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PostPosted: Jun 13, 2015 9:46 pm 
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I would appreciate any comments about mixtures containing di, tri, and tetrafluorides
Does the overall barns of the salt mixture go down since you have more fluorine? (difluoride, trifluoride, tetrafluoride )
Is it the overall barns of the salt mixture or is it the individual elements that dictate moderating ability?
Could a mixture of difluorides, trifluorides or tetrafluorides allow thermal breeding of thorium?

Magnesium difluoride: melting point =1263C, Boiling point =2260C, 0.063 barns (magnesium), 0.009 barns (fluorine)

Bismuth trifluoride: melting point = 649C, boiling point = 900C, 0.032 barns (bismuth), 0.009 barns (fluorine)

Lithium-7 fluoride: melting point =845C, boiling point = 1676C, 0.033 barns (lithium), 0.009 barns (fluorine)

Zirconium tetrafluoride: melting point= 910C, boiling point= ?, 0.18 barns (zirconium). 0.009 barns (fluorine)

Beryllium difluoride: melting point= 554C, boiling point 1169C, 0.010 barns (beryllium), 0.009 barns (fluorine)


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PostPosted: Jun 13, 2015 10:45 pm 
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For a long time I looked at MgF2 as a replacement for BeF2, but the melting temperatures of all the mixtures that contained MgF2 were too high.

BiF3 is too unstable in a fluoride salt mixture and will attack the structural alloy of the reactor.

The others you mention have been successfully used in the MSRE.


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PostPosted: Jun 18, 2015 10:18 am 
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Thanks Kirk,
I was interested in using Magnesium difluoride as a solid moderator/structural element in a MSR. I am worried that it would decrease thermal breeding too much to be useful.
My thought was to form the fuel channels, blanket, primary and secondary coolant loops, and heat exchangers from magnesium difluoride as a monolithic structure. Since it is a solid at the operating temperature of most MSR designs it can provide support to thin walled structures.
I would think that you would have to seperate the Magnesium Difluoride from the Flibe to prevent elevation in the melting point of the primary circuit, so the channels should be lined with a thin alloy coating or cvd layer of silicon carbide or pyrocarbon.
The lining may be able to be formed at the same time you make the monolith. Make the reactor channels out of salt, coat with alloy or other coating, form magnesium difluoride around form, dissolve the form with water (solvent), continue to run water(solvent) in the channels to make sure that there are no leaks, then dry and fill with Flibe. Since the monolith is manufactured on a form that is also made of salt you can compress the salt to get low defects in the monolith.
If you have a situation with an unexpected temperature spike, the temperature spike would melt Flibe containment, expose the magnesium difluoride, which will dilute the Flibe, raise the melting point and stop the chain reaction via dilution. Should add an additional layer of safety.
There goes that patent.


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PostPosted: Jun 18, 2015 10:38 am 
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The monolith of MgF would not have to be just salt. Graphite powder and MgF could also form the monolith if better moderation is needed around the core(may be able to absorb graphite swell since salts can "flow" under pressure). Carbon fibers could add some strength if needed. But may be easier to get NRC to approve the idea if it is a single molecule.


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PostPosted: Jun 18, 2015 12:17 pm 
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I would think that salts would not be good in tensile strength. So I would think that you would need to combine it with Graphite fiber like suggested above. It could be layered up on a form like when making an airplane wing and then fused together like is done with thermal-plastics composites in a very high temperature (possible pressurized) kiln. Or perhaps in a set of forms/mold in a vacuum kiln. Pump out the air, heat to the melting temp of the salt and allow enough time for the salt to wet all of the fiber. Then back fill with Helium to compress any voids.

This could give you the structural strength of the carbon fiber and the chemical and nuclear properties of the salt. I fear that the salts alone will flow under the heat, pressure and forces, even below their melting point. But the combo could be a winner.


Last edited by michael.runyan on Jun 18, 2015 1:38 pm, edited 1 time in total.

