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

by OC2513121

Department

Nuclear Engineering TechnologyCourse Code

NUC K117Professor

AbelChapter

12This

**preview**shows half of the first page. to view the full**2 pages of the document.**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 eﬃciently!

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 diﬀerent!

•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=lnEi−lnEf

ζ=ln Ei

Ef

N=

ln Ei

Ef

ζ

ζ

Ei

Ef

ζΣs

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