Reactor Theory (Reactor Operations)
Subcritical multiplication is the phenomenon that accounts for the changes in
neutron flux that takes place in a subcritical reactor due to reactivity changes.
It is important to understand subcritical multiplication in order to understand
reactor response to changes in conditions.
DEFINE the following terms:
Subcritical multiplication factor
Given a neutron source strength and a subcritical system of
known keff, CALCULATE the steady-state neutron level.
Given an initial count rate and keff, CALCULATE the final
count rate that will result from the addition of a known amount
Given count rates vs. the parameter being adjusted,
ESTIMATE the value of the parameter at which the reactor
will become critical through the use of a 1/M plot.
Subcritical Multiplication Factor
When a reactor is in a shutdown condition, neutrons are still present to interact with the fuel.
These source neutrons are produced by a variety of methods that were discussed in Module 2.
If neutrons and fissionable material are present in the reactor, fission will take place. Therefore,
a reactor will always be producing a small number of fissions even when it is shutdown.
Consider a reactor in which keff is 0.6. If 100 neutrons are suddenly introduced into the reactor,
these 100 neutrons that start the current generation will produce 60 neutrons (100 x 0.6) from
fission to start the next generation.
The 60 neutrons that start the second generation will
produce 36 neutrons (60 x 0.6) to start the third generation. The number of neutrons produced
by fission in subsequent generations due to the introduction of 100 source neutrons into the
reactor is shown below.