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 Post subject: Re: Lithium-7
PostPosted: Aug 15, 2014 1:07 pm 
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Jim L. wrote:
If memory serves, the government does have stockpiles of Li that has Li-7 greater than natural but below the 99.xx% target.


I inquired about this. The stockpile, which was once quite considerable, was sold off some time ago.


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 Post subject: Re: Lithium-7
PostPosted: Aug 15, 2014 2:45 pm 
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Kirk Sorensen wrote:
E Ireland wrote:
If you have an MSR or other reactor that would otherwise be a breeder can you trade in that fissile breeding for irradiating lithium targets to burn off the Li-6?


No.

Why not? As I understand it the primary reason you can't use natural lithium is that the 6Li will capture far too many neutrons, indeed you have indicated that this is the case yourself.

So why can't you just expose natural lithium to an intense neutron flux and 'burn off' the troublesome isotope?


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 Post subject: Re: Lithium-7
PostPosted: Aug 15, 2014 3:22 pm 
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Quote:
So why can't you just expose natural lithium to an intense neutron flux and 'burn off' the troublesome isotope?


Because it is obviously far too heroic an approach for such a simple task?

Apart from that, Li breeder blankets need more Li6 than Li7 to be efficient. Exactly what we don't need.


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 Post subject: Re: Lithium-7
PostPosted: Aug 15, 2014 3:31 pm 
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E Ireland wrote:
So why can't you just expose natural lithium to an intense neutron flux and 'burn off' the troublesome isotope?


Pray tell, what would this putative "intense neutron flux" be? If I had such a thing I would stick thorium in front of it and make uranium-233 rather than waste it on lithium that can be separated far more effectively other ways.

I think very few people realize the value of neutrons. They're hard to get. Incredibly expensive. Don't waste them on dumb stuff. And dumb stuff is anything other than fission or breeding.


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 Post subject: Re: Lithium-7
PostPosted: Aug 15, 2014 4:25 pm 
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That's very true Kirk. A large nuclear reactor only makes about 10 kg of neutrons per year.

Think of it this way. 10 kg of neutrons is needed to make 500 million dollars worth of electricity. That's $50 million a kg. And that's ignoring the fact that almost all of that 10 kg is needed to sustain criticality.


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 Post subject: Re: Lithium-7
PostPosted: Aug 15, 2014 4:29 pm 
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We can also compare to the value of burning Li6. 10 kg of neutrons would burn at most 60 kg of Li6. That's a rather hefty $8 million a kg of troublesome isotope burned!

Even if my estimate of $200/kg Li7 turns out to be $2000/kg, it is still only 1/4000th the cost of transmutation.


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 Post subject: Re: Lithium-7
PostPosted: Aug 15, 2014 8:00 pm 
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Cyril R wrote:
That's very true Kirk. A large nuclear reactor only makes about 10 kg of neutrons per year.

Think of it this way. 10 kg of neutrons is needed to make 500 million dollars worth of electricity. That's $50 million a kg. And that's ignoring the fact that almost all of that 10 kg is needed to sustain criticality.


And the "miracle" of neutron multiplication means that you can get those 10 kg of neutrons starting from just one, if you send that one into a critical arrangement of material and tend it carefully. Another poorly understood fact by our neutrons-produced-by-accelerator friends.


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 Post subject: Re: Lithium-7
PostPosted: Aug 16, 2014 2:52 am 
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Kirk Sorensen wrote:
Pray tell, what would this putative "intense neutron flux" be? If I had such a thing I would stick thorium in front of it and make uranium-233 rather than waste it on lithium that can be separated far more effectively other ways.

A fast neutron reactor is the only otherwise useful device producing spare neutrons. PFBR has been indeed designed to use them to make U-233.
U-233 is also a very valuable isotope. Too bad that the Americans are spending money and effort to destroy it. It is best used as fuel in a reactor, preferably an LFTR or a fast reactor.
PS- PFBR is having prototype problems and may be commissioned only later this year or even next year.


