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 Post subject: Uranium hexafluoride
PostPosted: Mar 09, 2015 6:43 pm 
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I've been musing about the potential uses of uranium hexafluoride for making a molten salt reactor, and would like to hear other's thoughts about it.

First it's important to note the very strange thing about the uranium fluorides. Normally under very high neutron flux any non-amorphous bonds will get rapidly destroyed, and what's left of the liquids will be the absolute lowest energy state it can possibly have, which is generally a salt. Uranium fluoride doesn't have such a clear minimum though. It has both the terafluoride and hexafluoride forms, but of which are happy being that way, and if a neutron blasts into either of them, changing it back to a trifluoride or pentafluoride, it will get rapidly oxidized back to the way it was, resulting in this strange situation that even in a molten salt reactor uranium tetrafluoride will stay uranium tetrafluoride and uranium hexafluoride will stay uranium hexafluoride (somebody correct me if I'm wrong!)

Generally people don't like designing things with uranium hexafluoride because it tends to corrode everything in sight and aerosolize and poison everyone and everything in the vicinity. Those are certainly downsides, but it also has some upsides. It's the form which uranium naturally has immediately after being enriched or chemically separated, and it's significant complexity and expense to change it to anything else, and by the same token it's ready to be purified back out. There's also the general principle that lower temperature salts are better, so why not go with the lowest temperature salt there is? Uranium hexafluoride will turn into a gas at a very low temperature, which is generally viewed as a problem, but can be useful.

My basic idea is to have the core be a pressurized chamber containing uranium hexafluoride with a bunch of elemental carbon which liquid UH6 flows through at the bottom. The pressure will be provided by the UF6 itself, and the thing as a whole will hit an extraordinarily stable temperature at the point where the pressure is such that it's just barely critical. If it gets hotter, more UF6 boils and the reaction slows down. If it gets colder, more UF6 liquifies and the reaction speeds up.

To build this into a fully functioning controllable system you need several chambers with valves between them. At the bottom is the core. Immediately above that is another chamber which liquid UF6 is collected in. Above that, connected by an intake and an exhaust with heat exchange (the exhaust wraps around the intake, to minimize thermal losses), is a cooling chamber which is basically a radiator: It has a lot of surface area at the top where gas cools off and liquifies, then rains down and collects at the bottom. The upper chamber is pressurized with some other gas to allow UF6 to become a liquid, probably helium or nitrogen or argon. That gas can of course go into the lower chambers as well, but they're typically kept under much higher pressure by extra UF6.

To turn the system off you open all the valves and the UF6 boils out (or maybe a safety valve blew and it did that on its own.) In this configuration the system will naturally cool off the core because it is, in fact, a liquid cooling system and hence can keep the decay of fission by-products in the core from raising the temperature uncontrollably. Getting it started again is a bit of a dance: First you seal off the upper chamber and let it cool off until it has a pool of liquid UF6 at the bottom. Then you close the valve between the two lower chambers and let the liquid UF6 flow down. Before that liquid has a chance to boil off you close the valve to the upper chamber, then open the valve between the two lower chambers for the UF6 flow down, and finally seal off the core and everything is running again.

Removing xenon require a periodic little dance where the valve between the two lower chambers is opened to allow the xenon to go to the middle chamber, then sealed back up again and the valve to the upper chamber is opened, then the heat exchanger does its thing while the xenon escapes into the upper chamber and you re-seal that again.

There are some obvious problems with this, aside from having to keep extremely toxic UF6 at a few hundred degrees C and roughly a hundred atmospheres of pressure, which doesn't sound like such a hot idea, you have to somehow clean the fission by-products out of the core, which might be most easily done by flowing some flibe through it to dissolve all of them, which raises the question of why not just use flibe working fluid to begin with. But I think this system is at least in principle workable and I've had a lot of fun thinking about it and would be interested in hearing anyone else's thoughts on its feasibility.


