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PostPosted: Feb 04, 2014 12:46 pm 
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There is already an existing thread in the Flouride Reactor Design board.

viewtopic.php?f=3&t=4303


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PostPosted: Feb 04, 2014 1:29 pm 
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Sorry. Thanks!


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PostPosted: Feb 05, 2014 1:52 am 
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Would moderator rods that can one way vent, and are inserted into the molten salt pool from above in the cover gas plenum space such that the rod top vents are out of the salt, be a work around to pressurization of a sealed rod? Though I suppose having hydrogen gas collecting in the plenum might be a bad idea too...


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PostPosted: Feb 05, 2014 5:33 am 
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Wouldn't magnesium hydride be more stable in a suitable casing? It could also carry a higher proportion of hydrogen than Zr.
Moderator aside, how would you get rid of fission products poisons in salt solution? ThCl4-233UCl4 salts, if used with 37Cl, can be distilled for recovery of fissile and fertile fuels.
Used fuel can, however, only be used in a U-239Pu cycle. It will need chloride/fluoride volatility and electrolysis in pyroprocessing.


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PostPosted: Feb 05, 2014 7:59 am 
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Interesting idea, but the melting point appears too low (650C).

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


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PostPosted: Feb 07, 2014 8:09 pm 
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David wrote:
...However for their claims of complete burn up while using a makeup feed of 1.8% LEU or SNF, they will need a conversion ratio of at least 0.98. ...

The other funny thing is that in fig 10, they show the U235 and Pu239 concentrations staying roughly equal to the initial values over 100 years, at 1% and 0.6% respectively.

For complete burn-up, they need to transmute and fission 54 atoms of U238 for every atom of U235 fissioned. The DMSR paper (ORNL-TM-7207) shows Pu239 having 4.5 times higher neutron absorption than U235, so I would expect the Pu239 concentration to be roughly 1%*54/4.5 = 12% compared to 1% U235.

It is hard to see how their superior moderation could make that much of a difference.

It would be believable if they were claiming near complete transuranic (TRU) burn-up, and discarding and replacing the uranium during reprocessing. Of course this would mean their claimed ore & SNF utilization efficiency was greatly overstated.

The DMSR paper also shows the neutrons per absorption for Pu239 to be only 1.79, which is again inconsistent with their claimed burn-up (and implied conversion ratio). High neutron yield with Pu required lots of fast fissions, which requires low moderation and high fissile inventory.


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PostPosted: Feb 07, 2014 10:54 pm 
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Nathan2go wrote:
High neutron yield with Pu required lots of fast fissions, which requires low moderation and high fissile inventory.

....or a bi-modal, thermal/fast spectrum, as shown in their Fig. 8 sim results:


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PostPosted: Feb 08, 2014 2:48 am 
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Cyril R wrote:
Interesting idea, but the melting point appears too low (650C).

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

A Zirconium capsule could retain everything inside.


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PostPosted: Feb 08, 2014 6:39 am 
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jagdish wrote:
Cyril R wrote:
Interesting idea, but the melting point appears too low (650C).

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

A Zirconium capsule could retain everything inside.


It wouldn't. Molten magnesium hydride is extremely corrosive (embrittling) to zirconium, and indeed any sort of metal.

Apart from that, zirconium is itself not compatible with fuel salts.

There are no metal claddings that will work with molten hydrides on one side and molten fuel salt on the other. There may in fact not be any ceramic composites even.


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PostPosted: Feb 09, 2014 11:32 am 
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jaro wrote:
"High neutron yield with Pu required lots of fast fissions, which requires low moderation and high fissile inventory" (Nathan)
....or a bi-modal, thermal/fast spectrum, as shown in their Fig. 8 sim results:

I think the big win with the bi-modal spectrum isn't the large amount of fast fissions (the absorption is 100x higher at thermal, so the fast peak is still pretty small as far a fissions).

The important thing is apparently the dip in neutron flux at middle energies, where all of those resonance absorptions happen. Here is a plot from "Aim High":
Attachment:
AimHigh-p21_neutron_production-Pu.png
AimHigh-p21_neutron_production-Pu.png [ 16.32 KiB | Viewed 1336 times ]

In the thermal region, neutron production per absorption is actually above 2. It's not obvious from this graph what the average neutron production is across the resonance region, but again in the DMSR paper, table 16 seems to be saying it is only 1/3 of the production in the thermal region.

So maybe it is possible to make a near-breeder with the plutonium cycle, but it still seems to me that the plutonium concentration must be much higher than that of the make-up fissile, U235.


