in the addition of negative
reactivity. Typically, in pressurized water reactors (PWR), the magnitude of this effect is
considerably less than that of a change in temperature. In two-phase systems such as boiling
water reactors (BWR), however, the effects of pressure changes are more noticeable because
there is a greater change in moderator density for a given change in system pressure.
A change in reactor power level can result in a change in reactivity if the power level change
results in a change in system temperature.
The power level at which the reactor is producing enough energy to make up for the energy lost
to ambient is commonly referred to as the point of adding heat. If a reactor is operating well
below the point of adding heat, then variations in power level produce no measurable variations
in temperature. At power levels above the point of adding heat, temperature varies with power
level, and the reactivity changes will follow the convention previously described for temperature
The inherent stability and power turning ability of a negative temperature coefficient are
ineffective below the point of adding heat. If a power excursion is initiated from a very low
power level, power will continue to rise unchecked until the point of adding heat is reached, and
the subsequent temperature rise adds negative reactivity to slow, and turn, the rise of reactor
power. In this region, reactor safety is provided by automatic reactor shutdown systems and