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 Post subject: Re: Uranium hexafluoride
PostPosted: Apr 30, 2015 10:44 pm 
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jagdish wrote:
A. Graphite has a low life and will add to nuclear waste.


Glassy carbon is both stronger than graphite and amorphous, so it might handle neutron flux much better and last a lot longer.


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 Post subject: Re: Uranium hexafluoride
PostPosted: May 01, 2015 4:58 am 
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I wonder why harder carbides like Be2C have not been tried. It could work up to 2000 degrees K.


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 Post subject: Re: Uranium hexafluoride
PostPosted: May 01, 2015 8:57 am 
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jagdish wrote:
I wonder why harder carbides like Be2C have not been tried. It could work up to 2000 degrees K.

Wouldn't that dissolve in FLiBe?

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 Post subject: Re: Uranium hexafluoride
PostPosted: May 01, 2015 9:29 am 
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KitemanSA wrote:
jagdish wrote:
I wonder why harder carbides like Be2C have not been tried. It could work up to 2000 degrees K.

Wouldn't that dissolve in FLiBe?


Yes, it sure will.


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 Post subject: Re: Uranium hexafluoride
PostPosted: May 02, 2015 9:37 pm 
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Getting back to the topic of uranium hexafluoride, I have another crazy idea.

It turns out that Neon has an extraordinarily low thermal neutron cross section, even without isotopic separation, which is quite a funny coincidence, given that it naturally is composed of three different isotopes. It also happens to be a noble gas, and not only that but it becomes supercritical at the extraordinarily low pressure of 26.3 atmospheres. On top of that, its density in fluid state is roughly comparable to carbon. The end result is that Neon has the density to possibly be a great moderator, if it fact it works well as a moderator, I'm not sure what values to read to guess at that.

So my new crazy idea is that you could have a thermal spectrum reactor with a supercritical neon and uranium-233 hexafluoride core, and some kind of thorium blanket. It could have a very simple and reliable safety mechanism with a valve which let some of the fluid go into another chamber if the pressure got too high.

The main potential advantage of this design would be that it might lead to a smaller and more compact reactor *if* neon can moderate better than carbon, which it probably can't, but it would be interesting to find out.


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 Post subject: Re: Uranium hexafluoride
PostPosted: May 05, 2015 12:57 am 
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Helium has a critical pressure of just over 2 atmospheres, has zero neutron cross-section until you get to really high energies, and is 1/5 of the mass and is thus a much better moderator. With either helium or neon though, you will need pressures far higher than 26 atmospheres in order to get reasonable density.


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 Post subject: Re: Uranium hexafluoride
PostPosted: May 06, 2015 1:25 am 
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Titanium48 wrote:
Helium has a critical pressure of just over 2 atmospheres, has zero neutron cross-section until you get to really high energies, and is 1/5 of the mass and is thus a much better moderator. With either helium or neon though, you will need pressures far higher than 26 atmospheres in order to get reasonable density.


Yes helium is appealing in some ways, but its density as a liquid is much less than that of neon, even when factoring in their atomic weights, which would seem to indicate that it's at a disadvantage in that regard. I can't find anything which gives good information about how compression works with supercritical fluids, although I've been assuming that the amount of pressure you need to maintain a particular density drops off, perhaps dramatically, when you go from a gas to a fluid. That could be horribly wrong though. It results in the counterintuitive notion that in some circumstances the pressure drops when the temperature goes up.

Is there anything which gives a good chart of how good of a moderator different isotopes are? I've been using nudat2 for thermal neutron cross section, which mostly follows the pattern that larger things have higher cross sections, but there are some notable massive exceptions, for example krypton-86, which has hardly any thermal neutron cross section for no apparent reason. The two data points I know for how good of a moderator something is are that hydrogen is a great moderator, while deuterium is almost invisible to neutrons. Those don't seem to indicate any coherent pattern at all.


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 Post subject: Re: Uranium hexafluoride
PostPosted: May 06, 2015 1:57 am 
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Both helium and neon will display nearly ideal gas behavior up to fairly high pressures, and then the density will begin increase more slowly than the pressure as the volume fraction occupied by atoms becomes significant. The substantial increase in density with modest increases in pressure that can occur in supercritical fluids only happens near the critical temperature - that is 647 K for water, but only 44 K for neon and 5 K for helium.


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 Post subject: Re: Uranium hexafluoride
PostPosted: May 12, 2015 7:06 pm 
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After going over this stuff with Kirk, I now understand moderators a lot better. For the properties needed for a gas here - critical point around 300 C or so and a good moderator - hydrazine fits the bill. The problem is that hydrazine looks an awful lot like water from a nuclear standpoint, and the whole thing looks like a candu but worse. Candus are really amazing contraptions, they're clearly superior to light water reactors from a design standpoint, although they don't get the inherent physical and chemical stability and thorium breeding of molten salt reactors. Being able to operate off of natural uranium is quite a trick though.

For supercritical turbines which don't need to be able to handle lots of neutron flux, it looks like taking iodine and mixing in some argon to get the critical point down a bit should easily beat carbon dioxide for density. If money is no object you can use krypton instead of argon, and if money is REALLY no object you can use xenon, although that brings very little marginal benefit. The question, of course, is whether steel can handle exposure to iodine at high temperature and pressure.


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 Post subject: Re: Uranium hexafluoride
PostPosted: May 14, 2015 12:50 am 
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Okay, I was 'mostly' wrong about hydrazine. The problem with Nitrogen-14 is that it has a high n,p cross section, and I was just looking at the n,y cross section. (This is a problem in a lot of the charts online. It's almost like there aren't very many people doing nuclear engineering on a daily basis.) Nitrogen-15 has a very low cross section, at least for n,y but getting that isotopically separated for something resembling water would be a bit ridiculous. Maybe not completely though - water has significantly better moderation than H2, so maybe the clumping together of n2h4 would make it act somewhat better than two separate h2o, which it's chemically similar to in terms of density and other physical properties.

There is a funny thing trick which might be useful though. Helium has an extremely low critical point, both in terms of temperature and pressure, so if you combine helium and heavy water you can make the combination have a critical point at a low temperature and pressure, making it potentially suitable as a moderator. It will of course have somewhat lower density than liquid heavy water, although possibly need less cladding because it's kept at a higher temperature. Since heavy water is a much better moderator than carbon, this trick might be useful for moderating an MSR.

The helium and water mixture has a bunch of other potential uses as well. It's useful enough to make me suspicious that here's some gotcha which makes it not work as expected. The very low critical point of helium is bizarre, because it's the most nonreactive of all the noble gases normally but suddenly likes being a fluid at higher pressures.


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 Post subject: Re: Uranium hexafluoride
PostPosted: May 16, 2015 9:10 pm 
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Another realization (and sorry for having a conversation with myself here): Hydrogen is almost as good at having a low pressure critical point, and its temperature is effectively the same when averaged out, and it is of course a vastly better moderator than Helium, so it would be better to use a combination of heavy hydrogen and heavy water. That would make a good moderator, with the obvious problem that it requires cladding, and cladding which can take a lot of pressure at that, and is really just trying to avoid having to keep a caldera cool, which isn't that much of a problem to begin with.


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