Energy From Thorium Discussion Forum

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PostPosted: May 11, 2011 1:05 pm 
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Okay, so that would be several meters. That seems no problem, more thermal buffer capacity, and NaF-BeF2 isn't too pricey. The pool can be used for temporary spent fuel cooling.

A deep pool does mean the fuel rods will have to have a very long non-active portion if they are to reach out of the top liquid level to vent to the hot cell directly...


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PostPosted: May 11, 2011 1:29 pm 
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Cyril R wrote:
Okay, so that would be several meters. That seems no problem, more thermal buffer capacity, and NaF-BeF2 isn't too pricey.

EXACTLY !! ....no way you could do that with FLiBe !

Cyril R wrote:
A deep pool does mean the fuel rods will have to have a very long non-active portion if they are to reach out of the top liquid level to vent to the hot cell directly...

Why ?
What's wrong with a few small bubbles coming out of the top (non-active) portion of SHORT fuel rods ??


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PostPosted: May 11, 2011 1:52 pm 
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Okay so that's what you meant. So there would have to be simple valves on the fuel rods, not directly open to the hot cell. The pressure in the rods has to be slightly above the 'fluoridostatic pressure' (ugh!) or there will be salt ingress into the rods. But then maybe that's no big deal with active carbon plugs in between the valve and the fuel.

There would be an extra benefit of molten salt scrubbing of trace I, Br and Cs that make it through the active carbon filter. But perhaps there would also be some Xenon and to a greater extent, Krypton, that would end up sticking with the fluoride melt so grabbing neutrons, that's why I thought you meant to have the rods sticking above the liquid level.


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PostPosted: May 11, 2011 1:55 pm 
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I'm also wondering how to deal with the excess reactivity for a deep burn. Non actinide poisons seem a waste. Is the Pu-240 in the spent nuclear fuel start plutonium enough 'burnable poison' to make this a small issue?


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PostPosted: May 11, 2011 2:32 pm 
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Cyril R wrote:
I'm also wondering how to deal with the excess reactivity for a deep burn.

With a good fuel shuffling strategy, I think this would be an excellent breeder -- very little excess reactivity required ! (like SFRs)
If by "deep burn" you mean going far beyond IFR fuel burnup levels without reprocessing, then it might be a problem.
The whole point of metal fuel is to make quick & easy reprocessing -- in some ways easier than fluoride fuel.


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PostPosted: May 11, 2011 3:45 pm 
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Salt seeking fission product capture on deep burn shouldn't be too bad, ORNL's DMSR only lost about 5% of the neutrons to fission product after 10 full power years.

One processing method that might work well without messing with a lot of chemistry is to just heat the metal up in an atmospheric one stage still at very high temperature, perhaps 2000 Celcius tungsten still. Boil off most fission products, leaving actinides in the still bottoms to be used again. Promethium, neodymium and praseodymium will stick with the actinides but not much else that is neutron hungry. Sm, Eu, Rb, Sr, Cs, I, Br, Kr, Xe, Sb, Se, Cd, Ba and Te can be removed this way. 2000 degrees C might be ok if its a simple atmospheric operation pot and pan in argon atmosphere. What do you think?


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PostPosted: May 11, 2011 4:15 pm 
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Sounds good to me - but I'm no chemist !
.....you need some of the real experts to chime in here.

The other thing to keep in mind is the fuel re-fabrication, post-reprocessing: It better not be a difficult-to-fabricate fuel form (despite some potential advantages).


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PostPosted: May 11, 2011 6:30 pm 
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I should have added that you should take advantage of the documentation of fuel (re-)processing for EBR-II & IFR -- much good work has been done on those projects ! ....the metal fuel commonality is compelling ! .....might even convince some of the pro-SFR die-hards to switch sides 8)


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PostPosted: May 12, 2011 2:59 am 
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Yes, that's also where I got the sodium-bonded metal fuel concept from. And some inspiration from Charles Forsberg’s paper on liquid salt cooled fast reactors:

http://arss.ornl.gov/reactor_design/ica ... _Paper.pdf

His conclusion is that up to 45% cost reduction could be possible with liquid salt coolant as compared to sodium using the S-PRISM as baseline design. Impressive!