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PostPosted: Feb 14, 2014 11:11 am 
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http://nextbigfuture.com/2014/02/transa ... -salt.html

And:

http://transatomicpower.com/white_paper ... _Paper.pdf

Critical design choices:

Salt:

'The vast majority of past work on molten salt reactors has used a lithium-beryllium-fluoride salt, called FLiBe.
Transatomic Power’s reactor instead uses LiF-based fuel salt. The salt dissolves a fuel consisting primarily of uranium and a heavy metal mix similar to typical spent nuclear fuel. One known drawback of this salt is that its melting point is higher than that of FLiBe, and thus the primary loop piping must be carefully designed to avoid cold spots that could restrict flow and induce freezing in the salt. We chose to accept this engineering challenge for two reasons.
The first reason is that FLiBe contains beryllium. A portion of the population is hypersensitive to this material, and even trace amounts of beryllium can induce the chronic lung disease berylliosis in these people.
We therefore choose a fuel salt that does not contain beryllium.
The second reason is that LiF-(Heavy metal)F4 is capable of containing a higher concentration of uranium than FLiBe salt.
Therefore,each liter of our fuel salt has a higher amount of uranium than would be possible using FLiBe. This salt composition thus helps us operate using low-enriched fuels, as well as spent nuclear fuel.' (page 9)

Moderator:

Zirconium hydride’s high hydrogen density allows it to achieve the same amount of thermalization as graphite in a much smaller volume.
The zirconium hydride moderator therefore allows us to significantly reduce the reactor core volume, thereby reducing the size and cost of the reactor vessel and the volume of fuel salt.
In Transatomic Power’s reactor, only about 50% of the core volume is moderator, which gives us room for five times more fuel salt in the same size core, allowing better performance, reduced enrichment, and lower cost.
Co-optimizing the core geometry with the new moderator and new salt formulation, we can drop the minimum
fuel enrichment level from 33% to 1.8%.
This efficiency also enables us to consume SNF. (page 11)

Fuel:

The TAP reactor’s primary innovations–a novel combination of moderator and fuel salt–can also be adapted for use with thorium. Transatomic Power believes that the thorium fuel cycle holds theoretical advantages over uranium in the long run because of its generally shorter half-life waste, its minimization of plutonium from the fuel cycle, and its greater natural supply. However, we chose to start with uranium for several reasons: (1) there is a great deal of spent nuclear fuel, and we want to harness its energy while reducing the risk of onsite SNF storage;(2) the industry already has a commercial fuel cycle developed around uranium , which makes it cheaper to use uranium as fuel is this design; (3) we already greatly eliminate waste; and (4) we already greatly expand the energy potential of existing uranium supplies.Thorium reactors do not contain plutonium, but they do have a potential proliferation vulnerability because of the protactinium in their fuel salt.
Protactinium has a high neutron capture cross section and therefore, in most liquid thorium reactor designs, it must be removed continuously from the reactor . The process for doing this yields relatively pure protactinium, which then decays into pure U-233.
By design, the pure U-233 is sent back into the reactor where it is burned as its primary fuel.
The drawback, however, is that U-233 is a weapons-grade isotope that is much easier to trigger than plutonium.It is possible to denature the U-233 by mixing it with other uranium isotopes, or modify the design to further reduce diversion risk , but further research is required to implement these anti-proliferation measures in thorium molten salt reactors. (pg23)


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PostPosted: Feb 14, 2014 12:29 pm 
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jagdish wrote:
Wouldn't magnesium hydride be more stable ... than Zr [hydride].


Much less so. The hydrogen pressures of the zirconium hydride in TRIGA fuel and of HfH are given in http://www.webetc.info/pnc/2006-Proceed ... l00350.pdf figure 8: less than 0.1 bar at 650°C. MgH2, which has been considered as a pure hydrogen source for cars, is at 1 bar near 300°C (which is, for that purpose, too high).

_________________
--- G. R. L. Cowan, former hydrogen-energy fan
Oxygen expands around B fire, car goes


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PostPosted: Feb 14, 2014 6:50 pm 
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DaveMart wrote:
Protactinium has a high neutron capture cross section and therefore, in most liquid thorium reactor designs, it must be removed continuously from the reactor .

This especially true in the epithermal region, which a bi-modal spectrum reactor seeks to minimize.....


Attachments:
Th232_Pa233_U233_(n,f)_(n,g)_epi_LINEAR.gif
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PostPosted: Feb 14, 2014 7:42 pm 
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Cyril R wrote:
jagdish wrote:
Cyril R wrote:
Interesting idea, but the melting point appears too low (650C).

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

A Zirconium capsule could retain everything inside.


It wouldn't. Molten magnesium hydride is extremely corrosive (embrittling) to zirconium, and indeed any sort of metal.

Apart from that, zirconium is itself not compatible with fuel salts.

There are no metal claddings that will work with molten hydrides on one side and molten fuel salt on the other. There may in fact not be any ceramic composites even.

How about Zirconium Carbide? You could have zirconium for containment and carbide for corrosion.


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PostPosted: Feb 16, 2014 4:46 pm 
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I'm surprised that people are arguing about the thermal failure temperature of the moderator rods in the TA concept. It's an MSR, and overtemp would just result in the freeze valve opening and draining the reactor to the dump tank away from the moderator. It would be relatively simple to make the freeze valve cooling system adequate to hold it frozen against heat conduction, but inadequate to halt a flow of hot salt. Picking the maximum temperature setting then becomes an exercise in calculating the heat conduction vs. temperature and setting the rating of the valve's cooling system.


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