Which substance is the best moderator for fission reactors?

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In summary: I would say that for thermal reactors, heavy water is the best moderator because it is the densest and does not readily absorb neutrons.
  • #1
ahaanomegas
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Which substance is the best moderator for fission reactors, if there is even a definition for "best"? My current physics book says heavy water. Do you agree? Please explain. :smile:
 
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  • #2
Protons are the best at slowing down neutrons, however they can also absorb neutrons (and form deuterons) in competition with the fuel. Heavy water or D2O slows down neutrons but does not readily absorb thermal neutrons, there it is considered a better moderator for thermal fission reactors.

Moderating ratio is a figure of merit for moderators.
http://www.tpub.com/content/doe/h1019v1/css/h1019v1_131.htm
 
  • #3
Astronuc said:
Protons are the best at slowing down neutrons, however they can also absorb neutrons (and form deuterons) in competition with the fuel. Heavy water or D2O slows down neutrons but does not readily absorb thermal neutrons, there it is considered a better moderator for thermal fission reactors.

Moderating ratio is a figure of merit for moderators.
http://www.tpub.com/content/doe/h1019v1/css/h1019v1_131.htm

It's not "the best".
Definition of "the best" is moderating ratio, so liquid D2 maybe "the best". D2 is gaseous state in environmental temperature. When D2 is compressed into liquid state, it would be "the best".
 
  • #4
law&theorem said:
It's not "the best".
Definition of "the best" is moderating ratio, so liquid D2 maybe "the best". D2 is gaseous state in environmental temperature. When D2 is compressed into liquid state, it would be "the best".
But that wouldn't work for a hot reactor. D2 doesn't compress to a liquid but for pressures well beyond what we normally achieve terrestrially.
 
  • #5
:( I thought this was a poll on who is our favourite PF mod...
 
  • #6
Astronuc said:
But that wouldn't work for a hot reactor. D2 doesn't compress to a liquid but for pressures well beyond what we normally achieve terrestrially.

Above critical temp (IIRC close to 40K) D2 has no liquid state per se.
 
  • #7
nikkkom said:
Above critical temp (IIRC close to 40K) D2 has no liquid state per se.
Supercritical fluid would seem more appropriate. The moderator needs a certain density.

One way to determine the superiority of deuterium to heavy water would be to determine the density of deuterium necessary to give the same moderator ratio as heavy water. From that density, then determine the pressure required. Then determine the necessary requirements of the pressure vessel and its material to contain that pressure.
 
  • #8
A dense solid compound of deuterium would be good, i.e. "heavy paraffin" or perhaps ZrD2. Remember that the moderator can be outside the reactor and doesn't have to also be the coolant (e.g. CANDU, RBMK, MAGNOX, etc).
 
  • #9
QuantumPion said:
A dense solid compound of deuterium would be good, i.e. "heavy paraffin" or perhaps ZrD2. Remember that the moderator can be outside the reactor and doesn't have to also be the coolant (e.g. CANDU, RBMK, MAGNOX, etc).
Since you brought up the idea of solid hydrides, which is being considered in some high temperature concepts, e.g., SCWR, the other complication of high pressure deuterium would be the propensity for the hydrogen to diffuse into the structural metals and forming hydrides. This would be bad news for steels or Inconels as it would undermine their structural integrity, especially when it comes to shutdowns. Probably the primary system would have to be cladding in Al or Cu or some other element that does not readily allow diffusion of hydrogen into the metal or formation of hydrides. However at the pressure required to maintain a supercritical deuterium fluid, I'm not sure there is a metal that could handle it.

Bare in mind, a moderated reactor has to be cooled, and there is only so much volume for the coolant, moderator, fuel and structural materials.

Certainly one could design an epithermal or fast reactor spectrum, but then there are issues of control and stability - depending on various design factors. Nevertheless, the OP was concerned about 'the best moderator'.
 

1. What is the role of a moderator in a scientific study?

A moderator in a scientific study is responsible for overseeing and managing the research process. This includes selecting and managing participants, collecting and analyzing data, and ensuring the study is conducted ethically and accurately.

2. What qualities should a good moderator possess?

A good moderator should have strong communication and interpersonal skills, as well as attention to detail and the ability to remain unbiased. They should also be knowledgeable about the subject matter and have good problem-solving abilities.

3. How do you determine which moderator is best for a specific study?

The best moderator for a specific study will depend on the research question, methodology, and target population. It is important to consider factors such as experience, expertise, and availability when selecting a moderator.

4. What are some common mistakes that moderators make in a study?

Some common mistakes that moderators make include bias in participant selection or data collection, failure to follow ethical guidelines, and misinterpretation of data. It is important for moderators to be aware of these potential pitfalls and take steps to avoid them.

5. How can a moderator ensure the accuracy and validity of a study?

To ensure the accuracy and validity of a study, a moderator should carefully plan and design the research, use reliable and valid measures, and follow ethical guidelines. They should also regularly review and analyze the data to identify any potential errors or biases.

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