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PostPosted: Feb 11, 2010 9:27 am 
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Graphite pebbles have a lot of potential advantages. In one two fluid design, the pebbles are filled with blanket salt. This means every pebble is a barrier, and so barrier maintenance is potentially easier. However, circulating pebbles turns out to be a bit tricky, and with a lot of pebbles all containing liquid blanket salt, broken pebbles seem like a big risk in a true two fluid design. Maybe in a 1 1/2 fluid design it would be less of a problem, but circulating hollow pebbles still sounds like a challenge.

However, if solid graphite pebbles were to be used in a two fluid design like David's tube in shell, things would be easier. The pebbles wouldn't circulate, but act as fairly static moderator. The simple graphite pebbles would last longer and be easy to replace. Because pebbles have a high void fraction, the traditional MSR graphite density could not be achieved (probably at least half the graphite density).

What do the people here think about it?

One thing that I was wondering about, is where to put the pebbles. In the blanket, in the core, or in both the blanket and the core? It seems convenient to only place the pebbles in the annular thorium fluoride blanket, and not inside the cylindrical core, to achieve high salt density in the core and easier pebble maintenance/replacement. I don't know enough about neutronics, so any comments on that are much appreciated...


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PostPosted: Feb 11, 2010 11:27 am 
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IF the pebbles are only in the blanket then they do not help protect the wall between the core and the blanket.
IF the pebbles are in the core and contain the fuel then we are approaching Dr. Peterson's design - which can protect the wall nicely and retains some of the advantages of a liquid but you do lose the ability to extract the offgases which absorb so many neutrons.


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PostPosted: Feb 11, 2010 11:41 am 
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Lars wrote:
IF the pebbles are only in the blanket then they do not help protect the wall between the core and the blanket.
IF the pebbles are in the core and contain the fuel then we are approaching Dr. Peterson's design - which can protect the wall nicely and retains some of the advantages of a liquid but you do lose the ability to extract the offgases which absorb so many neutrons.


I'm not so sure about fast neutron damage always being worse than thermal neutron damage, there are conflicting reports about this and it strongly depends on the material used. But it could be a drawback of a blanket only pebbles design.

The pebbles don't contain fuel. They are moderator only. Solid amorphous graphite. This is a LFTR concept. Which is why I'd like to avoid pebbles in the highly radioactive fuel loop, unless we can be sure the pebbles last the life of the plant. Then we may want to put pebbles in both the blanket and core?

Pebbles are an interesting solution to the graphite swelling problem, as they behave like a liquid, so just like for the salt, one could design expansion vessels/tubes where the pebbles migrate towards to accomodate swelling while keeping void (ie fuel) fraction almost fixed.


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PostPosted: Feb 11, 2010 12:20 pm 
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OK. So they do not serve to protect the core/blanket wall.
As far as thermal versus fast damage ORNL was most worried about He generation in their distinctly thermal design.
This was dominated by the two step process 58Ni(n,gamma) then 59Ni(n,alpha). A faster spectrum makes this become much less significant.
There is another process 58Ni(n,alpha) that kicks in around 500keV and this was the main source of He for the French fast design.
Which makes me wonder about a spectrum that is in between.

But back to your idea. Graphite in the blanket would serve to slow the neutrons down. The slower spectrum will make all cross-sections larger so less blanket salt is required to absorb the neutrons. However, the cross-section of the fissile will grow dramatically faster than anything else - which is not good in the blanket. It means that we have to keep the u233/th232 ratio much lower to keep fission in the blanket rare. So one result is that we have to process the blanket faster to keep the u233 concentration down. I'm not sure how big an issue this is - the original 2-fluid ORNL designs were thermal and so they faced this issue and did not identify it as the reason to stop work on the 2-fluid design. But they also generally assumed they could process things at a pretty high rate.

Another effect of graphite in the blanket would be that any neutrons that hit the exterior wall would be slow so they are much easier to absorb and stop and are easier on that wall.

