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PostPosted: Nov 30, 2012 12:48 pm 
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KitemanSA wrote:
If we use a reasonable fractional distillation unit, the condensates should never solidify, except maybe some precip in the still bottoms.


This requires very high temperatures in the hotter fractions (thorium fluoride).

What's the point of fractional distillation anyway? We don't want seperate products. We just want the fission products in one bucket, and the fuel salt in another. A simple single stage distillation unit is the appropriate technology for such an application.


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PostPosted: Nov 30, 2012 1:37 pm 
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Some FPs distill out BEFORE FLiBe and Thorium. So I would like to put those into a separate pot. So at least TWO stages. Besides, I'd kind of like to do as MUCH separation as possible to make future removal of stable FPs simpler. And if I can get Sr almost alone in the bottoms, perhaps a 90Sr STG becomes more practical.

By the way, where does Pa fit in the temperature range?

Hmm. on further search, looks like maybe it becomes PaF5 during the FV step and comes out at about 500C. Not sure I understood that abstract as well as I would like.

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Last edited by KitemanSA on Nov 30, 2012 2:11 pm, edited 1 time in total.

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PostPosted: Nov 30, 2012 2:02 pm 
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We do have Zr and Cs that come out before the Li, Be, U, and Th. Eventually we will need to pull out some of these to keep them from building up too much. We can tolerate an awful lot of them though so no hurry. Here are a few approaches that we might compare the complexity&cost of.
a) Pull the most volatile things into a separate bucket to be better sorted later - something like whatever Zr&Cs we can get while pulling out less than 1% of the Li and less than 0.1% of the U. We can decide far into the future if it is worthwhile mining the results to get the residual Li and U out. In any event, the leakage here is small enough that there is no negative economic impact to waiting to recover the residual amounts.
b) Use some separate process done perhaps once per decade
c) I recall another suggestion to use oxygen to separate Zr out. ORNL used 5% Zr in MSRE as a safety oxygen getter. They found that they did not suffer oxygen leakage into the reactor so the Zr wasn't needed and was deleted for MSBR. This suggests that by adding some oxygen we could get the Zr to precipitate out.


For Pa extraction. I think the easy thing is to distill once as soon as reasonable (around 1 day to reduce the decay heat so it isn't a problem). Then store the stuff for a year. Then distill a second time. By that time the Pa will have decayed to U. (Except Pa231 with half-life of 32k years. There will only be trace amounts of this. We need to see how much to know if it is something we need to bother with.)


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PostPosted: Nov 30, 2012 2:17 pm 
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KitemanSA wrote:
Some FPs distill out BEFORE FLiBe and Thorium. So I would like to put those into a separate pot. So at least TWO stages


This complicates the still design, for no real gain in neutronic efficiency. Also the upper stage would likely accumulate BeF2 which you want to keep rather than remove.

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Besides, I'd kind of like to do as MUCH separation as possible to make future removal of stable FPs simpler. And if I can get Sr almost alone in the bottoms, perhaps a 90Sr STG becomes more practical.


This complicates the still, especially the heat removal part, at a time that you really don't want it - maximum decay heat & radiation. If the purpose is to get valuables out later, then that is best done with a seperate still at a later time (perhaps offsite to get the required economies of scale for such a campaign).


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PostPosted: Nov 30, 2012 2:19 pm 
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Since my assumption is that there will be a fluoride volatility step first, the next step should be fractional distillation of EVERYTHING, SeF6 to LaF3. Ok, many of the triFs would probably be stuck together in the bottoms, but a lot can be separated fairly easily, no?

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PostPosted: Nov 30, 2012 2:26 pm 
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Lars wrote:
We do have Zr and Cs that come out before the Li, Be, U, and Th. Eventually we will need to pull out some of these to keep them from building up too much. We can tolerate an awful lot of them though so no hurry.


ORNL never planned on removing the CsF. It's no problem to have several decades of this in the fuel salt. At the end of the reactor life, centralized processing is probably a realistic option to remove and further seperate fission products.

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c) I recall another suggestion to use oxygen to separate Zr out. ORNL used 5% Zr in MSRE as a safety oxygen getter. They found that they did not suffer oxygen leakage into the reactor so the Zr wasn't needed and was deleted for MSBR. This suggests that by adding some oxygen we could get the Zr to precipitate out.


