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PostPosted: Dec 26, 2010 1:35 pm 
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ORNL did do an excellent job of grabbing Pu using fluorination. Trouble is, the vessel. Can we create a vessel that can handle 650C F2? If we made a vessel out of thorium metal would the ThF4 form a protective film or would it form a powder that would fall away leaving our vessel to be dissolved by the F2?

Could we diamond coat something to serve as the vessel? Can we make the coating sufficiently flawless? At least here there is no neutron flux to convert the diamond into black carbon.


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PostPosted: Dec 26, 2010 5:13 pm 
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Lars wrote:
ORNL did do an excellent job of grabbing Pu using fluorination. Trouble is, the vessel. Can we create a vessel that can handle 650C F2? If we made a vessel out of thorium metal would the ThF4 form a protective film or would it form a powder that would fall away leaving our vessel to be dissolved by the F2?

Could we diamond coat something to serve as the vessel? Can we make the coating sufficiently flawless? At least here there is no neutron flux to convert the diamond into black carbon.


But you can't grab Am and Cm this way. Plus, PuF6 is really corrosive. It actually has a positive free energy of formation, which is bizarre. Doesn't sound like something we want to be making or having to handle and contain.

650 C fluorine, thats asking a lot. The diamond coating is only stable in fluorine up to a point (far lower than 650 C). In some ORNL documents they say 400 C which sounds doable for diamond. What is the point of such high temperatures when you're fluorinating oxide fuel? The oxide will always be solid anyway, and the volatiles are all gasses wether its 400 C or 650 C.

Thorium metal, that's promising too. If its thick enough it might do. If Kirk can't get radioactive materials then this is out of the question of course. If there's not too much liquid contact then nickel will do also, it forms protective NiF2. Same with copper.


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PostPosted: Dec 27, 2010 5:19 am 
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Lars wrote:
An alternate path may be to use HF to form UF4 then vacuum distill - likely the Np is still NpF3 which has low volatility. This path would result in some fission products (Cs, and Zr) coming along that would need to be separated out as well. Seems like this should work but I'm cautious since ORNL never mentions this and they were considering using both fluorination and vacuum distilling.

But the main question is why do this? What is the benefit?


Well, HF is less corrosive than free F2. You can skip the most corrosive flame fluorination step, and resultant corrosive UF6 and PuF6, plus less deconversion later on. Use cheaper structural materials such as copper. You can't hydrofluorinate tungsten, though.

HF converts UO2 up to UF4 allright, but wouldn't that also push Np and Pu mostly up to tetrafluorides? This would then come out in the vacuum still? Could you use this as online hydrofluorinator followed by distillation to send most Np and Pu back into the core as tetrafluorides?


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PostPosted: Dec 27, 2010 12:03 pm 
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HF isn't going to change the oxidation state of the fuel. The U, Np and Pu are all present as the +4 oxides AnO2, so they all become AnF4 + H2O with HF. PuF4 is rather corrosive, though, as it 'wants' to be PuF3 and give the spare F to something else. Nothing like as bad as F2, though. If you wanted to be selective, you could add something reducing (magnesium?) and have UF4 + PuF3, with the Np as mostly NpF4 or NpF3 as desired. Clean U/Pu split is easy, Np harder from both (short of Purex)


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PostPosted: Dec 27, 2010 2:24 pm 
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Luke wrote:
HF isn't going to change the oxidation state of the fuel. The U, Np and Pu are all present as the +4 oxides AnO2, so they all become AnF4 + H2O with HF. PuF4 is rather corrosive, though, as it 'wants' to be PuF3 and give the spare F to something else. Nothing like as bad as F2, though. If you wanted to be selective, you could add something reducing (magnesium?) and have UF4 + PuF3, with the Np as mostly NpF4 or NpF3 as desired. Clean U/Pu split is easy, Np harder from both (short of Purex)


So you can make U, Np and Pu from spent fuel rods with HF all in one step as tetrafluorides, and then distill them to grab them. Then put them all in Jaro's HW-MSR. Or reduce with Mg to further distill U from Pu.

