Energy From Thorium Discussion Forum

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PostPosted: Sep 18, 2011 5:41 am 
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As a variation, I suggest Th-Pu-Be carbide fuel to ensure moderation and carbon or SiC cladding. If it can be done in pebbles, it should be possible in whatever fuel configuration.
FNaBe could be the coolant as suggested.


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PostPosted: Sep 18, 2011 5:54 am 
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jagdish wrote:
As a variation, I suggest Th-Pu-Be carbide fuel to ensure moderation and carbon or SiC cladding. If it can be done in pebbles, it should be possible in whatever fuel configuration.
FNaBe could be the coolant as suggested.


The main issue with carbide fuel in this reactor would be, how are you going to reprocess it? With metal fuel it is easier. Metal fuel also gets the highest thermal conductivity and heavy metal density which combines to give an excellent fast fission bonus.


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PostPosted: Oct 08, 2011 6:26 am 
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Per Peterson's AHTR design uses a very efficient heat exchanger. I was quite impressed by the low temperature drop across the heat exchangers.

It appears that for a salt cooled reactor that has no size restriction on the heat exchanger, a drop of only 14 degrees Celcius is possible over the primary, and another 15 degrees Celcius over the secondary (to helium).

This is quite impressive. Based on the above I'm guessing at the following drops for my salt cooled fuel rod design.

Let's start backwards on what is needed in temperature.

To make steam at 540 C is our goal.

With a typical 25 C drop over a modern steam generator we're into 565 Celcius in the NaNO3-KNO3 loop.

Then a drop of 15 Celcius over the nitrate to fluoride secondary loop we're at 580 Celcius for the secondary loop.

And that means with another 15 Celcius drop in the primary it is 595 Celcius peak primary coolant salt temperature.

The primary salt inlet to the fuel rods then becomes about 450-500 Celcius.

NaF-BeF2 melting point (57-43 mole%): 340 C. So this works just dandee.

This is an advantage a salt cooled design has over a salt fuelled reactor. Operating temperature of 595 Celcius allows all sorts of cheaper and easier to work with stainless steels to be used for the pool and heat exchangers.


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PostPosted: Oct 08, 2011 7:16 am 
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According to this study of coolant salts:

http://www.ornl.gov/~webworks/cppr/y2006/rpt/124584.pdf

About the only problem is the viscosity of NaF-BeF2 eutectic. Roughly 20 pa/s @ 500 C and 12 pa/s @ 600 C. Somewhat high, but not unreasonable (will probably require 2-3x more pumping power than the AHTR FLiBe 600-700 C but still much less pumping power than PWRs).


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PostPosted: Jan 04, 2012 6:15 am 
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Here's an inpile test of the triplex SiC cladding material:

http://www.ms.ornl.gov/IEA_workshop/pdf ... R_Clad.pdf

Looks like excellent performance. Oddly enough the optimal operating temperature is around 1000 degrees Celsius. The material loses very little strength with temperature, which means that the optimal operating temperature is near self annealing temperature (which is very hit). That's a big advantage compared to zircalloy, whose strength is too low to be operated at self annealing temperature, resulting in damage not healing out.


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PostPosted: Jan 05, 2012 6:03 am 
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Nice ! ....thanks !


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PostPosted: Jan 05, 2012 2:10 pm 
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This new information on point defect swelling got me thinking about the operating temperature. If the ideal temperature for the SiC is 1000 degrees Celsius, and we need SiC in core load bearing members anyhow, then we may need to up the operating temperature a bit. In fact considering this new constraint, a ~900 degrees Celsius outlet temperature would be optimal. The SiC would very much like this temperature, though the vessel would have to be made either of SiC itself or internally insulated alloy.

Think that's crazy? Actually it's being done all the time in aluminium production. They use even higher temperatures fluoride baths and also have to deal with crazy electrical currents. The reactor is surprisingly similar to such an aluminium production cell, and is even simpler in many ways (no crazy currents or molten aluminium to suck out). I need to talk to someone who works in such an aluminium electrolysis plant. They'd know how to build this reactor.

http://en.wikipedia.org/wiki/Hall-H%C3%A9roult_process

Jaro might be keen to point out, that the Hall-Hérault pumps out the liquid aluminium using... siphons. And, that a fume hood is used to remove the nasties.


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PostPosted: Jan 06, 2012 6:43 am 
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Actually, Jaro likes the high operating temps with SiC structures, to permit the use of carrier-free fuel salt (ie. UF4/UF3 eutectic).
Another possibility might be molten U metal - possibly with a slightly lowered m.p. in the case of U-Si eutectic....


