Resonance absorbtion of neutrons

In summary, the equation Fa=Φav∫∑a(E) dE gives the number of neutrons absorbed in a resonance region per cm3/sec, and it is dependent on the temperature. The flux spectrum is determined by the fissile source, lattice design, and moderator type and density, and is in thermal equilibrium with the coolant in an LWR. The resonance absorption is affected by the vibration of atoms, which is dependent on fuel temperature, and the bounds of the integral should be above and below the resonance. The flux spectrum is a result of the slowing down process and cannot be controlled with great accuracy in a reactor.
  • #1
oksuz_
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Hi,

As you may know, the following equation gives the number of neutrons absorbed per cm3/sec in a resonance region.

Faav∫∑a(E) dE

So, it is said in Lamarsh, introduction to nuclear engineering, that Φav depends on temperature. I do not get this because I know that the flux is something we control. Or should we think just whenever we tune the flux, let's say 1 MeV, the ambient temperature affects its energy?

Thank you in advance.
 
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  • #2
Temperature of what? If neutron temperature, then neutron flux in the resonance region of the material is affected.
 
  • #3
oksuz_ said:
Fa=Φav∫∑a(E) dE
That's a general equation for absorption over dE.

oksuz_ said:
I do not get this because I know that the flux is something we control. Or should we think just whenever we tune the flux, let's say 1 MeV, the ambient temperature affects its energy?
The flux spectrum is determined by the fissile source, lattice design and moderator type and density. In an LWR, the neutrons are moderated by the coolant (light water), and it is with water, not the fuel, with which the neutrons are more or less in thermal equilibrium. Resonance absorption occurs at energies above the thermal equilibrium of the moderator. The resonances are broadened by the vibration of the atoms, primarily U-238 and Pu-240 (Pu-240 in irradiated fuel), and the vibration is dependent on fuel temperature, which is dependent on linear power and coolant temperature.
Typical neutron energy spectrum - https://www.researchgate.net/figure/259717796_fig2_Fig-2-Neutron-source-spectrum-from-typical-LWR-reactor

Fission neutrons are born with energies in the low MeV range (0.1 to 10 MeV), with a most probable energy slightly below 1 MeV, and a mean about 2 MeV. One should be able to find a fission neutron energy spectrum in Lamarsh.

See also - https://t2.lanl.gov/nis/publications/madland1.pdf
 
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  • #4
I believe you are referring to the flux shape near a resonance, and the bounds of the integral should be above and below the resonance.
The high resonance cross section will cause the flux to "dip" and you will get a lower flux corresponding to the peak of the cross section in the resonance.
If you assume a flat flux shape in the resonance region, you will over-estimate the resonance absorption.

It is not quite true that we "control" the flux. We can control certain things, like the fuel material, moderator material, and size of the materials, but we cannot control the flux spectrum with great accuracy. (At least in a reactor.) The flux spectrum will be a result of the slowing down process. I believe you are studying slowing down process by your question.
 

1. What is resonance absorption of neutrons?

Resonance absorption of neutrons is a phenomenon that occurs when a neutron with a specific energy is absorbed by a nucleus, causing the nucleus to become unstable and emit radiation. This process is known as resonance because it occurs at specific energy levels that correspond to the energy levels of the nucleus.

2. How does resonance absorption of neutrons work?

Resonance absorption of neutrons works by the neutron interacting with the nucleus of an atom. When the energy of the neutron matches the energy level of the nucleus, the neutron is absorbed and the nucleus becomes unstable, emitting radiation in the process.

3. What are the applications of resonance absorption of neutrons?

Resonance absorption of neutrons has many applications in various fields such as nuclear power, nuclear medicine, and materials science. It is used to control the rate of nuclear reactions, produce radioactive isotopes for medical purposes, and study the structure and properties of materials.

4. How is resonance absorption of neutrons measured?

Resonance absorption of neutrons can be measured using a variety of techniques such as neutron spectroscopy, neutron activation analysis, and neutron diffraction. These methods allow scientists to determine the energy levels and behavior of neutrons in different materials.

5. What factors affect resonance absorption of neutrons?

The resonance absorption of neutrons is affected by factors such as the energy and speed of the neutrons, the composition and structure of the material, and the presence of other particles or radiation. Additionally, the shape and size of the nucleus also play a role in the absorption process.

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