Energy From Thorium Discussion Forum

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PostPosted: Dec 24, 2008 6:28 am 
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Hello, All !

One simple question (I suppose, at least for you)
I read in many docs about thorium molten salts reactors (Kirk' s slides, for example) that with LFTR we can produce more than 11 TWh of net electricity per tonn of thorium (only heavy metal fraction, I guess). It' s a very impressive number, but where does exactly come from? For example, a single gram of uranium 235 (I don't know about 233) produces, if completely fissionated, 0,95 MWday of thermal energy. Moreover, the fission capture ratio in a thermal spectrum is 0,9 for 233 and 0,8 for 235, so a great fraction of fuel can be burned but we know not 100%

I suspect, but I could be wrong, that you consider that the fraction of uranium 233 not fissionated is easily converted in 234 and 235 afterthat, which is again a fissile element; so, at last the burned fraction is (1- (1-0,9)*(1-0,8)) ~ 0,98 which is the most available number.

So, it needs a net thermal efficiency of at least 50% (if I understand correctly, easily achievable in molten salt reactor with gas turbines energy conversion) to produce > 11 TWh of electricity from a single tonn of ore thorium, assuming a production of 0,95 MWd of heat per gram of 233 (maybe you know the exact number, I'd appreciate very much to know it)

Thanx very much in advance


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PostPosted: Dec 24, 2008 7:37 am 
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Well, work from rough equivalence and the fact that 1 metric tonne = 1 Mg

Thus, complete burnup produces 0.95 TWd per tonne - as 24 h = 1 d, that comes out to 22.8 TWh. After conversion to electricity, 48.2% efficiency would be required to reach the figure you've quoted, 49.2% after that 2% loss to higher isotopes of U.

The working rule of thumb around here seems to be 1 t thorium = 1 GWe-a of juice, or 8.766 TWh, requiring 38.4% and 39.2% efficiencies before and after 2% losses to higher isotopes.

Edited to fix unit muckup.

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PostPosted: Dec 24, 2008 9:07 am 
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Alex P wrote:
So, it needs a net thermal efficiency of at least 50% (if I understand correctly, easily achievable in molten salt reactor with gas turbines energy conversion) to produce > 11 TWh of electricity from a single tonn of ore thorium, assuming a production of 0,95 MWd of heat per gram of 233 (maybe you know the exact number, I'd appreciate very much to know it)
Fissioning U-233 and U-235 release about the same amount of energy. See http://en.wikipedia.org/wiki/Talk:Burnu ... _burnup.3F for a bunch of links and numbers.


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PostPosted: Dec 24, 2008 11:52 am 
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For near term developments we generally assume 40% efficiency thermal to electricity.
It depends on how high a temperature one wishes to run the reactor at. The molten salt is capable of going quite high (in fact higher is better) - but it is hard on the structure (reactor walls, pipes, pumps, turbines, etc.) so it comes down to developments in material science. Even at 40% efficiency LFTR is such a huge win that I think it is prudent to go for the more modest operating temperature initially and not delay deployment waiting for improvements in material science. Long way to say we should plan on 40% for now with a future goal of 50%.

Your value of 0.95 MWth-day/g seems a tad low - Lamarsh ("Introduction to Nuclear Engineering" John Lamarsh pg 89) uses 1.05 MWth-day/g. This is equivalent to 200 MeV per fission. There is a bit more energy released but in the form on neutrinos so it simply flies right through everything and contributes no thermal energy to us.

We generally presume keeping the transuranics inside a reactor until they are completely fissioned so there is no 10% or 2% loss here. This is done to reduce transuranic wastes. The waste flow is around 80 grams per year or roughly 0.008% - not much loss here. There is some discussion about stopping after u235 (either as Np237 or Pu238). If this were done then we should include the 2% loss you mentioned.


Using 1.05 MWth-day/g and 40% efficiency and complete burnup one comes to 920kg per GWe-year.


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PostPosted: Dec 24, 2008 6:01 pm 
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Welcome to the forum!

