Reactor Theory (Reactor Operations)
When a reactor is started up with unirradiated fuel, or on those occasions when the reactor is
restarted following a long shutdown period, the source neutron population will be very low. In
some reactors, the neutron population is frequently low enough that it cannot be detected by the
nuclear instrumentation during the approach to criticality. Installed neutron sources, such as
those discussed in Module 2, are frequently used to provide a safe, easily monitored reactor
startup. The neutron source, together with the subcritical multiplication process, provides a
sufficiently large neutron population to allow monitoring by the nuclear instruments throughout
the startup procedure. Without the installed source, it may be possible to withdraw the control
rods to the point of criticality, and then continue withdrawal without detecting criticality because
the reactor goes critical below the indicating range. Continued withdrawal of control rods at this
point could cause reactor power to rise at an uncontrollable rate before neutron level first
becomes visible on the nuclear instruments.
An alternative to using a startup source is to limit the rate of rod withdrawal, or require waiting
periods between rod withdrawal increments. By waiting between rod withdrawal increments,
the neutron population is allowed to increase through subcritical multiplication. Subcritical
multiplication is the process where source neutrons are used to sustain the chain reaction in a
reactor with a multiplication factor (keff) of less than one.
The chain reaction is not
"self-sustaining," but if the neutron source is of sufficient magnitude, it compensates for the
neutrons lost through absorption and leakage. This process can result in a constant, or
increasing, neutron population even though keff is less than one.
Estimated Critical Position
In the first chapter of this module, 1/M plots were discussed. These plots were useful for
monitoring the approach to criticality and predicting when criticality will occur based on
indications received while the startup is actually in progress. Before the reactor startup is
initiated, the operator calculates an estimate of the amount of rod withdrawal that will be
necessary to achieve criticality. This process provides an added margin of safety because a large
discrepancy between actual and estimated critical rod positions would indicate that the core was
not performing as designed. Depending upon a reactor's design or age, the buildup of xenon
within the first several hours following a reactor shutdown may introduce enough negative
reactivity to cause the reactor to remain shutdown even with the control rods fully withdrawn.
In this situation it is important to be able to predict whether criticality can be achieved, and if
criticality cannot be achieved, the startup should not be attempted.
For a given set of conditions (such as time since shutdown, temperature, pressure, fuel burnup,
samarium and xenon poisoning) there is only one position of the control rods (and boron
concentrations for a reactor with chemical shim) that results in criticality, using the normal rod
withdrawal sequence. Identification of these conditions allows accurate calculation of control
rod position at criticality. The calculation of an estimated critical position (ECP) is simply a
mathematical procedure that takes into account all of the changes in factors that significantly
affect reactivity that have occurred between the time of reactor shutdown and the time that the
reactor is brought critical again.