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 Post subject: Lithium-7
PostPosted: Aug 31, 2007 2:23 pm 
It looks like most of the government produced enriched Lithium was prepared using the Li-Hg process or gas diffusion using the Y-12 plant. Where will we get the multiple tons of 99.999% enriched Li-7 for our MSRs?


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 Post subject: Re: Lithium-7
PostPosted: Sep 03, 2007 7:31 pm 
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Andrew W. Mangold wrote:
It looks like most of the government produced enriched Lithium was prepared using the Li-Hg process or gas diffusion using the Y-12 plant. Where will we get the multiple tons of 99.999% enriched Li-7 for our MSRs?


A very good question. I recently spoke with Charles Forsberg at ORNL and he mentioned how early methods for producing enriched Lithium were very costly. He did not feel that producing large quantities of Li-7 should be too difficult though. There is such a large relative mass difference between Li-6 and Li-7 that we should be able to use chemical techniques since even the bonding behavior will have enough difference. I can`t see it costing more than 1000$ a kg even by the most expensive techniques so it will be costly but still only few million dollars for the few tonnes needed.


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PostPosted: Sep 19, 2007 8:46 pm 
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The nice thing about Li7 is that it comprises 92.58% of natural lithium -- so its really more of a process of REMOVING Li6.

By contrast, heavy water comprises only something like one part in 7000 of ordinary water.
The great expense is incurred in the initial steps, where huge quantities of water must be processed.
Once a concentration of ~1% is reached, it gets a lot easier -- various techniques can be used, which are much more efficient at isotope separation than chemical effects or diffusion, but which would be too energy-intensive to apply at low concentrations (for example MVLIS - Molecular Vapour LASER Isotope Separation).

With a high concentration affair like lithium, MVLIS would be a perfect choice.

Same goes for other possible MSR-related enrichment requirements, such as zirconium and lead.

Of course its even better if no enrichment at all is required -- as in the case of fluorine, carbon and bismuth.

.


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PostPosted: Nov 07, 2007 4:28 pm 
This may be a stupid question, but why can't we eliminate the 6Li by just fluxing natural lithium with neutrons? For instance, pipe it through the reactor core in the form of Li-37Cl, scavenge the hydrochloric acid, reuse the 37Cl, and sell the tritium? Enough passes through the core and you'd get rid of the 6Li while liberating some energy at the same time and maybe even saving some wear on the graphite. Any chance this could be feasible, or that it would be cheaper than chemical isotope separation?

(Or get the fusion people interested in 6Li as a breeder material again and let them pay for the separation).


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PostPosted: Nov 07, 2007 4:38 pm 
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Abgrund wrote:
This may be a stupid question, but why can't we eliminate the 6Li by just fluxing natural lithium with neutrons? For instance, pipe it through the reactor core in the form of Li-37Cl, scavenge the hydrochloric acid, reuse the 37Cl, and sell the tritium? Enough passes through the core and you'd get rid of the 6Li while liberating some energy at the same time and maybe even saving some wear on the graphite. Any chance this could be feasible, or that it would be cheaper than chemical isotope separation?

(Or get the fusion people interested in 6Li as a breeder material again and let them pay for the separation).


Basically the problem is we don`t have that many neutrons to spare in most designs. We`d need to transmute close to a tonne of Li6 to get enough enriched Li7 for many MSR designs. Might not be an entirely unreasonable proposal though.

Related to your point though is how much enrichment we need in the first place. Some people quote 99.995 or even 99.999% Li7. Older ORNL documents seem fine with 99.99% and we could probably get by with far less as the Li6 would simply burn off in the reactor as you describe. Sure we loose a few extra neutrons this way but 99.9% might be a lot cheaper than 99.999%

Splitting the cost with the fusion folks who want Li6 is a good idea but I doubt they will be in serious need of a large supply for several decades. I`m hoping MSRs will be out to market long before them!


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PostPosted: Nov 08, 2007 1:09 pm 
Is there any conceivable way of scavenging HF directly from the fuel-salt mixture, or neutralizing it? I'm not a chemist by any means, but it seems that if HF is more reactive with the pipe walls than LiBeF is, there might be some other reaction that would withdraw the tritium from circulation or bind it in a harmless form without reacting with the salt. If H can be filtered out, the system is self-correcting, which would reduce the initial cost and provide another level of safety.


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PostPosted: Nov 08, 2007 2:36 pm 
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Abgrund wrote:
Is there any conceivable way of scavenging HF directly from the fuel-salt mixture, or neutralizing it? I'm not a chemist by any means, but it seems that if HF is more reactive with the pipe walls than LiBeF is, there might be some other reaction that would withdraw the tritium from circulation or bind it in a harmless form without reacting with the salt. If H can be filtered out, the system is self-correcting, which would reduce the initial cost and provide another level of safety.


