Graphite Moderator Explained: Carbon's Atomic Structure & Neutron Moderation

In summary: This means that carbon has a high scattering cross section, which means that it will slow down the neutrons more than other materials.
  • #1
Johnleprekan
57
0
Can someone explain to me, in layman's terms, why carbon (especially graphite) has a high scattering cross section? What is it about carbon's atomic structure that makes it a good neutron moderator?
 
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  • #2
High compared to what?
To slow down neutrons, it is useful to have targets with a mass similar to the neutron mass:
- If you throw an elastic ball on a wall (~heavy nucleus), it will bounce back with the same energy
- If you throw it on another ball (~light nucleus), both will receive a part of the kinetic energy
- And if you hit a fly with it (~electron), it will just continue to fly in the initial direction

Hydrogen has the best mass. But protium (1 proton, 0 neutrons) can absorb the neutron, and deuterium in large amounts is expensive.
Helium is a gas, which is bad for the required density.
Lithium has some safety issues, and the additional neutron absorption problem (in Li-6)
No idea about beryllium. It is toxic, but you don't want to get in contact with anything in the reactor core anyway.
Boron is fine if you want to absorb neutrons... ;)
Carbon (especially graphite) is cheap and solid within a useful range of temperatures
 
  • #3
Carbon has a high binding energy per nucleon, which is slightly higher than than of He-4.
http://www.nndc.bnl.gov/chart/reCenter.jsp?z=5&n=5 (click on BE/A to see binding energy pre nucleon, and use Zoom 1)

It has a relatively low absorption cross-section, and atomically/chemically it has a high melting point and reasonable atomic density, as compared to those properties of the neighboring elements.

Be and BeO are good moderators too, but there is the issue of toxicity.


One also has to be concerned with chemical stability in addition to neutron absorption.
 
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  • #4
Thank you.
 
  • #5
One of the first reactors in Britain was the air-cooled Windscale reactor built in 1950-51 It had a solid graphite core, with metallic uranium fuel cartridges inserted into channels in the graphite. Wigner predicted that graphite, when bombarded by neutrons, could build up stored potential energy ("Wigner energy") in atomic crystalline dislocations. Periodically the dislocations had to be annealed out by running the air-cooled reactor at reduced power to gradually raise the reactor core temperature and slowly anneal the graphite. The annealing process is exothermic, and the stored energy is released as as heat. In 1957, the annealing process overheated the core, causing the graphite to overheat and catch fire, leading to the uncontrolled release of radioactivity into the environment. This was the most serious reactor accident in British history. So although graphite is a good neutron moderator, it has serious shortcomings when used in a reactor. See http://en.wikipedia.org/wiki/Windscale_fire
 
  • #6
Johnleprekan said:
Can someone explain to me, in layman's terms, why carbon (especially graphite) has a high scattering cross section? What is it about carbon's atomic structure that makes it a good neutron moderator?

The distance between carbon atoms in graphite's crystalline structure is about the same as the wavelength of a thermal neutron.
 

1. What is a graphite moderator and how does it work?

A graphite moderator is a type of material used in nuclear reactors to slow down the speed of neutrons. The atomic structure of graphite allows it to effectively slow down the neutrons as they pass through, making them more likely to cause fission in the reactor.

2. Why is carbon used as the material for the moderator?

Carbon is used as the material for the moderator because of its atomic structure. Carbon atoms have a low atomic mass and are arranged in a hexagonal lattice, which makes them ideal for slowing down neutrons without absorbing them.

3. How does the atomic structure of carbon affect its ability to moderate neutrons?

The hexagonal lattice structure of carbon allows for the neutrons to easily pass through and interact with the carbon atoms. This results in the neutrons being slowed down, without being absorbed, as they bounce off the carbon atoms.

4. What other materials can be used as moderators in nuclear reactors?

Besides graphite, other materials such as heavy water (deuterium oxide) and light water (regular water) can be used as moderators. However, these materials are less efficient in slowing down neutrons compared to graphite.

5. Are there any disadvantages to using graphite as a moderator?

One of the main disadvantages of using graphite as a moderator is its susceptibility to radiation damage. As the neutrons pass through the graphite, they can cause structural changes, making the graphite more brittle and less effective as a moderator over time. Additionally, graphite can also catch fire in certain conditions, which can be a safety concern.

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