Rf
f
1 x 1013
neutrons
cm2 sec
0.1 cm1
1 x 1012
fissions
cm3 sec
Rf
f
f
Rf
1.29 x 1012
fissions
cm3 sec
3 x 1013
neutrons
cm2 sec
0.043 cm1
Reactor Theory (Neutron Characteristics)
DOE-HDBK-1019/1-93
REACTION RATES
Rev. 0
Page 19
NP-02
The reaction rate calculated will depend on which macroscopic cross section is used in the
calculation. Normally, the reaction rate of greatest interest is the fission reaction rate.
Example:
If a one cubic centimeter section of a reactor has a macroscopic fission cross section of
0.1 cm , and if the thermal neutron flux is 10 neutrons/cm -sec, what is the fission rate
-1
13
2
in that cubic centimeter?
Solution:
In addition to using Equation (2-6) to determine the reaction rate based on the physical properties
of material, it is also possible to algebraically manipulate the equation to determine physical
properties if the reaction rate is known.
Example:
A reactor operating at a flux level of 3 x 10 neutrons/cm -sec contains 10 atoms of
13
2
20
uranium-235 per cm . The reaction rate is 1.29 x 10 fission/cm . Calculate
and
.
3
12
3
f
f
Solution:
Step 1:
The macroscopic cross section can be determined by solving
Equation (2-6) for
and substituting the appropriate values.
f