Another effect is that it becomes more cost effective to absorb a higher percentage of the neutrons in the blanket since the thorium in the salt is more effective at absorbing slower neutrons.

One BIG concern is that if there is a big break in the plumbing for the blanket salt you will drain the neutron absorber from the blanket. With the graphite present it will slow down and reflect many neutrons back toward the core. In normal operations the blanket salt will absorb most of them. With the blanket salt drained they will go back to the core. In other words, if you get a dramatic break in the blanket plumbing and drain the blanket salt the reactivity of the core will go up. This can not be allowed. The design would need to somehow guarantee that no matter what the reactivity of the core does not go up in any accident scenario.


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PostPosted: Feb 11, 2010 3:19 pm 
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Thanks Lars, that's good insight. The loss of blanket salt accident sounds quite serious. In some cases, like a catastrophic breach of the outer wall, the pebbles are likely to be lost quickly as well. Though in the case of a major plumbing failure, like in the bottom cold blanket salt inlet returning from the blanket HX, that could be a problem.

I have some ideas. Like, perhaps we can design the blanket processing/HX inlet and outlet to be high enough to make any failure lose very little salt level in the first place. Like a recent CANDU calandria passive cooling loop design.

And we could design the outer wall to be highly absorbtive of neutrons, using the right material and geometry, so that much of the neutron flux would be absorbed at the outer wall during a major loss of blanket fluid? According to David LeBlanc, the tube and shell design has an outer wall that sees very little neutron flux at all during normal operation, so perhaps this is acceptable.

It also seems that very fast loss of blanket salt is a major event, so we'd have the burnable neutron poison injection system. That's not passive safety of course...

Hmm. Don't other graphite moderated designs have a similar problem, when the graphite is in the core?


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PostPosted: Feb 11, 2010 3:44 pm 
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Yes, as I try to include in all my presentations on tube within tube design, you don`t want graphite or other good neutron reflecting material in the blanket zone or you end up with reactivity problems if the blanket salt drains or even just gets hotter and less dense.

For the core, using graphite is always a serious option and pebbles certainly have some big advantages but they don`t really help with the core to blanket barrier issue. Even a core with graphite moderator you still need some sort of physical barrier to the separate the fuel and blanket salts (any Two Fluid or 1 and 1/2 Fluid needs barrier material). A graphite core of logs might make things a little easier because we could have a simple metal cladding wrapped around it that would need no structural strength of its own). My google tech talk also shows a method to use individual graphite logs bunched together as the barrier but too complicated to describe here.

So in general, pebbles versus logs is always going to be an interesting trade off of pros and cons. In terms of radiation damage, I still don`t know if anyone has a good idea of how long a pebble would be last. The expansion beyond original size is no longer of structural concern for the core (like it would be for logs) but it the pebble starts to crack due to expansion then we do have a big problem. ORNL seemed to be on the fence regarding this in their early studies with pebbles (which seemed always to be a Plan B that never got too deep a look).

David L.


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PostPosted: Feb 11, 2010 3:53 pm 
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If the graphite is exposed to the fuel salt then expansion beyond the original size is a concern. Namely, some of the Xe would tend to get trapped inside the graphite rather than get sparged out and would steal neutrons. Not an issue if the graphite has a metal cladding but then we get back to concerns over damage to the wall - though without the strength requirement so we can be more aggressive.


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PostPosted: Feb 11, 2010 4:01 pm 
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David, there are a bunch of ways we can deal with the reactivity issue of a moderating blanket with small pebbles in the event of a blanket fluid loss. I've mentioned some already.

Here's another idea: have a pressure closed (like a springed) large diameter valve or other mechanism at the bottom of the blanket, which opens wide when the pressure of the blanket fluid is reduced below safe levels, dumping the remaining blanket fluid and pebbles (they should be small, like marbles) into a dump tank below.

Here's an image that shows cooling inlet and oulets at the top:

Image

Using such a system, we don't need any plumbing below normal blanket fluid level.


