Effects of high energy alpha on materials

Discuss the technical details of an "open source" community-driven design of a polywell reactor.

Moderators: tonybarry, MSimon

nferguso
Posts: 55
Joined: Sat May 17, 2008 3:43 am

Effects of high energy alpha on materials

Post by nferguso »

Looking around on the web I have found some discussion of the effects of high energy neutrons on materials - dislocations, embrittlement, swelling, transmutation to radioactive isotopes. Can someone say what effect comparably energetic alpha radiation has?

drmike
Posts: 825
Joined: Sat Jul 14, 2007 11:54 pm
Contact:

Post by drmike »

It won't be anywhere near as bad as neutron damage because the penetration depth is far less. Neutrons always get absorbed and cause chemical changes, alphas rarely do that.

The outer layer that gets first impact will have similar stresses, but the bulk material will be able to hold up. Regular maintanance will then be to polish up that outer layer - and it may only be a few microns thick. If you have to do that once every 6 months and replace things once every 25 years it's not too bad.

Bottom line is that alphas are much, much easier to deal with.

93143
Posts: 1131
Joined: Fri Oct 19, 2007 7:51 pm

Post by 93143 »

I would add that there does seem to be an additional problem with alphas, due to the fact that they don't penetrate very far. Most of their energy is deposited near the surface, which can cause liberation of surface atoms, possibly in large numbers. This is referred to as the sputtering problem.

At 80% grid transparency and zero ion loss, one alpha impact would have to release an average of 5/3 boron-11 atoms and 5/3 light hydrogen atoms for the reactor to be self-fueling. Any more than that and the core will flood. Any other material besides protium or boron-11 and the core will be poisoned.

Pulsing may enable it to be cleared. Neutronic fuels may still have this problem; neutrons tend to go deep, but D+D can release protons.

Mike Holmes
Posts: 308
Joined: Thu Jun 05, 2008 1:15 pm

Post by Mike Holmes »

Would this only be a problem for the test reactor? That is, if we're taking the energy from the machine as a generator by decellerating the alphas to create electrical current for use, then won't the impacts be lessened or negated nearly entirely?

As usual I'm probably misunderstanding something here...

Mike

drmike
Posts: 825
Joined: Sat Jul 14, 2007 11:54 pm
Contact:

Post by drmike »

It would be ideal if all the energy could be extracted before impact. We'll have to see how well we can do that!

dch24
Posts: 142
Joined: Sat Oct 27, 2007 10:43 pm

Post by dch24 »

Mike Holmes wrote:Would this only be a problem for the test reactor? That is, if we're taking the energy from the machine as a generator by decellerating the alphas to create electrical current for use, then won't the impacts be lessened or negated nearly entirely?
93143 mentioned 80% grid transparency. This means 80% of the alphas get outside the MaGrid and can be used for generating power by decelerating them.

That still leaves the 20% that smash into the MaGrid, causing heating, sputtering, and (if it's a neutron not an alpha particle) radioactivity. Remember that there will be a small number of neutrons even in aneutronic fusion (p-B11 fusion).

20% of a 100MW fusion reaction is a LOT of power barraging the MaGrid.

MSimon
Posts: 14332
Joined: Mon Jul 16, 2007 7:37 pm
Location: Rockford, Illinois
Contact:

Post by MSimon »

dch24 wrote:20% of a 100MW fusion reaction is a LOT of power barraging the MaGrid.
We have a solution: a LOT of heat transfer.

About 100 to 400 gpm of cooling water per grid. Not too bad. (multiply by 4X to get l/min - close enough for now).
Engineering is the art of making what you want from what you can get at a profit.

ravingdave
Posts: 650
Joined: Wed Jun 27, 2007 2:41 am

Post by ravingdave »

MSimon wrote:
dch24 wrote:20% of a 100MW fusion reaction is a LOT of power barraging the MaGrid.
We have a solution: a LOT of heat transfer.

About 100 to 400 gpm of cooling water per grid. Not too bad. (multiply by 4X to get l/min - close enough for now).
:)
Hee hee hee hee.... I hate being a nuisance, but there's still the idea of using the MagGrid AS part of the collector grid. At first I thought it might work, then I thought it wouldn't, now i'm not sure.