Purex is not nice. We want the processes to be like a fresh babies diaper. Dry and clean, not wet and messy.

PuF4 may be corrosive but isn't as bad as PuF6. It could probably be contained well in C, Cu or W. These elements do not want to become a fluoride more than PuF4 wants to become PuF3. Containing PuF6 must be impossible. It will become PuF5 and then PuF4 anyway, just on the basis of its own alpha activity smashing off the unstable extra fluorine.

What about Am and Cm? They're even less stable as tetrafluorides than plutonium. Will they be made as +4 if you saturate with HF?


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PostPosted: Dec 27, 2010 3:22 pm 
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Could one of you chemists please explain to us non-chemists why it is that PUREX generates so much contaminated waste?

-Iain


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PostPosted: Dec 27, 2010 3:58 pm 
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Cyril R wrote:
Lars wrote:
An alternate path may be to use HF to form UF4 then vacuum distill - likely the Np is still NpF3 which has low volatility. This path would result in some fission products (Cs, and Zr) coming along that would need to be separated out as well. Seems like this should work but I'm cautious since ORNL never mentions this and they were considering using both fluorination and vacuum distilling.

But the main question is why do this? What is the benefit?


Well, HF is less corrosive than free F2. You can skip the most corrosive flame fluorination step, and resultant corrosive UF6 and PuF6, plus less deconversion later on. Use cheaper structural materials such as copper. You can't hydrofluorinate tungsten, though.

HF converts UO2 up to UF4 allright, but wouldn't that also push Np and Pu mostly up to tetrafluorides? This would then come out in the vacuum still? Could you use this as online hydrofluorinator followed by distillation to send most Np and Pu back into the core as tetrafluorides?

I expect the PU and Np will form tri-fluorides and not come out in the vacuum still.

The main question is what is the value in removing the uranium from the spent fuel?
It does reduce the mass and volume (say 20x) but does not reduce the heat load, radiation, radioactive danger, etc.


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PostPosted: Dec 27, 2010 4:01 pm 
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iain wrote:
Could one of you chemists please explain to us non-chemists why it is that PUREX generates so much contaminated waste?

-Iain


A non-chemists answer - because it was designed to produce very pure plutonium. The goal had nothing to do with clean processing of spent fuel and everything to do with producing plutonium with as little radioactive fission products as possible.


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PostPosted: Dec 27, 2010 4:07 pm 
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Lars wrote:
ORNL did do an excellent job of grabbing Pu using fluorination. Trouble is, the vessel. Can we create a vessel that can handle 650C F2? If we made a vessel out of thorium metal would the ThF4 form a protective film or would it form a powder that would fall away leaving our vessel to be dissolved by the F2?

Could we diamond coat something to serve as the vessel? Can we make the coating sufficiently flawless? At least here there is no neutron flux to convert the diamond into black carbon.

The outer structure of most furnaces is constructed of materials which wouldn't be able to stand the temperature inside. The trick is to use insulating porous firebricks.

The obvious material to build something to withstand attack by hot fluorine is a fluoride. I think it should be possible to create insulating bricks of a suitable high-melting fluoride salt to line the inside. The vapors leaving the chamber can be cooled by mixing with cold fluorine gas. A flow of cold fluorine can also be used to protect specific sensitive points like the roof directly above the combustion or a camera port (with a window of calcium fluoride glass?)


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PostPosted: Dec 27, 2010 4:17 pm 
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True, uranium recovery only would not be worth much. But why would Np and Pu stay mostly as trifluorides? Hydrogen fluoride is a lot less stable than NpF4 and PuF4 so it will be an oxidant to PuF3 and NpF3. NpF4 and PuF4 are only slightly less stable than UF4. I think this is important if you follow the SNF hydrofluorination by vacuum distillation, as NpF4 and PuF4 are volatile under vacuum at 1100 C.


