Reactor Theory (Reactor Operations)
Determine the initial value of keff for the core.
1 ( 0.01000)
Determine the final value of keff for the core. The final value of reactivity
will be -500 pcm (-1000 + 500).
1 ( 0.00500)
Use Equation (4-4) to determine the final count rate.
Notice from this example that the count rate doubled as the reactivity was halved (e.g., reactivity
was changed from -1000 pcm to -500 pcm).
Use of 1/M Plots
Because the subcritical multiplication factor is related to the value of keff, it is possible to
monitor the approach to criticality through the use of the subcritical multiplication factor. As
positive reactivity is added to a subcritical reactor, keff will get nearer to one. As keff gets nearer
to one, the subcritical multiplication factor (M) gets larger. The closer the reactor is to
criticality, the faster M will increase for equal step insertions of positive reactivity. When the
reactor becomes critical, M will be infinitely large. For this reason, monitoring and plotting M
during an approach to criticality is impractical because there is no value of M at which the
reactor clearly becomes critical.