If alphas are leaving the wiffleball with it's negative potential, and moving towards the MagGrid at it's positive potential, won't they be deccelerate by the gradient between the two ? And once they strike the MagGrid, won't they pull a couple of electrons out of it and become neutrals ?

Maybe I don't know as much about this design as others, but i'm thinking that with enough parameter juggling, the idea might work.


David

MSimon
Posts: 14332
Joined: Mon Jul 16, 2007 7:37 pm
Location: Rockford, Illinois
Contact:

Post by MSimon »

Fore efficient collection the grid should be charged to particle MeV/particle charge.

However, if you do that the energy output goes negative due to the energy imparted to the pB11.

You will get some help. i.e. (grid voltage * particle charge) of energy per particle collected. However, 90% or more of the particle energy will appear at the grids as heat.
Engineering is the art of making what you want from what you can get at a profit.

nferguso
Posts: 55
Joined: Sat May 17, 2008 3:43 am

Post by nferguso »

I gather then that (at the risk of being too deterministic) the particles hit the magnets more like beanbags than golf balls? They don't bounce off with most of their energy but instead pick up a couple of electrons and, what, drift around?

MSimon
Posts: 14332
Joined: Mon Jul 16, 2007 7:37 pm
Location: Rockford, Illinois
Contact:

Post by MSimon »

nferguso wrote:I gather then that (at the risk of being too deterministic) the particles hit the magnets more like beanbags than golf balls? They don't bounce off with most of their energy but instead pick up a couple of electrons and, what, drift around?
Some will drift around. Some will get implanted in the surface of the grid.
Engineering is the art of making what you want from what you can get at a profit.

Jccarlton
Posts: 1747
Joined: Thu Jun 28, 2007 6:14 pm
Location: Southern Ct

Post by Jccarlton »

You are in the molecular flow region. There are some tricks you can play around with to ensure that alpha particles only go in the directions you want and eliminate backscatter and sputtering. For instance you could make the grids as bank of short thin walled tubes. or layered sheet metal grids. If you do it right, everything that bounces in can't bounce back out.

Mike Holmes
Posts: 308
Joined: Thu Jun 05, 2008 1:15 pm

Post by Mike Holmes »

Oh, now I wish I could recall where I saw a quote, but I can't. Somebody speaking of the Polywell process said that the neutrons produced would be few enough, and maybe more importantly, unenergetic enough, that the half-lives of anything created by neutron impacts would be short enough that one would be able to sfely walk into the room with the generator only a few minutes after it was shut down.

Could have been pure speculation, but I recall the source seeming to know what they were talking about. Anybody know from whence I'm recalling this quote?

Anyhow, for water cooling... do we then use the heat energy in the water to turn a turbine? Thus capturing some percentage of that 20% of otherwise "lost" energy? And cooling it in the process for reuse?

Um... "regenerative breaking" for a polywell engine? If you will?

I know, I know... "engineering details..."

Are the products of "sputtering" ionic? Or can they just be "sucked up" along with the alphas turned into HE? To avoid "poisoning the well" (that's too good an analogy not to use)?

Mike

MSimon
Posts: 14332
Joined: Mon Jul 16, 2007 7:37 pm
Location: Rockford, Illinois
Contact:

Post by MSimon »

Turbines and the associated auxiliaries are expensive long lead time items.

Better to use the "waste" as process heat.
Engineering is the art of making what you want from what you can get at a profit.

Keegan
Posts: 206
Joined: Fri Aug 17, 2007 6:29 am
Location: Brisbane, Australia

Post by Keegan »

Mike Holmes wrote:Oh, now I wish I could recall where I saw a quote, but I can't. Somebody speaking of the Polywell process said that the neutrons produced would be few enough

Hey Mike H. Yes the p + B11 reaction can produce neutrons in side reactions.
Fortunately this need not apply to polywell as evident in this excellent thread


Nferguso i consider the alpha first wall problem largly solved. All one needs to do is find a suitable material that is resistant to alphas. Currently im a fan of Diamond Coating a metal reactor. As evident below diamond will have a low cross section with alphas, we can coat the reactor interior with existing technology (leaving open the tantalising prospect of self healing reactors), and is optically transparent, leaving gammas and x rays to be absorbed by the underling metal structure.

Anyone got any better ideas ?

note this cross section is for "plain" Carbon. Not SP3 Diamond

Image
Purity is Power

Post Reply