Energy From Thorium Discussion Forum

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PostPosted: Apr 02, 2012 6:05 pm 
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So, I know that a MSR solves the Xenon absorption problem, but in one of Kirk's videos I remember seeing that Protactinium is a very absorptive of neutrons, could we be facing a big problem like Xenon in a LFTR if we don’t separate the Protactinium? I know that some people advocate not separating the Protactinium to help with the proliferation concerns is this really an issue, and what will it do to the neutron budget?


P.S. I am new to nuclear energy and don’t have an education in it so layman's terms would be much appreciated.


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PostPosted: Apr 03, 2012 3:10 am 
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Dr. Leblanc has looked at this in some detail. Charles Barton (who has recently been de-zombified) has a good post on it on his blog:

http://nucleargreen.blogspot.com/2011/0 ... ology.html


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PostPosted: Apr 03, 2012 5:39 pm 
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Thank you for the article Cyril, I found the numbers on neutron loses to be quite helpful in raping my head around the problem (it seems that Protactinium isn’t that extreme of a neutron absorber).

Quote:
Single Fluid design with a 3 day Pa removal time, Pa losses are 0.0017 out of 2.23 neutrons

Single Fluid design WITHOUT Pa removal, Pa losses are 0.05 out of 2.23

2 Fluid design with lots of blanket salt, NO Pa removal (ORNL 1467) 0.0079 out of 2.22 (0.36%)

2 Fluid design with less blanket salt but Pa removal (ORNL 4528) 0.0002 out of 2.22.


but if you could answer one last question regarding Pa separation.

Of the different designs available which one is preferred/better for dealing with Protactinium?


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PostPosted: Apr 03, 2012 6:32 pm 
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I believe it is liquid metal exchange. ORNL assumed LME was using bismuth as the metal. Their reports make it sound like extracting protactinium was easy compared to separating thorium or plutonium from the fission products.

I am a bit mystified in that the ORNL experimental results I find are pretty discouraging regarding LME with fluorides and bismuth but the plans for MSRE and MSBR both take it as a given and presume it works well.

I would also note that there are good results using LME with chlorides and aluminum. I would if switching from bismuth to aluminum would solve the problems.


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PostPosted: Nov 08, 2012 9:30 pm 
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Given all the talk of recovering the FLiBe from the fuel salt via distillation, would that work to remove the PaF4(?) from the blanket salt?

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PostPosted: Nov 08, 2012 11:02 pm 
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Not so well. Neither ThF4 nor PaF4 are particularly volatile and will stay in the still during distillation.
Second, you have to do this pretty fast for it to matter - less than 10 days to turn the whole salt.
Third, some designs have thorium in the fuel salt. In this case, the neutron flux is much lower and the neutron losses by capture on Pa are also lower.
Finally, the blanket salt isn't very expensive so by doubling the blanket salt you can cut the neutron losses to Pa capture in the blanket in half AND you also capture more of the neutrons before the hit the wall saving you both the neutrons and wear and tear on the wall.


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PostPosted: Nov 09, 2012 12:10 am 
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What I think you are saying is that when distilling the blanket salt, the FLiBe would come off first and the Th and Pa would stay in the bottoms. This sounds like a simple way to isolate the Pa for a while without having to greatly increase the FLiBe volume. After all, if there is going to be one still in the plant, why not two?

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PostPosted: Nov 09, 2012 4:24 am 
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KitemanSA wrote:
What I think you are saying is that when distilling the blanket salt, the FLiBe would come off first and the Th and Pa would stay in the bottoms. This sounds like a simple way to isolate the Pa for a while without having to greatly increase the FLiBe volume. After all, if there is going to be one still in the plant, why not two?


But then you also remove the Th, and you want to put it back into the blanket. Otherwise your blanket would be just FLiBe. You'd have to add new thorium fluoride to the still product quite rapidly. Perhaps it is feasible though.


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PostPosted: Nov 09, 2012 5:48 am 
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Given that Thorium is so dang cheap, keeping a bunch with the bred Pa and U233 should not be a significant expense. The dwell time off line can then be adjusted to assure the minimum desirable dusting of U232.

Hmmm. Given that one path to U232 production is the (n,2n) reaction, does the extra neutron help breed more U233?

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PostPosted: Nov 09, 2012 7:50 am 
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KitemanSA wrote:
Given that Thorium is so dang cheap, keeping a bunch with the bred Pa and U233 should not be a significant expense.


Yes, but you have to add new thorium in the FLiBe you've recovered, to go back in the blanket. I don't know it that's such a big issue, but adding thorium in these large amounts means adding it as the fluoride, which is solid, which is annoying.


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PostPosted: Nov 09, 2012 9:08 am 
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Cyril R wrote:
KitemanSA wrote:
Given that Thorium is so dang cheap, keeping a bunch with the bred Pa and U233 should not be a significant expense.
Yes, but you have to add new thorium in the FLiBe you've recovered, to go back in the blanket. I don't know it that's such a big issue, but adding thorium in these large amounts means adding it as the fluoride, which is solid, which is annoying.
Only until the dwell time has passed. At that time, the remaining bottoms (after fluoridation) can be returned to the blanket with the FLiBe. So basically, the extra Th needed is related to the concentration, the process rate, and the dwell time. It is a simple optimization routine.

The benefit may not outweigh the cost, in which case...

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PostPosted: Nov 09, 2012 3:36 pm 
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KitemanSA wrote:
Cyril R wrote:
KitemanSA wrote:
Given that Thorium is so dang cheap, keeping a bunch with the bred Pa and U233 should not be a significant expense.
Yes, but you have to add new thorium in the FLiBe you've recovered, to go back in the blanket. I don't know it that's such a big issue, but adding thorium in these large amounts means adding it as the fluoride, which is solid, which is annoying.
Only until the dwell time has passed. At that time, the remaining bottoms (after fluoridation) can be returned to the blanket with the FLiBe. So basically, the extra Th needed is related to the concentration, the process rate, and the dwell time. It is a simple optimization routine.

The benefit may not outweigh the cost, in which case...


The blanket turnover rate to the still is quite high, and the Pa holdup time must be quite large, to be effective.

I wonder how we can add large amounts of ThF4 powder to the returning FLiBe product. It´ll have to be some sort of mechanical contraption, it seems complicated.


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PostPosted: Nov 09, 2012 4:21 pm 
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I bet just making the blanket twice as large would be less costly than provisioning for so rapid processing of the blanket to help with isolating Pa.


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PostPosted: Nov 09, 2012 4:42 pm 
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As I said
KitemanSA wrote:
So basically, the extra Th needed is related to the concentration, the process rate, and the dwell time. It is a simple optimization routine.

The benefit may not outweigh the cost, in which case...
Until someone does the numbers in an understandable way, all bets are off! :mrgreen:

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PostPosted: Nov 10, 2012 6:02 am 
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I like the idea of the bigger blanket better. You get the same functional advantage in terms of Pa isolation. And there are added advantages, such as a large thermal store that you can use for decay heat absorption. I imagine some sort of PRACS running some fuel salt in tubes through the blanket, which is your passive heat exchanger for the core fluid. Then the core salt inventory can be very small.

Also, since the blanket has less neutron flux, it could probably use a NaF (Alkali/F something eutectic) cheap carrier salt without hurting the breeding too much. Thorium is cheap too. Such a blanket will also shield the gamma and neutron radiation very well, reducing shielding costs in the hot cell.


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