The average fission yield of U-233 is 191 MeV (excluding neutrinos), which corresponds to 20.0 TWh thermal per ton Th-232. So the electric yield is this, less unburned fuel, less thermodynamic conversion losses. LFTR has a closed fuel cycle, and it will have roughly complete fuel burnup - first factor is ~1. The second factor depends on what the heat engine is, and what temperature its heat source is (which depends on the reactor temperature). A molten fluoride-salt core will have a much higher temperature than a liquid-water moderated core, so the conversion efficiency of an LFTR could be twice as good as LWRs. 50% is an oft-repeated figure for a high-temperature Brayton cycle (Kirk wrote an applet, you can play around with parameters). 11 TWh electric / ton corresponds to 55% efficiency.


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PostPosted: Dec 25, 2008 6:43 am 
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Quote:
Welcome to the forum!


Thanks, even if I' am not really a new member, this is indeed my second post !

Quote:
Well, work from rough equivalence and the fact that 1 metric tonne = 1 Mg

Thus, complete burnup produces 0.95 TWd per tonne - as 24 h = 1 d.....Your value of 0.95 MWth-day/g seems a tad low - Lamarsh ("Introduction to Nuclear Engineering" John Lamarsh pg 89) uses 1.05 MWth-day/g. This is equivalent to 200 MeV per fission. There is a bit more energy released but in the form on neutrinos so it simply flies right through everything and contributes no thermal energy to us.


A couple of points :

First, you both didn' t consider the "alpha" factor (i.e. the fission to capture ratio), that's about 0,9 for 233 and 0,8 for 235 or 0,98 in total, if you consider uranium 235 production (and fission) for multiple neutron absorptions from thorium 232
Second, although I understand is of little importance, I don' t think that 1,05 MWd per gram, even completely fissionated, is a correct value, according to this
http://www.kayelaby.npl.co.uk/atomic_an ... 4_7_1.html
is 0,919 for 235 and 0,914 for 233 (excluding neutrinos) - by the way 200 MeV for 235 is 0,95 MWd/g....

Quote:
Fissioning U-233 and U-235 release about the same amount of energy. See http://en.wikipedia.org/wiki/Talk:Burnu ... _burnup.3F for a bunch of links and numbers.


Thanks, Bill. Contrary to my previous belief, I finally understand that the fission energy release per gram for 233 and 235 uranium isotopes is definitely almost identical

So, at last, with the figure of 0,91 MWd per gram of uranium 233, and including the correct alpha values for both 233 and 235, it takes a thermal efficiency of at least about 52% for producing more than 11 TWh/year per tonn of ore thorium....by the way, is it physically and technically possible with improved Brayton closed cycles/gas turbines and typical delta T of MSR to achieve net efficiencies in the range of 50-55% ? Just curious, which is the max temperature achievable in the hot leg in LFTR or chloride version?


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PostPosted: Dec 25, 2008 10:29 am 
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Actually activation isn't a loss in a burner/breeder like LFTR - the higher actinides are (I think?) all subsequently fissioned, or activated further to something that is fissioned (U-235, Pu-239, Am-241, etc.). Their average fission energies are a bit different, but the total error in assuming it's all U-233 should be small.

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Second, although I understand is of little importance, I don' t think that 1,05 MWd per gram, even completely fissionated, is a correct value,

I think you're right, it looks like a conversion error somewhere. As you point out, 200 MeV/nucleon is closer to 0.95 MWd/gram than 1.05. I'm using 191 MeV <-> 0.92 MWd/g. (197 MeV includes 6 MeV of neutrinos which don't contribute to reactor heat).

Google Calculator is useful for quick conversions.


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PostPosted: Dec 25, 2008 7:57 pm 
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fnord wrote:
..., 48.2% efficiency would be required to reach the figure you've quoted, 49.2% after that 2% loss to higher isotopes of U.



So I didn't consider it, did I?

I think max hot leg temps are materials-limited, Alex P.

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PostPosted: Dec 26, 2008 1:48 pm 
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fnord wrote:
fnord wrote:
..., 48.2% efficiency would be required to reach the figure you've quoted, 49.2% after that 2% loss to higher isotopes of U.