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PostPosted: Dec 27, 2010 5:35 pm 
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Cyril R wrote:
...Hydrogen fluoride is a lot less stable than NpF4 and PuF4 so it will be an oxidant to PuF3 and NpF3....
Sorry, no. HF will not do this conversion, because you have to put the hydrogen somewhere. It works from the oxides, because the H can go onto the O to make water, but there isn't enough energy in the system to drive the H back to being hydrogen. However, as I said above, the Np/Pu in spent fuel are likely present as the dioxides, as was the U they were made from, so the initial product is likely to be mostly tetrafluorides. For the higher actinides, the +4 state is unstable, so AmF3 / CmF3 it is, with something else, (fission product or vessel or UF4 --> UF5 conversion) picking up the spare fluorines.

On PUREX, Lars certainly has one good answer. Another perspective is that PUREX is dilute. It has to be as water and, to a slightly lesser extent, kerosine, are good moderators. Once the fission product poisons are split off, the system is rather too close to being an aqueous homogeneous reactor. The fissile material, especially the Pu, has to be kept dilute and spread out in long, thin, pipes, to ensure it is well away from criticality. The large volumes then expose a lot of kerosine / butyl phosphate to radiation. The radiolysis products cause process issues - difficulty in getting clean settling and separation of aqueous and organic layers - and constitute a substantial waste problem.


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PostPosted: Dec 27, 2010 9:26 pm 
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Lars wrote:
The main question is what is the value in removing the uranium from the spent fuel?
It does reduce the mass and volume (say 20x) but does not reduce the heat load, radiation, radioactive danger, etc.

That's the sell right there.

Until the NWPA of 1982 is amended we can't remove activity from a storage site--at least as far as I know. But maybe we can remove the uranium and stable fission products.


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PostPosted: Dec 28, 2010 3:03 pm 
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Is the idea to reduce the amount that goes into dry cask storage by putting a fluorinator local at each power plant site?


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PostPosted: Dec 29, 2010 8:07 pm 
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iain wrote:
Could one of you chemists please explain to us non-chemists why it is that PUREX generates so much contaminated waste?

-Iain


I am not here to bash PUREX, although very clearly better chemistry exists, but...

PUREX is a solvent extraction process which relies on differential solubility in a system of immiscible solvents and a variety of extraction complexing agents. The immiscible solvents for most of the history of PUREX have simply been kerosene and aqueous (and corrosive) nitric acid. The complexing agents (added to kerosene) are typically organophoshates, most commonly TBP, tributyl phosphate.

The chemistry is designed to procure plutonium primarily, and very little attention has been paid to recovering valuable fission products, since they are present in low concentrations. The prime focus is to separate plutonium from uranium. There is very little focus on recovering the rest of the valuable elements, including some very wonderful elements like Americium, neptunium, and curium.

The raffinates, again nitric acid, need to be neutralized, and regrettably, this is most often done with sodium hydroxide. This leaves sodium nitrate salts in the raffinates, which complicates the recovery of cesium and rubidium and generally just makes a mess.

TPB is subject to slow radiolytic degradation, and it is also not recovered, although sometimes the kerosene is.

Many other "designer" extraction agents are available beyond TBP, but a discussion of that would take a very, very, very long time and I'm not going to do it..

I believe that some solvent extraction techniques may be useful in the future, but we need to get rid of the kerosene/TPB system.

All that said the problems with PUREX are definitely, in my mind, over rated.


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PostPosted: Dec 30, 2010 2:22 am 
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The traditional PUREX process is definitely messy, but aqueous processes don't have to be if making bombs isn't the goal. Most of the uranium can be removed by crystallization of uranyl nitrate from the nitric acid solution, with no radiolysis-prone organic solvents required. By controlling the redox potential, Pu (and Np) can be co-crystallized with the U as Pu(VI), left with the fission products as Pu(IV), or separated later in a second crystallization step. Rather than dumping in NaOH to neutralize the mother liquor and bulking up the fission products with a bunch of NaNO3, the mother liquor could be distilled to recover the nitric acid (BP = 120°C) for recycle. Further heating will convert most nitrates to oxides and release the nitrogen as nitrogen oxides, which could be converted to nitric acid and recycled as well. Criticality concerns do require scale up to take the form of more small scale equipment rather than larger equipment, but is such a highly modular plant a bad thing?


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