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PostPosted: Jan 06, 2012 6:59 am 
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UF4/UF3 eutectic has the problem of too high a UF3 ratio, causing chemical problems of extreme reduction. What's more, high UF3 isn't stable at these elevated temperatures. Higher UF4 in the mixture is too high melting and both UF4/UF3 and UF4 have unfavorable heat transfer properties compared to the lower Z fluoride salts. I think you'll want LiF-UF4, with 27% UF4, or NaF-UF4, with 28% UF4, for a workable power reactor.

Molten U metal is neutronically attractive but a materials and fire hazard nightmare. If the U is solid and enclosed in SiC fuel rods, in a deep below grade pool with lots of NaF-BeF2 on top of it, that seems very safe and workable to me.


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PostPosted: Jan 06, 2012 7:11 am 
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The older document (from 2000) on temperature windows for fusion reactor materials has more to say on SiC irradiation:

http://www.fusion.ucla.edu/ITER-TBM/Doc ... window.pdf

Quote:
Void swelling is considered to be of particular
importance for SiC (and also Cu alloys, which
were shown to be unattractive fusion structural
materials in Section 3). Fig. 4 summarizes the
swelling data for irradiated monolithic SiC [64–
67]. Early irradiation studies by Price and
coworkers suggested that SiC had negligible swelling up to 1100°C, and peak void swelling
was reported to occur at 1300–1500°C [68]. In
contrast, two recent studies have reported significant
swelling in SiC-based materials at an irradiation
temperature near 1000°C [64,66]. Further
work is needed to accurately establish the temperature
range for void swelling in SiC. The maximum
operating temperature for SiC:SiC due to
void swelling concerns is taken to be 990940°C,
pending resolution of the apparent conflict between
the previous accepted void swelling trend
and the two recent studies. Radiation-induced
matrix microcracking and strength degradation
might impose further limits on this maximum
operating temperature, but irradiation data on
appropriate advanced SiC:SiC composites are not
yet available.


Looking at figure 4, 850-900 Celsius looks to be an optimal temperature for SiC. This would require coolant temperatures in excess of 800 degrees Celsius.

The newer studies show that the triplex SiC cladding has sufficient strength after irradiation. If it's good enough for a PWR then it will certainly be good enough for a fluoride salt.


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PostPosted: Jan 07, 2012 2:02 pm 
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SiC cladding is basically required for reactor vessel and piping. A coolant could be cleaned by batches if the fuel has only thorium casing..


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PostPosted: Jan 08, 2012 3:57 am 
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jagdish wrote:
SiC cladding is basically required for reactor vessel and piping.


I'm not sure if it would be possible to fabricate large vessels from SiC. But we don't need to, since the reactor wall only sees the cold leg temperature of about 700 degrees Celsius. The cladding, core internals, riser, pump and heat exchanger tubes have to be preferably all made out of SiC composites. Though everything outside the core but operating at the hot leg temperature could also be made out of some superalloy, as there are no neutrons there. For example the riser has negligible stresses other than its own weight, could easily be a metal.


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PostPosted: Jan 10, 2012 4:17 am 
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SiC is only required for cladding or enamel to avoid corrosion and the metal could take the stresses. Coating of even large areas should be feasible.


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PostPosted: Jan 10, 2012 5:25 am 
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jagdish wrote:
SiC is only required for cladding or enamel to avoid corrosion and the metal could take the stresses. Coating of even large areas should be feasible.


Certainly, coatings can be used to improve corrosion resistance of alloys out of flux, where using composites isn't allowed or impractical. In particular, the vessel, heat exchanger, and pump internals.

I’m thinking to combine this idea with internally insulating the vessel. Think low thermal conductivity graphite plates lining the vessel interior. This would reduce the temperature of the vessel alloy during normal operation and importantly also during any accident. Even during a severe accident, the vessel does not heat up. The vessel could be passively cooled by the hot cell (ie ducted open to the hot cell). The graphite insulating plates could be laced with boron-10 to stop any neutrons hitting the vessel, increasing vessel life.


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PostPosted: Jan 10, 2012 9:53 am 
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Properties of thorium metal from ORNL:

http://www.ornl.gov/info/reports/1965/3445601336962.pdf

Clearly thorium metal is greatly superiour to uranium as a fast reactor fuel. Thorium can run much hotter without swelling and has a higher thermal conductivity. Unlike U-Pu it also achieves negative coolant void in a fast reactor as we can see here:

http://www.osti.gov/energycitations/pro ... id=6747565


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