Reactor Theory (Reactor Operations)
Shutdown margin is the instantaneous amount of reactivity by which a reactor is subcritical or
would be subcritical from its present condition assuming all control rods are fully inserted except
for the single rod with the highest integral worth, which is assumed to be fully withdrawn.
Shutdown margin is required to exist at all times, even when the reactor is critical. It is
important that there be enough negative reactivity capable of being inserted by the control rods
to ensure complete shutdown at all times during the core lifetime. A shutdown margin in the
range of one to five percent reactivity is typically required.
The stuck rod criterion refers to the fact that the shutdown margin does not take credit for the
insertion of the highest worth control rod. The application of the stuck rod criterion ensures that
the failure of a single control rod will not prevent the control rod system from shutting down
During reactor operation, numerous parameters such as temperature, pressure, power level, and
flow are continuously monitored and controlled to ensure safe and stable operation of the reactor.
The specific effects of variations in these parameters vary greatly depending upon reactor design,
but generally the effects for thermal reactors are as follows.
The most significant effect of a variation in temperature upon reactor operation is the addition
of positive or negative reactivity. As previously discussed, reactors are generally designed with
negative temperature coefficients of reactivity (moderator and fuel temperature coefficients) as
a self-limiting safety feature. A rise in reactor temperature results in the addition of negative
reactivity. If the rise in temperature is caused by an increase in reactor power, the negative
reactivity addition slows, and eventually turns the increase in reactor power. This is a highly
desirable effect because it provides a negative feedback in the event of an undesired power
Negative temperature coefficients can also be utilized in water cooled and moderated power
reactors to allow reactor power to automatically follow energy demands that are placed upon the
For example, consider a reactor operating at a stable power level with the heat
produced being transferred to a heat exchanger for use in an external closed cycle system. If
the energy demand in the external system increases, more energy is removed from reactor system
causing the temperature of the reactor coolant to decrease.
As the reactor temperature
decreases, positive reactivity is added and a corresponding increase in reactor power level