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Chapter 12

NUC K117 Chapter Notes - Chapter 12: Neutron Temperature, Electronvolt, Increment And Decrement Operators


Department
Nuclear Engineering Technology
Course Code
NUC K117
Professor
Abel
Chapter
12

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I. Neutron Moderation!
1. Fission “Fast” Neutrons!
At 2 MeV, they quickly enter the water flowing through the core and begin
colliding with atoms in H2O and any other material they encounter!
2. Fun Facts!
Physical collisions between bodies transfer energy!
Collisions between bodies of equal mass transfer the most energy!
A neutron and proton (hydrogen nucleus) have almost exactly same mass!
Water is therefore a good ‘moderator’ to slow down neutrons eciently!
3. Thermal Neutrons!
Energy of < 1 eV (started at 2 MeV)!
Average of 6 moderator collisions (H2)!
Average of 19 moderator collisions in water !
At thermal equilibrium with core!
4. Fission cross sections are dierent!
Fast rarely cause fission!
Slow : = 505 barns!
Fast : = 1 barn!
5. Water is used because!
Large scattering cross section!
Small absorption cross section!
Facilitates maximum energy transfer per collision!
It’s a great heat-transfer medium!
6. Forms of the “Log Mean Energy Decrement” equation!
!
!
= Average Logarithmic Energy Decrement!
= Average Initial Neutron Energy!
= Average Final Neutron Energy!
N = Number of Collisions!
7. Macroscopic Slowing Down Power!
MSDP = !
0n1
U235
0n1
σthermal
0n1
σfast
ζ=lnEilnEf
ζ=ln Ei
Ef
N=
ln Ei
Ef
ζ
ζ
Ef
ζΣs
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