So I didn't consider it, did I?


My humble conclusion is that the exact number is slightly higher : if one tonn of thorium = 0.91 MWday, alfa factor for 233 * alfa factor for 235 = 0,1*0,2 = 0,02, that means a total 0,98 fission to capture ratio,then it takes a net thermal efficiency of at least about 51% to produce more than 11 TWh for tonn of thorium

Quote:
I think max hot leg temps are materials-limited, Alex P.


I don't really know if an efficiency of ~ 51% can realisticly reached, even with closed cycle gas turbine applications...in an other thread
http://www.energyfromthorium.com/forum/viewtopic.php?f=17&t=417
someone supposed the max hot leg temp = 1200 °C, that means, assuming the real efficiency = 2/3 of Carnot ideal efficiency = 2/3 * (1473 - 40)/1473 = 52,5 % (I supposed 40 °C for atmospheric air), slightly above the efficiency needed


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PostPosted: Dec 26, 2008 9:41 pm 
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I think the best efficiency would be the one of a supercritical steam turbine: 44%..

And the fission to capture (which is the amount of neutrons transformed into gamma's, not the amount of neutrons absorbed) ratio alfa means that alpha/(1+alpha) is the fission to total absorption ratio, which is used in the 4 factor formula for k_inf. But it also means that alpha/(1+alpha) U-233 are fission and 1/(1+alpha) are transformed into U-234.

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PostPosted: Dec 27, 2008 7:13 am 
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STG wrote:
I think the best efficiency would be the one of a supercritical steam turbine: 44%..


Yes, almost certainly, but I meant rather very efficient gas turbine Brayton (closed) cycles. I think it's not impossible with them to achieve efficiency in the range of 50-55% for typical MSR delta temps. Aside pure efficiency, this may have at least two indirect advantages, first we can use air and low cost-low dimensions dry towers for cooling , easing power plants siting; second, it' s quite easy (with pratically no or little loss in the power production) to develop low temperature thermal applications like seawater desalination and district heating (or cooling)


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PostPosted: Dec 27, 2008 9:38 am 
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Alex P wrote:
STG wrote:
I think the best efficiency would be the one of a supercritical steam turbine: 44%..

Yes, almost certainly, but I meant rather very efficient gas turbine Brayton (closed) cycles. I think it's not impossible with them to achieve efficiency in the range of 50-55% for typical MSR delta temps. Aside pure efficiency, this may have at least two indirect advantages, first we can use air and low cost-low dimensions dry towers for cooling , easing power plants siting; second, it' s quite easy (with pratically no or little loss in the power production) to develop low temperature thermal applications like seawater desalination and district heating (or cooling)


you'll always need a secondary circuit to limit the dispersion of tritium and other diffusive radioactive substances, which will always give a loss on delta T. Furthermore a lof of the MSBR original safety concepts relied on the heat rejection by water boiling, because it's the most fail safe removal method. So you'll always need a water source...

And not that I know that much about gas turbines, but I've never seen a concept that uses air. Mostly supercritical CO2 or He are foreseen as a working fluid. On the low temperature utilization I have to agree, but a lot of reactor concepts are foreseen for this...Besides I think that for some of these applications solar is as good as nuclear, as the countries which need this type of applications are normally quite sunny....

Furthermore I worry about your financial impact. Because what you might lose on constructional costs, you might gain on your safety analysis...

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PostPosted: Dec 27, 2008 5:24 pm 
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STG wrote:
you'll always need a secondary circuit to limit the dispersion of tritium and other diffusive radioactive substances, which will always give a loss on delta T. Furthermore a lof of the MSBR original safety concepts relied on the heat rejection by water boiling, because it's the most fail safe removal method. So you'll always need a water source...


