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PostPosted: Nov 26, 2009 12:35 am 
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We really need a good way to remove samarium from the core salt (LiF-BeF2-UF4-fission products) while the reactor is online and operating. It needs to be quite preferential for samarium and as small and simple as possible.

I'm not thinking distillation here.

Any ideas? Seriously, I'm casting the net wide here for any chemists who might have a good idea.


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PostPosted: Nov 26, 2009 10:48 am 
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Kirk Sorensen wrote:
We really need a good way to remove samarium from the core salt (LiF-BeF2-UF4-fission products) while the reactor is online and operating. It needs to be quite preferential for samarium and as small and simple as possible.

I'm not thinking distillation here.

Any ideas? Seriously, I'm casting the net wide here for any chemists who might have a good idea.


I think I have something somewhere on this. I'll look into it after the holidays. In non-aqueous solutions, samarium has redox chemistry and can be reduced to a II oxidation state.

(Sm (II) is used in organic chemistry to make cyclopropanes and other cool things.)


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PostPosted: Nov 26, 2009 11:04 am 
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NNadir wrote:
I think I have something somewhere on this. I'll look into it after the holidays. In non-aqueous solutions, samarium has redox chemistry and can be reduced to a II oxidation state.


I didn't see samarium trifluoride listed on the free-energies-of-formation table that I often reference, so I don't know how stable it is in the salt. But if it could be preferentially reduced from the salt and extracted (and I have no idea how not being any sort of chemist) then that would be highly advantageous to the operation of the reactor.

The interest in samarium is pretty simple. If you think about your two top neutron-eating fission products in the reactor, they are xenon and samarium. Thank goodness, xenon is so easy to get out during operation. It will basically come out whether you want it to or not, you just improve the effectiveness. But samarium on the other hand isn't in any hurry to go anywhere. It doesn't form a volatile fluoride under fluorination like so many other fission products do, and ironically the isotope that causes you the most trouble neutronically (Sm-149) is actually stable and non-radioactive. Then there's Sm-149's older brother Sm-151, which has a 90-year half-life and has about 1/3rd the absorption cross-section of Sm-149.


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PostPosted: Nov 26, 2009 11:24 am 
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Here's some more information on rare-earths in general from the paper "MOLTEN-SALT REACTOR CHEMISTRY", pg 151-152. Remember that this is in the context of a one-fluid reactor, with thorium mixed in the salt that makes separation from rare-earths (like samarium) difficult. It may be possible to do things with a two-fluid reactor that simply won't work in a one-fluid reactor.

Quote:
Separation of Rare Earths

There is no real doubt that the rare-earth elements, which form fluorides that are more stable than ThF4 will be present in MSBR fuel sent to the processing plant.

The limited solubility of these trifluorides (though sufficient to prevent their precipitation under normal MSBR conditions) has suggested a possible recovery scheme. When a LiF-BeF2-ThF4-UF4 melt (in the MSBR concentration range) that is saturated with a single rare-earth fluoride (LaF3, for example) is cooled slowly, the precipitate is the pure simple trifluoride. When the melt contains more than one rare-earth fluoride the precipitate is a (nearly ideal) solid solution of the trifluorides. Accordingly, addition of an excess of CeF3 or LaF3 to the melt followed by heating to effect dissolution of the added trifluoride and cooling to effect crystallization effectively removes the fission product rare earths from solution. It is likely that effective removal of the rare earths and yttrium (along with UF3 and PuF3) could be obtained by passage of the fuel through a heated bed of solid CeF3 or LaF3. However, the price is almost certainly too high; the resulting fuel solution is saturated with the scavenger fluoride (LaF3 or CeF3, whose cross section is far from negligible) at the temperature of contact.

Similar solid solutions are formed by the rare-earth trifluorides with UF3. It might, accordingly, be possible to send a side-stream (with the 233Pa and 233U removed by methods described above) through a bed of UF3 to remove these fission product poisons; 238UF3 would presumably be used for economic reasons. The resulting LiF-BeF2-ThF4 solution would be saturated with UF3 after its passage through the bed. This 238U would have to be removed (for example, by electrolytic reduction into molten Bi or Pb) before the salt could be returned to the cycle. The UF3 bed could be recovered by fluorination to separate the uranium and rare earths. While this process probably deserves further study, the instability of UF3 in melts with high UF3/UF4 ratios and the ease with which uranium alloys with most structural metals would tend to make application of such a process unattractive.