Adding oxygen seems like a shame after all the trouble to keep it out. Having an oxygen supply system then means we have to do all sorts of safety analysis if there's a leak. It seem much easier just to bypass some fuel salt over a sublimation trap to get ZrF4 out. This is a simple physical process based on the sublimation behaviour of ZrF4, and allows almost zero uranium loss.


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PostPosted: Nov 30, 2012 2:28 pm 
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KitemanSA wrote:
Since my assumption is that there will be a fluoride volatility step first, the next step should be fractional distillation of EVERYTHING, SeF6 to LaF3. Ok, many of the triFs would probably be stuck together in the bottoms, but a lot can be separated fairly easily, no?


If we have a one-fluid distillation step that can recover thorium, it will recover virtually all of the uranium. So you can skip the fluoride volatility part, with all the annoying sorber, desorber beds, associated radioactive solids handling, not to mention toxic and corrosive UF6, F2, explosive embrittling H2, HF and associated aqeous scrubbers, etc. etc.

Such a still is all that a single fluid reactor needs. Two fluid is forced to use fluoride volatility.


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PostPosted: Nov 30, 2012 2:30 pm 
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KitemanSA wrote:
Since my assumption is that there will be a fluoride volatility step first, the next step should be fractional distillation of EVERYTHING, SeF6 to LaF3. Ok, many of the triFs would probably be stuck together in the bottoms, but a lot can be separated fairly easily, no?


Doing fluoride volatility first is ORNL's baseline plan. I think it is also interesting to consider what would happen if we reversed the order or even skipped on-site fluoride volatility all together.

But to continue on the thought of fluoride volatility then distillation.
As best I can tell there are a whole raft of rare earths that travel together and would become solids if we finish distilling everything. Handling the solids seems like a pain. If we have thorium in the salt then I'm thinking that we keep enough thorium in the still bottoms that it serves as the solvent to keep things liquid until the day arrives that we want to glassify the remaining fission products. I don't know how to separate the Sr from the rest of the fission products. IF we could separate Cs, Sr, and the actinides then the rest cools pretty quickly and can be decently disposed of. I think this would be good PR if we can do it without much expense. It doesn't have to be perfect - removing 90% of the Cs and Sr would drop the heat load as much as waiting 100 years.


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PostPosted: Nov 30, 2012 2:42 pm 
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Cyril R wrote:
KitemanSA wrote:
Since my assumption is that there will be a fluoride volatility step first, the next step should be fractional distillation of EVERYTHING, SeF6 to LaF3. Ok, many of the triFs would probably be stuck together in the bottoms, but a lot can be separated fairly easily, no?


If we have a one-fluid distillation step that can recover thorium, it will recover virtually all of the uranium. So you can skip the fluoride volatility part, with all the annoying sorber, desorber beds, associated radioactive solids handling, not to mention toxic and corrosive UF6, F2, explosive embrittling H2, HF and associated aqeous scrubbers, etc. etc.

Such a still is all that a single fluid reactor needs. Two fluid is forced to use fluoride volatility.

Sounds right. Have you looked at the numbers to see that the still won't separate uranium from thorium any better than the blanket already has them? This argument would also apply to 1.5 fluid reactors.

For the blanket we can skip the sorber/desorber beds, since we are moving stuff from the blanket to the core there should be almost no fission products in the blanket in the first place. If there are, then it is fine to move them from the blanket to the core. This also eliminates the radioactive solids handling. You still have to deal with UF6, vessel corrosion, handling of F2, etc that while known are a bit of a pain.

Note that with fluoride volatility of the core salt we can recycle the plutonium onsite and reduce the actinide flow to around 1.5kg/GWe-yr of Am,Cm. Without it we have almost 20kg/GWe-yr of plutonium - mostly Pu238 in the waste flow. This is a pretty big heat load and complicates storage and transportation back to the central facility.


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PostPosted: Nov 30, 2012 6:55 pm 
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Quote:
For the blanket we can skip the sorber/desorber beds, since we are moving stuff from the blanket to the core there should be almost no fission products in the blanket in the first place. If there are, then it is fine to move them from the blanket to the core. This also eliminates the radioactive solids handling. You still have to deal with UF6, vessel corrosion, handling of F2, etc that while known are a bit of a pain.


