Energy From Thorium Discussion Forum

It is currently Jul 19, 2018 2:37 am

All times are UTC - 6 hours [ DST ]




Post new topic Reply to topic  [ 59 posts ]  Go to page Previous  1, 2, 3, 4
Author Message
PostPosted: Sep 25, 2012 1:32 pm 
Offline

Joined: Jul 28, 2008 10:44 pm
Posts: 3065
jagdish wrote:
Crystalline carbides like Be2C have to be only physically contained. There is no absorption of gases as in graphite. Minor porosity will not have any adverse effect. They are closer to diamond than graphite in this respect.
It may be possible to sinter it with Be metal and clad it in SiC.

If Be2C gets exposed to fluorides it may exchange the carbon for fluorides - this needs to be understood and then understand the rate at which this happens and if that causes a difference in physical size which could be problematic. Just a bunch of things to check out not knowledge that any of these things are show stoppers.


Top
 Profile  
 
PostPosted: Sep 25, 2012 2:21 pm 
Offline

Joined: Jul 14, 2008 3:12 pm
Posts: 5048
What is the advantage of beryllium carbide over beryllium metal for this application? Seems to me that adding carbon just increases the swelling in a neutron flux, and maybe reduces beryllium atom density as well?


Top
 Profile  
 
PostPosted: Dec 30, 2012 3:16 pm 
Offline

Joined: Jan 21, 2008 9:12 pm
Posts: 308
Location: idaho falls
djw1 wrote:
Guys,

Our 1 GWe design goes through 50 tons of moderator a year at steady state,
New graphite will cost at least $20/kg and god knows how much to dispose of.
Where I come from, a milion dollars per year is real money,

IF this works, it means we only have to put the old graphite thru the last step
in the moderator production process. IF it works, it should save money.

Jack


We've discussed in situ graphite annealing several times in the past. In my opinion it needs much more consideration than it's been getting because it would simultaneously address several of the issues being raised about LFTRs.

1) it would greatly reduce the mass/volume of C waste generated (probably at least 50 fold)
2) in situ graphite annealing would render David's simple/cheap/"clean" tube-in-shell 2 salt reactor practical because it would solve its otherwise almost insuperable "barrier material" problem

Implementation would involve:
1) drain both salt streams
2) fill/flush both the tube & its surrounding tank/shell with inert gas
3) run electricity lengthwise through the tube to heat it to roughly 2000C for a few minutes
4) allow the tube to cool to roughly 600C & run the salt streams back into both it and the tank/shell

this should not take more than about one work shift to accomplish

I'm ATTACHING a little spreadsheet which shows both why & how it should work.


Attachments:
In situ graphite tube annealing calcs.xls [16.5 KiB]
Downloaded 154 times

_________________
Darryl Siemer
Top
 Profile  
 
PostPosted: Dec 31, 2012 4:36 am 
Offline

Joined: Jul 14, 2008 3:12 pm
Posts: 5048
Using Darryl's referenced calculator (thanks - very useful gadget!) and graphite emissivity of 0.9, 22 m2 surface area, 2000 C radiator and 600 C blackbody, the calculator gives about 30 MWt lost.

But, the blackbody is not a blackbody, it is a polished Hastelloy N wall having a high IR reflectivity. This should cut thermal losses to well under 10 MWt.

This isn't so bad. However, we must determine what such a severe heatup does to the mechanical properties of the graphite. Even if annealing heals all damage, will the mechanical moduli be the same as the virgin material? There could well be thermal embrittlement from the temp cycling.


Top
 Profile  
 
PostPosted: Dec 31, 2012 9:29 am 
Offline

Joined: Jun 05, 2011 6:59 pm
Posts: 1332
Location: NoOPWA
Dumb question... why do we care if the graphite swells? Can't we compensate by changing the molality of the fuel salt?

_________________
DRJ : Engineer - NAVSEA : (Retired)


Top
 Profile  
 
PostPosted: Dec 31, 2012 9:00 pm 
Offline

Joined: Apr 19, 2008 1:06 am
Posts: 2225
Swelling of graphite is only a nuisance as far as moderation is concerned. There may be a small reduction due to reduced density. Other, more serious, problems could be:-
1. Reduced structural strength.
2. Catching fire as at Sellafield in the UK.
Hardness like diamond and higher molecular density were behind my suggestion for Be2C.
You could also:-
Provide Ar as cover gas as in sodium coolant.
Provide a cladding to moderator blocks.
Best of all, go for a fast spectrum and avoid the moderator requirement altogether.


Top
 Profile  
 
PostPosted: Jan 01, 2013 6:08 am 
Offline

Joined: Jul 14, 2008 3:12 pm
Posts: 5048
jagdish wrote:
Swelling of graphite is only a nuisance as far as moderation is concerned. There may be a small reduction due to reduced density. Other, more serious, problems could be:-
1. Reduced structural strength.
2. Catching fire as at Sellafield in the UK.
Hardness like diamond and higher molecular density were behind my suggestion for Be2C.
You could also:-
Provide Ar as cover gas as in sodium coolant.
Provide a cladding to moderator blocks.
Best of all, go for a fast spectrum and avoid the moderator requirement altogether.


Be2C is not very strong either. And much more flammable than graphite!!

Graphite won't catch fire as at Sellafield. The irradiation temperature is high enough to anneal out virtually all Wigner energy buildup.

Be2C is almost certainly not compatible with molten salts, and will swell under neutron flux (from the carbon that's in there). Might as well use beryllium metal itself.


Top
 Profile  
 
PostPosted: Jan 01, 2013 7:34 am 
Offline

Joined: Jun 05, 2011 6:59 pm
Posts: 1332
Location: NoOPWA
Does it every STOP swelling? If so, does it ever stop losing strength?