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 Post subject: Re: Lithium-7
PostPosted: Aug 16, 2014 6:10 am 
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jagdish wrote:
Kirk Sorensen wrote:
Pray tell, what would this putative "intense neutron flux" be? If I had such a thing I would stick thorium in front of it and make uranium-233 rather than waste it on lithium that can be separated far more effectively other ways.

A fast neutron reactor is the only otherwise useful device producing spare neutrons. PFBR has been indeed designed to use them to make U-233.
U-233 is also a very valuable isotope. Too bad that the Americans are spending money and effort to destroy it. It is best used as fuel in a reactor, preferably an LFTR or a fast reactor.
PS- PFBR is having prototype problems and may be commissioned only later this year or even next year.


Even if you have excess neutrons, you are still competing for breeding fissile material (or startup fissile inventory). That's no competition with enrichment - fissile is worth tens of thousands of dollars per kg.

Kirk is absolutely right here. Transmutation schemes in general are just silly. Even long lived FP transmutation with neutrons is crazy, because you forego the value of tens of thousands of dollars per kg fissile material. It follows that the price of transmuting anything with neutrons must be at least tens of thousands of dollars per kg.


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 Post subject: Re: Lithium-7
PostPosted: Aug 18, 2014 8:04 pm 
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I would approach the problem of isotopic separation of Lithium 7 from Li 6, without using mercury, by using CENTRIFUCAL SEPARATION similarly to how it is done with U 238 and U 235. But there will need to be a few adaptions and I think it would actually be EASIER and CHEAPER than with U 235. In addition, NO CHEMISTRY, Li metal in and Li metal out.

Material properties comparison between U-235 Vs. Li 7 with regard to Centrifugal separation:
To start with, for centrifugal separation to work, we need to be dealing with a gas. This is done with Uranium by forming UF6. U238-F6 has a mass of 352 atomic units (au) and U235-F6 has a mass of 349au, a difference of only 0.85%. This of course, as we all know, makes separating them very difficult. Ok, let’s look at lithium. Unfortunately, Lithium does not have a convenient gas compound like uranium and any gas compound that one might come up with would be so heavy that the 1 au difference would become lost in isotopic uncertainty of the other elements. So, we must work with lithium alone in the gas state. This however is a big advantage, because the mass of Li 7 is 7au and the mass of Li 6 is 6au, a difference of 14.28%, much better than the 0.85% of uranium. Therefore separation should be much easier and more efficient for lithium than with uranium. Another big advantage is that we the want the isotope that is already the majority.
Now of course the obvious issue is that UF6 is gas and standard temperature and pressure, which makes it “easier” to work with and of course lithium is a solid at standard conditions. So, let’s look at how to work with Lithium. At standard pressure lithium vaporizes at 1330C, which is very toasty, and would be hard to work with. However, in a high vacuum, lithium vaporizes at less than 500C. Making equipment that can operate at 500C and in a vacuum is quit do-able. In addition, in a high vacuum, the operating gas (lithium vapor) would be by orders of magnitude the dominate gas; make interference by other gases a minimal issue. Not the case if operating at standard pressure, so one would either need to operate in a vacuum anyway or in helium. Heaver gases would mix in and one would not get separation and hydrogen would be too reactive. High vacuum is the best option in any case.

I have 2 design approaches in mind to talk about; “centrifugal gas spiral” and the “spinning can spiral”. Both are feeds and pick-ups would be basically the same. The feed would be a heated chamber that liquid lithium is injected into at a controlled rate; the lithium would flash to vapor and be jetted out of the chamber. The pick-up would be tubes that would collect the separated material at strategic locations and would lead into a chamber held at temperature that would condense the lithium vapor to a liquid. The “centrifugal gas spiral” would be easier and cheaper because it have very few moving parts, but is likely not the most efficient. Basically, the lithium vapor would be jetted at high speed into a spiral channel shaped chamber of reducing diameter (to keep the G force up as the gas slows down). The gases would be separated and collect at the end of the spiral. The “spinning can spiral” would be more difficult and expensive to build but is likely to be more efficient. It would be a spiral channel shaped chamber of increasing diameter that is spinning at high speed. The lithium vapor would be injected into and pick-up like before. With the spinning system, the time and G force applied to the gas could be much greater and therefore get a better effect.

