Energy From Thorium Discussion Forum

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PostPosted: Sep 12, 2010 9:36 pm 
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bwebb wrote:
I have performed the pressure drop calculations and they are indeed less than 1 psi as presented in our public presentation. As reported in the natural circulation literature, the primary flow rate is dependent on the elevational head between the centers of the reactor core and the steam generator. Further, much time and analysis has been spent on mitigating form and friction pressure losses within the system and its components. NuScale has several patent applications in process, so I can not elaborate at this time.

NuScale has several engineering positions currently open for thermal mechanical engineers.

I am interested in thorium fuel and how it maybe used in our reactor system. I would like to discuss its potential benefits.



Brent,

Sorry about this thread going way off topic, this happens often in any discussion group (please talk about fluorination elsewhere guys...)

As I mentioned finding expert advice on using thorium within PWRs might be hard on this site but you never know who might wander by and read things. Thorium in PWRs is a good way to modestly improve uranium utilization which might be very important for you work since obviously if you stick to regular PWR UO2 fuels you are going to need a significant amount more uranium per GWe-year (mainly with your tiny cores and increased neutron leakage). This could be a simple way to bring your numbers more back in line with the big core PWRs.

I wonder as well if thorium might be a way to lower your starting inventory of U235. Thorium has a larger thermal neutron cross section that U238 but in most cases I've studied, the average effective cross section is lower than U238 because of U238's stronger resonance cross sections. Thus a mix of LEU and thorium in an otherwise identical fuel pin might actual require less U235 to have the same net reactivity (just thinking out loud here). ThO2 is also well known to be superior to UO2 for a wide range of thermal and stability properties.

David LeBlanc


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PostPosted: Sep 12, 2010 10:25 pm 
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David wrote:
[ I wonder as well if thorium might be a way to lower your starting inventory of U235. Thorium has a larger thermal neutron cross section that U238 but in most cases I've studied, the average effective cross section is lower than U238 because of U238's stronger resonance cross sections. Thus a mix of LEU and thorium in an otherwise identical fuel pin might actual require less U235 to have the same net reactivity (just thinking out loud here). ThO2 is also well known to be superior to UO2 for a wide range of thermal and stability properties.

David LeBlanc


Of course the issue with this is that you'll need higher uranium enrichment, which will cost you. And with less capture in the Th than the U238, you produce less fissile. I think that you'll have to do the detailed physics to see if this will work out.

This paper will give you some idea of how the seed-blanket concept works:

http://www.iaea.org/inisnkm/nkm/aws/fnss/fulltext/te_1319_11.pdf

The uranium enrichment in this case is 20%. I have seen other studies where it's 10%.

There's also some information in this standard IAEA document:

http://www-pub.iaea.org/mtcd/publications/pdf/te_1450_web.pdf


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PostPosted: Sep 12, 2010 11:26 pm 
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Brent, Lighbridge developed seed/blanket configuration for PWRs. The uranium savings are about 10% natural uranium, so rather modest. There are other advantages though.

See http://www.ltbridge.com/assets/Technical_Fact_Sheet.pdf and
http://www.ltbridge.com/technologyservi ... itsthorium


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PostPosted: Sep 12, 2010 11:46 pm 
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Cyril R wrote:
Owen T wrote:
Cyril R wrote:
...
Isn't crystal growing a very slow process? Wouldn't you need a huge bath with numerous crystal seeds?


Yes, it's pretty slow. But the rate of U production is quite slow, too. It seems like a pretty straightforward process (after you finish tearing your hairs out debugging it) consisting of simple mechanical manipulation. Assuming it can be made to work I think it should be an attractive option. I believe the initial concentration of the material should be relatively high for this to work well so it should be used on a distillation fraction rather than the whole liquid.

It could work for other materials too like recovering thorium in a single fluid reactor.


Oh, so that would be for the still bottoms, ok. Something funny will probably happen to the different salt compositions when you chill it. For example, cooling FLiBeU just above freezing point gets you a 89 at% LiF-BeF2 phase, and a seperate 11 at% LiF-UF4 phase. Maybe we can use this to our advantage, as pre-reprocessing, getting the volumous LiF-BeF2 phase out with a centrifuge or something. I bet if there are a lot of different fission product fluorides things will get very complicated, though. But any reprocessing scheme that doesn't involve any chemistry change I find very interesting.


In the case of thorium recovery I think it may be necessary to use very high temperature distillation to further fractionate what would be "still bottoms" in the regular vacuum distillation scheme before attempting any kind of trick like this.


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PostPosted: Sep 13, 2010 12:28 am 
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Let's continue the vacuum distillation discussions in the reprocessing section.


