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Reactor Theory (Nuclear Parameters) DOE-HDBK-1019/2-93
REACTIVITY
Example:
Calculate the reactivity in the reactor core when keff is equal to 1.002 and 0.998.
Solution:
The reactivity for each case is determined by substituting the value of keff into
Equation (3-5).
r
keff 1
keff
1.002 1
1.002
0.001996
r
keff 1
keff
0.998 1
0.998
0.0020
Units of Reactivity
Reactivity is a dimensionless number. It does not have dimensions of time, length, mass, or any
combination of these dimensions. It is simply a ratio of two quantities that are dimensionless.
As shown in the calculation in the previous example, the value of reactivity is often a small
decimal value. In order to make this value easier to express, artificial units are defined.
By definition, the value for reactivity that results directly from the calculation of Equation (3-5)
is in units of Dk/k. Alternative units for reactivity are %Dk/k and pcm (percent millirho). The
conversions between these units of reactivity are shown below.
1%
D k
k
0.01
D k
k
1 pcm 0.00001
D k
k
Another unit of reactivity that is used at some reactors is equivalent to 10-4 Dk/k. This unit of
reactivity does not have a unique name. Special units for reactivity that do have unique names
are dollars and cents. These units and their applications will be described in a later chapter.
Rev. 0
NP-03
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