Doppler effect neutron absorption

In summary, the resonance absorption of U-238 and Pu-240 helps to stabilize the reactor, and it prevents over-power events.
  • #1
girts
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Just a quick question I got in my mind while reading up about fission, the so called Doppler effect which manifests itself in fertile material like U238, I read it helps control reactor stability , hence the majority of fuel consists of U238, as the fuel heats up at some point, U238 starts to absorb more neutrons and hence less neutrons are available for fission which decreases the further rise of reactivity,

now firstly I assume this is a natural physical phenomenon that just so happens to serve in our favor in a reactor core?
secondly tell me please what exactly is the "magic" trick here, is it that while the temperature is lower in the fuel the nucleus of U 238 has lower motion and hence a smaller cross section so only neutrons of specific energy get absorbed while others are not and when the temp rises the target nucleus kind of "moves" more and hence it somehow can accept a wider range of neutrons energies and absorb them?
 
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  • #2
girts said:
. . . . while reading up about fission, the so called Doppler effect which manifests itself in fertile material like U238, I read it helps control reactor stability, hence the majority of fuel consists of U238, . . . .
Not hence. Natural uranium is predominantly U-238 (~0.992) with some U-235 (~0.007), with traces of U-234. U-235 is fissile, so commercial LWR fuel is enriched (up to 4.95%) in order to operate for a given energy generation over a given time (cycle). U-238 and many heavy isotopes (not usually found in a core) absorb neutrons in the so-called 'resonance' region of the neutron spectrum, that is in the high eV to ~ 1 MeV. Pu-240 is another isotope in LWR fuel that provides resonance absorption, but it is formed from transmutation of Pu-239, which is formed from U-238 through neutron absorption and subsequent beta decay.

girts said:
a natural physical phenomenon that just so happens to serve in our favor in a reactor core?
secondly tell me please what exactly is the "magic" trick here, is it that while the temperature is lower in the fuel the nucleus of U 238 has lower motion and hence a smaller cross section so only neutrons of specific energy get absorbed while others are not and when the temp rises the target nucleus kind of "moves" more and hence it somehow can accept a wider range of neutrons energies and absorb them?
In resonance absorption, a high-Z nucleus, which preferentially absorbs neutrons of specific energy (resonance), helps control reactivity in the core, and it helps prevent over-power events (e.g., reactivity insertion accident (RIA)), where a control rod is inadvertently ejected (PWR) or dropped (BWR) from a core, which leads to a power excursion. When the power increases, the fuel temperature increases, and as the fuel temperature increases the atomic vibrations increase. The vibrational increase in U-238 and Pu-240 means that the resonances broaden allowing more neutrons to be capture, which prevents the power from increasing.

Decreases in fuel and moderator density also contribute to a reduction in core/fuel reactivity.
 
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1. What is the Doppler effect neutron absorption?

The Doppler effect neutron absorption is a phenomenon where the frequency of neutrons changes when they are absorbed or emitted by an atom that is moving relative to the observer. This change in frequency is due to the relative motion between the neutron and the atom, and it can either increase or decrease the energy of the neutron.

2. How does the Doppler effect neutron absorption affect neutron beams?

The Doppler effect neutron absorption can cause a broadening or narrowing of the energy distribution of neutron beams. If the absorbing atoms are moving towards the neutron beam, it will increase the energy of the neutrons, resulting in a broader energy distribution. Conversely, if the absorbing atoms are moving away from the neutron beam, it will decrease the energy of the neutrons, resulting in a narrower energy distribution.

3. What factors can influence the Doppler effect neutron absorption?

The Doppler effect neutron absorption is influenced by the relative velocity between the neutron and the absorbing atom, the mass of the absorbing atom, and the angle of incidence of the neutron beam. It is also affected by the temperature of the absorbing material, as higher temperatures can cause atoms to move faster, resulting in a stronger Doppler effect.

4. How is the Doppler effect neutron absorption used in scientific research?

The Doppler effect neutron absorption is an important tool in the study of materials and their properties. By analyzing the changes in the energy distribution of neutron beams, scientists can gain insights into the structure and behavior of atoms and molecules. This is particularly useful in fields such as nuclear physics, materials science, and chemistry.

5. Are there any practical applications of the Doppler effect neutron absorption?

Yes, the Doppler effect neutron absorption has several practical applications. It is used in medical imaging techniques such as positron emission tomography (PET) and magnetic resonance imaging (MRI). It is also used in industrial processes, such as in the production of neutron radiography images and in neutron activation analysis for detecting trace elements in materials.

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