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PostPosted: Sep 29, 2012 9:35 am 
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Looking at some Ellingham diagrams of fluorides...

http://showard.sdsmt.edu/MET320/Handout ... _12_06.pdf

...this suggests a potential way to seperate the fission product fluorides from each other.

There is a fairly large dG between the different fluorides, and for some fluorides it also looks like temperature could be exploited to reduce the required element.

The idea is fairly simple: apply increasing current (with perhaps a LiF based electrolyte) or add magnesium in the amount required to reduce the fission product fluorides. In a batch-wise operation, increasing the redox more and more to reducing, the different elements should come out fairly neatly. They can then be drained from below or above the molten fluoride bath surface, depending on their specific gravity. Magnetic elements could also be seperated using an electromagnet suspended above the fluoride pool, and again temperature might be used to an advantage in partioning.


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PostPosted: Sep 29, 2012 12:28 pm 
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The PNNL work on NF3 is doing something similar.
www.pnnl.gov/publications/

and search on pnnl-20775

They are focusing on spent LWR fuel
but, talking to them, they think the same approach
will work even better for FP fluorides, and UF4.

Problem is our fuelsalt melting temperature is so high
that even at the freezing point, a lot of U will come out with the FP.


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PostPosted: Sep 29, 2012 12:45 pm 
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djw1 wrote:
The PNNL work on NF3 is doing something similar.
http://www.pnnl.gov/publications/

and search on pnnl-20775

They are focusing on spent LWR fuel
but, talking to them, they think the same approach
will work even better for FP fluorides, and UF4.

Problem is our fuelsalt melting temperature is so high
that even at the freezing point, a lot of U will come out with the FP.


There won't be much U in the FP fluoride mix, because the feed would be distillation bottoms. UF4 is more volatile than LiF under vacuum so is removed about 99.9% from that feed.

The idea is to partition the FPs for marketing them.


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PostPosted: Sep 29, 2012 4:12 pm 
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If you distill first, a lot of the low boiling FP
wont end up in the bottoms.


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PostPosted: Sep 29, 2012 5:04 pm 
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djw1 wrote:
If you distill first, a lot of the low boiling FP
wont end up in the bottoms.


Which ones? CsF yes, but Cs can't be marketed much. Send back to the reactor. Ditto for RbF. ZrF4 also boils out, but is easily seperated via sublimation trapping. Ok to have some in the reactor as well.

Seperating strontium from the lanthanides would be useful, it can be used in RTGs as fluoride. But perhaps barium will contaminate it.


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PostPosted: Sep 29, 2012 10:04 pm 
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Cyril R wrote:
There won't be much U in the FP fluoride mix, because the feed would be distillation bottoms. UF4 is more volatile than LiF under vacuum so is removed about 99.9% from that feed.

Do you have a reference for this?


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PostPosted: Sep 30, 2012 4:31 am 
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IFR system, with further modifications, may turn out to be convenient and economical.
1. Leach out heat producing Cs & Sr as chlorides in aqueous solution. They could be used together as RTG fuel on land or Sr separated as insoluble fluoride.
2. Separate Th or U as volatile chlorides. U-233 from thorium fueled reactors can be reused as fissile feed.
3. Recover Pu and TRU's from uranium fueled reactors by electrolysis. They can be used as fissile feed in fast or thorium reactors.
Remaining are mostly fission products and can be discarded. Base metals could be recovered for value.


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PostPosted: Sep 30, 2012 5:59 am 
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Lars wrote:
Cyril R wrote:
There won't be much U in the FP fluoride mix, because the feed would be distillation bottoms. UF4 is more volatile than LiF under vacuum so is removed about 99.9% from that feed.

Do you have a reference for this?


No reference, because ORNL always put a fluorinator first, that typically removed more than 99.99% of the uranium. But looking at how much LiF distills out and then considering that UF4 is a tad more volatile under vacuum, it should be in the ballpark of 99.9%. Boiling point of UF4 under vacuum is less than 1100 C and the still operates at 1100 C.


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PostPosted: Sep 30, 2012 10:20 pm 
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UCl4 boils below 800C at atmospheric temperature and ThCl4 at 921C. If you start with irradiated UO2 or thorium, the chloride is a better idea. UF4 is best treated electrolytically, as in IFR processing. Alternatively, the uranium and plutonium could be oxidized to hexafluorides and removed.


Last edited by jagdish on Oct 05, 2012 3:10 am, edited 1 time in total.

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PostPosted: Oct 04, 2012 4:47 pm 
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Cyril R wrote:
Lars wrote:
Cyril R wrote:
There won't be much U in the FP fluoride mix, because the feed would be distillation bottoms. UF4 is more volatile than LiF under vacuum so is removed about 99.9% from that feed.

Do you have a reference for this?


