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 Post subject: Re: Uranium hexafluoride
PostPosted: Mar 26, 2015 11:30 pm 
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Higher temperatures might be good for Russia, Canada, Alaska and Scandinavian countries. Overall, the higher temperatures and the CO2 could result in higher biomass production for food and fuel.
Uranium hexafluoride. enriched to required level and reduced to trifluoride/tetrafluoride or mixed with plutonium/TRU salts, could be used as fuel in the MSR.


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 Post subject: Re: Uranium hexafluoride
PostPosted: Mar 27, 2015 6:44 pm 
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rgvandewalker wrote:
We discussed alternate turbine fluids in the conversion section. At first, SF6 looks really, really good: Inert, heavy, good heat transfer, compressible, etc.
What came out after some research (some accelerator physcists looked into it for reasons of their own) is that at fairly modest temperatures (~500C)
fluoride gasses start to dissociate into fluoric acids, fluoryl radicals and etc.
They reform very fast, but the hot gas tends to eat things and be very poisonous.
So, then the issue is, which high-temperature, high-strength, anticorrosive material do you trust for your piping,
and even more critically, your first turbine stage, which is both hot and highly stressed!?


Yes that appears to be the problem. Do fluorocarbons have the same problem? In the aforementioned paper they suggest C4F10, although I have to wonder if C6F14 might be better, because it has slightly higher density, hopefully lower pressure necessary to become supercritical (although I can't find any data on that) And it isn't a greenhouse gas because, technically speaking, it isn't a gas at normal earth conditions (although it become one at only slightly hot temperatures so it will comfortably stay that way in a turbine when it's running).


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 Post subject: Re: Uranium hexafluoride
PostPosted: Apr 01, 2015 7:29 pm 
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Yes, fluorocarbons have similar problems. Alas. CO2 was the fall-back position in that discussion, as well.


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 Post subject: Re: Uranium hexafluoride
PostPosted: Apr 01, 2015 10:05 pm 
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Fluorocarbons are used as lubricants in ICEs a and are liely to be harmless. Right boiling points may be the main consideration.


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 Post subject: Re: Uranium hexafluoride
PostPosted: Apr 01, 2015 10:05 pm 
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Fluorocarbons are used as lubricants in ICEs a and are liely to be harmless. Right boiling points may be the main consideration.


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 Post subject: Re: Uranium hexafluoride
PostPosted: Apr 04, 2015 12:46 am 
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rgvandewalker wrote:
Yes, fluorocarbons have similar problems. Alas. CO2 was the fall-back position in that discussion, as well.


Argh.

It turns out someone's already done most of the work of making a nice list of critical points on wikipedia:

http://en.wikipedia.org/wiki/Critical_p ... modynamics)#Table_of_liquid.E2.80.93vapor_critical_temperature_and_pressure_for_selected_substances

My guess from the above referenced paper is that xenon would be the ideal substance but it's disregarded because of its cost. Going over numbers, it looks like Krypton's physical properties are similar, being only a little bit less dense in liquid form and having a critical point with a somewhat lower pressure. Carbon dioxide has a much lower density as a liquid. Anybody know if krypton would work better than carbon dioxide in principle? Having a 'gas' whose density is much greater than water's sounds very appealing.

And this is a really fun paper on what ideally could be used, although the author should maybe put down the crack pipe. Sulfuric acid as a supercritical working fluid. I'll get right on that for you.

http://web.mit.edu/rsi/www/pdfs/papers/ ... 5-ianr.pdf

The upshot is that ferric chloride could in principle hit 55% efficiency at 1200K. It has the annoying property of being a solid at room temperature, but that can be worked around. I assume the real problem is corrosion. Anybody know if there are any metal alloys which could handle that kind of exposure?


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 Post subject: Re: Uranium hexafluoride
PostPosted: Apr 04, 2015 3:45 am 
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From corrosion viewpoint, why not go for Argon? It is chemically as inert as helium. It is the cheapest of inert gases. Its density is reasonably close to air or carbon dioxide.


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 Post subject: Re: Uranium hexafluoride
PostPosted: Apr 04, 2015 5:10 pm 
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jagdish wrote:
From corrosion viewpoint, why not go for Argon? It is chemically as inert as helium. It is the cheapest of inert gases. Its density is reasonably close to air or carbon dioxide.