Thorium’s high melting point means you need a higher temperature crucible for the injection molding step, otherwise no problems were observed when it was tested in the EBR-II.


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PostPosted: May 12, 2011 7:56 am 
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Hmm, the power density of the LMFBR is around 500 MW thermal/cubic meter. That seems a bit excessive.

Even with 250 MW thermal/cubic meter and a 20 cubic meter core (quite small) this gets you 5000 MW thermal, 2300 MWe. Wow!

How much are we going to lose to protactinium capture with these kind of power densities, even with fuel shuffling? I’d like to avoid too much out of core fuel sitting idle half a year waiting for protactinium to decay, because plutonium is not cheap.


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PostPosted: May 15, 2011 6:35 pm 
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Cyril R wrote:
How much are we going to lose to protactinium capture with these kind of power densities, even with fuel shuffling? I’d like to avoid too much out of core fuel sitting idle half a year waiting for protactinium to decay, because plutonium is not cheap.

Suppose you arrange your core similar to the way one would a "two-fluid" MSR -- with driver fuel and breeding fuel segregated in separate zones ?
For example, the Th fertile material could be in several rows of assemblies surrounding the fissile-laden inner core.
The inner rows of Th rods could be shuffled to the outer, reduced flux rows, on a prescribed schedule.
The inner core fertile might consist of only U238, while the outer would have only enough to denature the bred U233.
Sound reasonable ?


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PostPosted: May 16, 2011 9:42 am 
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Good, except that the outer core won't have U238. CANDUs make weapons grade plutonium and this is allowed because there is no seperation from other actinides. Similarly U233 would be allowed because it will always stay with thorium.

Possibly the inner core wouldn't have any U238, either; it can be started with spent nuclear fuel plutonium as part of the waste eating/thorium economy transitioning objective. Though the seed could also be started as LEU metal fuel, thorium has better thermal and physical properties.


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PostPosted: May 16, 2011 9:52 am 
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It appears Magnox reactors, that use metal fuel, reprocess over 80% fissile content plutonium.

http://www.world-nuclear.org/info/inf15.html

Perhaps the UK would be a good place to build this reactor, with their metallic fuel experience.


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PostPosted: May 16, 2011 2:26 pm 
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Cyril R wrote:
Good, except that the outer core won't have U238. CANDUs make weapons grade plutonium and this is allowed because there is no seperation from other actinides.

First of all, CANDUs don't make weapons grade plutonium as part of the normal SNF discharge stream: http://www.nuclearfaq.ca/cnf_sectionF.htm#x2

Secondly, not only is there no separation (or reprocessing, as implied in MSR or IFR-style fuel management), but the entire fuel handling chain is designed & built so as to make such processing extremely difficult without major plant re-engineering, which in turn is not allowed under IAEA-monitored nonproliferation agreements.

Consequently, it seems to me that if the intention is to include fuel processing, then the Th should include U238 for fissile denaturing.
Unless I misunderstood something, I don't see how you hope to get around that political constraint....

Cyril R wrote:
It appears Magnox reactors, that use metal fuel, reprocess over 80% fissile content plutonium.

Magnox SNF Pu is somewhat closer, but still not quite weapons-grade (moreover, no-one is marketing Magnox reactors - they are being phased out).


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PostPosted: May 17, 2011 2:37 am 
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Seperation of uranium would be very difficult without 'major plant engineering' in this molten salt cooled reactor.

Plus, while SNF candu fuel is less weapons grade, candus can shuffle fuel around, so you can just pretend that some fuel is defect after low burnup, pull it out and get the good plutonium out.

Anyone with the capability of removing uranium would be left with very good bomb grade plutonium and few fission products. So you've got a proliferation path right there, and Pu-239 is far better (proven) weapons grade material than U233.


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