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

[Johnleprekan]

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?

 

[mfb]

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

 

[Astronuc]

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.

 

[Johnleprekan]

Thank you.

 

[Bob S]

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

 

[QuantumPion]

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.