Energy From Thorium Discussion Forum

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PostPosted: Dec 19, 2013 12:11 am 
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In case of Zirconium cladding, it would be preferable to chlorinate instead of fluoridating. ZrCl4 sublimes at 331C. Thorium and uranium tetra-chlorides can be distilled away. Only Plutonium and other trans-uranics, if present due to irradiation of 20%LEU, have to be electrolytic-ally separated from fission products.
With proper design of bore and thickness, metal thorium could double as container and fertile fuel to the fertile feed. Surface could be further oxidised or fluoridated to a passive layer.


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PostPosted: Dec 19, 2013 9:43 am 
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As I understand it thorium is currently so cheap that people would be paying to get rid of it even with a significant amount being required for nuclear power use.
This would seem to indicate it is probably not worth recovering from the waste stream, especially as it can serve as a nice thermal diluent for the fission products in the short term.

As to plutonium and so forth and the heavier actinides, recovering them from the waste in the short term is going to be very expensive.
It appears that a large majority of the fissile material in the spent fuel is the two fissile isotopes of Uranium, and thus attempting to implement the cheapest possible way of recovering the uranium seems reasonable.

Metal based systems would produce uranium metal which would then have to be fluorinated in a seperate step to produce a feed suitable for reenrichment, and it will likely be far more contaminated with fission products with all the attendant problems with occupational exposure to workers in the plant.

Chloride Volatility has the same issue but doeos have the advantage of avoiding decladding which does appear to be required for the fluoride mechanism.
How hard is it to convert uranium hexachloride to hexafluoride? That is rather outside my experience as a chemist.
Mechanical decladding might be the best bet for the fluoride volatility process, although it might be cheaper just to use a diamond tipped saw to slice the fuel rods in half lengthways.

Reenrichment will also tend to extract 237U from the fuel alongside the waste 238U, which will likely tend to suppress Np production in the long term.
It would be interesting to see what the equilibrium uranium isotopic composition is in this system.


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PostPosted: Dec 19, 2013 9:59 am 
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Ha! uranium is currently so cheap as to make reenrichment of heavily contaminated U232 bearing U nonviable.

Recovering TRUs is not necessary if you don't let them pass to the waste stream.

I don't like diluting with Th as it complicates future recovery


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PostPosted: Dec 19, 2013 10:48 am 
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The IAEA ran studies on reenrichment of uranium and decided it was cheaper than fresh uranium if the reprocessing had already occured.

Whilst it appears that in the first case that any kind of reprocessing is out of the question, a very low cost reprocessing step might be able to breakeven, but it could not consist of any attempted solvent seperation or anything like that as yas you know that will put the cost of the product through the roof rather rapidly.

It all depends on what sort of burnup we are attempting.
If we go with 50GWd per tonne of fuel that means that roughly all the initial fissile would have been fissioned away if we neglect breeding, meaning we will end up with a product with rather large amounts of U-233 in it.
IThis sort of thing needs properly modelling though.

As to complicating recovery of the material, since the only fissiles in the waste will be small amounts of plutonium, it might be possible to hydrothermally convert it to oxide and then run it through a conventional extraction scheme some time in the future.
I doubt the fluoride would be able to reach criticality even if it is stored in 44 gallon drums, although that might not be a good idea for heat dispersal reasons.


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PostPosted: Dec 19, 2013 1:19 pm 
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The IAEA may be right - for U/Pu cycle U.

Th cycle U contains U232, in increasing amounts if you're planning on multiple re-enrichment steps.

Its not clear to me that enrichers will even accept it. In fact, enrichers are quite skittish about U234 in reprocessed uranium from U/Pu cycle. Enrichers are not used to having to deal with gamma radiation. They're used to alpha radiation, which doesn't even get out of the centrifuge.


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PostPosted: Dec 19, 2013 3:08 pm 
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Well I would assume the spent fuel uranium would still be significantly enriched, perhaps to the extent of 5-10% (1-2% of the fuel element), which means you would only need to enrich by two-four times, which is rather lower than the enrichment extent needed for normal fuel.

It is plausible that the enrichment facility could be integrated with the reprocessing plant, would require something like 8.5kSWUs per tonneU, which is roughly 1.7kSWU per tonne of spent fuel.
That isn't that much, even a 2000t/yr plant would only need 3.4MSWU/yr to reenrich the entire spent uranium stream (assuming 5% feed).
That is something like a quarter of the enrichment capability of the plant at Paducah, which means it is not implausible for the reprocessor to own it.

EDIT:
It would appear that the 232U would end up entirely in the enriched feed, concentrating it to four times its spent fuel concentration, I assume eventually the 232U concentration would reach equilibrium, what sort of value would that be? Assuming that the reenriched uranium is blended with fresh enriched uranium prior to fuel fabrication.
(Would it be cheaper to fabricate 4 units of slightly contaminated fuel or 1 unit of contaminated fuel [although I doubt it would be as nasty as MOX fabrication] and 3 units of fresh fuel?)


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PostPosted: Dec 19, 2013 10:51 pm 
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If you are using PuFFF/ThF4 fuel that is the basis of this thread, where does the "reenrichment" come in?