Good point, the blanket avoids the beds.

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Note that with fluoride volatility of the core salt we can recycle the plutonium onsite and reduce the actinide flow to around 1.5kg/GWe-yr of Am,Cm. Without it we have almost 20kg/GWe-yr of plutonium - mostly Pu238 in the waste flow. This is a pretty big heat load and complicates storage and transportation back to the central facility.


Are you sure we can recycle plutonium without fluorinating, say, the fluorine spargers, fluorinator recirc salt lines, etc. ? Some parts can be protected by frozen layer of salt (the fluorinator wall) but others probably can't be protected like this.

20 kg of Pu238 makes about 0.01 MWt. Not so bad! And little gamma radiation coming from this as well (one of the reasons Pu238 is such a good RTG - needs very little shielding).

I had hoped to use the still bottoms productively, if that includes more hot heads, Pu238 or otherwise, then great. By draining them all in a tank at the bottom of the buffer salt pool we discussed. After 30 years at full power, it ends up being about 10 MWt at steady reactor operation (though drops off rapidly if we shut down the plant).


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PostPosted: Nov 30, 2012 7:04 pm 
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Cyril R wrote:
Are you sure we can recycle plutonium without fluorinating, say, the fluorine spargers, fluorinator recirc salt lines, etc. ? Some parts can be protected by frozen layer of salt (the fluorinator wall) but others probably can't be protected like this.


Am I certain? No. But the documentation on fluorinators taking UF4 to UF6 state that it is an exothermic process and the temperature in the "flame" gets to 1100C. The "falling drops" document from ORNL shows that the residual plutonium when the temperature is 650C and the particles are small is tiny - this is where I got the <100Grams/GWe-yr from.


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PostPosted: Nov 30, 2012 7:18 pm 
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Lars wrote:
Cyril R wrote:
Are you sure we can recycle plutonium without fluorinating, say, the fluorine spargers, fluorinator recirc salt lines, etc. ? Some parts can be protected by frozen layer of salt (the fluorinator wall) but others probably can't be protected like this.


Am I certain? No. But the documentation on fluorinators taking UF4 to UF6 state that it is an exothermic process and the temperature in the "flame" gets to 1100C. The "falling drops" document from ORNL shows that the residual plutonium when the temperature is 650C and the particles are small is tiny - this is where I got the <100Grams/GWe-yr from.


It's exothermic because fluorine will react with most anything. There must be excess fluorine to make PuF6. PuF6 is a bit dubious in it's free energy of formation, appears very unstable. It's probably one of the best fluorinating molecules available, which is bad news for any process lines, sparger mouths and such. Are we going to make these things out of platinum? Or is there only a liquid in the bottoms, that needs protection?


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PostPosted: Nov 30, 2012 8:16 pm 
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Would graphite work?
(Or a wilder thought - thorium - does ThF4 form a protective film like Al2O3 or is the rate of dissolving the walls slow enough that we can accept it and put the ThF4 into the reactor?)


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PostPosted: Nov 30, 2012 9:23 pm 
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Cyril R wrote:
Or is there only a liquid in the bottoms, that needs protection?

The ORNL experiments worked much better by making tiny drops of the salt and injecting them at the top while F2 gas flowed upward. The liquid at the bottom would not contain uranium or plutonium or excess F2 so I don't think the liquid section is the problem area.

Certainly the very top of the vessel where you have hot F2,UF6,PuF6 gas hitting the wall and turning the corner to go down into the condenser would be a challenging area to protect.


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PostPosted: Dec 01, 2012 4:00 am 
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Lars wrote:
Cyril R wrote:
Or is there only a liquid in the bottoms, that needs protection?

The ORNL experiments worked much better by making tiny drops of the salt and injecting them at the top while F2 gas flowed upward. The liquid at the bottom would not contain uranium or plutonium or excess F2 so I don't think the liquid section is the problem area.

Certainly the very top of the vessel where you have hot F2,UF6,PuF6 gas hitting the wall and turning the corner to go down into the condenser would be a challenging area to protect.


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.

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).


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