Tritium control is a major issue and probably one of the areas that still needs the most work. I don`t think any of us is much of an expert on this front though. Bottom line is that it shouldn`t be too big of a problem but will need a fair bit of money thrown at the problem. Recent Russian work mentions it will bubble out with the Xenon and other gases but I`m sure some will migrate through the heat exchanger. The traditional coolant salt is NaF-NaBF4 which according to ORNL work has some tricks to get out tritium but I don`t know the details.

Any salt with LiF or BeF2 will produce tritium upwards of the levels seen in a heavy water CANDU. I think many people think that tritium control costs a fortune in CANDUs but from what I`ve heard they don`t end up spending that much on the issue. It will be very different for MSR designs but shouldn`t be any sort of show stopper. Salts without LiF or BeF2 are attractive as you would only get a small amount of tritium from fissions I think. However the salts with Li or Be are very attractive.


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PostPosted: Nov 08, 2007 5:45 pm 
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David wrote:
I think many people think that tritium control costs a fortune in CANDUs but from what I`ve heard they don`t end up spending that much on the issue.

You probably don't hear much about it because its mainly an issue that arises later in the life of a plant: It takes years for tritium levels to build up, but eventually you have to deal with it, because at some point the containment building de-humidification system can no longer suck up enough contaminated moisture from the air, to keep tritium below regulatory levels for worker exposure.
At that point, the moderator can be cleaned at a detritiation plant. But these are certainly not cheap.


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 Post subject:
PostPosted: Jan 10, 2008 5:45 pm 
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You may want to consider the fact that lithium-7 also produces tritium when it is bombarded with neutrons. It does not have as high a cross section as lithium-6 and it's cross section is greater with fast neutrons than thermal. Of course, there will always be some fast neutrons, and either way it's still high enough to be significant, even if it's not as high as Li-6.

Don't make the same mistake they made during Castle Bravo! http://en.wikipedia.org/wiki/Castle_bravo. The designers did not account for lithium-7 reacting to produce tritium and the yield of the test turned out to be nearly three times what was expected!


I doubt it would be as big a deal in a thermal reactor, but still something that should be considered!


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PostPosted: Jan 10, 2008 6:06 pm 
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drbuzz0 wrote:
You may want to consider the fact that lithium-7 also produces tritium when it is bombarded with neutrons. It does not have as high a cross section as lithium-6 and it's cross section is greater with fast neutrons than thermal. Of course, there will always be some fast neutrons, and either way it's still high enough to be significant, even if it's not as high as Li-6.

Don't make the same mistake they made during Castle Bravo! http://en.wikipedia.org/wiki/Castle_bravo. The designers did not account for lithium-7 reacting to produce tritium and the yield of the test turned out to be nearly three times what was expected!


I doubt it would be as big a deal in a thermal reactor, but still something that should be considered!


There was somewhere already a discussion on Lithium. The point is that Li-7 only will create tritium when bombarded with fast neutrons, because the (n,n't) reaction has a threshold energy. Furthermore the (n,n't) reaction can be considered as inelastic scattering explaining the higher yield of castle bravo


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 Post subject:
PostPosted: Jan 10, 2008 8:28 pm 
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Well obviously using lithium_7 is going to reduce the amount of tritium produced because it requires more fast neutrons but you'll still have fast neutrons even in a thermal reactor. You may reduce it by a lot, but as long as there is lithium 7 there is going to be at least a small amount of tritium produced and it's worth at least considering.


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 Post subject:
PostPosted: Jan 11, 2008 4:19 am 
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Tritium will always be produced. Even due to the presence of Beryllium as jaro already pointed out once. But also in tertiary fission events you will end up with tritium...


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 Post subject:
PostPosted: Jan 15, 2008 10:06 pm 
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STG wrote:
Tritium will always be produced. Even due to the presence of Beryllium as jaro already pointed out once. But also in tertiary fission events you will end up with tritium...


Good point.


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 Post subject: Re: Lithium-7
PostPosted: May 01, 2008 1:29 am 
Li-7 ("depleted lithium") is already fairly common, isn't it? I believe current LWR power plants often use Li-7-OH to tweak the coolant pH - the Li-6 is undesirable as it causes increased tritium production.

The government in the US produces enriched Li-6 for for thermonuclear weapons - the "depleted lithium" is sold off as the unwanted byproduct of this process.


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 Post subject: Re: Lithium-7
PostPosted: May 03, 2008 11:35 pm 
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enochthered wrote:
Li-7 ("depleted lithium") is already fairly common, isn't it? I believe current LWR power plants often use Li-7-OH to tweak the coolant pH - the Li-6 is undesirable as it causes increased tritium production.

The government in the US produces enriched Li-6 for for thermonuclear weapons - the "depleted lithium" is sold off as the unwanted byproduct of this process.


Yeah, during the cold war there was quite a lot of lithium enrichment going on, especially for the enormously high yield weapons in the early 1960's. I'd be curious to find out if all the 'depleted lithium' was sold off or whether the DOE is still sitting on a big pile of it.


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