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PostPosted: Sep 13, 2010 8:34 am 
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I think that Thorium as the fertile in addition to U238 could improve the burn-up significantly and maintain reactivity for a much longer fuel cycle. 232Th has twice the capture thermal cross section of 238U so it will convert twice as quickly. The sacrifice in U238 fast fission is a minor effect. A lower power density in the core will reduce the Proactinium losses without the need to process. Lower power density also makes passive safety systems much less expensive.

The quest for higher power densities has led to the unintended consequence that accidents can happen very quickly. I think the LFTR strategy of higher temperatures, high power densities, and complex, continuous processing to maximize breeding is naive. Solid fuel with a zirconium cladding that provides a barrier to fission product release and becomes a permanent waste receptacle is a very practical design. Eliminating that fission product barrier and circulating molten salt which solidifies at high temperatures will require an enormous engineering and licensing effort to go commercial. I believe LFTR is the solution to a problem that doesn't exist. We will not run out of uranium before the existing plants finish their useful life and existing plants are far safer than any other energy source. The only problem with nuclear now is the retroactively designed safety system costs.

Core design is very sophisticated and well understood now and I'm sure there are ways that Thorium could supplement typical LWR fuel. However, the existing LWR's have been optimized around LEU UO2 fuel at a pretty fast burn-up and there just isn't enough to save at current uranium costs to justify all the engineering and licensing work to add Thorium. For a new reactor design like NuScale's, Thorium might be worth considering.


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PostPosted: Sep 13, 2010 8:39 am 
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ondrejch wrote:
Brent, Lighbridge developed seed/blanket configuration for PWRs. The uranium savings are about 10% natural uranium, so rather modest. There are other advantages though.

See http://www.ltbridge.com/assets/Technical_Fact_Sheet.pdf and
http://www.ltbridge.com/technologyservi ... itsthorium



Wow, I had not noticed that fact sheet before, just full of info. I knew the once through seed and blanket didn't save too much uranium but didn't realize it was such a tiny amount (10%). This is a negligible money saver and would it in fact be canceled out by the modest increase in enrichment SWUs? Less to enrich but going to almost 20% will likely be more SWUs in total (anyone know for sure?).

So the biggest reasons are a gain in proliferation resistance (poorer Pu isotopes) and less volume and long term radiotoxicity of the spent fuel (substantial for the last two items). This is of course by higher burnups which come at their own price of taking much longer to certify the fuel.

Hmmm, I wonder how much fissile U they end up throwing out and what percentage it is at discharge (i.e. the U233+U235 content as a percent of U). Reprocessing by THOREX would be a huge expense but I wonder how viable fluoride volatility of the spent solid fuel would be to recover this uranium resource, perhaps it would be reactive enough to use in PWRs directly? Similar to how PWR spent fuel uranium can be used in CANDUS. (Ooops, I'm talking about fluorination now)

David L.


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PostPosted: Sep 13, 2010 9:21 am 
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Quote:
I think that Thorium as the fertile in addition to U238 could improve the burn-up significantly and maintain reactivity for a much longer fuel cycle. 232Th has twice the capture thermal cross section of 238U so it will convert twice as quickly.


Quote:
The quest for higher power densities has led to the unintended consequence that accidents can happen very quickly. I think the LFTR strategy of higher temperatures, high power densities, and complex, continuous processing to maximize breeding is naive.


Moliterno,


Yes thorium can increase burn up but the "effective" cross section of thorium is almost identical to uranium in a PWR spectrum (U238 absorbs epithermal neutrons much more than Th). The Radkowsky Seed and Blanket method appears to have just about an identical U235 content as a PWR core because of this.

I won't debate your opinion that LFTR sucks, that is your opinion. The only thing I'll point out is that Molten Salt Reactors can take many forms so be careful with your blanket statements. Most designs are much lower power density than PWRs and some do not employ continuous fission product removal beyond diverting Xenon and Krypton that bubble out. PWRs and new innovations like NuScale would be a welcome alternative to more coal plants but a big hurdle is their large capital costs. Yes, we can't prove it yet as research funding is almost non-existent but all indications point a substantial ability for MSRs to drop capital costs.

David LeBlanc


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PostPosted: Sep 13, 2010 9:22 am 
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Enriching to 20% will increase the SWU's 44% versus typical 4% enrichment at typical tails concentration. Since you only use 90% of the uranium, the SWU's increase 30% overall. With current enrichment costs about equal to uranium costs, the total fuel cost impact will be less than 10% increase. If the fuel loading can be rearranged to lengthen the refueling cycle or eliminate it, this cost increase can be easily justified. I believe a lower power density core that does not need shuffling or on-site refueling could drastically lower overall cost. The additional first cost and interest cost of fuel that lasts 10 years instead of 5 would be more than offset by increasing availability by 5% and reducing refueling costs by 80%.