Removal of rare-earth ions (and other ionic fission-product species) by use of cation exchangers has always seemed an appealing possibility. The ion exchanger would, of course, need (a) to be quite insoluble, (b) to be extremely unreactive (in a gross sense) with the melt, and (c) to take up rare-earth cations in exchange for ions of low-neutron cross section. The bed of CeF3 described above functions as an ion exchanger; it fails to be truly beneficial because it is too soluble in the melt.

Unfortunately, not many materials are truly stable to the LiF-BeF2-ThF4-UF4 fuel mixture. Zirconium oxide is stable (in its low temperature form) to melts whose Zr4+/U4+ ratio is in excess of ~3, and UO2-ThO2 solid solutions are stable at equilibrium U/Th ratios. It is conceivable that sufficiently dilute solid solutions of Ce2O3 in these oxides would be stable and would exchange Ce3+ for other rare earth species. Intermetallic compounds of rare earths with moderately noble metals (or rare earths in very dilute alloys with such metals) seem unlikely to be of use because they are unlikely to be stable toward oxidation by UF4. Compounds with oxygenated anions (such as silicates and molybdates) are decomposed by the fluoride melt; they precipitate UO2 from the fuel mixture. It is possible that refractory compounds (such as carbides or nitrides) of the rare earths, either alone or in solid dilute solution with analogous uranium compounds, may prove useful.


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PostPosted: Nov 26, 2009 11:57 am 
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Would you be content to remove the samarium together with the plutonium?
(Or remove the Np earlier?)
If so, then I think we can do the job fine with distillation.
I just don't know how to separate the samarium from the plutonium.


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PostPosted: Nov 26, 2009 12:00 pm 
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Kirk Sorensen wrote:
NNadir wrote:
I think I have something somewhere on this. I'll look into it after the holidays. In non-aqueous solutions, samarium has redox chemistry and can be reduced to a II oxidation state.


I didn't see samarium trifluoride listed on the free-energies-of-formation table that I often reference, so I don't know how stable it is in the salt. But if it could be preferentially reduced from the salt and extracted (and I have no idea how not being any sort of chemist) then that would be highly advantageous to the operation of the reactor.

The interest in samarium is pretty simple. If you think about your two top neutron-eating fission products in the reactor, they are xenon and samarium. Thank goodness, xenon is so easy to get out during operation. It will basically come out whether you want it to or not, you just improve the effectiveness. But samarium on the other hand isn't in any hurry to go anywhere. It doesn't form a volatile fluoride under fluorination like so many other fission products do, and ironically the isotope that causes you the most trouble neutronically (Sm-149) is actually stable and non-radioactive. Then there's Sm-149's older brother Sm-151, which has a 90-year half-life and has about 1/3rd the absorption cross-section of Sm-149.


This is also true of europium to a lesser extent. Eu-151 has a absorption cross section on the order of 7000 barns for thermal neutrons and Eu-152 more than 10,000. In fact, 153, 154, and 155 all have large cross sections. 151 and 153 are stable, the others all have half-lives n the order of yearss.

There IS volatization chemistry of europium known, exotic but realistic.

This is all off the top of my head, so don't hold me to it.


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PostPosted: Nov 26, 2009 1:01 pm 
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Pulling both samarium and europium out would be great. Neodymium removal is also interesting since Nd stabilizes pretty quickly and there's quite a bit of it in the salt.


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PostPosted: Nov 26, 2009 1:27 pm 
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Selective reduction of Sm3+ to Sm2+ looks difficult

CRC Handbook 74th Ed data:-
Sm3+ + e- = Sm2+ E = -1.55 V

U4+ + e- = U3+ E = -0.607 V

So any process that forms Sm2+ will reduce UF4 to UF3 first, and if you make any Sm2+ it will react with UF4 to give UF3 and Sm3+ again.

Of course, you could fluorinate out all the U first, but I'm guessing you are looking for something that can run at a substantially higher rate than the regular reprocessing system, and drive the Sm residence time down low enough that most of it doesn't capture.

There is nothing inherently impossible about distilling 3m^3/day of salt, so you process the whole lot every week, but the still would be as big as the reactor, and probably end up as expensive. if it had to be built to run for 60 years unattended in a hot cell.


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PostPosted: Nov 26, 2009 5:02 pm 
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What would happen if we cool the salt to just above freezing?

If the trifluorides have limited solubility won't they tend to precipitate out?