No reference, because ORNL always put a fluorinator first, that typically removed more than 99.99% of the uranium. But looking at how much LiF distills out and then considering that UF4 is a tad more volatile under vacuum, it should be in the ballpark of 99.9%. Boiling point of UF4 under vacuum is less than 1100 C and the still operates at 1100 C.

At 1 atm the boiling point is 1417C and it is less under vacuum. Darryl challenged me on the temperature and I couldn't back up where I got it. I think I got it from you but it would be helpful to have a reference.


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PostPosted: Oct 05, 2012 3:03 am 
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Ok Lars, if you're just looking for the actual volatility curves, they are of course to be found in the ORNL distillation work:

http://moltensalt.org/references/static ... L-3791.pdf

Figure F-3a, p. 113 and F-3b, p. 114.

As you can see, UF4 is significantly more volatile than LiF above 0.1 mmHg, and the relative volatility difference becomes bigger for the milder vacuums.


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PostPosted: Oct 21, 2012 9:52 am 
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Cyril R wrote:
djw1 wrote:
If you distill first, a lot of the low boiling FP
wont end up in the bottoms.


Which ones? CsF yes, but Cs can't be marketed much. Send back to the reactor. Ditto for RbF. ZrF4 also boils out, but is easily seperated via sublimation trapping. Ok to have some in the reactor as well.

Seperating strontium from the lanthanides would be useful, it can be used in RTGs as fluoride. But perhaps barium will contaminate it.
Wikipedia wrote:
137Cs beta decays to barium-137m (a short-lived nuclear isomer) then to nonradioactive barium-137, and is also a strong emitter of gamma radiation. 137Cs has a very low rate of neutron capture and cannot be feasibly disposed of in this way, but must be allowed to decay.
Wouldn't 137Cs do well in the food irradiation market? Also, while putting it back won't necessarily hurt the LFTR, it won't do much to eliminate the stuff either.

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PostPosted: Oct 21, 2012 5:25 pm 
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137Cs has a 30 year half life. The right strategy here is to contain it for 300 years. This is not a serious engineering challenge. It should be contained in a form that wants to stay put - so chemically lock it up as CsF. The heat and radiation given off from 137Cs and 90Sr (and 238Pu from a long term LFTR) are problematic for deep geological burial. So, either separate these or wait quite a while before burial. Almost all of the 137Cs comes through 137Xe with its 4 minute half-life so we should collect about 75% of the 137Cs in the offgas system. This much could easily be combined with fluorine and stored in containers in the buffer salt. The remaining Cs tends to go back into the salt in the distillation process. Eventually, we need to clean the Cs out of the fuel salt and at that stage it should be easy to grab the Cs to add to containers in the buffer salt. Net, I think it is reasonable to plan that virtually all the Cs can be separated and placed into containers separate from the bulk of the fission products headed for burial.

The 238Pu would be extracted together with the plutonium either at the reactor or at the central processing site (depending on which paranoia is greater in the public mind).

But the 90Sr so far is stuck in the fission products and will be the main heat load in the glassified wastes.


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PostPosted: Oct 23, 2012 9:51 am 
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Lars wrote:
137Cs has a 30 year half life. The right strategy here is to contain it for 300 years. This is not a serious engineering challenge. It should be contained in a form that wants to stay put - so chemically lock it up as CsF. The heat and radiation given off from 137Cs and 90Sr (and 238Pu from a long term LFTR) are problematic for deep geological burial. So, either separate these or wait quite a while before burial. Almost all of the 137Cs comes through 137Xe with its 4 minute half-life so we should collect about 75% of the 137Cs in the offgas system. This much could easily be combined with fluorine and stored in containers in the buffer salt. The remaining Cs tends to go back into the salt in the distillation process. Eventually, we need to clean the Cs out of the fuel salt and at that stage it should be easy to grab the Cs to add to containers in the buffer salt. Net, I think it is reasonable to plan that virtually all the Cs can be separated and placed into containers separate from the bulk of the fission products headed for burial.

The 238Pu would be extracted together with the plutonium either at the reactor or at the central processing site (depending on which paranoia is greater in the public mind).

But the 90Sr so far is stuck in the fission products and will be the main heat load in the glassified wastes.
Sorry, it seems to me that 137Cs and 90Sr are just too dang useful to throw away.

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PostPosted: Oct 23, 2012 10:01 am 
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You are very welcome to take the both the 137Cs and 90Sr. The 137Cs we can likely give you mixed together will all other isotopes of Cs and other decay products from Kr and Xe. A second batch of Cs comes from the distiller combined with fluorination so it would contain a mix of Cs and Zr with a bit of Be and Li. The 90Sr is mixed with all the other salt seekers. Any idea how to separate them? It would be good news for me if you could because then the remaining fission products would have a short half-life so they could be glassified and buried without any concern about heat load underground.


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