Reactor Theory (Reactor Operations)
DOE-HDBK-1019/2-93
SUBCRITICAL MULTIPLICATION
SUBCRITICAL MULTIPLICATION
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.
EO 1.1
DEFINE the following terms:
a.
Subcritical multiplication
b.
Subcritical multiplication factor
EO 1.2
Given a neutron source strength and a subcritical system of
known keff, CALCULATE the steady-state neutron level.
EO 1.3
Given an initial count rate and keff, CALCULATE the final
count rate that will result from the addition of a known amount
of reactivity.
EO 1.4
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.
Generation
1st
2nd
3rd
4th
5th
6th
7th
8th
9th
10th
11th
12th
Neutrons
100
60
36
22
13
8
5
3
2
1
0
0
Rev. 0
NP-04
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