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PostPosted: Dec 01, 2012 8:42 am 
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Cyril R wrote:
There will also have to be some sort of recirculation line at the top to go down again, to let the gas take another pass and improve reaction effectiveness. Even graphite is going to suffer from fluorination by F2. But if you have a gas phase you can use Monel or similar metal that forms protective NiF2, CuF2 layers. This should work. I think there will be a problem if you have both a very oxidising F2, PuF6 environment, combined with liquid salts because the liquid salts will dissolve the NiF2, CuF2 layer. Frozen salt can work for the main fluorinator tube but not so well for smaller process piping and such. Frozen salt also likely isn't gas tight, so that you still have to make the entire thing out of Monel. But that's ok, it's quite small.

The recirc gas will be F2, it doesn't want to be bubbled through the liquid. Can we inject the F2 above the liquid and have a "sealing" liquid that is lighter than the salt that separates the liquid from the protective layers?

OR if we do vacuum distill first then could we tolerate losing the Ni&Cu that is right at the surface level? It does two things: a) adds Ni&Cu to the liquid - but if we did vacuum distill first then the fluorination the remainder isn't of high value and so we are just adding a bit of Ni&Cu to the waste stream b) removes a bit of Ni&Cu from the vessel - as long as the removal rate leaves us with an acceptable lifetime this is ok.

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One promising route to take, that is both protective and gas tight, is to use noble metal coatings. Some noble metals don't form fluorides (these have a much lower free energy of formation than even carbon tetrafluoride). Noble metal coatings on carbon steel are likely cheaper than Monel too, and don't take up as much space as a frozen wall of salt so can be used in the process lines (such as the recirc line).


This does sound promising.


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PostPosted: Dec 01, 2012 9:03 am 
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Can we get a nobel metal coating on graphite to stay there up to ~1500C?

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PostPosted: Dec 01, 2012 9:13 am 
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KitemanSA wrote:
Can we get a nobel metal coating on graphite to stay there up to ~1500C?


The primary deposition process for the platinum group metals is typically electrolytic deposition, and graphite's high conductivity should allow deposition of noble metal quite well. Diffusion of the structural metal also isn't an issue with graphite. However I think noble metals will bond better to a metal, for reasons similar to what creates a possible substrate diffusion problem (ah well, we can't always have cake and eat it).

Seems like something like this could be demonstrated with a fairly low budget, though.

I think 1500 C would be asking a bit much. Why such a high temperature in the still?


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PostPosted: Dec 01, 2012 9:17 am 
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Cyril R wrote:
I think 1500 C would be asking a bit much. Why such a high temperature in the still?
The better to isolate the Sr in the bottoms. The more the merrier! ;)

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PostPosted: Dec 01, 2012 9:25 am 
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Lars wrote:
The recirc gas will be F2, it doesn't want to be bubbled through the liquid. Can we inject the F2 above the liquid and have a "sealing" liquid that is lighter than the salt that separates the liquid from the protective layers?


Isn't PuF6 going to come along the recirc line? It's very volatile.

Let me get this straight. The recirc line takes unreacted F2 and some PuF6 and injects this back to the lower portion of the column. This return gas would be above the liquid pool at the bottom, right? If the F2 and PuF6 must go through the liquid bottoms, this will be a tough environment for any material. If they don't go through the liquid bottoms, it's essentially a one pass fluorinator, PuF3 that falls into the fluorinator bottoms stays there and won't have another chance at fluorinating. What's the efficiency per pass?

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OR if we do vacuum distill first then could we tolerate losing the Ni&Cu that is right at the surface level? It does two things: a) adds Ni&Cu to the liquid - but if we did vacuum distill first then the fluorination the remainder isn't of high value and so we are just adding a bit of Ni&Cu to the waste stream b) removes a bit of Ni&Cu from the vessel - as long as the removal rate leaves us with an acceptable lifetime this is ok.


The liquid fluoride will flux the protective CuF2 and NiF2 off very rapidly, if there's F2 and PuF6 mixed in the liquid, producing a quite rapid dissolution of the wall. Freezing the salt around the wall seems feasible for this bottom part of the fluorinator, though. The fluorine sparger line coming in from the bottom could be a problem, but it needn't come in from the bottom, it could come in from the top or side.