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PostPosted: Nov 26, 2009 5:20 pm 
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Luke wrote:
Of course, you could fluorinate out all the U first, but I'm guessing you are looking for something that can run at a substantially higher rate than the regular reprocessing system, and drive the Sm residence time down low enough that most of it doesn't capture.


I'm really hoping to find some way to do it that is relatively unobtrusive--of course, such a way may not exist! Nevertheless, I'm really hoping to find something that would be the next "layer" of reprocessing beyond gas sparging to remove noble-gases (xenon particularly).


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PostPosted: Nov 26, 2009 5:42 pm 
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I found this, it might be relevant.

Cathodic behaviour of samarium(III) in LiF–CaF2 media on molybdenum and nickel electrodes



Looks like it may be the foundation of a process.


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PostPosted: Nov 26, 2009 5:55 pm 
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Yes, something along these lines would be very attractive--some kind of cathode (I think) stuck in the salt that operates at just the right voltage to suck samarium (and only samarium) out of the salt.


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PostPosted: Nov 26, 2009 8:27 pm 
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My greatest doubts about LFTR have been about Samarium and other Lanthanides lingering on in the fuel after sparging of Xe and Kr. From webelements I find that fluorides of these elements have higher melting points than Thorium and Uranium. It is likely that their solubility would be lower and it may be possible to selectively crytallise/precipitate them out (along with Sr) by keeping the solution at a certain temperature. May need practical checking. Technetium could be evaporated and sparged out.


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PostPosted: Nov 27, 2009 9:26 pm 
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Well, looking around quickly - also skimming through the paper referenced by DV82XL - it would seem that the propensity for the electrochemical reduction of Sm(III) is complicated by the Sm(II) oxidation state, and that it is not easy to electrochemically reduce compounds in the presence of lithium, except using Ni electrodes, during which intermetallic species with Ni are formed. With other metallic electrodes reduction to the metal is problematic.

DV82XL’s reference refers to an LiF/CaF system and not LiF/BeF. Although Be and Ca are formally cogeners, they are very different elements in many ways.

(Nickel is the magic metal of nuclear science - isn't it? - but there is a level on which this information might be a bit disturbing actually.)

Recent Russian work with chloride salts of heavier elements, sodium, potassium, etc is somewhat different, and Sm metal can be obtained via molten salts. One interesting thing observed by the Russians was the well defined II oxidation of americium in molten salt mixtures.

Cool, very cool. I love that. I recall reading about Am(II), but not recently. Mostly in my mind I tend to focus on Am(III) and Am(V).

The situation with europium is analogous, and the existence of Am(II) extends further one’s faith in the periodic system. If I recall correctly, Am(II) is not stable in aqueous solutions, whereas Eu(II) – but not Sm(II) – is.

Jagdish's idea is nice, but probably impractical in the sense that crystallizations are generally very sensitive to the nature of the solvent which will be complex owing to differing levels of burn-up in the salts.

However the phase change method is appealing on one level. There are and will be inhomogenities owing to phase changes in molten salt systems to be sure, and one idea is precipitation using a reagent. Crystallization/precipitation by the addition of a reagent is certainly one relatively simple possibility. Tungsten samarate salts can be precipitated out of certain molten salts under certain conditions, for instance, also very cool. If one looks at computational phase diagrams in these kinds of systems, or experimental data, one recognizes that a variety of chemical species are usually present in equilibrium and that temperature does, in fact, play a profound role.

That much can be exploited in very positive ways I think, and just not for removing things that one doesn’t want in a reactor.

In general, phase behavior has some risks associated with it too, I think, but nothing too scary.

I have seen work that suggests phase based separations are possible. It may or may not offer something assuming one does end up with emulsions. In fact, in the MSRE, I believe there were issues with plating out of noble metals. This is phase behavior.

However if you asked me to set up an experiment that was most likely to succeed, I’d end up trying first another multiphasic system, extraction, probably using metallic aluminum as an extractant. (Molten lithium metal is also sometimes discussed in this context.) There are certainly other possibilities, in fact, many other possibilities. We live in the golden age of solvents. Extractions are clean, simple, painless and fast if one can avoid emulsions.

By the way, these elements do exert vapor pressures. I always have liked distillation as a separation method. It would be cool – but I have no idea if it’s possible or realistic – to observe salt or metal azeotropes, which may exist.

My own vision of MSRs, which is somewhat different than other systems I’ve seen, takes another approach entirely than what I’ve suggested here.

Looking it over, quickly and lazily scanning some papers on the subject, I think Sm/Eu accumulation is an issue, but not a big deal probably.


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