Energy From Thorium Discussion Forum

It is currently Nov 13, 2018 5:11 pm

All times are UTC - 6 hours [ DST ]




Post new topic Reply to topic  [ 59 posts ]  Go to page 1, 2, 3, 4  Next
Author Message
 Post subject: Self-healing of graphite
PostPosted: Sep 12, 2012 11:31 am 
Offline

Joined: Apr 28, 2011 10:44 am
Posts: 250
Excuse me if this has been discussed before.

In a 2005 ORNL paper, Carbon-Carbon composite salt-cooled electric space heaters,
Forsberg, Williams, Holcomb et al (all big names) toss out the following comment
concerning graphite

"is is known that most types of radiation damage are reversed by treatment
at temperatures near 2000C. Theoretical considerations and indirect experimental evidence
suggest that at very high temperatures there may be sufficient self-healing
of the carbon materials to enable extreme burn up and long-term operations
at temperatures between 1500C and 2000C."

Does anybody know what "indirect experimental evidence" (oxymoron?)
they are talking about?


Top
 Profile  
 
PostPosted: Sep 12, 2012 1:45 pm 
Offline

Joined: Jul 14, 2008 3:12 pm
Posts: 5039
We know that graphite self heals because at MSRE temperatures there wasn't any buildup of Wigner energy in the graphite. It was annealed out. Yes, I think that's what it is called, technically, "annealing". If you Google you should find lots of interesting stuff.

Operating the reactor hotter won't help us, of course. The annealing is more powerful at higher temperatures - but so is the mobility of carbon atoms in dislocating away from the lattice! It is counterintuitive that the annealing would become more powerful than the damaging radiation at some point. Nature favors chaos.

But we did discuss earlier the high temperature annealing of graphite.

I went ahead and Googled already. Some interesting papers turn up:

http://www.ems.psu.edu/~radovic/1955/pa ... 55_149.PDF

http://www-pub.iaea.org/MTCD/publications/PDF/ngwm-cd/PDF-Files/paper%207%20(Botzem).pdf

Not really any talk about ultra high temperature annealing. But since graphite is produced, in the first place, at elevated temperatures to get the desired properties - maybe even the crystal lattice? - this could be the "indirect experimental evidence" that the heavyweight VIP researchers are talking about?

It makes sense that, without radiation to damage it (in an inerted oven for example) the graphite should anneal very well. Whether this will also shrink it back again is the big question!


Top
 Profile  
 
PostPosted: Sep 12, 2012 3:23 pm 
Offline

Joined: Apr 28, 2011 10:44 am
Posts: 250
Were probably going to replace the moderator
as soon as it starts to swell above its original volume,
so we really dont need to shrink it.
We just need to get the atoms back into the lattice
where they belong.

If all we had to do to the used moderator
was take it out and put in an oven at graphitization temp for a while,
wow. Seems to good to be true.

We need a solid state guy to comment.


Top
 Profile  
 
PostPosted: Sep 12, 2012 3:49 pm 
Offline

Joined: Jul 28, 2008 10:44 pm
Posts: 3060
Cyril R wrote:
We know that graphite self heals because at MSRE temperatures there wasn't any buildup of Wigner energy in the graphite. It was annealed out. Yes, I think that's what it is called, technically, "annealing". If you Google you should find lots of interesting stuff.

True that Wigner energy isn't an issue for us as it gets released around 300C (IIRC). However, the graphite still suffers radiation damage in MSRE. Atoms still get displaced and force the planes of graphite to separate. It just that the stored energy of the stress is released at our temperatures.
Quote:
Operating the reactor hotter won't help us, of course. The annealing is more powerful at higher temperatures - but so is the mobility of carbon atoms in dislocating away from the lattice! It is counterintuitive that the annealing would become more powerful than the damaging radiation at some point. Nature favors chaos.