_________________
DRJ : Engineer - NAVSEA : (Retired)


Top
 Profile  
 
PostPosted: Jan 01, 2013 1:43 pm 
Offline

Joined: Jul 14, 2008 3:12 pm
Posts: 5048
KitemanSA wrote:
Does it every STOP swelling? If so, does it ever stop losing strength?


Probably not, based on both theory and inpile experiment. It just keeps swelling uniformly till the strength loss or xenon absorption become unacceptable. Or just the dimensions itself - the core can only fit so much graphite! From a perspective of restraint and anti-vibrations, it always seemed very difficult to me to accomodate both shrinkage (initially for most graphites) and then swelling.


Top
 Profile  
 
PostPosted: Jan 01, 2013 3:53 pm 
Offline

Joined: Apr 28, 2011 10:44 am
Posts: 249
I think ORNL's biggest concern was
the possibility of cracking when it begins to swell
which would result in salt penetration and possible hot spots.

Also graphite themal conductivity can drop drastically
once you get into the swelling stage.

Jack


Top
 Profile  
 
PostPosted: Feb 17, 2013 12:22 pm 
Offline

Joined: Apr 28, 2011 10:44 am
Posts: 249
Cyril R wrote:
djw1 wrote:


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?


Probably old news, but
stumbled across this in ORNL/TM-12925. The brackets are my comments.
From ORNL/TM-12925, page 22
2
3 Neutron bombardment produces atomic displacements
4 which result in vacancy-interstitial pairs.
5 Vacancies migrate to to dislocations and collapse;
6 interstitials combine into ordered planes
7 which assume a graphite structure with stacking faults,
8 The result is that individual crystallites contract
9 in the a-axis direction and expand in the c-axis direction.
10 This is reflected in the initial macroscopic bulk contraction
11 of the polycrystalline graphite,
12 followed by bulk expansion as damage proceeds.
13 [Non sequitur. More likely the interstitials migrate
14 to pores early in the process.]
15 Eventually, the expansion induces macroscopic cracking
16 of the graphite until mechanical disintegration occurs.
17 During this progression, all of the thermo-mechnical properties
18 are affected and radiation-activated creep occurs,
19 resembling in some detail its high temperature thermal analogue.
~


Top
 Profile  
 
PostPosted: Feb 17, 2013 1:57 pm 
Offline

Joined: Jan 21, 2008 9:12 pm
Posts: 308
Location: idaho falls
Cyril R wrote:
Using Darryl's referenced calculator (thanks - very useful gadget!) and graphite emissivity of 0.9, 22 m2 surface area, 2000 C radiator and 600 C blackbody, the calculator gives about 30 MWt lost.

But, the blackbody is not a blackbody, it is a polished Hastelloy N wall having a high IR reflectivity. This should cut thermal losses to well under 10 MWt.

This isn't so bad. However, we must determine what such a severe heatup does to the mechanical properties of the graphite. Even if annealing heals all damage, will the mechanical moduli be the same as the virgin material? There could well be thermal embrittlement from the temp cycling.


Your questions can only be addressed by doing experiments & seeing what happens. Since we can't do them ourselves, about all we can do is lobby DOE's lead NE lab (INL) (or maybe it's academic collaborators) to give high priority to this sort of work. It isn't rocket science & INL's been irradiating graphite-based (HTGR) fuel in ATR for many years. All they'd have to do is repeatedly measure those characteristics of a graphite (or whatever) tube in non destructive fashion, re anneal & re irradiate.

_________________
Darryl Siemer


Top
 Profile  
 
PostPosted: Jan 04, 2015 10:25 am 
Offline
User avatar

Joined: Jul 14, 2011 9:22 pm
Posts: 31
Location: Forest Park, GA
There's a patent on just such an annealing process for reducing the dimensional swelling of heavily irradiated graphite.

It was filed in 1962, and granted in 1965 by 3 guys, at least one of whom appears to be out of Brookhaven.

https://drive.google.com/viewerng/viewe ... 183166.pdf

I'm still working my way through it, but I believe he's suggesting a small reactor to create a low neutron flux against the graphite, in a regime being alternated with relatively low temperature annealing (150 C or 250 C).

I'd be much obliged if someone could once-over it. Maybe it's just what's needed; maybe it's a good start for further inquiries.

Also, I believe these methods were tested in the Brookhaven Graphite Research Reactor (BGRR) as he indicates actual trials where graphite samples were reduced in dimension.


Top
 Profile  
 
PostPosted: Jan 04, 2015 5:06 pm 
Offline

Joined: Jan 21, 2008 9:12 pm
Posts: 308
Location: idaho falls
I read the patent. The conditions generating the dimensional changes that it would/could address are quite different than what the core tank wall of a MSBR/MSFR would experience - the only common feature would be that both changes are caused by fast moving neutrons. Consequently, it is basically irrelevant 'cept for the fact that it addresses a genuinely important issue (in situ repair of a reactor's guts rather than disassembly/discard)

I really like the concept of being able to make the reactor's barrier tank out of C fiber reinforced, pyrolytic C sealed, graphite because it should be reasonably cheap/simple to make in any shape desired. However, it would only be practical if it could also be in situ cheaply/simply/quickly annealed which would take much higher temperatures (e.g. 2000 vs 150 C) than those addressed by the patent. This in turn would require the sorts of experimental facilities that DOE possesses but has to date refused to use.

_________________
Darryl Siemer


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

All times are UTC - 6 hours [ DST ]


Who is online

Users browsing this forum: No registered users and 2 guests


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