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PostPosted: Jun 18, 2015 1:37 pm 
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Michael,
I agree, I would use the higher melting point fluoride salts as a base for a salt/fiber composite if needed for strength. Additionally, the composition of the salts could be changed near the ultimate reactor vessel (pool type) to decrease moderating ability or to act as a reflector to prolong the life of the reactor vessel.
Use of salts as structure may allow some high melting temperature salts to be included as reflectors. A solid block of salt inside a sealed reactor vessel solves some of the issues with pool type reactors with regards to earthquakes, while still giving you thermal mass to ride out a loss of integrity in the primary loop.


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PostPosted: Jun 18, 2015 2:08 pm 
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Michael,
Since the structure would be monolithic and constrained by the reactor vessel many of the forces would be compression rather than tensile. Adding additional mass of salt above the primary reactor loops would add additional compressive forces if needed to balance any forces within the loops.


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PostPosted: Jun 18, 2015 7:17 pm 
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Kirk,
Sorry, I got excited hearing that you had looked at MgF and tied my secondary idea with my first post. I had intended to post the use of high temperature fluorides as structural materials in a separate post. Since it doesn't really go with the title, feel free to fork to a separate post if you think it needs to be separated.
Mike


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PostPosted: Jun 19, 2015 1:07 pm 
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michaelw wrote:
Michael,
Since the structure would be monolithic and constrained by the reactor vessel many of the forces would be compression rather than tensile. Adding additional mass of salt above the primary reactor loops would add additional compressive forces if needed to balance any forces within the loops.

Michael W.;
I would be very careful about saying “only in compression”, that is a very rare condition in structural members, especially when you start looking that the forces within the member. Example, take a cube and put a weight on top of it. We would like to say that the cube is in compression, but if I look at the cube itself, parts of the cube are in tension, the parts holding the shape of the cube. If the cube was made of wet sand, then the sand cube would crush, because it has very little tensile strength (the surface tension of the water). A quick mental test of “only in compression” would be “Could I use dry sand?” So in the cube example, the outside of the cube would fail this test, but some of the volume of the inside might pass.

I think that in the case of the salt it will manifest itself as cracks and/or it will start to flow. So, I think that the parts are all going to be composite in nature.

I was also thinking that one could incorporate “hard points” in the structure by imbedding metal (like hastelloy n) components or lattice for added support and making connection to other parts. A method often done in composite aircraft, for example.


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PostPosted: Jun 19, 2015 8:58 pm 
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Michael:
Thanks for the comments. I'm sorry for the confusion about the use of the salt to hold the reactor loops in compression. I didn't mean to imply the only forces would be compression, rather that compression via the salt would support the structural integrity of the reactor allowing for thinner cross sections to be applied to the design, similar to the use of the blanket salt at higher hydrostatic pressure places the primary salt tubes under compression.
An accurate quote of my text is "many of the forces would be compression rather than tensile", sorry it was unclear.
As i have thought about it more, a monolith may be adequate if you use a system like Terrestrial Energy and replace the core every 7 year or so and have sufficient redundancy in the heat exchangers.
Breaking the reactor into sections that can be hooked together and replaced if needed may be more practical if the reactor has a 60 year life. Each part could be supported by salt if it required it and be free standing if not required. A layer of salt near the reactor containment could contain all reactor components with only the final loop to the powerhouse outside the reactor. The thermal mass and dilution effect of the salt are nice features that would come in handy in a loss of primary containment.
I would be interested in a graphene- salt composite and its behavior under nuclear flux, but it will require graphene production to scale to viable quantities to allow for nuclear certification (or is it already certified since graphite is?). Graphene has very interesting barrier qualities when mixed with plastics( decreases gas permeability). I wonder what the tritium mobility would be in a graphene-salt composite?


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PostPosted: Jun 19, 2015 10:49 pm 
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Michael W.:
I was wondering about the issue of a suitable material to be between the core salt and the blanket salt. When you started talking about using a salt as a structural member and if coupled with even standard graphite fiber, I was thinking that this class of material could be a possible choice for core salt – blanket salt wall and possible even for heat exchangers. Assuming that the corrosion resistance, neutron transparency and structural strength are good enough.

Sorry, no idea about the Tritium mobility, but I do know that Kirk stated that he expects that the Tritium will end up in the Supercritical CO2 system where he plans to scrub it out, “because any Hydrogen in the CO2 system will be Tritium”.


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