The quotation discussed treatment of radiation exposed graphite so I take this to refer to some remedial action rather than operating the reactor at this temperature. Operating at 2000C would indeed be some feat!
Quote:
It makes sense that, without radiation to damage it (in an inerted oven for example) the graphite should anneal very well. Whether this will also shrink it back again is the big question!

That was a question I posed to the graphite expert at ONRL. His response assumed that we had to first grind the graphite into extremely fine powder, add some binder, then apply both high temperature and high pressure. It did not give me the impression that one could simply apply high temperature to the graphite pieces and have the high temperature alone move the displaced atoms back into the planes from whence they came. It would be wonderful if we could come up with a machine that could take a graphite log and simply heat it or both heat&pressure to return it to the proper shape. Even better if the machine could be small enough that we could do that on-site at a slow pace (the logs are around 40 cm in diameter and 5 meters long). I think that we would have around 50 of them and around 4 years to get the job done so about 25 days per log. The question is whether we need to grind them up first (and thus have fine radioactive dust to jam machinery with).


Top
 Profile  
 
PostPosted: Sep 12, 2012 4:48 pm 
Offline

Joined: Jul 14, 2008 3:12 pm
Posts: 5039
djw1 wrote:
Were probably going to replace the moderator as soon as it starts to swell above its original volume, so we really dont need to shrink it. We just need to get the atoms back into the lattice where they belong.


Getting the atoms back into the lattice will probably shrink it. That's what I meant. If it doesn't shrink it, there's not much point to use this for easy recycling. If it needs grinding up then it's much less attractive to do it onsite.

I would like to know exactly what happens to the graphite. The atoms get smashed out of the lattice, then take up space between crystal planes, forcing the planes further apart than they were. Something like that I can imagine. But why does it first shrink (at least many graphites do)? Does the radiation also do something to the position of the lattices - maybe pushing them into closer to theoretical density, since the graphite is produced at much lower than theoretical density? Or is it the dislocated carbon atoms that go into these manufactured voids first, and then when they can't fit there, go and wedge out the lattice planes further apart?


Top
 Profile  
 
PostPosted: Sep 12, 2012 4:56 pm 
Offline

Joined: Jun 05, 2011 6:59 pm
Posts: 1334
Location: NoOPWA
I envision a system wherein robot arms connect high power electrical leads to the top and bottom of each graphite column and during low demand times dumps a goodly bit of power into said column to anneal it. The heat generated will go back into the salt and about 45% will be recaptured. Hmmm? Or do we need high pressure too?

_________________
DRJ : Engineer - NAVSEA : (Retired)


Top
 Profile  
 
PostPosted: Sep 12, 2012 6:45 pm 
Offline

Joined: Apr 28, 2011 10:44 am
Posts: 250
This important conversation desperately needs an expert.

Re Lars comment, I'm certainly no expert
but I cant imagine pressure is the key.
The lattice we want is the low energy arrangement,
jiggling the atoms enough should get them there.

AFAIK, the graphitization furnaces dont use pressure.

If pressure is important, than Forsberg, Holcomb, Williams
were blowing smoke.

I'm wondering if we asked the right question of the ORNL expert.


Top
 Profile  
 
PostPosted: Sep 15, 2012 2:26 pm 
Offline

Joined: Nov 01, 2011 2:15 am
Posts: 73
The simplest approach is to limit the plant's power density so that graphite elements will last the lifetime of the plant. 30 years should be a long enough target.


Top
 Profile  
 
PostPosted: Sep 15, 2012 2:41 pm 
Offline

Joined: Jul 28, 2008 10:44 pm
Posts: 3060
Simple yes but expensive. Basically you are requiring 8-10x more fissile for this option.


Top
 Profile  
 
PostPosted: Sep 15, 2012 7:49 pm 
Offline

Joined: Apr 28, 2011 10:44 am
Posts: 250
Simple and bogus. For a given temperature, the amount of moderator used up
is proportional to power. The graphite waste will be the same.

