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Chapter 13

NUC K117 Chapter 13: Neutron Life Cycle


Department
Nuclear Engineering Technology
Course Code
NUC K117
Professor
Abel
Chapter
13

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I. Neutron Life Cycle!
1. !
The above physical factors are encompassed into the ke ‘Six Factor
Formula’ as simple ratios.!
By multiplying the ratios together, we get a unitless value of ke that reflects a
multiplication factor for change in neutron population from one generation to
the next!
2. All values below: All factors over 1 is a contributor !
3. Fast Fission Factor ( ) !
 !
About 2% of U235 fissions caused by fast neutrons!
Therefore, for a 2% reactor a typical value is: 1.02!
Commonly a breeder reactor!
Not really needed
4. Fast Non-Leakage ( Lf )!
 !
Fast neutrons begins slowing down through collisions with moderator!
Some fast neutrons leak out of the core and are ‘lost’ to the fission process!
Lf is expressed in terms of fast neutrons that DO NOT leak out of the core!
Typically value: Lf .97!
5. Resonance Escape Probability ( )!
 !
As fast neutrons begin slowing down, they are subject to ‘resonance capture’
by the fuel!
These neutrons do not cause fission, and therefore ‘escape’ the fission
process!
Typical value .87!
6. Thermal Non-Leakage ( LTH )!
 !
Some thermal neutrons also leak out of the core and are not available to
cause fission!
kef f =ϵLfρLth fη
ϵ
ϵ=Number of fast Neutrons produced by All Fission
Number of fast Neutrons produced by Ther m al Fission
ϵ
Lf=Number of Fast Neutrons that do NOT L eak
Number of Fast Neutrons produced by All Fission}
ρ
ρ=Number of Neutrons that Reach T her m al Energy
Number of Fast Neutrons that Start to Slow Down
ρ
LTH =Number of Neutrons that Reach T her m al Energy
Number of Fast Neutrons that Start to Slow Down
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