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PostPosted: Feb 11, 2010 4:08 pm 
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Lars wrote:
If the graphite is exposed to the fuel salt then expansion beyond the original size is a concern. Namely, some of the Xe would tend to get trapped inside the graphite rather than get sparged out and would steal neutrons. Not an issue if the graphite has a metal cladding but then we get back to concerns over damage to the wall - though without the strength requirement so we can be more aggressive.


All the more reason to not put the graphite in the core, and make the graphite easy to replace.

You'll want to avoid cladding as much as possible. This can be done with graphite, we should capitalize on that strength. Hastalloy N cladding just eats neutrons (we're trying to avoid that with graphite in the first place!) and adds chromium in the melt, unless you want to use a non-proven clad, which adds significant development risk.


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PostPosted: Feb 11, 2010 4:10 pm 
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By the way, is a graphite pebble bed really a good reflector? It seems that it would not reflect very well at all, such a leaky configuration?


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PostPosted: Feb 11, 2010 4:26 pm 
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Without a blanket salt I would expect it looks like less dense graphite - and graphite I think is one of the best reflectors available. So the fact that it is pebbles, or powder or solid, or logs I don't think matters. Likewise I don't think it matters whether it is graphite or diamond. I think the controlling factor is just the number of atoms between the core and the outside wall.


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PostPosted: Feb 11, 2010 4:31 pm 
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Lars wrote:
Without a blanket salt I would expect it looks like less dense graphite - and graphite I think is one of the best reflectors available. So the fact that it is pebbles, or powder or solid, or logs I don't think matters. Likewise I don't think it matters whether it is graphite or diamond. I think the controlling factor is just the number of atoms between the core and the outside wall.


So, in the loss of blanket fluid event, what portion of the neutrons would be reflected back into the core? What energy level would they be at?


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PostPosted: Feb 11, 2010 5:01 pm 
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David wrote:
In terms of radiation damage, I still don`t know if anyone has a good idea of how long a pebble would be last. The expansion beyond original size is no longer of structural concern for the core (like it would be for logs) but it the pebble starts to crack due to expansion then we do have a big problem. ORNL seemed to be on the fence regarding this in their early studies with pebbles (which seemed always to be a Plan B that never got too deep a look).

David L.


Compressive stength itself would be governed strongly by the size of the pebbles. Graphite has very poor strenghts, so they have to be big enough to be strong, but small enough to be manageable. Preferably smaller than a tennis ball, if possible as small as marbles.

The experience with PBMR and AHTR would be quite useful to this design. Their challenges are mostly related to dynamic pebble management, whereas our pebbles are static. They just sit in their bed, are given room to expand, and that's it. Cracking of the pebbles would not be a big issue if they were in the blanket, and they are easily replaced. Pump out the blanket fluid and use some kind of vacuum cleaner to suck out all the pebbles, then put new pebbles in followed by the blanket salt going back in. The temporary blanket salt holding tank also serves as an emergency dump tank for the blanket/pebble mixture, and can be used for initial heating of the blanket salt for startup.

Core pebbles would just soak up fission products, creating more radwaste, and maintenance headaches. Logs are not easily replaced without opening the core, which is not nice at all.

Don't get me wrong David, I really like the simplicity of your tube in shell salt only design.


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PostPosted: Feb 11, 2010 5:29 pm 
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Cyril,
What is the advantage of having pebbles in the blanket versus just having a thicker blanket with more blanket salt?


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PostPosted: Feb 11, 2010 5:43 pm 
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Lars wrote:
Cyril,
What is the advantage of having pebbles in the blanket versus just having a thicker blanket with more blanket salt?


I'm not sure, which is why I made a thread about it. Less absorbtions in carrier salt comes to mind. FLiBe isn't anywhere near graphite in moderating ratio. LiF-ThF4 is obvisously even worse on its own. I was hoping the enrichment level could be lower. Simplicity of blanket salt only sound very enticing also. And easy to make inherently safe...


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