I don' t know if it' s really paramount, according to the Fuji doc:
http://www.energyfromthorium.com/pdf/Fu ... ES2007.pdf
"The gaseous fission-products such as Kr, Xe, and tritium, mostly produced from 7Li, can be
continuously removed from the fuel-salt during reactor operation. Therefore, the possibility of an
environmental release of radioactivity can be significantly decreased in an accident. Daily tritium
release could be reduced to less than 3x10 10 Bq (1 curie) per day in the FUJI "

Quote:
And not that I know that much about gas turbines, but I've never seen a concept that uses air. Mostly supercritical CO2 or He are foreseen as a working fluid. On the low temperature utilization I have to agree, but a lot of reactor concepts are foreseen for this...Besides I think that for some of these applications solar is as good as nuclear, as the countries which need this type of applications are normally quite sunny....


Maybe for desalination, certainly not for district heating, and even for desalination I have a lot of doubts, the process is very energy intensive and needs anymore a very energy density and non intermittent energy source

Air is to be used for cooling the power plants through small and low cost dry towers, not as working fluid for gas turbine, where helium or nitrogen should be the best choice

Quote:
Furthermore I worry about your financial impact. Because what you might lose on constructional costs, you might gain on your safety analysis...


My humble opinion is the safety issues for a Msr (at least for the thermal fluoride version, I can't say for the fast chloride version) are less and less stringent than those needed for an ordinary, well know light water or heavy water reactor, at least on a theoretical basis. What of this will show pratical, it' s all to be seen


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PostPosted: Dec 27, 2008 6:13 pm 
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Alex P wrote:
I don' t know if it' s really paramount, according to the Fuji doc:
http://www.energyfromthorium.com/pdf/Fu ... ES2007.pdf
"The gaseous fission-products such as Kr, Xe, and tritium, mostly produced from 7Li, can be
continuously removed from the fuel-salt during reactor operation. Therefore, the possibility of an
environmental release of radioactivity can be significantly decreased in an accident. Daily tritium
release could be reduced to less than 3x10 10 Bq (1 curie) per day in the FUJI "


I tend to know quite well how T is formed in the salt...For Xe and Kr I couldn't care less because those are huge atoms which have won't migrate that fast. T migration is a problem already described a lot and different techniques to preven diffusion trough heat exchangers are known. Purging of the primary salt with T could do in a small reactor, though I prefer to keep T emission as low as possible.

Alex P wrote:
Therefore, the possibility of an environmental release of radioactivity can be significantly decreased in an accident.


Depends on the type of accident.

Alex P wrote:
My humble opinion is the safety issues for a Msr (at least for the thermal fluoride version, I can't say for the fast chloride version) are less and less stringent than those needed for an ordinary, well know light water or heavy water reactor, at least on a theoretical basis. What of this will show pratical, it' s all to be seen


I still doubt that...a first look could show that it's like that. And i fear for some effects. But your tought is the same as in the early days of PWR technology...

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PostPosted: Dec 27, 2008 7:25 pm 
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Alex P wrote:
I don' t know if it' s really paramount, according to the Fuji doc:
http://www.energyfromthorium.com/pdf/Fu ... ES2007.pdf
"The gaseous fission-products such as Kr, Xe, and tritium, mostly produced from 7Li, can be
continuously removed from the fuel-salt during reactor operation. Therefore, the possibility of an
environmental release of radioactivity can be significantly decreased in an accident. Daily tritium
release could be reduced to less than 3x10 10 Bq (1 curie) per day in the FUJI "


The ORNL reports show 18-19% of the T is captured by the He bubbling system. The remainder travels through the heat exchanger walls. The salt chosen for the secondary heat loop is selected expressly to capture and hold the T (rather than let it travel into the turbine gas). The trapped T is the released into a cover gas above the secondary salt and sequestered. Overall, they expected release of 0.2% of the T to the atmosphere.


STG wrote:
I still doubt that...a first look could show that it's like that. And i fear for some effects. But your tought is the same as in the early days of PWR technology...


The safety analysis I have seen include loss of pumps, injection of U233 instead of Th232, a leak between the fuel and blanket salts, and a leak between the fuel and secondary salts. Assuming you are talking about public safety (worker safety needs a much more detailed design before even a first cut analysis can be done) are there other issues you have concern with? (The one I missed in the safety analysis was decay heat dissipation).


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