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PostPosted: Dec 20, 2013 5:38 am 
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KitemanSA wrote:
If you are using PuFFF/ThF4 fuel that is the basis of this thread, where does the "reenrichment" come in?


It doesn't. But you then need a feed of reprocessed Pu in stead, and a way to recover Pu from Th without getting FPs (almost impossible with fluoride, easy with metal).


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PostPosted: Dec 20, 2013 8:09 am 
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Cyril R wrote:
KitemanSA wrote:
If you are using PuFFF/ThF4 fuel that is the basis of this thread, where does the "reenrichment" come in?


It doesn't. But you then need a feed of reprocessed Pu in stead, and a way to recover Pu from Th without getting FPs (almost impossible with fluoride, easy with metal).

Fluoride Volatility, no? PuF6 condenses @ ~62C. Getting the thorium is more difficult, but Th is cheap. Using FV, we get all the U, Np, and Pu... unless someone messed up the wiki on FV.

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PostPosted: Dec 20, 2013 9:43 am 
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But fluoride volatility recovery of PuF6 is hard. It has around zero Gibbs free energy of formation which indicates that your fluorinator will be "recovered" before Pu will be as PuF6 at least. There is some hope that a passivation fluoride layer can be maintained but I have serious doubts about the wetted (lower) section where passivation can't occur. Corrosion making a hole in the fluorinator bottom is very bad.

And we can't recover am and cm this way, which you'll make a bunch of if you're starting on Pu.


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PostPosted: Dec 20, 2013 7:14 pm 
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Once you have reactors running with good conversion ratio, Chloride volatility will produce enough fissiles without needing re-enrichment. Th-U233 cycle is better as the fissile U-233 can also be extracted as volatile chloride. This is, of course, synergistic with liquid chloride reactor.
For solid fluoride fuels, you work on U-Pu239 cycle, burn the cladding with chlorine, and carry out the reprocessing with fluoride volatility. PuF6 is volatile but ThF4 is not.


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PostPosted: Dec 20, 2013 8:48 pm 
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The recovery percentage of plutonium was impressive in an experiment called "falling drops" and was the basis of deciding that we could get down to around 100grams of plutonium flowing in the waste stream together with fission products. But Cyril is right that the vessel is a challenge. 650C F2 gas is very aggressive so you'll need to select the vessel material carefully. Among the ideas:
a) use a wall coated in frozen fuel salt, actively cool the wall to keep the wall covered. This was ORNL's plan.
b) look to a vessel with tungsten or other moly - we aren't restricted by neutron exposure here.
c) see if any material forms a passivation layer in fluorine like aluminum does in oxygen.

It seems like we haven't tried very hard to see if this work so there is a reasonable chance to find a solution.

Am and Cm likely won't be collected this way (at least I've not heard whether Am or Cm form a gaseous fluoride). I'm not sure if we actually know. So, I have more confidence in using this method to extract plutonium formed in a thermal LFTR to send to a fast MSR for burning so that we create less Am and Cm.


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PostPosted: Dec 21, 2013 4:03 am 
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Tungsten and moly will fluorinate well before you get PuF6 (and if you get it the PuF6 will make WF6 in the bottoms). In fact, gold, platinum and graphite will fluorinate before you get to PuF6. It's that bad.

Passivation won't work in the liquid bottoms zone.

So I think we're down to one option, maintaining frozen layer. Not sure if that is practical with small piping like drain and feed lines. If a little pipe or nozzle corrodes, then you have one hell of a mess.


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PostPosted: Dec 24, 2013 7:11 pm 
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Aluminum oxidizes very easily but the oxide forms a tight seal that protects the aluminum from further oxidation. I wonder if this happens for tungsten or moly or thorium(?) .

The falling drops experiment that ORNL did had upflowing hot (650C) F2 gas and downflowing small (50u?) drops so that the surface area to volume ratio was quite high. I believe the hot gas is inserted above the surface of the liquid because ORNL had earlier found that bubbling the gas through the liquid was not very effective even for removing uranium. They used Hastalloy-N for the vessel and did suffer significant corrosion. I thought moly was more resistant to fluorine than Hastalloy is.


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PostPosted: Dec 24, 2013 7:31 pm 
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Yes, nickel and copper form protective fluoride coatings. But these are soluble in molten salts, unlike aluminium oxide which is insoluble in water. So the passivation for fluoride will work in gas space but not under fluoride salt. With falling drops it starts to get dubious already, and the drops have to fall into some pool or line at some point, which will have the worst corrosion (extremely oxidising, plus dissolving salt all around).

Mo and W are more resistant than Hastelloy. However, their preferred fluoride oxidation state is the +6 state, which is volatile. So they do not form protective coatings, quite the opposite occurs. They just float away upon fluorinating, exposing more metal for reaction. It should be noted that no metal resists fluorination from hot fluorine. Mo gets you about 200 kJ/gramF in its fluorination with F2, to MoF6. That is a lot of reaction enthalpy. So passivation is the only option for something as aggressive as PuF6 production. And I worry about passivation under a fluoride salt.


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