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PostPosted: Dec 01, 2012 9:25 am 
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For the still can't we use a graphite vessel?


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PostPosted: Dec 01, 2012 9:26 am 
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KitemanSA wrote:
Cyril R wrote:
I think 1500 C would be asking a bit much. Why such a high temperature in the still?
The better to isolate the Sr in the bottoms. The more the merrier! ;)


It might seperate Sr better but at these temperatures won't some of the rare earths (the worst neutron poisons in the still) come along in the product line?


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PostPosted: Dec 01, 2012 9:29 am 
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Lars wrote:
For the still can't we use a graphite vessel?


Luke from the forum said we could. He's suggested a vacuum chamber with the graphite or carbon composite still completely enclosed in the vacuum vessel. This way no good vacuum seal on the actual still itself is needed. I think you'd want boron carbide balls or blocks in the still bottoms in that case, to make sure no criticality is every remotely possible.

If we use graphite for the still we don't need a noble metal coating, that's true.


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PostPosted: Dec 01, 2012 9:35 am 
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Cyril R wrote:
KitemanSA wrote:
Cyril R wrote:
I think 1500 C would be asking a bit much. Why such a high temperature in the still?
The better to isolate the Sr in the bottoms. The more the merrier! ;)


It might seperate Sr better but at these temperatures won't some of the rare earths (the worst neutron poisons in the still) come along in the product line?
Not with fractional distillation.

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PostPosted: Dec 01, 2012 9:38 am 
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Cyril R wrote:
I think you'd want boron carbide balls or blocks in the still bottoms in that case, to make sure no criticality is every remotely possible.

I wonder if the still we need is even large enough to contain a critical mass?


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PostPosted: Dec 01, 2012 9:47 am 
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KitemanSA wrote:
Cyril R wrote:
I think 1500 C would be asking a bit much. Why such a high temperature in the still?
The better to isolate the Sr in the bottoms. The more the merrier! ;)
Oh, and I forgot the best reason. All the fluorides listed in the wiki article on fluoride volatility have melting points below 1500C. No solidification except with the UF6 grouping.

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PostPosted: Dec 01, 2012 9:59 am 
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How would SiC do as a liner?

They also seem to suggest that graphENE is impervious to near everything. Might that work? Can you put graphENE on graphITE? :roll:

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PostPosted: Dec 01, 2012 10:22 am 
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Lars wrote:
Cyril R wrote:
I think you'd want boron carbide balls or blocks in the still bottoms in that case, to make sure no criticality is every remotely possible.

I wonder if the still we need is even large enough to contain a critical mass?


It might not be a problem. The ORNL still was about 150 liters if memory serves. Not much at all, and in a pure U233 cycle there would be many kg of neutron poison for every kg of plutonium. This will not be the case if the reactor is started on Pu or mixed TRUs, something to consider in the still design I think for a TRU startup. A 1.5 or 1 fluid design would have lots of thorium which helps here.


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PostPosted: Dec 01, 2012 10:48 am 
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KitemanSA wrote:
Not with fractional distillation.

If the geometry of the still gets too complicated it may be difficult to produce out of some materials such as graphite. If we need things like reflux lines, multiple condensers, etc. that are typically used in fractional distillation, this really complicates a number of things. Unlike petroleum distillation, we have a heat source that can't be turned off which may make reflux difficult. I think 1500 degrees C piping is going to be tough as well.


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PostPosted: Dec 01, 2012 11:38 am 
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Hmm, ok, so apparently adding a high surface area, low pressure drop packing material in the still increases the seperation efficiency for a given number of distillation steps:

http://www.indiamart.com/mistermesh-wir ... ducts.html

They have it available in nickel, copper, Monel and Hastelloy!

This may allow 2 distillation stages with good seperation without reflux.

Has any group done R&D on multistage distillation for MSRs? ORNL seemed very focused on simple, single stage vacuum stills without rectification/reflux lines. But then they didn't have any carbon composites at their disposal...


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