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 Post subject: Re: Uranium hexafluoride
PostPosted: Mar 09, 2015 7:38 pm 
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Better still, go for a ThCl4-233UCl4 fuel. The fissile 233U is liquid from 590-791 C. It will boil off into vapor space if and when things get hot. Use 37Cl to reduce other nuclear activity.


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 Post subject: Re: Uranium hexafluoride
PostPosted: Mar 09, 2015 9:34 pm 
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Not sure what you're proposing. Thorium should only be in the blanket, not the core, and both thorium tetrachloride and uranium tetrachloride have high melting points even at standard pressure.


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 Post subject: Re: Uranium hexafluoride
PostPosted: Mar 10, 2015 2:57 pm 
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One thing I worry about with these reactors are "pressurization transients" where you suddenly increase pressure that then turns UF6 gas to a liquid and then goes supercritical. The "evaporation" argument doesn't fly because we have a fixed volume. Can't let the FPs just float into the environment!

One advantage of a liquid is that its incompressible, it won't care about the operating pressure. Liquids are also great coolants whereas gas are insulators.

Hex reactors are nasty. Forget it.


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 Post subject: Re: Uranium hexafluoride
PostPosted: Mar 10, 2015 3:36 pm 
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A UF6 reactor would have to be supercritical (in the fluid sense, not w.r.t neutron multiplication) anyways - Tc = 503 K (230°C). Reference


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 Post subject: Re: Uranium hexafluoride
PostPosted: Mar 11, 2015 2:58 am 
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Titanium48 wrote:
A UF6 reactor would have to be supercritical (in the fluid sense, not w.r.t neutron multiplication) anyways - Tc = 503 K (230°C). Reference


Very interesting, thanks.

So are we looking at operating pressures well above 50 bar then? If the supercritical pressure is 46 bar.

Not sure if this solves anything. Actually that pun may be a problem - supercritical UF6 has to be a good solvent for reactor vessels and heat exchangers. We can't make everything out of carbon and carbon itself is likely attacked at this redox window.

Supercritical fluids (or gasses if you will) are actually quite compressible. They have gas like and liquid like properties, mostly like a gas physically and more like a liquid chemically. There may still be a pressure transient problem.

But I must admit these reactors are academically fascinating things. Scary and impractical, but fun.


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 Post subject: Re: Uranium hexafluoride
PostPosted: Mar 13, 2015 8:02 pm 
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Titanium48 wrote:
A UF6 reactor would have to be supercritical (in the fluid sense, not w.r.t neutron multiplication) anyways - Tc = 503 K (230°C). Reference


Ooooh that explains the problem. Leads to another idea though, since the really super efficient CO2-based generator is based on supercritical CO2, and UF6 is of course much heavier than that, maybe the working fluid could run directly through the generator, leading to very high efficiency in the direct conversion and no need for a heat exchange loop in the middle.

I'm of course kidding... sort of.


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 Post subject: Re: Uranium hexafluoride
PostPosted: Mar 14, 2015 2:29 am 
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Bram wrote:
Titanium48 wrote:
A UF6 reactor would have to be supercritical (in the fluid sense, not w.r.t neutron multiplication) anyways - Tc = 503 K (230°C). Reference


Ooooh that explains the problem. Leads to another idea though, since the really super efficient CO2-based generator is based on supercritical CO2, and UF6 is of course much heavier than that, maybe the working fluid could run directly through the generator, leading to very high efficiency in the direct conversion and no need for a heat exchange loop in the middle.

I'm of course kidding... sort of.


I hope you're kidding because if not, that would be a quick way to tell the regulator, "we're not really serious, we're just having fun here, don't bother reviewing our design". Even without a regulator this concept would be totally unworkable because of the very high pressure of the CO2 cycle and carryover of fission products and other nasties into the turbine-generator.

Direct cycle works for BWR because of a combination of favorable features... ultra low leak cladding, steam separators and dryers removing solid and liquid stuff that does get out of the cladding, and ease of demineralizing to clean up the coolant.