The concept of a plant life is also bogus or at least artificial, As Weinberg pointed out,
plant life should be forever. A proper design simply changes out
components as they wear out. See hydro or the B-52.


Top
 Profile  
 
PostPosted: Sep 15, 2012 11:32 pm 
Offline

Joined: Nov 01, 2011 2:15 am
Posts: 73
Any initial MSR demonstration plant will have low power density and long graphite life. For a commercial plant, if graphite elements are designed to be replaceable maybe a 10 year replacement cycle would not be too objectionable.


Top
 Profile  
 
PostPosted: Sep 16, 2012 7:19 am 
Offline

Joined: Dec 26, 2007 11:45 am
Posts: 191
djw1 wrote:
Simple and bogus. For a given temperature, the amount of moderator used up
is proportional to power. The graphite waste will be the same.

The concept of a plant life is also bogus or at least artificial, As Weinberg pointed out,
plant life should be forever. A proper design simply changes out
components as they wear out. See hydro or the B-52.


I understand the analogy and generally agree with it. But it does seem worth mentioning that a B-52 is not maintained with remotely operated machines in a hazardous environment. Almost every part may be replaced multiple times during the airplane's service - but not the basic airframe. When it reaches its end of life the plane is dismantled. The equivalent part of a reactor is likely to be the primary reactor vessel.

Some types of processing can reach an equilibrium composition that can, theoretically, last forever with ongoing processing and addition of fuel. Others have a limited lifetime before some part of the salt needs to be discarded or processed offline. An extreme example of the latter is the DMSR, of course. For reactors that have a limited salt batch lifetime it makes sense if it is matched to the moderator lifetime.


Top
 Profile  
 
PostPosted: Sep 16, 2012 10:27 am 
Offline

Joined: Jul 28, 2008 10:44 pm
Posts: 3060
Owen T wrote:
I understand the analogy and generally agree with it. But it does seem worth mentioning that a B-52 is not maintained with remotely operated machines in a hazardous environment. Almost every part may be replaced multiple times during the airplane's service - but not the basic airframe. When it reaches its end of life the plane is dismantled. The equivalent part of a reactor is likely to be the primary reactor vessel.

Some types of processing can reach an equilibrium composition that can, theoretically, last forever with ongoing processing and addition of fuel. Others have a limited lifetime before some part of the salt needs to be discarded or processed offline. An extreme example of the latter is the DMSR, of course. For reactors that have a limited salt batch lifetime it makes sense if it is matched to the moderator lifetime.

In a single fluid system the primary reactor vessel can be protected down to an arbitrarily low level of neutron flux by incorporating more replaceable boron shield material between the core and the vessel. I see no reason why technically we couldn't design for 200 years and I suspect it would work out economically too. But I suspect the real lifetime limit is going to be changing regulations and how long a given plant will be grandfathered in. 40 years seems to be the norm expected for the gen II plants and 80 years for gen III+. Seems like 80 years is long enough that we should expect sufficiently dramatic improvements that an 80 year old power plant should be retired.


Top
 Profile  
 
PostPosted: Sep 16, 2012 2:17 pm 
Offline

Joined: Apr 28, 2011 10:44 am
Posts: 250
IMHO the reactor vessel is just another component
that in a proper design should be replacable.

We've gotten way off topic and its my fault.
And we still dont know if the moderator can
be re-graphitized without grinding it down.

Mea culpa.


Top
 Profile  
 
PostPosted: Sep 16, 2012 10:08 pm 
Offline

Joined: Nov 01, 2011 2:15 am
Posts: 73
No good. At around 1200 degrees C fluoride salt will start to evaporate. This is way hotter than how we want to operate the reactor.


Top
 Profile  
 
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 59 posts ]  Go to page 1, 2, 3, 4  Next

All times are UTC - 6 hours [ DST ]


Who is online

Users browsing this forum: No registered users and 1 guest


You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot post attachments in this forum

Search for:
Jump to:  
cron
Powered by phpBB® Forum Software © phpBB Group