Energy Self-Shielding: Explaining the Concept

[middlephysics]

Can someone explain the concept to me? As far as I know, in some energy interval there are materials which have resonance absorption. The book then goes on to say that energy self shielding does something to combat this absorption but I am not clear on how and hope someone can clear this up for me.

 

[QuantumPion]

Neutron absorption rate is proportional to flux. At resonance peaks, there is a reduction in flux due to the large absorption cross section. Therefore, there are less total absorptions over that energy range as flux is depressed. Thus, the resonance peak shields itself.

 

[middlephysics]

This is what I thought, but this is basically like saying "There are less absorptions because the absorptions reduce the flux"... well ok, but this is a tautology I don't see any "shielding" per say

edit: to understand where I'm coming from- it's like saying: a bunch of army men walk onto a mine field and some get blown up by mines. Now there are less army men to step on mines, so they've been "shielded".

 

[Astronuc]

This is what I thought, but this is basically like saying "There are less absorptions because the absorptions reduce the flux"... well ok, but this is a tautology I don't see any "shielding" per say

edit: to understand where I'm coming from- it's like saying: a bunch of army men walk onto a mine field and some get blown up by mines. Now there are less army men to step on mines, so they've been "shielded".

It's more along the lines that there are less mines on which personnel would step.

The removal of neutrons of a given energy range (window) by absorption means the flux at that energy level (over that range) is reduced so that the rate of aborption decreases as compared to flux measured at the higher level.

The shielding can be seen if one looks radially into a cylindrical fuel pin or plate or sphere. There is more absorption (an Pu/TU production) at the surface than interior of the fuel. Pu and TU build up on the periphery of a fuel pellet or plate or sphere more so than in the interior. There is more 'shielding' as enrichment increases.

 

[middlephysics]

It's more along the lines that there are less mines on which personnel would step.

The removal of neutrons of a given energy range (window) by absorption means the flux at that energy level (over that range) is reduced so that the rate of aborption decreases as compared to flux measured at the higher level.

Thanks for the response. I've heard it enough, I understand it, I feel OK about it, so I'll accept it.

Just seems like the resonance absorpers have stolen a bunch of neutrons at a specific range and now they can't really steal anymore, but if "shielding" is the way to see it.. ok

 

[QuantumPion]

Thanks for the response. I've heard it enough, I understand it, I feel OK about it, so I'll accept it.

Just seems like the resonance absorpers have stolen a bunch of neutrons at a specific range and now they can't really steal anymore, but if "shielding" is the way to see it.. ok

Don't think of the flux as time-varying. The neutrons can't be at those energy range, because if they were, they would have been absorbed! It sounds like circular definition because it is - that is the nature of differential equations.

 

[QuantumPion]

And to address your minefield example, it would be more like the soldiers know there's a minefield there so they don't walk over it to begin with. Therefore the minefield is shielded from the soldiers.

 

[QuantumPion]

Or alternatively, in the case of spatial self-shielding, imagine a minefield with mines that set off only if the soldier weighs more than 200 lbs. The soldiers march through the minefield trying to get to the object at the center. All of the heavy soldiers get blown up, so only the thin soldiers make it to the center. The center is shielded from heavy soldiers and so no mines get set off there.