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PostPosted: Sep 13, 2010 9:33 am 
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David wrote:
Quote:
I think that Thorium as the fertile in addition to U238 could improve the burn-up significantly and maintain reactivity for a much longer fuel cycle. 232Th has twice the capture thermal cross section of 238U so it will convert twice as quickly.


Quote:
The quest for higher power densities has led to the unintended consequence that accidents can happen very quickly. I think the LFTR strategy of higher temperatures, high power densities, and complex, continuous processing to maximize breeding is naive.


Moliterno,


Yes thorium can increase burn up but the "effective" cross section of thorium is almost identical to uranium in a PWR spectrum (U238 absorbs epithermal neutrons much more than Th). The Radkowsky Seed and Blanket method appears to have just about an identical U235 content as a PWR core because of this.

I won't debate your opinion that LFTR sucks, that is your opinion. The only thing I'll point out is that Molten Salt Reactors can take many forms so be careful with your blanket statements. Most designs are much lower power density than PWRs and some do not employ continuous fission product removal beyond diverting Xenon and Krypton that bubble out. PWRs and new innovations like NuScale would be a welcome alternative to more coal plants but a big hurdle is their large capital costs. Yes, we can't prove it yet as research funding is almost non-existent but all indications point a substantial ability for MSRs to drop capital costs.

David LeBlanc



Please don't characterize my opinion that LFTR "sucks". It is a great concept and the less ambitious designs are very promising. I am just discouraged that so much effort on this site seems to be directed to trivial issues and very dangerous ideas that distracts us from real progress. Amateurs that promote fanciful ideas can really harm the promotion of nuclear energy in general and willingness to consider new designs in particular.


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PostPosted: Sep 13, 2010 11:24 am 
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Moliterno wrote:
Enriching to 20% will increase the SWU's 44% versus typical 4% enrichment at typical tails concentration. Since you only use 90% of the uranium, the SWU's increase 30% overall. With current enrichment costs about equal to uranium costs, the total fuel cost impact will be less than 10% increase. If the fuel loading can be rearranged to lengthen the refueling cycle or eliminate it, this cost increase can be easily justified. I believe a lower power density core that does not need shuffling or on-site refueling could drastically lower overall cost. The additional first cost and interest cost of fuel that lasts 10 years instead of 5 would be more than offset by increasing availability by 5% and reducing refueling costs by 80%.



That sounds about right. I think the Radkowsky claims of about equal fuel cycle cost take into account the savings on fuel fabrication (since they get longer burnups).

I'd agree that lower power density cores have good advantages. However the Nuscale approach claims to have the same power density as conventional PWR cores (just much smaller per unit and shorter core and overall pin lengths). I also agree that the savings from lower power densities could possibly off set the increased interest charges on your first core (i.e. roughly double the fissile content if you halve the power density). However when one starts talking 10 tonnes or more of U235 per GWe to start (versus about 3 tonnes years ago or 5 more recently with higher burnups) it gets hard to imagine a rapid and large fleet expansion without a huge increase in U mining and more importantly enrichment facilities.

David L.


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PostPosted: Sep 13, 2010 1:13 pm 
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David,

I agree with all you have said. I'm guessing that NuScale is proposing as little change as possible and that is why the power density is close to existing modern PWR's. However, it would be a simple change to double the core length and use even more standard fuel assemblies at half the rate of burn-up to double the refueling cycle. Also, it would give some systems twice as much time to respond to transients.

I suspect that Lightbridge could optimize a NuScale core design with Thorium that would reduce the cycle cost. But first, NuScale needs to reassure the regulators that they are not trying anything new except building passive safety features.


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PostPosted: Sep 14, 2010 9:17 am 
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Quote:
Please don't characterize my opinion that LFTR "sucks".


Sorry, point taken...

David L.


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PostPosted: Sep 14, 2010 10:20 am 
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David wrote:
U238 absorbs epithermal neutrons much more than Th

I'm not so sure that's true David: Rather, it seems that the U238 resonances are slightly shifted (towards lower energy) relative to Th, but otherwise they're pretty much the same.

In the graph I posted on another thread, this wasn't very clear, because of the log-log scale.
If you change the vertical scale to linear, you can see it much more clearly.
Also, the wider base of the first U238 resonance is again an optical illusion caused by the horizontal log scale.

Nevertheless, as you imply, this DOES make a difference for reactors that are not fully thermal spectrum -- including PWRs -- because much of the neutron flux ends up in these resonant absorbtion energies, instead of getting past them quickly, and into the low-absorbtion thermal region - especially for U238.
I recall you making reference to this effect on a number of occasions, as being responsible for the substantially better neutron economy of wide lattice pitch HW reactors like CANDU.


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PostPosted: Sep 14, 2010 10:51 am 
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Thanks Jaro. The pro-LWR way to think of it ; ) would be to say it makes sense to use mostly thorium as fertile rather than U238 in for a BWR or PWR. :lol:


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