NEUTRON LIFE CYCLE
DOE-HDBK-1019/2-93
Reactor Theory (Nuclear Parameters)
The next step in the analysis is to consider the number of neutrons that are absorbed in the
intermediate energy level. The probability of escaping this resonance absorption (p) is stated
as follows.
p
number of neutrons that reach thermal energy
number of fast neutrons that start to slow down
720
900
0.80
The number of neutrons entering the thermal energy range is now represented by the quantity
No f
p.
After reaching thermal energies, 100 neutrons leak from the core. The value for t can be
calculated by substitution of the known values in the definition as shown below.
t
number of thermal neutrons that do not leak from reactor
number of neutrons that reach thermal energies
620
720
0.861
The number of thermal neutrons available for absorption anywhere in the core is represented by
the quantity No f
p t.
Figure 1 indicates that 125 neutrons were absorbed in non-fuel materials. Since a total of 620
thermal neutrons were absorbed, the number absorbed by the fuel equals 620 - 125 = 495.
Therefore, the thermal utilization factor can be calculated as follows.
f
number of thermal neutrons absorbed in the fuel
number of thermal neutrons absorbed in any reactor material
495
620
0.799
NP-03
Rev. 0
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