Argon would work fine but isn't nearly as good as Krypton. Krypton's density in grams per cubic centimeter in liquid form (which I'm guessing is reasonably representative of the density it would have in supercritical form, which in turn is indicative of how efficient the engine will be) is 2.4 grams per cubic centimeter, as opposed to 1.4 for Argon, 0.8 for carbon dioxide, and 1.0 for water. Krypton is about 100 times as expensive as Argon, which is a factor to consider but when you're looking at a generator for a power plant that kind of expense is clearly worth it for a few more percent efficiency. Xenon's density is 2.94 as a liquid, which is more than Krypton's but the efficiency gains in exchange for the stratospheric price of Xenon probably aren't worth it. Xenon also might get a bit corrosive at high temperatures and pressures. Krypton appears to remain inert even then, so it's unlikely you'll have to periodically shut down the engine to scrape kryptonite out of it. (Yes I know any compounds formed would decompose again at lower temperatures, I just couldn't resist telling that joke.)


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 Post subject: Re: Uranium hexafluoride
PostPosted: Apr 05, 2015 11:50 pm 
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Krypton's neutron cross section is about 7,000 times that of the carbon in CO2 ( 25 compared to 0.0035 barns ) and the Oxygen's cross section is much less again - 0.00019.
http://www.ncnr.nist.gov/resources/n-le ... nts/o.html
Considering that there is far more experience in using CO2, and it's much cheaper, you can see why steam or CO2 have been used much more than Helium or any other noble gas.


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 Post subject: Re: Uranium hexafluoride
PostPosted: Apr 06, 2015 10:10 am 
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jon wrote:
Krypton's neutron cross section is about 7,000 times that of the carbon in CO2 ( 25 compared to 0.0035 barns ) and the Oxygen's cross section is much less again - 0.00019.
http://www.ncnr.nist.gov/resources/n-le ... nts/o.html
Considering that there is far more experience in using CO2, and it's much cheaper, you can see why steam or CO2 have been used much more than Helium or any other noble gas.


Hopefully that isn't an issue! The generator should be separated from the core by a heat exchanger or two and running a closed cycle, so the amount of neutrons it's absorbing should be de minimis.

The main issue appears to be experience. Mechanical engineers are used to using Krypton approximately, umm, never, so are understandably a bit circumspect about jumping full-on into using it at high temperatures and pressures.


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 Post subject: Re: Uranium hexafluoride
PostPosted: Apr 07, 2015 3:54 am 
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Krypton absorbs too many neutrons and is not suitable inside the reactor core. Problems are much less in secondary coolants.
Argon's absorption cross-section can be compared to that of hydrogen in water and sodium coolant in fast reactors. It is used as inert atmosphere above sodium in fast reactors. It could be a candidate for gas coolant and definitely as a secondary coolant..


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 Post subject: Re: Uranium hexafluoride
PostPosted: Apr 07, 2015 2:52 pm 
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For me a lighly presurized UCL4-BeCl2 fuel and LiF-BeF2-THF4 blanket with graphite moderator in between and leakage detection between fluids and dump tanks in both fluids looks like a very good compromise to be an achievable technology break even reactor candidate.


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 Post subject: Re: Uranium hexafluoride
PostPosted: Apr 29, 2015 2:27 am 
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Torres wrote:
For me a lighly presurized UCL4-BeCl2 fuel and LiF-BeF2-THF4 blanket with graphite moderator in between and leakage detection between fluids and dump tanks in both fluids looks like a very good compromise to be an achievable technology break even reactor candidate.


Yes, others have made that observation as well, although nobody seems to yet claim to have a fully fleshed out design ready for build experiments.

It also may be possible to use small pebbles in flibe in the core with a flibe+thorium blanket, and later replace the core with flibe+U233.


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 Post subject: Re: Uranium hexafluoride
PostPosted: Apr 29, 2015 2:45 am 
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Torres wrote:
For me a lighly presurized UCL4-BeCl2 fuel and LiF-BeF2-THF4 blanket with graphite moderator in between and leakage detection between fluids and dump tanks in both fluids looks like a very good compromise to be an achievable technology break even reactor candidate.

There are just two problems with the idea.
A. Graphite has a low life and will add to nuclear waste.
B. Li has 7 percent neutron poison Li-6 and has to be made 99.995 percent pure Li-7. There are no facilities at present.


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 Post subject: Re: Uranium hexafluoride
PostPosted: Apr 29, 2015 9:06 am 
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There is a team that believes it possible to recycle nuclear graphite into new nuclear graphite now. It involves pulverising it (bench scale tests using a kitchen blender) and mixing it in with some new pitch and graphitising it.

So graphite is no longer a nuclear waste product assuming a graphite life shorter than that of the reactor its in. (And maybe not even then, assuming we assume the reactor programme continues beyond the first generation of plants).


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