What elements are used to prevent losing neutrons?

[Physicsissuef]

Main Question or Discussion Point

what elements are used to prevent losing neutrons and what are the ways of preventing losing of neutrons in nuclear power plant and atomic bomb?

 

[Astronuc]

what elements are used to prevent losing neutrons and what are the ways of preventing losing of neutrons in nuclear power plant and atomic bomb?

The cores of nuclear plants and nuclear weapons (pits) are two different animals.

Nuclear fuel uses relatively low enrichment, typcially just less the 5% enriched these days in most modern LWRs, or slightly enriched in CANDU's, as compared to nuclear weapons, which are based on highly enriched Pu-239 (~94% Pu-239).

The fuel in the cores have been design with low absorbing Zr-based alloys, while the structural materials are typically stainless steels of the 300 series (e.g. 304, 316 . . .). The core is optimized geometrically, and the power distribution is then optimized to reduce in the assemblies on the exterior of the core in order to reduce neutron leakage (loss). Some older plants had steel pads designed to reflect fast neutrons back into the core. In some exotic core design Be and compounds of Be have been proposed, however Be is a very toxic compound.

In nuclear weapons, the pits are high enriched and then compressed to much greater densities very quickly in order to maximize yield (energy from the fissile reaction). There is no reflector for reflecting neutrons since nuclear weapons are on a very fast time scale based on prompt neutrons.

 

[Physicsissuef]

And how can we prevent losing neutrons with neutron capture?

 

[vanesch]

And how can we prevent losing neutrons with neutron capture?

If neutrons are captured, they are lost!

 

[Physicsissuef]

In my book it says that we can prevent neutron loss with neutron capture. Maybe it thinks about neutron-mirrors?

 

[vanesch]

In my book it says that we can prevent neutron loss with neutron capture. Maybe it thinks about neutron-mirrors?

Sounds funny. Maybe they have non-standard definitions of what neutron capture means. If not, put the book in the dustbin

 

[Physicsissuef]

Is neutron mirror that first absorb neutron and then released? After how many minutes it releases the neutron?

 

[vanesch]

Is neutron mirror that first absorb neutron and then released? After how many minutes it releases the neutron?

No, a neutron mirror is a material on which neutrons scatter elastically, without being absorbed. In fact, the properties of a neutron mirror are close to those of a moderator.

 

[Physicsissuef]

So the only way to prevent loss of neutrons is with neutron scattering, or neutron capture of the next generation of uranium atoms in chain reaction, right?

 

[vanesch]

So the only way to prevent loss of neutrons is with neutron scattering, or neutron capture of the next generation of uranium atoms in chain reaction, right?

Yes.

 

[Delok]

I heard that Tungsten carbide makes a good neutron reflector.

By the way, does anyone knows where i could get more precise data about this material, especially about Neutron reflection rates per mm thickness and so on? I consider using it for a project of mine.

 

[Astronuc]

I heard that Tungsten carbide makes a good neutron reflector.

By the way, does anyone knows where i could get more precise data about this material, especially about Neutron reflection rates per mm thickness and so on? I consider using it for a project of mine.

The best reflector has a high fast removal cross-section with a low absorption cross-section. I believe W-184 has the lowest absorption cross-section of the W isotopes, and is about 30.6% of natural tungsten. One needs to look at ENDF/B data or the Barn book.

One drawback is the density. Iron has also been used as a reflector in thermal (LWRs)reactors.

I'm assuming one is referring to thermal/epithermal spectrum.