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 Post subject: Re: Uranium hexafluoride
PostPosted: Mar 14, 2015 4:08 am 
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Bram wrote:
Not sure what you're proposing. Thorium should only be in the blanket, not the core, and both thorium tetrachloride and uranium tetrachloride have high melting points even at standard pressure.

UCl4 melts at 590C. ThCl4-UCl4 eutectic would be lower melting. Additional 'thinners' could be added if requited.
Thorium tetrachloride also melts at 770C unmixed. They could also be dissolved in salts. Silicates are safe in chlorides but not in the fluorides.


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 Post subject: Re: Uranium hexafluoride
PostPosted: Mar 16, 2015 12:02 am 
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jagdish wrote:
Bram wrote:
Not sure what you're proposing. Thorium should only be in the blanket, not the core, and both thorium tetrachloride and uranium tetrachloride have high melting points even at standard pressure.

UCl4 melts at 590C. ThCl4-UCl4 eutectic would be lower melting. Additional 'thinners' could be added if requited.
Thorium tetrachloride also melts at 770C unmixed. They could also be dissolved in salts. Silicates are safe in chlorides but not in the fluorides.


UCl4 has been seriously proposed for making a fast spectrum reactor to burn natural uranium or lightly enriched. It probably is much easier to handle than liquid sodium. But it's a very different animal than what I proposed at the beginning.


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 Post subject: Re: Uranium hexafluoride
PostPosted: Mar 16, 2015 12:51 am 
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Cyril R wrote:
Bram wrote:
Titanium48 wrote:
A UF6 reactor would have to be supercritical (in the fluid sense, not w.r.t neutron multiplication) anyways - Tc = 503 K (230°C). Reference


Ooooh that explains the problem. Leads to another idea though, since the really super efficient CO2-based generator is based on supercritical CO2, and UF6 is of course much heavier than that, maybe the working fluid could run directly through the generator, leading to very high efficiency in the direct conversion and no need for a heat exchange loop in the middle.

I'm of course kidding... sort of.


I hope you're kidding because if not, that would be a quick way to tell the regulator, "we're not really serious, we're just having fun here, don't bother reviewing our design". Even without a regulator this concept would be totally unworkable because of the very high pressure of the CO2 cycle and carryover of fission products and other nasties into the turbine-generator.


Well this is a forums post, not a serious proposal. And yes, having a radioactive substance complete with fission by-products running directly through a high pressure turbine isn't such a hot idea. But in all seriousness hexafluorides are heavy low-temperature gases, and might be useful for closed cycle turbines. This paper seems to agree:

http://users.ugent.be/~mvbelleg/literat ... 0cycle.pdf

It mentions a bunch of possible working fluids, with carbon dioxide being the preferred candidate due mostly to familiarity. Sulfur hexafluoride and Perfluoropropane have much higher density and lower pressure at the critical point than CO2, although one wonders if they might tend to decompose at high temperatures, which might get a bit unpleasant.


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 Post subject: Re: Uranium hexafluoride
PostPosted: Mar 23, 2015 7:31 pm 
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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!?


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 Post subject: Re: Uranium hexafluoride
PostPosted: Mar 24, 2015 4:29 pm 
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Location: Newport Beach, CA
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!?


http://en.wikipedia.org/wiki/Mark_50_torpedo

Apparently the Navy built a torpedo that uses SF6 and Li to power a closed-cycle Rankine. But I think the actual working fluid is steam.


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 Post subject: Re: Uranium hexafluoride
PostPosted: Mar 24, 2015 9:06 pm 
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SF6 is a ludicrously powerful greenhouse gas though.
Which is why its use in the EU is banned for all uses other than high voltage electrical switchgear (it has an insanely high breakdown voltage).


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 Post subject: Re: Uranium hexafluoride
PostPosted: Mar 26, 2015 6:54 pm 
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Yeah, I forgot to mention the greenhouse issue. But, really, it might be an advantage. If you look at maps, the barren parts are the cold parts. Obviously the whole planet needs to be warmer. If the hot parts were a problem, they'd be barren. 8)


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