This approach would be very cheap to operate. No chemistry, the only cost other than the raw material would be a modest amount of energy the run the vacuum system, heat for the chamber and vaporize the mass of lithium a few times.

Sorry already long, I will stop there. Thoughts? Problems I missed or did not cover?


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 Post subject: Re: Lithium-7
PostPosted: Aug 18, 2014 11:30 pm 
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I have only thoughts. Two of them, in fact.
1. Centrifuge the liquid Lithium. Liquid is also a fluid but takes much less space. Power requirement may be higher.
2. Convert to anode rays of Li ions and separate in a mass spectrum. Can be separated in one pass.


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 Post subject: Re: Lithium-7
PostPosted: Aug 19, 2014 1:54 am 
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jagdish wrote:
1. Centrifuge the liquid Lithium. Liquid is also a fluid but takes much less space. Power requirement may be higher.


This shouldn't work at all. The liquid has too much interaction with itself, and those forces (diffusion etc.) should overwhelm separative forces. As a result the enrichment should be zero (not small but really zero).

Like Michael says, we are restricted to gasses.


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 Post subject: Re: Lithium-7
PostPosted: Aug 19, 2014 2:03 am 
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michael.runyan wrote:
I would approach the problem of isotopic separation of Lithium 7 from Li 6, without using mercury, by using CENTRIFUCAL SEPARATION similarly to how it is done with U 238 and U 235. But there will need to be a few adaptions and I think it would actually be EASIER and CHEAPER than with U 235. In addition, NO CHEMISTRY, Li metal in and Li metal out.



Thoughts? Problems I missed or did not cover?


I've suggested vacuum Li centrifuge trains before. While it should work technically, it does not look economical.

First problem is the vacuum. This means thick walled centrifuges, which means heavy centrifuges, which is bad for high RPM precision machinery. Next problem is the high temperature - that then needs an even thicker centrifuge wall. It also needs quite clever seals and bearings (read costly seals and bearings).

All of this is compounded by the ultra low density of Li vapor in a vacuum. While UF6 is a gas at low pressure, the pressure in the centrifuge is kept quite high to get a good gas density.

If the cost of the centrifuge train scales anywhere nearly proportional to gas density, then this quickly becomes very expensive.

According to Kirk you need very roughly the same SWUs for 5% LEU as you'd need for enriched Li7. So separative work is roughly similar, despite the better separation factor of Li.


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 Post subject: Re: Lithium-7
PostPosted: Aug 19, 2014 2:12 am 
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Perhaps more interesting (cheaper, simpler) than centrifuges is vacuum distillation. The vapor pressure difference between Li6 and Li7 is significant. We avoid all the high RPM precision machinery (not to mention restricted materials of construction).

This would require many stages. Fractional distillation might be interesting here. There is no nuclear requirement (not even a faint radiation like from UF6).

This would be my shortlist of lithium seperation approaches to explore further.

- multistage vacuum distillation
- Li crown ether (liquid-liquid) extraction.
- laser enrichment (this one less attractive I think, due to cost and complexity).


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 Post subject: Re: Lithium-7
PostPosted: Aug 19, 2014 9:10 am 
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Another point about centrifuges is the extremely high rpms used, I believe for UF6 the early models were around 50,000 rpm and the latest models are at 100,000 rpm. Even accounting for the better relative mass differences with Li, you are going to need a pretty high rotational speed.
As far as economics, using the crown ether separation method just involves pumps and tank vessels for the most part - and I would think those would be cheaper than any of these other methods. A full cascade would be 400 stages, not thousands, IIRC.


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